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Ligand design and the synthesis of reactive organometallic complexes of tantalum for dinitrogen activation Johnson, Samuel Alan 2000

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LIGAND DESIGN AND THE SYNTHESIS OF REACTIVE ORGANOMETALLIC COMPLEXES OF TANTALUM FOR DINITROGEN ACTIVATION by SAMUEL ALAN JOHNSON B. Sc. (Hon.), McMaster University, 1995 A thesis submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 2000 © Samuel Alan Johnson, 2000 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada Date DE-6 (2/88) A B S T R A C T The organometallic chemistry of tantalum with the macrocyclic [P2N2] ligand (where [P2N2] = PhP(CH 2 SiMe2NSiMe 2 CH2)2Ph)) and the acyclic tridentate [NPN] ligand (where [NPN] = PPh(CH2SiMe2NPh)2) is explored. The complex [P2N2]TaMe3 is accessed by the reaction of [P2N2]Li2(C 4 H 8 0 2 ) with TaMe3Cl2. Photolysis with an UV source produces [P2N2]Ta=CH2(Me), with the elimination of methane. Attempts to deprotonate {[P2N2]TaMe2}+ do not provide a chemical route to [P2N2]Ta=CH2(Me). In the presence of ethylene [P2N2]Ta=CH2(Me) is slowly converted to [P2N2]Ta(C2H4)Et, with [P2N2]Ta(C2H4)Me observed as a minor product. Pure [P2N2]Ta(C2H4)Et can be synthesized by the hydrogenation of [P2N2]TaMe3 in the presence of PMe3, followed by the addition of ethylene. Crystallographic and NMR spectral data indicate the presence of a (3-agostic interaction between the ethyl group and tantalum center in [P2N2]Ta(C2H4)Et. Partially deuterated analogues of [P2N2]Ta(C2H4)Et show a large isotopic perturbation of resonance for both the (3-protons and the a-protons of the ethyl group, indicative of an equilibrium between a (3-agostic and an a-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the 'H environments of the ethyl and ethylene ligands. The hydrogenation of [P2N2]TaMe3 affords the dinuclear Ta(IV) hydride ([P2N2]Ta)2(fl-H)4. This diamagnetic tetrahydride fails to react with many reagents including ethylene and carbon monoxide; however, upon addition of iodomethane, {([P2N2]Ta)2(|i-H)4}+T is produced as a paramagnetic green crystalline solid. Ab initio calculations using density functional theory were performed in an attempt to further understand the influence of the macrocyclic ligand in the bonding in these complexes. The reaction of [NPN]Li2(THF)2 with TaMe3Cl2 produces [NPN]TaMe3. The hydrogenation of [NPN]TaMe3 yields the tetrahydride ([NPN]Ta)2(|i-H)4. This hydride reacts with dinitrogen with the loss of H 2 to produce ([NPN]Ta(u:-H))2(|>V:r)2-N2), which contains N 2 bound in the unprecedented side-on end-on dinuclear bonding mode. A ii prelimary study of the reactivity of this complex with B(C6F5)3, AlMe3, HB(C6F 5) 2 , PhCH2Br, 9-BBN, C1B(C6F5)2 and PhO=CH is reported. iii TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES ix LIST OF FIGURES xiii GLOSSARY OF TERMS xxi ACKNOWLEDGEMENTS xxvii Chapter 1: The Activation of Dinitrogen and Ligand Design 1 1.1 Dinitrogen Chemistry 1 1.1.1 Introduction 1 1.1.2 The Nature of the Metal-Dinitrogen Bond 2 1.2 Early Transition Metal Dinitrogen Complex Formation and Reactivity 4 1.2.1 Synthesis of Dinitrogen Complexes 4 1.2.2 Reactivity of Dinitrogen Complexes 7 1.2.3 Toward the Catalytic Functionalization of Dinitrogen 11 1.3 Summary of Previous Work in the Fryzuk Laboratory 12 1.3.1 The [PNP] Hybrid Ligand Design 12 1.3.2 The [P2N2] Ligand Design 14 1.4 Side-On versus End-On Bonding 18 1.5 Scope of this Thesis 21 1.6 The Organometallic Chemistry of Tantalum 23 1.7 References 28 Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 40 2.1 Introduction 40 2.2 Attempted Synthesis of [P2N2]TaCl3 46 (i) Reaction of [P2N2]Li2-(C4H802) and [P 2N 2]Li r(C 4H 80) with TaCl5 46 iv (ii) Synthesis of the ether-free ligand precursor [P2N2]Li2 48 (iii) Synthesis of the less strongly reducing ligand precursor [P 2 N 2 ]Mg(C 4 H 8 0) 49 (iv) Synthesis of the less strongly reducing ligand precursor [P 2N 2]Zn 51 (v) Reaction of [P 2 N 2 ]H 2 with TaCl 5 54 2.3 Attempted Synthesis of [P 2N 2] Complexes of Low-Valent Tantalum Halides... .55 (i) Reaction of [P 2 N 2 ]Li 2 with Ta 2 Cl 6 (SC 4 H 8 )3 56 (ii) Reaction of [P 2 N 2 ]Li 2 with TaCl 4 (NC 5 H 5 ) 2 56 2.4 Synthesis and Characterization of [P2N2]TaMe3 57 (i) Synthesis and Structure of [P2N2]TaMe3 57 (ii) Variable Temperature N M R Spectroscopy of [P 2N 2]TaMe 3 (5) 60 2.5 Synthesis and Characterization of [P 2N 2]Ta=CH 2(Me) (6) 64 (i) Photolysis of [P 2N 2]TaMe 3 and Isolation of [P 2N 2]Ta=CH 2(Me) 64 (ii) Structure of [P 2N 2]Ta=CH 2(Me) (6) 66 (iii) Fluxional Processes in [P2N2]Ta=CH2(Me) 69 2.6 Synthesis and Deprotonation of {[P 2N 2]TaMe 2 }+X" (X = B F 4 \ B(C 6F 5) 4") 75 (i) Synthesis of {[P 2N 2]TaMe 2} +X" (X" = B F 4 \ B(C 6F 5) 4~) 76 (ii) Attempted Deprotonation of {[P2N2]TaMe2}+ BF 4 " with LiNPr 2 ' and NaN(SiMe 3) 2 78 (iii) Attempted Deprotonation of {[P 2N 2]TaMe 2} + X with Me 3 P=CH 2 (X" = B F 4 \ B(C 6F 5) 4") 80 (iv) Decomposition of {[P 2N 2]TaMe 2} + BF 4 " 81 2.7 Attempted In Situ Synthesis and Reaction of [P 2 N 2 ]TaCl 3 85 2.8 Summary and Conclusions 86 2.9 Experimental 86 2.9.1 General Procedures 86 2.9.2 Materials 87 2.9.3 Synthesis and Reactivity of Complexes 88 2.10 References 95 v Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex 100 3.1 Introduction 100 3.1.1 Introduction to Agostic Interactions 101 3.2 Reactivity of [P2N2]Ta=CH2(Me) (6) with Ethylene 102 (i) NMR Spectral Data for [P2N2]Ta(C2H4)Et 104 (ii) Isotopic Perturbation of Resonance 106 (iii) NMR Spectral Data for Carbon-13 Labeled [P2N2]Ta(1 3C2H4) l 3Et I l l (iv) NMR Spectral Data for [P2N2]Ta(C2H4)Me (13) 112 3.3 Alternate Synthesis and Structure of [P2N2]Ta(C2H4)Et (12) 113 3.4.Mechanism of the Reaction of Ethylene with [P2N2]Ta=CH2(Me) (6) 116 3.5 Exchange of Proton Environments in Ethyl Complex 12 121 3.6 Summary and Conclusions 123 3.7 Future Work 124 3.8 Experimental 128 3.8.1 General Procedure 128 3.8.2 Materials 128 3.8.3 Synthesis, Characterization and Reactivity of Complexes. 128 3.9 References 135 Chapter 4: Dinuclear Tantalum Hydrides Complexes 140 4.1 Introduction 140 4.2 Hydrogenation of [P2N2]TaMe3 141 4.3 Partial Hydrogenation of [P2N2]TaMe3 146 4.4 Reactivity of ([P2N2]Ta)2(p-H4) as a Reducing Agent 149 (i) Synthesis and Identification of {([P2N2]Ta)2(p-H)4}"T 149 (ii) Magnetism of {([P2N2]Ta)2(p-H)4}T 155 4.5 Bonding Considerations and Density Functional Theory Calculations 157 4.6 Summary and Conclusions 167 4.7 Experimental 168 4.7.1 General Procedures 168 vi 4.7.2 Materials 168 4.7.2 Synthesis and Characterization of Complexes 169 4.7.3 Calculations 171 4.8 References 172 Chapter 5: A New Bonding Mode for Dinitrogen 176 5.1 Introduction 176 5.2 Synthesis and Characterization of the [NPN] Ligand Precursor 180 5.3 Synthesis of [NPN]TaMe3 184 5.4 Hydrogenation of [NPN]TaMe3: Synthesis of ([NPN]Ta)2(u-H)4 186 5.5 Reaction of ([NPN]Ta)2(u-H)4 with Dinitrogen .188 5.5.1 Solid-State Molecular Structure of ([NPN]Ta(|l-H))2N2 (22) 190 5.5.2 Characterization of the Side-on End-on Bonding Mode by l 5 N NMR Spectroscopy 193 5.5.3 Significance of the Reaction of Hydride 21 with N 2 194 5.5.4 Bonding in the Side-on End-on Mode 196 5.6 Why Does Complex 22 Contain the Side-On End-On Bonding Mode 200 5.6.1 Reaction with Propene 200 5.6.2 Synthesis of ([NPN]NbCl)2(ji-T|l:Ti1-N2) 205 5.7 Reactivity of ([NPN]Ta(|i-H))2(|i-r|1:ri2-N2) (22) 208 5.7.1 Reaction of ([NPN]Ta(ji-H))2(M--tl1:T|2-N2) (22) with Lewis Acids 210 5.6.2 Synthesis of ([NPNlNbClMli-VV-^) 205 i) Reaction with B(C 6F 5) 3 210 ii) Reaction with AlMe3 213 5.7.2 Reaction of 22 with Benzylbromide 216 5.7.3 Reaction of 22 with Secondary Boranes 220 i) Reaction of 22 with 9-Borabicyclo[3.3.1]nonane 220 ii) Reaction with HB(C 6F 5) 2 224 5.7.4 Reaction of 22 with C1B(C6F5)2 227 5.8 Reaction of 22 with PW>CH 233 vii 5.9 Summary and Conclusions 237 5.10 Experimental 240 5.10.1 General Procedures 240 5.10.2 Materials 240 5.10.3 Synthesis and Characterization of Complexes 240 5.8 References 254 Chapter 6: Future Work 260 6.1 Introduction 260 6.2 Further Study of the Side-on End-on Bonding Mode of Dinitrogen 261 6.3 Further Reactivity of Dinitrogen Complex 22 262 6.4 Further Reactivity of Functionalized Derivatives of Complex 22 265 6.5 Organometallic Tantalum Chemistry 266 6.6 Experimental 277 6.6.1 General Procedures 277 6.6.2 Materials 277 6.6.2 Synthesis and Characterization of Complexes 277 6.7 References 280 Appendix One: X-ray Crystal Structure Data 283 Appendix Two: Density Functional Theory Calculation Data 327 viii List of Tables Table Title Page Table 1.1 N-N bond lengths and stretching frequencies for N-N triple, 4 double, and single bonds, as well as a selection of dinitrogen compounds. Table 2.1 Early transition metal halide complexes of the [P2N2] ligand 41 prepared to date in the Fryzuk laboratory. Table 2.2 Selected bond lengths, angles and dihedral angles in [P2N2]Zn (3). 53 Table 2.3 Selected bond lengths, angles and dihedral angles in [P2N2]TaMe3 59 (5). Table 2.4 Selected bond lengths, angles and dihedral angles in 68 [P2N2]Ta=CH2(Me) (6). Table 3.1 Selected bond lengths, angles and dihedral angles for 116 [P2N2]Ta(C2H4)Et (12). Table 3.2 Selected bond lengths, angles and dihedral angles for complex 17. 126 Table 4.1 Bond lengths, angles, and dihedral angles for complex 14. 143 Table 4.2 Bond lengths, angles, and dihedral angles for complex 18. 154 Table 4.3 Selected bond lengths and energies for the ab initio DFT geometry 161 optimizations of 14A. Table 5.1. Selected bond lengths and angles for [NPN]Li2(C4H80)2 (19). 183 Table 5.2. Selected bond lengths and bond angles for [NPN]TaMe3 (20). 185 Table 5.3. Selected bond lengths, bond angles, and dihedral angles for 191 ([NPN]TaH)2N2 (22). Table 5.4. Selected bond lengths, bond angles and dihedral angles for the ab 197 initio DFT geometry optimization of the model complex [(H3P)(H2N)2Ta(|i-H)]2(Li-ri1 :r,2-N2) (22A). Table 5.5. Selected bond lengths, bond angles and dihedral angles for 203 ([NPN]Ta(CH2CH2CH3))2(|i-r|1 :rj '-N2) (23). ix Table 5.6. Selected bond lengths and bond angles for ([NPN]NbCl)2(u-r|1 :r\1 - 207 N2) (24). Table 5.7. Selected bond lengths and bond angles for ([NPN]Ta(p- 212 H))2N2B(C6F5)3 (25). Table 5.8. Selected bond lengths and angles for ([NPN]Ta(p-H))2(N2)AlMe3 215 (26). Table 5.9. Selected bond lengths and angles for 220 [NPN]Ta(u-r|1 :ri2-N2CH2Ph)(p-H)2TaBr[NPN] (27). Table 5.10. Selected bond lengths and bond angles for 226 ([NPN]Ta(p-H))2N2BH(C6F5)2 (29). Table 5.11. Selected bond lengths and bond angles for 231 ([NPN]Ta)2(p-H)(|i-Cl)(N2BH(C6F5)2 (31). Table 5.12. Selected bond lengths and bond angles for ([NPN]Ta(p-H))2(p- 236 PhCCH) (32). Table 5.13. Summary of 1 5 N NMR spectral data. Chemical shifts are 239 referenced relative to nitromethane by setting an external I 5 NH4C1 reference in D 2 0 at -352.9 ppm. The J N p value refers to the coupling of N(5) to the frans-disposed phosphine. The abbreviation Ta2(p-N2) refers to dinitrogen complex 22. Table 6.1. Selected bond lengths, bond angles and dihedral angles for 269 complex 16. Table 6.2. Selected bond lengths, bond angles and dihedral angles for 275 complex 35. Table A l . l . Crystal data and structure refinement for [P2N2]Zn (3), 283 [P2N2]TaMe3 (5) and [P2N2]Ta=CH2(Me) (6). Table A1.2. Crystal Data and Structure Refinement for ([P2N2]Ta)2(u-H)4 (14) 284 and {([P2N2]Ta)2(p-H)4}+r (18). x Table A1.3. Crystal Data and Structure Refinement for [P2N2]Ta(C2H4)Et (12) 285 and 17. Table A1.4. Crystal data and structure refinement for [NPN]Li2(C4H80)2 (19), 286 [NPN]TaMe3 (20), and ([NPN]Ta(uJ-H))2(|i-ril:ri2-N2) (22). Table A1.5. Crystal data and structure refinement for ([NPN]TaPr')2(pi-'ql:ri1- 287 N2) (23), ([NPN]NbCl)2(uxn^V-N2) (24), and ([NPN]Ta(p-H))2(|LL-r|1 :r)2-N2B(C6F5)3) (25). Table A1.6. Crystal data and structure refinement for ([NPN]Ta)2(p-H)2(p> 288 V:r| 2-N 2)-AlMe3 (26), [NPN]Ta(|i-ri1:r|2-N2CH2Ph)(fi-H)2TaBr[NPN] (27), and ([(NPN]2TaH)2(p-N2(HB(C6F5)2) (29). Table A1.7. Crystal data and structure refinement for ([(NPN]Ta)2(p-H)(p-Cl) 289 N2(HB(C6F5)2) (31) and ([NPN]Ta)2(p-H)2(p-PhCCPh) (32). Table A1.8. Crystal data and structure refinement for [P2N2]TaH3(PMe3) (16), 290 [P2N2]Ta(K3(CPP)-0-C6H4PP(C6H5)) (35). Table A1.9. Positional parameters and B(eq) (A 2 ) for [P2N2]Zn (3). 291 Table A L I O . Positional parameters and B(eq) for [P2N2]TaMe3.0.5(C6H14) (5). 293 Table A l . l l . Positional parameters and B(eq) for [P2N2]Ta=CH2(Me) (6). 294 Table A1.12. Positional parameters and B(eq) for [P2N2]Ta(C2H4)Et (12). 295 Table A1.13. Positional parameters and B(eq) for 17. 296 Table A1.14. Positional parameters and B(eq) for ([P2N2]Ta)2(u-H)4 (14). 297 Table A1.15. Positional parameters and B(eq) for {([P2N2]Ta)2(p-H)4}+r (18). 299 Table A1.16. Positional parameters and B(eq) for [NPN]Li2.(C4H80)2 (14). 300 Table A1.17. Positional parameters and B(eq) for [NPN]TaMe3 (20). 301 Table A1.18. Positional parameters and B(eq) for ([NPN]Ta(p-H))2(N2) (22). 302 Table A1.19. Positional parameters and B(eq) for ([NPN]TaPr')2(|i-N2) (23). 304 Table A1.20. Positional parameters and B(eq) for ([NPN]NbCl)2(p-N2) (24). 306 xi Table A1.21. Positional parameters and B(eq) for ([NPN]Ta(p:-H))2(u- 307 N 2B(C 6F 5) 3 (25). Table A1.22. Atomic coordinates (xlO4) and equivalent isotropic displacement 312 parameters (x 103) for ([NPN]Ta(u-H))2(N2AlMe3) (26). Table A1.23. Positional parameters and B(eq) for [NPN]Ta(|^-r|l:'ni2- 315 N2CH2Ph)(u-H)2TaBr[NPN] (27). Table A1.24. Atomic coordinates (xlO4) and equivalent isotropic displacement 317 parameters (x 103) for ([NPN]Ta(u-H))2(N2(HB(C6F5)2) (29). Table A1.25. Positional parameters and B(eq) for ([NPN]Ta)9(u-H)(u- 320 C1)(N2BH(C6F5)2) (31). Table A1.26. Positional parameters and B(eq) for ([NPN]Ta(u-H))2(u-PhCCH) 322 (32). Table A1.27. Positional parameters and B(eq) for [P2N2]TaH3(PMe3) (16). 324 Table A1.28. Positional parameters and B(eq) for [P2N2]Ta(K3(CPP)-o- 326 C6H4PP(C6H5)) (35). Table A2.1. Method, Z-matrix and initial parameters for model complex 14A. 327 Table A2.2. Final Parameters (A and degrees) and Energy (Ha) for Model 328 Complex 14A. Table A2.3. Method, Z-matrix and initial parameters for model complex 22A. 328 Table A2.4. Final Parameters (A and degrees) and Energy (Ha) for Model 330 Complex 22A. xii Figure Figure 1.1 Figure 1.2 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 List of Figures Title The most common mononuclear and dinuclear bonding modes observed for dinitrogen, where M is a transition metal. Lines between atoms are meant to approximate bond order; no account of valencies, formal charges or oxidation states is implied. Approximate orbital overlaps involved in the bonding of dinitrogen to metals (M) in the side-on dinuclear (top) and end-on dinuclear (bottom) modes. ORTEP depiction of the solid-state molecular structure of complex 3, [P2N2]Zn. ORTEP representation of the solid-state molecular structure of [P2N2]TaMe3, 5. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. Variable-temperature 3IP{ 'H} NMR spectra of [P2N2]TaMe3, 5. Eyring plot for the exchange of ~ P environments in [P2N2]TaMe3, (5) (R2= 0.9914). Depiction of the fluxional behaviour of capped trigonal-prismatic 5 in solution. ORTEP representation of the solid-state molecular structure of [P2N2]Ta=CH2(Me), 6. Hydrogen atoms and silyl methyl groups are omitted and only the ipso carbon of the phenyl rings attached to the phosphine donors are shown. The unshaded thermal ellipsoids of the disordered methyl and methylene group indicate that these atoms were refined with only isotropic thermal parameters. Eyring plot for the exchange of silyl methyl environments in [P2N2]Ta=CH2(Me) (6). Page 3 19 53 59 61 62 64 67 71 xiii Figure 2.8 The effect of Gaussian multiplication on the Ta=CH2 (left) and Ta- 73 CH 3 (right) resonances in the 500 MHz 'H NMR spectrum of 6 at 300K. The uppermost traces were processed using an exponential decay function. The lower traces were processed using Lorentz-Gaussian enhancement and clearly show the small 1.2 Hz coupling between these signals, as well as larger coupling to 3 1 P. Figure 2.9 The effect of temperature on the methylidene resonance in the 500 74 MHz 'H NMR spectrum of [P2N2]Ta=CH2(Me), 6. Figure 2.10 Representations of two possible orientations of the Ta=CH2 double 75 bond in [P2N2]Ta=CH2(Me), 6. Competition with the it-donor amido ligands for the dxy orbital disfavours B . Figure 2.11 Fluxional processes in [P2N2]TaMe2F, 9. 79 Figure 3.1 Depiction of the three-centre two-electron bonding model for an 102 ethyl group with a [3-agostic interaction (left) and the half-arrow convention used to depict such an interaction (right). Figure 3.2 Depiction of the fluxional nature of the (3-agostic interaction that 106 results in the C s symmetry of complex 12 on the NMR timescale. The intermediate species shown here is drawn with an oc-agostic interaction. Figure 3.3 The ethyl region of the 'H NMR (500 MHz, 295 K) spectrum of 108 [P2N2]Ta(C2H4)Et, 12, before (top) and after (bottom) C 2 D 4 was added. Figure 3.4 The ethyl region of the ! H NMR (500 MHz, 255 K) spectrum of 110 [P2N2]Ta(C2H4)Et (12) in which the ethyl group is -75% deuterated. Figure 3.5 ORTEP diagram of the solid-state molecular structure of 115 [P2N2]Ta(C2H4)Et, 12, as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. xiv Figure 3.6 Two possible mechanisms for the formation of [P2N2]Ta(C2H4)Me, 117 13. The starred carbon atoms in the products illustrate where carbon-13 labels would appear in the products if the reaction was performed with " C 2 H4. Figure 3.7 Two possible reaction mechanisms for the formation of 119 [P2N2]Ta(C2H4)Et (12) from the reaction of ethylene with [P2N2]Ta=CH2(Me). Figure 3.8 Depiction of the mechanism believed to exchange the proton 122 environments of the ethyl and ethylene ligands of species 12. Figure 3.9 A portion of the EXSY spectrum (x = 0.4 seconds) of 123 [P2N2]Ta(C2Fi4)Et, illustrating four cross-signals between the ethyl and ethylene ligands, and cross-signals between pairs of silyl-methyl environments. Figure 3.10 ORTEP depiction of the solid-state molecular structure of 17 as 125 determined by X-ray crystallography. Figure 4.1 ORTEP diagram of the solid-state molecular structure of 142 ([P2N2]Ta)2(p-H)4 (14) as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. Figure 4.2 Depiction of "staggered by 90°" and "eclipsed" ligand 146 arrangements. Figure 4.3 Cyclic voltammogram of ([P2N2]Ta)2(p-H)4 (14). 152 x v Figure 4.4 ORTEP diagram of the solid-state molecular structure of the 154 cationic fragment of {([P2N2]Ta)2Qi-H)4}+r (18) as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. The bridging hydride ligands were not located. Figure 4.5 Measured molar magnetic susceptibilities, %, (A) and effective 156 magnetic moments, ji^ ff, (o) versus temperature for {([P2N2]Ta)2(u-H)4}+r (18). Figure 4.6 A plot of the effective magnetic moment (jxeff) versus temperature 157 for compound 18 from 2-50 K. The circles represent the data points and the bold line a model using the Curie-Weiss law with the parameters g = 1.86, TIP = 1670xl0"6emumor', and 0 = -0.11 K. Table 4.7 A depiction of the overlap of metal-based orbitals with the bridging 158 hydride ligands to generate delocalized molecular orbitals for complex 14 along with the appropriate symmetry labels for its respective point group (DT). Figure 4.8 Depiction of the metal-metal cj-bond interaction in the HOMO of 162 tetrahydride 14A. The PH 3 and NH 2 donors are labeled by P and N respectively. The frontmost NH 2 ligand on each tantalum centre obscures the view of the rear NH 2 ligand. Figure 4.9 Illustration of the amide lone-pair orbital overlaps for model 14A 163 when the H-N-Ta-Ta dihedral angle is 180° (a), and when the H-N-Ta-Ta dihedral angle is 90° (b), depicted in the N-Ta-N plane. The PH 3 donors and bridging hydrides are omitted. xvi Figure 4.10 Four possible linear combinations of the nitrogen lone-pair orbitals 164 in 14A, in D2 symmetry, depicted in the N-Ta-N plane, along with their symmetry labels. The P H 3 donors and bridging hydrides are not shown. Figure 4.11 Depictions of the molecular orbitals containing three of the four 165 possible linear combinations of the amido "lone-pair" p-orbitals for tetrahydride 14A. The front NH 2 groups and their jc-donor lone-pair orbitals obscure the view of the back NH 2 groups. Figure 4.12 Depiction of the overlap of the fourth symmetry combination of the 166 amido "lone-pair" p-orbitals with the metal orbitals involved in 8-bonding through the hydride ligands for tetrahydride 14A. Figure 4.13 Depiction of the two -^interactions between metal centres mediated 167 by the bridging hydrides ligands for the tetrahydride model complex 14A. Figure 5.1 Depiction of the anticipated effect on the vacant orbitals available 177 in the tetrahydride complex 14 upon removal a phosphine donor from the [P2N2] ligand. Figure 5.2 ORTEP depiction of the solid-state molecular structure of 183 [NPN]Li2-(C4H80)2 (19) as determined by X-ray crystallography. Figure 5.3 ORTEP depiction of the solid-state molecular structure of 185 [NPN]TaMe3 (20) as determined by X-ray crystallography. Figure 5.4 The 31P{'H} NMR spectrum (202.46 MHz) of the product of the 289 reaction of 21 under N 2 gas. xvii Figure 5.5 ORTEP depiction of the solid-state molecular structure of 191 ([NPN]TaH)2N2 (22) as determined by X-ray crystallography. The two bridging hydrides were not located. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.6 Depiction of an isosurface of the HOMO for model complex 22A 198 (left), and an illustration clarifying the metal-dinitrogen overlaps in the HOMO (right). Table 5.7 Depiction of an isosurface of the HOMO-1 for model complex 22A 198 (left), and an illustration clarifying the metal-dinitrogen overlaps in the HOMO-1 (right). Figure 5.8 Simplified depictions of the orbital overlaps in the two rj-bonding 199 interactions between the dinitrogen moiety and the tantalum centres in 22A. Figure 5.9 ORTEP depiction of the solid-state molecular structure of 203 ([NPN]Ta(CH2CH2CH3))2(M.-ril:ril-N2) (23) as determined by X-ray crystallography. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.10 ORTEP depiction of the solid-state molecular structure of 207 ([NPN]NbCl)2(li-ril:ri1-N2) (24) as determined by X-ray crystallography. Figure 5.11 Two simplified depictions of the bonding in 22. 209 xviii Figure 5.12 A depiction of the polynuclear titanium dinitrogen complex 210 reported by Pez et al. is labeled as A; the charge on this fragment is unknown and may be neutral or uninegative. Removal of a titanium fragment known to form a THF adduct produces the hypothetical fragment shown as B, in which the dinitrogen bonding mode resembles the bonding in complex 22. Figure 5.13 ORTEP depiction of the solid-state molecular structure of 212 ([NPN]Ta(p-H))2N2B(C6F5)3 (25) as determined by X-ray crystallography. The silyl methyl groups and fluorine atoms are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.14 ORTEP depiction of the solid-state molecular structure of 215 ([NPN]Ta(p-H))2N2AlMe3 (26) as determined by X-ray crystallography. The bridging hydrides were not located. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.15 ORTEP depiction of the solid-state molecular structure of 219 [NPN]Ta(p-ri':ri2-N2CH2Ph)(p-H)2TaBr[NPN] (27) as determined by X-ray crystallography. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. xix Figure 5.16 ORTEP depiction of the solid-state molecular structure of 226 ([NPN]Ta(p-H))2N2BH(C6F5)2 (29) as determined by X-ray crystallography. The bridging hydrides and the hydrogen bound to boron were not located. The silyl methyl groups and fluorine atoms are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.17 ORTEP depiction of the solid-state molecular structure of 231 ([NPN]Ta)2(p-H)(p-Cl)(N2BH(C6F5)2) (31) as determined by X-ray crystallography. The bridging hydride ligand was not located. The silyl methyl groups and fluorine atoms are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 5.18 ORTEP depiction of the solid-state molecular structure of 235 ([NPN]Ta(p-H))2(u-PhCCH) (32) as determined by X-ray crystallography. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Figure 6.1 ORTEP depiction of the solid-state molecular structure of 269 [P2N2]TaH3(PMe3) (16) as determined by X-ray crystallography. Figure 6.2 Variable-temperature 3IP{ 'H) NMR spectra (C 6D 6, 202.49 MHz) of 272 complex 34. Figure 6.3 ORTEP depiction of the solid-state molecular structure of 275 [P2N2]Ta(K3(CPP)-o-C6H4PP(C6H5)) (35) as determined by X-ray crystallography. xx GLOSSARY OF TERMS The following abbreviations, most of which are commonly found in the literature, used in this thesis. o A Angstrom Anal. analysis Ar aryl (or argon) B e q equivalent isotropic thermal parameter 9-BBN 9-borabicyclo[3.3.1]nonane, H B C 8 H I 4 B. M. Bohr magneton br broad Bu" n-butyl group, -CH2CH2CH2CH3 Bur tertiary butyl group, -C(CH3)3 1 3 C carbon-13 C Curie constant C a carbon atom in the a position Cp carbon atom in the (3 position Calcd calculated CCD charge coupled device COSY correlated spectroscopy (NMR experiment) cm centimetres Cp cyclopentadienyl ( C 5 H 5 ) Cp' substituted cyclopentadienyl groups, including C5H5 cryst crystal Cy cyclohexyl d doublet dd doublet of doublets ddd doublet of doublet of doublets dq doublet of quartets dt doublet of triplets deg (or °) degrees xxi dippe 1,2-bis(diisopropylphosphino)ethane °C degrees Celsius 2 D or D deuterium 1- D one dimensional 2- D two dimensional DFT density functional theory D M E 1,2-dimethoxyethane digly diglyme (CH 3OCH2CH20CH 2CH 2OCH3) dmpe l,2-bis(dimethylphosphino)ethane dn numbers of J-electrons dn n-deuterated dppe 1,2-bis(diphenylphosphino)ethane ECP effective core potential EPR electron paramagnetic resonance Et ethyl group, - C H 2 C H 3 eV electron Volt E X S Y exchange spectroscopy (2-D N M R spectrum) l 9 F fluorine-19 G Gauss G M Gaussian multiplication A G + Gibb's energy of activation g grams g electron g value GC-MS gas chromatography/mass spectrometry hv photon AH* enthalpy of activation ' H proton {lH} proton decoupled Ha Hartrees HOMO highest occupied molecular orbital xxii HOMO-n the molecular orbital that is n orbitals lower in energy than the HOMO Hz Hertz, seconds" I nuclear spin IPR isotopic perturbation of resonance IR infrared n T JAB n-bond scalar coupling constant between nuclei A and B k rate constant K Kelvin kJ kiloJoules 6 L i lithium-6 7 L i lithium-7 L neutral two-electron donor L B line broadening In natural logarithm L U M O lowest unoccupied molecular orbital M central metal atom (or molar, when referring to concentration) M + parent ion m meta m multiplet (NMR spectroscopy) mm millimetres m M millimolar Me methyl group, -CH3 mle mass/charge (mass spectrometry unit) Mes mesityl group, -2,4,6-Me3C6H 2 mg milligram(s) M H z megaHertz mL milliliter mmol millimole M O molecular orbital mol mole xxiii MS mass spectrometry nm nanometres no. number 1 4 N nitrogen-14 l 5 N nitrogen-15 N M R nuclear magnetic resonance [NPN] diamidophosphine ligand PhP(CH 2SiMe 2NPh) 2 o ortho ORTEP Oakridge Thermal Ellipsoid Plotting Program p para p pentet 3 1 P phosphorus-31 Ph phenyl group, - C 6 H 5 R[PNP] amidodiphosphine ligand, N(SiMe 2 CH 2 PR 2 ) 2 [P 2N 2] diamidodiphosphine ligand, PhP(CH 2 SiMe 2 NSiMe 2 CH 2 )2PPh ppm parts per million Pr' isopropyl group, -CH(CH 3 ) 2 q quartet R hydrocarbyl substituent 9 R" coefficient of determination for a linear regression ROMP ring opening metathesis polymerization reflns reflections (X-ray crystallography) AS* entropy of activation SQUID superconducting quantum interference device s singlet sh shoulder (UV-visible spectroscopy) syst system t triplet tt triplet of triplets tq triplet of quartets 1 8 'Ta tantalum-181 xxiv T temperature in Kelvin or °C TIP temperature-independent paramagnetism Tp* hydrotris(3,5-dimethylpyrazolyl)borate THT tetrahydrothiophene (C 4H 8S) U(eq) equivalent isotropic displacement parameter (one third of the trace of the orthogohalized Ujj tensor). UV-VIS or ultraviolet/visible UV-visible v br very broad vt virtual triplet V unit cell volume VSEPR valence-shell electron pair repulsion VT variable temperature W\n width at half height w weak X halide substituent Z asymmetric units per unit cell x-dn Complex x has n number of H atoms replaced by "H atoms x-!5N2 Complex x has a l 5N2 labeled dinitrogen moiety e molar extinction coefficient T)n n-hapto X m a x absorbance maximum (UV-VIS spectroscopy) p bridging or absorption coefficient (X-ray crystallography) p density p c a i C calculated density x mixing time (EXSY N M R experiment) 0 Weiss constant X wavelength 5 chemical shift in ppm K ( X Y Z ) hapticity (atoms X , Y, and Z are coordinated to the metal) xxv effective magnetic moment molar magnetic susceptibility corrected for diamagnetic contributions xxvi ACKNOWLEDGEMENTS This thesis might never have been completed if not for the many helpful individuals at the University of British Columbia who assisted in its creation. I would like to thank my supervisor, Professor Michael D. Fryzuk for providing inspiration in the area of dinitrogen activation while allowing endless freedom in the research I chose to pursue. Many thanks go to past and present members of the Fryzuk group, including Bruce MacKay, Chris Kozak, Mike Petrella, Michael Shaver, James Corkin, Lara Morello, Chris Carmichael and Drs. Jason Love, Paul Duval, Danny Leznoff, Garth Giesbrecht, Annie Schneider, Jim Kickham, Frederic Naud, Guy Clentsmith, Fran Kerton, and Laleh Jafapour for their support, friendship and insightful discussions. I am especially grateful for the unselfish guidance of Dr. Jason Love during my initial tenuous encounters with air-sensitive chemistry. The U B C support staff have provided extraordinary aid in this undertaking. I appreciate the help of Mr. Peter Borda (elemental analyses), Ms. Elizabeth Varty (illustration), Steve Rak and Brian Ditchburn (glassblowing), as well as the diligent mechanical shop and electronics personnel. The expert work of Steve Rettig in producing crystal structure solutions was so impressive that it could not be taken for granted. Steve will be missed for more than his immense skill as a crystallographer; his kindness, humour and inspiration will be long remembered. The ever-tolerant Brian Patrick deserves thanks for solving several of the crystal structures presented in this thesis, as does Rene Lachicotte (University of Rochester). I am grateful to Victor Sanchez for his assistance with SQUID measurements and Professor Robert Thompson for informative discussions on the subject of magnetism. Thanks are also due to Nick Burlinson, for his frequent and often unsolicited help in N M R spectroscopy experiments, and his patient teaching skills. A special thanks is due to Cerrie Rogers for her tireless support and assistance. I appreciate the financial support from the NSERC of Canada (PGS A and PGS B) as well as the University of British Columbia (UGF). xxvn Chapter One: The Activation of Dinitrogen and Ligand Design Chapter One The Activation of Dinitrogen and Ligand Design 1 . 1 Dinitrogen Chemistry 1.1.1 Introduction Molecular nitrogen (N 2) surrounds us as the major component of the atmosphere here on Earth. We breathe this molecule constantly, but because of its extreme inertness, dinitrogen is not involved in our metabolism. However, nitrogen is an essential element in the chemistry of life, and the initial source of all the nitrogen in organisms is molecular nitrogen. Only a few organisms are capable of utilizing this plentiful source of nitrogen. The process by which N 2 is incorporated into biological systems is referred to as nitrogen fixation and involves the nitrogenase enzymes, which contain a metal-sulfido cluster at the active site. The recent X-ray crystal structure1 of one such enzyme cofactor has inspired increased interest in the mechanism by which nitrogenase converts N 2 to ammonia.2"7 1 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Since the discovery of the first dinitrogen complex [(H3N)5Ru(N2)] + in 1965, 8 ' 9 the coordination chemistry of this simple molecule has flourished. Some of this coordination chemistry of N 2 was aimed at modeling what was believed to be the active site of nitrogenase. 4 ' 5 ' 1 0 - 2 0 Additionally, there is continued interest in developing new kinds of reactivities for coordinated N 2 in an attempt to achieve a different goal: the discovery of new catalytic processes for the fixation and functionalization of dinitrogen.21 The coordination chemistry of dinitrogen has received frequent literature reviews. 2 2" 2 9 Dinitrogen's limited reactivity, and the harsh conditions required for non-enzymatic systems to convert N 2 into useful nitrogen-containing compounds,30 are in contrast with the reactivities of other small molecules. The gases CO, H 2 , O? and ethylene undergo a variety of reactions catalyzed by transition metals under relatively mild conditions and therefore find use in a number of industrial processes.31 Numerous efforts have been made to develop synthetic metal-based catalysts to functionalize molecular nitrogen under mild conditions. 3 2 , 3 3 Although some progress has been made in this area, these systems remain intriguing curiosities and are commercially impract ica l . 2 2 ' 2 3 ' 3 4 ' 3 5 The conversion of molecular nitrogen to nitrogen-containing compounds catalytically under mild conditions remains one of the loftier goals in chemistry. 1.1.2 The Nature of the Metal-Dinitrogen Bond The dinitrogen molecule is poorly suited to act as a ligand. Compared to isoelectronic CO, N 2 is both a poorer o-donor and a weaker ^-acceptor.36 Dinitrogen is also more difficult to reduce, and it lacks a dipole moment. Nevertheless, dinitrogen compounds have been prepared for almost every transition metal. The bonding mode of N 2 is highly dependent on the metal centre(s), the oxidation state of the metal, and the ligand environment used. Some of the most common mononuclear and dinuclear37 bonding modes of dinitrogen to transition metals are shown schematically in Figure 1.1. The side-on mononuclear bonding models omitted from Figure 1.1 because only a metastable example of this bonding mode has been characterized in the solid-state.38"41 2 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Weak Activation Strong Activation M N ^ = N M = N = N End-on Mononuclear A B M N ^ ^ N M M N = N M M = = N N = M End-on Dinuclear C D E / I K / T \ A\ M M M M M M Side-on Dinuclear N N N M ^ M F G H I Figure 1.1. The most common mononuclear and dinuclear bonding modes observed for dinitrogen, where M is a transition metal. Lines between atoms are meant to approximate bond order; no account of valencies, formal charges or oxidation states is implied. The bonding of N 2 to a metal can be categorized by the ability of the metal centre(s) to reduce or "activate" the N - N bond, via donation of electron density into the 71* orbitals of the dinitrogen moiety. The degree of backbonding is typically measured by the weakening of the N - N bond, observed both from the increase in the bond length (usually determined by X-ray crystallography) compared to free N 2 , and from the decrease of the N - N stretching frequency (from infrared and Raman spectroscopy). The bond length of free N 2 is 1.0975 A , 4 2 ' 4 3 and is the reference point for the discussion of formal oxidation states in dinitrogen complexes. A comparison of N - N bond lengths and stretching frequencies of some organic compounds is given in Table 1.1, along with some examples of dinitrogen complexes. 3 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Table 1.1. N - N bond lengths and stretching frequencies for N - N triple, double, and single bonds, as well as a selection of dinitrogen compounds. Compound N - N Bond Length 0 (A) N - N stretching frequency (cm"1) N 2 gas 42.43 1.0975 2331 PhN=NPh 4 3 " 4 6 1.255 1442 H 2 N - N H 2 43,47,48 1.449 1111 CpFe(dippe)(N2)+ 49 1.13(1) 2112 {[(Me 3SiNCH2CH2)3N]Mo}2(N 2)Mg(THF)2 5 0 1.195(13) 1719 {[Pr i 2PCH 2SiMe 2)2N]Zr(Cl) }2(Li-ri2:ri2-N2) 5 1 1.548(7) 731 Although exceptions exist, most late transition metal dinitrogen complexes contain weakly activated dinitrogen and only rarely undergo reactivity that results in the functionalization of dinitrogen. 2 3 ' 2 5 Strong activation of the N - N bond requires a strongly reducing metal centre and is most commonly observed with the early transition metals. As a result of the strongly reducing nature of these metals, the N2 unit can be considered (N2)2~ or (N2)4". The formalism used is determined by the N - N bond length, N - N stretching frequency, and the apparent oxidation state of the metal(s). 1.2 Early Transition Metal Dinitrogen Complex Formation and Reactivity 1.2.1 Synthesis of Dinitrogen Complexes The most common route to early transition metal dinitrogen complexes is by the reduction of a transition metal complex with a strong reducing agent, such as Na, K, or Mg, in the presence of N2. Usually the complex being reduced is a metal halide, and the side product of the reduction is a salt. For example, the reduction of TaCl3(=CHCMe3)(PMe3)2 4 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design with Na/Hg generates the dinitrogen complex [(TaCl(=CHMe3)(PMe3)2]2(|i-N2) and NaCl, as shown in equation 1.1.52 The exact mechanism of dinitrogen complex formation is unknown, but it is not unreasonable to imagine the intermediacy of a reduced tantalum species such as TaCl(=CHCMe 3)(PMe3)2 which then binds dinitrogen. The species TaCl(=CHCMe3)(PMe3)4 can be isolated and also reacts with dinitrogen to produce [(TaCl(=CHMe3)(PMe3)2]2(u--N2). The bonding mode of dinitrogen in this species is end-on dinuclear (mode E in Figure 1.1), a bonding mode which is typical for the group 5 metals. 2 3 ' 5 3 " 6 4 The dinitrogen unit in this complex and related group 5 dinitrogen complexes has been described as a bridging diimido l igand. 6 5 ' 6 6 4 Na/Hg M E 3 P P M e 3 N 2 M e . C H C - 1 | ^ C H C M e 3 2 TaCI3(=CHCMe3)(PMe3)2 „ K, ^, > 3 ~~~\Ta=N N = T a [1.1] -4 NaCl C | ^ | | X C | Me 3P PMe 3 Borohydrides have also been used as reducing agents for transition metal halide starting materials. For example, the reaction of NaHBEt3 with a niobium halide under dinitrogen gas generates an end-on dinuclear dinitrogen complex, as shown in equation 1.2.5^ The reduction step probably generates the intermediate (Cy2N ) 3 Nb, a niobium(III) species that could bind and reduce dinitrogen by virtue of its reducing nature and coordinative unsaturation. Cy H^W 2 N . H B B , C y ^ M ( * v..Nb=N N=Nb [1.2] ,.-Nb n " 2 N a C I \ 2 Dinitrogen complexes can also be prepared from non-halide starting materials. The reduction of a dinuclear niobium(IV) calixarene complex with Na generates a reduced dinuclear species that, when exposed to dinitrogen, forms an anionic end-on dinuclear dinitrogen complex, as shown in equation 1.3.67 The long N - N bond distance in this 5 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design example (N-N = 1.390(17) A) is consistent with its description as formally a bridging (N 2) 4" moeity. [1.3] N-N = 1.390(17) A The activation of dinitrogen by the early transition metals almost always involves considerable reduction of the dinitrogen moiety. There are few examples where the formation of a dinitrogen complex occurs from a high valent early transition metal complex without the addition of a strong reducing agent. One example is provided by (C 5 Me 4 H) 2 TiH, which reacts with N 2 to produce the dinuclear dinitrogen complex [(C,Me4H)2Ti]2(p-ri l:ri'-N2) and one equivalent of H 2 (Scheme 1.1).68 The loss of H 2 from hydride complexes to form dinitrogen complexes is not uncommon among the late transition metals, but is most unusual for the early transition metals. 2 3- 2 5 The N 2 bond in this example is only moderately activated, with an N - N distance of 1.170(4) A. Consequently, the N 2 moiety is weakly bound, and this complex loses N 2 under vacuum. 6 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.1. 1.170(4) A 1.2.2 Reactivity of Dinitrogen Complexes In the past, a large emphasis was placed on the synthesis of dinitrogen compounds, and the bonding of the dinitrogen moiety. With the exception of protonation reactions, relatively little other reactivity has been reported.69 This is possibly because the number of systems that react in a controlled manner with functionalization of the N 2 unit are scarce; only the group 6 complexes M(N 2 ) 2 (PR 3 ) 4 (where M = Mo, W) have shown clean stoichiometric modification of the coordinated dinitrogen. 2 3 ' 2 9 In recent years, increasing emphasis has been placed on expanding the reactivity of coordinated dinitrogen. In strongly activated dinitrogen complexes, the N 2 unit is formally described as (N2)2~ or (N2)4", so it comes as no surprise that it is often susceptible to electrophilic attack. By far the most studied reaction of coordinated N 2 with an electrophile involves the reaction with protic acids, such as HCI; this reaction should have relevance to biological nitrogen fixation. The products that can result even under very mild protonation conditions are N H 3 , N 2 H 4 and free N 2 ; the other possible product, N 2 H 2 , is not stable and disproportionates to N 2 and N 2 H 4 . The metal-containing products of these reactions are rarely recovered. 7 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design In a few cases, clean reactivity with other strongly electrophilic reagents has been observed. 2 3 ' 7 0 - 7 4 An example of the reaction of a strongly activated dinitrogen complex with MesSiCl is shown in Scheme 1.2.50 This dinitrogen complex is prepared by the reaction of [N 3 N]MoCl (where [N 3N] = N(CH 2 CH 2 NSiMe 3 ) 3 ) with Mg, and contains a formally (N 2) 2" dinitrogen moiety. The terminal nitrogen reacts with Me 3 SiCl to from a Si-N bond. Scheme 1.2. Me3Si p | \ ? 2 A . - ^ - M o — N t L \ -;N SiMe3 R = SiMe3 Mg(THF)2 THF 2 Mg Me3Si \ Me 3S ^ V N n i . MgCI2 / N SiMec SiMe3 Me3Si Me3Si \ I / </ 1 2 Me3SiCI SiMe3 MgCI2(THF)2 The reactivity of dinitrogen complexes has been studied with a variety of electrophiles, such as Me 3 SiCl , Me l or HCI. In addition, double bond metathesis reactivity has also been demonstrated to occur in end-on dinuclear complexes.5 3 An example is shown in equation 1.4, where the end-on dinitrogen complex whose synthesis was shown previously in equation 1.3 is reacted with an aldehyde, PhCHO, to give PhCH=N-N=CHPh and a niobium oxide. 6 7 This can also be considered an example of a reaction of activated dinitrogen with an electrophile. 8 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design [1.4] Well characterized transition metal complexes that are capable of cleaving the N=N bond have recently been discovered. 7 5" 7 9 The use of bulky amide ligands has allowed for the preparation of coordinatively unsaturated molybdenum complexes of the formula Mo[N(R)Ar] 3 (R=C(CD 3 ) 2 CH 3 ; Ar = 3,5-C 6 H 3 Me 2 ) ; 7 5 the steric bulk of the ligands prevents the strong metal-metal multiple bond formation typical of coordinatively unsaturated Mo(III). 8 0 Cooling Mo[N(R)Ar] 3 under N 2 at -35 °C for several days produced the dinuclear end-on bridged N 2 intermediate {[N(R)Ar] 3 Mo} 2 ( | l - r i l : r i l -N 2 ) (Scheme 1.3). At 28 °C this species decomposes in a first-order process to generate the Mo(VI) nitride complex Mo(=N)[N(R)Ar]. This process has been examined in detail, and the activation parameters and l 5N-isotope effect are known. Calculations as well as symmetry considerations suggest a "zigzag" transition state in the N - N bond cleaving step. 7 6 ' 8 1 9 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.3. Bu Ar r, t \ BU' \ N ' i . . . N , ' Ar / \ Ar Bu< N N Bu •—\* y / N Ar \ Ar ,..Mo Bur N 5 Ar \ Ar 1/2 Bu r Bur. Bu ' v N " / / N Ar \ Ar /Ar il BV N I I N Ii Mo N Bu r Ar •1/2 Ar ' Ar Bu r \ K ( A r / N \ B U B u \ M ^ A / N \ / N-Ar / Ar Bu' -Bu r Clearly, a driving force for the cleavage of the strong N = N bond and formation of the nitride Mo(=N)[N(R)Ar] is the formation of a comparably strong M s N bond. To the extent that the activation of N 2 can be measured by N - N bond lengthening upon coordination, complete cleavage of the N - N bond would appear to be the ultimate example of N 2 activation; however, its utility in the catalytic functionalization of dinitrogen has yet to be demonstrated. 10 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 1.2.3 Toward the Catalytic Functionalization of Dinitrogen The previous sections described the formation of early transition metal dinitrogen complexes, and their reactivities. Unfortunately, it is not obvious how the observed reactivities can be combined to create a commercially viable catalytic cycle that functionalizes dinitrogen at ambient temperatures and pressures. Little progress has been made towards the goal of creating catalytic processes similar to those available for other small molecules such as CO and C2H4. Whereas other small molecules such as CO, alkenes and alkynes undergo migratory insertion into metal-alkyl, metal-hydride and metal-acyl bonds, the same transformation has never been observed for N 2 . 3 1 The reaction of alkenes and alkynes with dinitrogen complexes generally results in the displacement of the coordinated N 2 unit and the incorporation of the 7r,-ligand.82-84 The formation of C-N bonds via reaction of these unsaturated molecules with N 2 has not yet been observed, and would be of great interest. Although a number of catalytic cycles have been observed with dinitrogen, none is currently thought of as commercially interesting; most rely on costly strong reducing agents to generate the dinitrogen complexes.25 The reactivity of these dinitrogen complexes with electrophiles offers few benefits compared to the use of cheaply available N H 3 , which also reacts with electrophiles. Methods other than harsh alkali metal reductions are required to generate dinitrogen complexes if a process is to be economically viable, as these reagents are relatively expensive; this is chiefly a problem with the early transition metals, as the majority of their dinitrogen complexes are made in this manner. The early metals also suffer the disadvantage of forming strong bonds to nitrogen in the functionalized products as well, which makes a catalytic cycle that does not involve both electrophiles and reducing agents difficult to imagine. As a consequence of the above issues, the catalytic functionalization of dinitrogen using early transition metals is clearly a challenging goal. New routes to reactive dinitrogen complexes are required, as well as new reactivities of coordinated dinitrogen for the catalytic functionalization of dinitrogen at ambient conditions to be realized. 11 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 1.3 Summary of Previous Work in the Fryzuk Laboratory 1.3.1 The [PNP] Hybrid Ligand Design Previous work in the Fryzuk laboratory has focused on the study of hybrid ligands that combine "hard" amide donors (NR 2 ) and "soft" phosphine donors ( P R 3 ) . 8 5 The first ligand design incorporating these very different donor atoms was the chelating amide diphosphine ligand N(SiMe2CH2PR2)2, abbreviated as R[PNP] (where R = Me, 'Pr, 'Bu, Ph) . 8 6 The synthesis of the lithiated ligand precursor R [PNP]Li is readily performed using commercially available HN(SiMe 2 CH 2 Cl) 2 . For phosphines bearing alkyl substituents, the lithiated ligand precursor is prepared in a single step using three equivalents of L iPR 2 ; two equivalents of L i P R 2 functionalize the chloromethyl sidearms in a metathesis reaction and one equivalent of L i P R 2 deprotonates the amide donor. This ligand is then conveniently attached to metal halides with the loss of lithium halide. The R[PNP] ligand synthesis and metal complex formation is shown in Scheme 1.4. Scheme 1.4. Me2 Me2 Me2 Me2 Me2 Me2 THF Si £\* / S i / S l \ N / - S l \ 3LiPFt2 / > \ UMX / >J x u. / " 2 U C I P h P - U X P — M — P -C| H ri\ -HPR 2 Pry ^ ' " R wy L n \ ' " R R R R R R = Me, Pr', Bu f R[NPN]MLn The coordination and organometallic chemistry of the R[PNP] ligand design has been studied in the Fryzuk laboratory for two decades. The longevity of this ligand design is in part due to its versatility with respect to the number of transition metals to which it can be attached.87 The R[PNP] ligand can be considered a "hybrid" ligand, by virtue of the very different nature of the amide and phosphine donors.85 The phosphine donors are excellent donors for the late transition elements, and allows the [PNP] ligand to be attached to the late transition metals; the chelating nature of the ligand enables the anionic amide donor to bind, despite the fact that the amide ligand is generally a poor ligand for the late transition elements. On the other hand, the anionic amide donor forms strong bonds to the early 12 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design transition elements, 8 8 ' 8 9 and the chelate effect assists the binding of the phosphine ligands, which are comparatively poor ligands for the early transition metals.9 0 The combination of these two very different donors endows complexes of the [PNP] ligand with unique reactivity. An example is the reduction of [PNP]ZrCl3, which is easily prepared from the reaction of [PNP]Li with Z r C l 4 . 9 1 The reduction of [PNP]ZrCl 3 is accomplished using Na/Hg amalgam under N 2 and resulted in the dinitrogen complex ([PNP]ZrCl) 2(p-ri 2:r| 2-N 2), as shown in Scheme 1.5.51 The N 2 moiety is bound in a side-on o mode, and the N - N bond length of 1.548(7) A is appreciably lengthened from the bond length in free dinitrogen. In fact, this N - N bond length is longer than the nitrogen-nitrogen o single bond distance of 1.46 A observed in hydrazine, and remains the longest N - N bond distance reported in the literature. Scheme 1.5. Cl Me2 The long N - N bond length in ([PNP]ZrCl) 2(p-r| 2:r| 2-N 2) is the result of the considerable reduction of the N 2 moiety by the two zirconium centres, so that the bridging N 2 unit can be treated formally as (N 2) 4" and the zirconium centres as Zr(IV). The reactivity of 13 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design the dinitrogen moiety should, therefore, involve reactions with electrophiles. Unfortunately, the dinitrogen complex ([PNP]ZrCl)2(|i-r| 2:ri2-N2) does not exhibited controlled reactivity with electrophiles to provide isolable products. It was postulated that this is due to a deficiency of the [PNP] ligand; despite the chelate effect, the phosphine donors can dissociate,92 and as a result, a number of reaction pathways are potentially available with electrophilic reagents, which can lead to a number of products. 1.3.2 The [P2N2] Ligand Design One possible approach to avoiding the dissociation of the phosphine side arms of the [PNP] ligand is to incorporate them into a macrocyclic array. This was accomplished by linking the two phosphine ligands with an additional disilazane moiety, as shown in Scheme 1.6.93 This synthesis is quite remarkable for two reasons: firstly, the preparation of macrocyclic ligands typically requires metal templates or high-dilution techniques; 9 4 - 9 6 secondly, despite the existence of two possible isomers of the ligand depending on the stereochemistry at the phosphorus centres, 9 7 ' 9 8 the cis isomer can be exclusively prepared by the appropriate choice of solvent and temperature. This thesis deals only with the cis isomer, previously named in the literature syn-[P2^2], (where [ P 2 N 2 ] = PhP(CH 2 SiMe 2 NSiMe 2 CH 2 ) 2 PPh); from here on the syn prefix will be omitted. A readily isolated and useful precursor to [P2N2] ligand complexes is the 1,4-dioxane adduct [P 2N2]Li2-(C 4H 802). 14 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.6. Me 2 Me 2 THF S k / - S i x 2 LiPHPh N \ » I / - 2 LiCI Ci H Cl H Me? Me? / N \ A p M P -V ' P h H L d = 1,4-dioxane Ph- kPh Me 2 Me 2 LH N P - ^ L i - P , Si Si Me 2 Me 2 [P 2N 2]Li 2.(C 4H 80 2) 4 BunLi Et 20 C4H 8 0 2 0 °C Cl |_| Cl I Si Me 2 Si Me 2 The reaction of [P2N2]Li2-(C 4H 802) with ZrCl 4 (THT) 2 results in the formation of [P2N2]ZrCl2, which can be readily reduced with potassium graphite (KCs) under N2 gas to give the dinitrogen compound ([P2N2]Zr)2(p-r)2:r|2-N2). The synthesis of this complex is shown in Scheme 1.7, along with two unique reactions of this dinitrogen complex." The first reaction is with hydrogen gas. Most dinitrogen complexes either do not react with H2, or react by H2 simply displacing the dinitrogen moie ty ; 1 0 0 - 1 0 2 the result is the formation of a hydride complex and the loss of N2 gas. As shown in the bottom left of Scheme 1.7, ([P2N2]Zr)2(p-ri 2:T) 2-N 2) reacts with hydrogen to produce an N - H bond and a bridging hydride. This reaction can be viewed as the heterolytic activation of H2 gas, with the H + bonding to nitrogen and the H" occupying a bridging location between the zirconium centres. The analogous reaction with the silane Bu"SiH3 is also consistent with this description, and is shown on the bottom left of Scheme 1.7. The heterolytic cleavage of Bu"SiH 3 results in the more electropositive silicon atom bonding with the side-on bridging dinitrogen moiety and a bridging hydride is produced. Once again, this transformation had never been observed before in a dinitrogen complex. 1 0 3 15 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.7. The [P2N2] ligand was originally designed to study zirconium dinitrogen chemistry; however, it has also been attached to other elements to determine the effect of this unique ligand environment on their dinitrogen chemistry and organometallic chemistry. 1 0 4" 1 0 7 Dinitrogen [P2N2] complexes of niobium and vanadium were readily prepared from 16 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design NbCl 3 (DME) and VC1 3(THF) 3 respectively. 1 0 6 ' 1 0 8 The synthesis of the niobium dinitrogen complex ([P2N2]Nb)2(p-ri l:ri l-N2) is shown in Scheme 1.8.1 0 8 Scheme 1.8. Cl The complexes ([P2N2]Nb)2(u,-N2) and ([P2N2]V)(p>N2) are both dinuclear complexes containing dinitrogen bound in end-on dinuclear mode, rather than the side-on bonding observed in ([P2N2]Zr)2(N2). This is not unusual because the end-on dinuclear mode is by far the more prevalent bonding mode for dinitrogen, particularly for the group 5 metals; only recently have other bonding modes been reported for strongly activated dinitrogen complexes of these elements. The difference of N 2 bonding in ([P2N2]Nb)2(p-N2) versus ([P2N2]Zr)2(N2) may be due to the increased strength of multiple bonds to the group 5 elements. The ability of the group 5 elements to form strong multiple bonds is characteristic of their chemistry. The greater ionic character of the zirconium-nitrogen bond relative to the group 5 metal-nitrogen bond may be another reason for the preference of the side-on bonding mode of dinitrogen in ([P2N2]Zr)2(p-N2). The side-on mode increases the number of 17 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design interactions between the positively charged metal centre and the negatively charged dinitrogen unit, which should be preferred as the bonding of the metal to the dinitrogen unit becomes increasingly ionic. The difference in bonding mode plays an important role in reactivity. The group 5 end-on bound dinitrogen complexes ([P2N2]Nb)2(|i-N2) and ([P2N2]V)2(u,-N2) do not undergo reactions with H2 or silanes in contrast to that observed in the side-on bound dinitrogen complex ([P2N2]Zr)2(|l-r]2:r)2-N2). 1.4 Side-On versus End-On Bonding Why is it that the side-on bound dinitrogen complex ([P 2N 2]Zr) 2(|i-r| 2:r| 2- N 2 ) reacts with reagents such as hydrogen or silanes and results in bond formation to nitrogen whereas end-on complexes are unreactive with these reagents? Although density functional calculations have been performed on the ([P2N2]Zr)2(u.-N2) system, 1 0 3 perhaps there is an easier, if more approximate, way to understand its ability to react with reagents such as H 2 to form an N - H bond. A comparison of the approximate orbital overlaps involved in side-on versus end-on bonding in dinuclear dinitrogen complexes is shown in Figure 1.2.36.109,110 The orbitals are labeled with respect to the nature of the metal-dinitrogen interaction. 18 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design End-On AL i i i l TT TT TT o-bonding Ti-bonding 7r.-b0n.ding Figure 1.2. Approximate orbital overlaps involved in the bonding of dinitrogen to metals (M) in the side-on dinuclear (top) and end-on dinuclear (bottom) modes. In the side-on dinuclear mode, 1 0 9 - 1 1 1 a metal-dinitrogen a-bonding overlap occurs between a 7t-bonding orbital of the dinitrogen fragment and a metal orbital. The two back-bonding interactions from the metal centre to the dinitrogen n -orbitals are of different symmetries; one is 7t-bonding and the other 8-bonding with respect to the metal-dinitrogen 19 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design interaction. Finally, there is a dinitrogen based molecular orbital that is approximately non-bonding and has no bonding interaction with the metal centre either. The bonding in the end-on dinuclear mode is quite d i f f e ren t . 5 7 ' 1 0 9 ' 1 1 2 - 1 1 5 Here, the non-bonding dinitrogen molecular orbital forms a a-bonding interaction with the metal centre. The two back-bonding interactions from the metal centre to the dinitrogen 7t*-orbitals are both ft-bonding with respect to the metal-dinitrogen interaction. No non-bonding interaction remains in the end-on bonding mode. How do these differences in orbitals contribute to the reactivity of bound dinitrogen? Consider the reaction of a side-on bound dinitrogen complex with H 2 gas, akin to the reaction shown in Scheme 1.8 (top). In the case of the side-on bound dinitrogen, this involves several changes in the molecular orbitals involved. Firstly, the M ( N 2 ) M fragment distorts such that it no longer lies in a plane. This allows the two metal centres to back-bond via two 7i-bonding interactions, rather than a 7t-bonding interaction and a 8-bonding interaction; for covalent bonding, the two 7i-bonding interactions should be more favourable, as 7i-bonds are usually stronger bonds than 8-bonds. Overall, four bonds still exist between the (N 2) 4" moiety and the two metal centres. A N - H bond could be formed with the dinitrogen moiety without severing a metal-nitrogen bond because of the presence of a nonbonding orbital in the side-on dinuclear bonding mode. The other hydrogen atom is converted to a bridging hydride ligand, and bridges the two metal centres. In total, no bonds between the dinitrogen moiety and the metal centre have been broken and a N - H bond and a bridging hydride bond are formed. Only a H-H bond is cleaved. The hypothetical reaction of H 2 with end-on bound dinitrogen is also shown in Scheme 1.8 (bottom). In the end-on bonding mode there is no occupied non-bonding orbital, so the reaction of H 2 across a M - N bond necessarily decreases the bond order of the M - N bond. Similar to the reaction of a side-on bound dinitrogen complex with H 2 , a new N - H bond is formed. A metal-hydrogen bond is also formed; however, it is more likely to be terminal than bridging because of the separation of the two metal centres in the end-on bonding mode, which is thermodynamically less favourable. Most importantly, the absence of a non-bonding orbital on the dinitrogen moiety results in the breakage of a metal-nitrogen 20 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design interaction when the N-H bond is formed. Overall, an N - H bond and a terminal hydride bond are formed, but a M - N 7t-bond is broken, and the H-H bond is cleaved. Clearly, the reaction of the side-on bound dinitrogen complex with hydrogen is more likely to be thermodynamically favourable than the reaction of the end-on bound dinitrogen complex, because it does not involve breaking a M - N bonding interaction. Although this is a simple model, it may help explain at a basic level why a side-on bound dinitrogen complex would be more reactive than an end-on bound dinitrogen complex to reagents such as H 2 . Scheme 1.8. M 1.5 Scope of this Thesis The long-term goal of this investigation of dinitrogen chemistry is to design catalysts capable of converting dinitrogen gas into more valuable nitrogen-containing compounds. To reach this lofty goal, new milder routes to dinitrogen complexes and less electrophilic reactivity patterns at the metal-bound dinitrogen moiety must be discovered. This thesis examines the chemistry of tantalum stabilized by chelating amido-phosphine ligands, towards the goal of generating easily prepared reactive tantalum dinitrogen complexes. 21 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design The end-on dinuclear mode is predominant for early transition metal dinitrogen complexes, but particularly for the group 5 metals. 5 3 ' 6 5 When this study was first undertaken, no example of a strongly activated dinitrogen complex of the 5 metals in a bonding mode other than the end-on dinuclear mode had been reported. The unique reactivity towards hydrogen and silanes noted for the zirconium side-on bound dinitrogen complex ([P 2 N 2 ] Z r) 2(|ul - r | 2 : r | 2 - N 2 ) made a tantalum complex with dinitrogen bound in the side-on bridging mode an intriguing target. It was anticipated that if the end-on bridging mode could be avoided, exciting new reactivities for tantalum dinitrogen complexes would result. At the beginning of this study, it was not clear how to design a tantalum dinitrogen complex that would avoid the end-on bridging mode. The ligand system initially studied was the [ P 2 N 2 ] ligand, originally designed for the study of zirconium dinitrogen chemistry. The tantalum analogue of the previously prepared niobium dinitrogen complex, ([P 2 N 2 ]Nb) 2 (u> r|1 :r| !-N2), proved impossible to synthesize due to the lack of a convenient tantalum starting material. Furthermore its importance was questioned, because it would almost surely display the same dinitrogen bonding mode and lack of reactivity at the as the niobium analogue. Instead of searching for synthetically involved routes to dinitrogen complexes of the [P2N2] ligand, the research presented in this thesis turned to the study of the organometallic chemistry of tantalum complexes of the [P2N2] ligand. These compounds in general were relatively stable, and often unreactive; however, their study gave a better understanding of the bonding in these species. From these data, a new amide phosphine ligand was then designed that was believed would generate reactive tantalum hydride complex, and reactive dinitrogen complexes. This thesis is divided into five chapters aside from the introduction. Chapter 2 describes attempts to attach the [ P 2 N 2 ] ligand to tantalum using TaCls, and the synthesis of [P 2 N 2 ] T a M e 3 . The photolytic decomposition of [P 2 N 2 ] T a M e 3 to produce the methyl methylidene complex [P 2 N 2 ]Ta=CH 2 (Me) is also described in Chapter 2. It was anticipated that the decomposition of [P 2 N 2 ]Ta=CH 2 (Me) in the presence of ethylene would yield [P 2 N 2 ] T a ( C 2 H 4 ) M e , in analogy to the chemistry observed for the archetypal methylidene complex (r) 5-C 5H 5) 2Ta=CH 2(Me); however, instead the major product was 22 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design [P2N2]Ta(C2H4)Et. The mechanism of the reaction of [P 2N 2]Ta=CH 2(Me) with ethylene and the agostic interactions and fluxional behaviour in [P 2N 2]Ta(C 2H 4)Et are the subject of Chapter 3. The hydrogenation of [P 2N 2]TaMe 3 yielded ([P 2N 2]Ta) 2(u-H) 4, a Ta(TV) complex containing a tantalum-tantalum bond; this complex and its reactivity are described in Chapter 4. The hydride ligands in this complex have proven to be remarkably inert. The reactivity of ([P2N2]Ta)2(p>H)4 is dominated by the ability of the tantalum-tantalum bond to act as a source of electrons, in essence as a reducing agent. Unfortunately, this hydride is too coordinatively saturated to bind reagents at the tantalum centre in this reaction. Chapter 5 discusses the modification of the [P 2N 2] ligand design to generate a reactive dinuclear Ta(IV) hydride, and its reactivity, which results in a reactive dinitrogen complex with a new side-on end-on dinuclear bonding mode. The reactivity of this dinitrogen complex is investigated, and dinitrogen in this bonding mode exhibits enhanced reactivity. Chapter 6 presents some possible future avenues of research into the dinitrogen complex introduced in Chapter 5, as well as other complexes and ligand systems whose reactivities have not yet been thoroughly investigated. 1.6 The Organometallic Chemistry of Tantalum Although the goal of this thesis was to discover new dinitrogen complexes of tantalum and explore their reactivity, access to dinitrogen chemistry was hindered by the difficulty in obtaining a convenient starting material. As will be detailed in Chapter 2, the only viable starting material was TaMe3Cl 2, and this led to the study of the organometallic chemistry of tantalum;116.117 therefore, a brief introduction to the organometallic chemistry of tantalum is in order. Tantalum, as a group 5 element, exhibits characteristic reactivities of both the group 4 and group 6 elements. Similar to the group 4 metals, the majority of stable organometallic tantalum complexes occur in its highest oxidation state, Ta(V). Although Ta(IV) and Ta(III) complexes are known, they are typically strongly reducing. Oxidation states lower than Ta(III) are usually only accessible with 7t-acceptor ligands. 1 1 8 Similar to the group 6 metals, and most unlike the group 4 metals, tantalum forms strong multiple bonds to a variety of heteroatoms;119 120-123 o n e 0 f m e m o s t thoroughly studied is the 23 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design tantalum-carbon double b o n d . 1 2 4 ' 1 2 5 The initial discovery of tantalum-carbon double bonds was serendipitous. The homoleptic alkyl TaMe.5 had been synthesized by the reaction of T a M e 3 C l 2 1 2 6 and two equivalents of MeLi , but was thermally unstable. 1 2 7- 1 2 8 A more sterically bulky alkyl was chosen in an attempt to synthesize a thermally stable homoleptic a l k y l ; 1 2 9 however, the reaction of T a C l 2 ( C H 2 C M e 3 ) 3 with two equivalents of L i C H 2 C M e 3 does not generate T a ( C H 2 C M e 3 ) 5 as anticipated, but rather T a = C H C M e 3 ( C H 2 C M e 3 ) 3 , as shown in equation 1.5. 1 3 0 ' 1 3 1 This species is believed to form from transient T a ( C H 2 C M e 3 ) 5 by a process known as a-elimination, which is influenced by steric crowding at the metal centre. 1 3 2 The species Ta=CHCMe 3 (CH 2 CMe 3 ) 3 formally contains a double bond from a high-valent metal centre to carbon; this important class of compounds have become known as alkylidenes, or Schrock-alkylidenes, as opposed to the chemically quite different metal-carbon double bonds formed by metals in low oxidation states, which are known as Fischer-carbenes. Cl Me3CCH2,„ I 2LiCH 2CMe 3 Me3CCH2„ ^CHCMe 3 T a - C H 2 C M e 3 2 > J a [-, . 5 ] M e 3 C C H 2 ^ | -2LJCI M e 3 C C H 2 ^ | Cl " M e 4 C CH 2CMe 3 Since their discovery, the formation of double bonds to carbon has been considered a characteristic reactivity of high-valent tantalum. Numerous tantalum alkylidenes have been synthesized and studied, and for some time alkylidenes were considered the "raison d'etre" of organometallic tantalum chemistry. 1 1 6 ' 1 1 7 For other transition metal elements, metal-carbon double bonded species are important in the catalysis of olefin metathesis and ring-opening metathesis polymerization ( R O M P ) . 1 3 3 Despite the prevalence of tantalum alkylidenes, they are only slightly effective as catalysts for homogeneous olefin metathesis reactions; many tantalum alkylidenes are completely unreactive for this p rocess . 1 1 6 ' 1 1 7 ' 1 3 3 The greatest practical impacts of tantalum alkylidenes have been in modeling olefin metathesis catalyst decomposition pathways, and to determine the effect of ancillary ligands on the metathesis activity of these tantalum alkylidenes. 1 3 4 The exceptional stability of tantalum alkylidenes allowed for the synthesis of the first reported methylidene complex, (r|5-C5H5)2Ta=CH2(Me), which illustrates that steric bulk is 24 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design not necessary to generate a stable alkylidene. The complex was synthesized by the deprotonation of the cationic species {(r^-CsHs^TaMei}"1" by Me3P=CH 2, as shown in Scheme 1.9. 1 3 5" 1 3 7 Scheme 1.9. Another feature of tantalum chemistry that resembles the chemistry of the group 6 metals 1 3 8 is the formation of metal-metal bonds. 1 3 9 Coordinatively unsaturated complexes of tantalum in oxidation states less than Ta(V) frequently form dinuclear complexes with tantalum-tantalum bonds. 1 4 0 " 1 5 2 In some cases these tantalum-tantalum bonds are reactive. 1 5 3 " 1 6 0 Two examples are shown in Scheme 1.10. The uppermost example demonstrates the reactivity of a tantalum-tantalum doubly-bonded dihydride towards hydrogen gas, to produce a tantalum-tantalum singly-bonded tetrahydride.161 Other hydrides that bridge tantalum-tantalum bonds undergo unique reactivity by virtue of the presence of both reactive hydrides and a reducing metal-metal b o n d . 1 6 2 ' 1 6 3 The lower example in Scheme 1.10 demonstrates a more curious reactivity; the reaction of a tantalum-tantalum doubly bonded species with ethylene results in C-H bond activation, the first example of C-H bond activation by a metal-metal b o n d . 1 6 4 ' 1 6 5 25 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.10. In general, tantalum has not proven to be as important in catalysis as many other transition elements, but one class of compounds where tantalum does seem to exhibit promising catalytic reactivity is its mononuclear hydride complexes. Complexes of the type C p ' 2 M H 3 (where M = Nb, Ta, and Cp' = C 5 H 5 , C 5 Me 5 , C 5 H 4 SiMe 3 , C 5 H 3 (SiMe 3 ) 2 ) are well studied, and display remarkable reactivity. 1 6 6 For example, these trihydride complexes are capable of activating C-H bonds, and catalyze H/D exchange between H 2 and CeD 6, as shown in Scheme 1.11.167-169 26 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Scheme 1.11. Alternative ligand designs have generated mononuclear tantalum hydrides that perform catalytic hydrogenations; particularly remarkable is the ability of these species to catalyze arene hydrogenation. 1 7 0 - 1 7 2 In the example shown in equation 1.6, the formation of a trihydride by the hydrogenation of a Ta(V) triakyl species also results in the hydrogenation of the four ortho disposed phenyl rings attached to the aryloxy ligand. 173,174 [1.6] 27 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design Recent times have seen a vast increase in the study of the organometallic chemistry of tantalum, and this brief overview cannot endeavour to review the vast repertoire of reactivity demonstrated by derivatives of tantalum. 1 1 6 ' 1 1 7 New reactivities of these tantalum complexes continue to be discovered as the number of early transition metal ligand designs expands; 1 7 5J 7 6 this increase in ligand designs is primarily due to their application to group 4 olefin polymerization, and the application of these ligand designs to tantalum chemistry is often secondary. 1 7 7 - 1 8 7 Regardless, the application of novel ligand designs to tantalum continues to open new frontiers of the organometallic chemistry of tantalum to exploration. 1.7 References 1) Kim, J.; Rees, D. C. Nature 1992, 360, 563. 2) Sellmann, D. Angew. Chem. Int. Ed. Engl. 1993, 32, 64. 3) Sellmann, D.; Fursattel, A. Angew. Chem. Int. Ed. Engl. 1999, 38, 2023. 4) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460. 5) Tyson, M . A. ; Coucouvanis, D. Inorg. Chem. 1997, 37, 3808. 6) Han, J.; Beck, K.; Ockwig, N . ; Coucouvanis, D. J. Am. Chem. Soc. 1999,121, 10448. 7) Smith, B. E.; Durrant, M . C ; Fairhurst, S. A. ; Gormal, C. A. ; Gronberg, K. L. C ; Henderson, R. A. ; Ibrahim, S. K.; Le Gall, T.; Pickett, C. J. Coord. Chem. Rev. 1999, 185-186, 669. 8) Allen, A . D.; Senoff, C. V. J. Chem. Soc, Chem. Commun. 1965, 621. 9) Senoff, C. V . J. Chem. Ed. 1990, 67, 368. 10) Richards, R. L . Coord. Chem. Rev. 1996,154, 83. 11) Richards, R. L. Pure & Appl. Chem. 1996, 68, 1521. 28 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 12) Richards, R. L. The Chemistry of Dinitrogen Reduction; Dilworth, M . J. and Glenn, A. R., Ed.; Elsevier: Amsterdam, 1991, pp 58. 13) Dance, I. J. Chem. Soc, Chem. Commun. 1998, 523. 14) Helleren, C. A. ; Henderson, R. A. ; Leigh, G. J. J. Chem. Soc, Dalton Trans. 1999, 1213. 15) Hughes, D. L. ; Leigh, G. J.; Mc Mahon, C. N . / . Chem. Soc, Dalton Trans. 1999, 909. 16) Stavrev, K. K.; Zerner, M . C. Chem. Eur. J. 1996, 2, 83. 17) Zhong, S.-J.; Liu, C.-W. Polyhedron 1996,16, 653. 18) Henderson, R. A. ; Leigh, G. J.; Pickett, C. J. Adv. Inorg. Chem. Radiochem. 1983, 27, 197. 19) Richards, R. L. New J. Chem. 1997, 21, 727. 20) Sellmann, D.; Fiirsattel, A. ; Sutter, J. Coord. Chem. Rev. 2000, 200-202, 545. 21) Leigh, G. J. Science 1998, 279, 506. 22) Gambarotta, S. J. Organomet. Chem. 1995, 500, 117. 23) Hidai, M . ; Mizobe, Y . Chem. Rev. 1995, 95, 1115. 24) Tuczek, F.; Lehnert, N . Angew. Chem. Int. Ed. Engl. 1998, 37, 2636. 25) Fryzuk, M . D.; Johnson, S. A. Coord. Chem. Rev. 2000, 200-202, 379. 26) Dilworth, J. R.; Richards, R. L. Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A. and Abel, E. W., Ed.; Pergamon Press: Oxford, 1982; Vol . 8, pp 1073. 27) Hidai, M . ; Mizobe, Y . Reaction of Coordinated Ligands; Braterman, P. S., Ed.; Plenum Press: New York, 1989; Vol . 2, pp 53. 28) Leigh, G. J. Science 1995, 268, 827. 29 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 29) Hidai, M . Coord. Chem. Rev. 1999,185-186, 99. 30) Harding, A . J. Ammonia Manufacture and Uses; Oxford University Press: London, 1959. 31) Parshall, G. W. Homogeneous Catalysis; 2nd ed.; Wiley: New York, 1992. 32) Kawaguchi, M . ; Hamaoka, S.-I.; Mori, M . Tetrahedron Lett. 1993, 34, 6907. 33) Komori, K.; Oshita, H. ; Mizobe, Y. ; Hidai, M . J. Am. Chem. Soc. 1989, 111, 1939. 34) Shilov, A . E. Russ. Chem. Rev. 1974, 43, 378. 35) Green, M . L. H. J. Chem. Soc, Dalton Trans. 1991, 575. 36) Yamabe, T.; Hori, K.; Minato, T.; Fukui, K. Inorg. Chem. 1980,19, 2154. 37) Henderson, R. A. Transition Met. Chem. 1990,15, 330. 38) Jeffery, J.; Lappert, M . F.; Riley, P. I. J. Organomet. Chem. 1979,181, 25. 39) Cusanelli, A. ; Sutton, D. / . Chem. Soc, Chem. Commun. 1989, 1719. 40) Cusanelli, A. ; Sutton, D. Organometallics 1996,15, 1457. 41) Fomitchev, D. V. ; Bagely, K. A.; Coppens, P. /. Am. Chem. Soc. 2000,122, 532. 42) Stoicheff, B. P. Canad. J. Phys. 1954, 82, 630. 43) Sutton, L. E. Tables of Interatomic Distances and Configuration in Molecules and Ions; Chemical Society Special Publication No. 11, Chemical Society: London, 1958. 44) Brown, C. J. Acta Cryst. 1966, 21, 146. 45) Mostad, A. ; Romming, C. Acta Chem. Scand. 1971, 25, 3561. 46) Bouwstra, J. A. ; Schouten, A. ; Kroon, J. Acta Cryst. C. 1983, 39, 1121. 47) Collin, R. L. ; Lipscomb, W. N . Acta Cryst. 1951, 4, 10. 30 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 48) Morino, Y . ; Iijima, T.; Murata, Y . Bull. Chem. Soc. Jpn. 1960, 33, 46. 49) de la Jara Real, A. ; Tenorio, M . J.; Puerta, M . C ; Valerga, P. Organometallics 1995, 14, 3839. 50) O'Donaghue, M . B.; Davis, W. B.; Schrock, R. R. Inorg. Chem. 1998, 37, 5149. 51) Fryzuk, M . D.; Haddad, T. S.; Rettig, S. J. J. Am. Chem. Soc. 1990, 112, 8185. 52) Rocklage, S. M . ; Turner, H. W.; Fellmann, J. D.; Schrock, R. R. Organometallics 1982,1, 703. 53) Rocklage, S. M . ; Schrock, R. R. /. Am. Chem. Soc. 1982,104, 3077. 54) Dilworth, J. R.; Henderson, R. A. ; Hills, A. ; Hughes, D. L. ; Macdonald, C ; Stephens, A . N . ; Walton, D. R. M . J. Chem. Soc, Dalton 1990, 1077. 55) Schrock, R. R.; Wesolek, M . ; Liu, A. H. ; Wallace, K. C ; Dewan, J. C. Inorg. Chem. 1988, 27, 2050. 56) Hao, S.; Berno, P.; Minhas, R. K.; Gambarotta, S. Inorg. Chim. Acta 1996, 244, 37. 57) Ferguson, R.; Solari, E.; Floriani, C ; Osella, D.; Ravera, M . ; Re, N . ; Chiesi-Villa, A. ; Rizzoli, C. J. Am. Chem. Soc. 1991,119, 10104. 58) Berno, P.; Gambarotta, S. Organometallics 1995,14, 2159. 59) Desmangles, N . ; Jenkins, H.; Ruppa, K. B.; Gambarotta, S. Inorg. Chim. Acta 1996, 250, 1. 60) Edema, J. J. H. ; Meetsma, A. ; Gambarotta, S. J. Am. Chem. Soc. 1989, 111, 6878. 61) Buijink, J.-K. F.; Meetsma, A. ; Teuben, J. H. Organometallics 1993,12, 2004. 62) Song, J.-L; Berno, P.; Gambarotta, S. /. Am. Chem. Soc. 1994,116, 6927. 63) Berno, P.; Hao, S.; Minhas, R.; Gambarotta, S. J. Am. Chem. Soc. 1994,116, 7417. 31 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 64) Ferguson, R.; Solani, E.; Floriani, C ; Shiesi-Villa, A. ; Rizzoli, C. Angew. Chem. Int. Ed. Engl. 1993, 32, 396. 65) Turner, H. W.; Fellmann, J. D.; Rocklage, S. M . ; Schrock, R. R.; Churchill, M . R.; Wasserman, H. J. J. Am. Chem. Soc. 1980,102, 7811. 66) Churchill, M . R.; Wasserman, H. J. Inorg. Chem. 1981, 20, 2899. 67) Zanotti-Gerosa, A. ; Solari, E.; Giannini, L. ; Floriani, C ; Chiesi-Villa, A. ; Rizzoli, C. / . Am. Chem. Soc. 1998,120, 437. 68) de Wolf, J. M . ; Blaauw, R.; Meetsma, A. ; Teuben, J. H. ; Gyepes, R.; Varga, V . ; Mach, K.; Veldma, N . ; Spek, A . L. Organometallics 1996,15, 4977. 69) Leigh, G. J. Acc. Chem. Res. 1992, 25, 177. 70) Chatt, J.; Heath, G. A. ; Leigh, G. J. J. Chem. Soc, Chem. Commun. 1972, 444. 71) Chatt, J. J. Organomet. Chem. 1975,100, 17. 72) Chatt, J.; Diamantis, A . A. ; Heath, G. A. ; Hooper, N . E.; Leigh, G. J. J. Chem. Soc. Dalton Trans. 1977, 688. 73) Takagahara, K.; Ishino, H.; Ishii, Y. ; Hidai, M . Chem. Lett. 1998, 897. 74) Takahashi, T.; Kodama, T.; Watakabe, A. ; Uchida, Y. ; Hidai, M . / . Am. Chem. Soc. 1983, 105, 1680. 75) LaPlaza, C. E.; Cummins, C. C. Science 1995, 268, 861. 76) LaPlaza, C. E.; Johnson, M . J. A. ; Peters, J. C ; Odom, A. L.; Kim, E.; Cummins, C. C ; George, G. N . ; Pickering, I. J. J. Am. Chem. Soc. 1996,118, 8623. 77) Cummins, C. C. J. Chem. Soc, Chem. Commun. 1998, 1777. 78) Caselli, A. ; Solari, E.; Scopelliti, R.; Floriani, C ; Re, N . ; Rizzoli, C ; Chiesi-Villa, A. J. Am. Chem. Soc. 2000,122, 3652. 32 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 79) Clentsmith, G. K. B.; Cloke, F. G. H. J. Am. Chem. Soc. 1999,121, 10444. 80) Hahn, J.; Landis, C. L. ; Nasluzov, V . A. ; Neyman, K. M . ; Rosch, N . Inorg. Chem. 1997, 36, 3947. 81) Cui, Q.; Musaev, D. G.; Svensson, M . ; Sieber, S.; Morokuma, K. J. Am. Chem. Soc. 1995, 117, 12366. 82) Vigalok, A. ; Ben-David, Y . ; Milstein, D. Organometalllcs 1996,15, 1839. 83) del Rio, I.; Gossage, R. A.; Hannu, M . S.; Lutz, M . ; Spek, A. L.; van Koten, G. Organometallics 1999, 18, 1097. 84) Nakamura, G.; Harada, Y. ; Arita, C.; H. , S.; Mizobe, Y. ; Hidai, M . Organometallics 1998,77, 1010. 85) Pearson, R. G. J. Chem. Ed. 1968, 45, 581. 86) Fryzuk, M . D.; MacNeil, P. A.; Rettig, S. J.; Secco, A . S.; Trotter, J. Organometallics 1982,1, 918. 87) Fryzuk, M . D. Can. J. Chem. 1992, 70, 2839. 88) Kempe, R. Angew. Chem. Int. Ed. Engl. 2000, 39, 469. 89) Lappert, M . F.; Power, P. P.; Sanger, A . R.; Srivastava, R. C. Metal and Metalloid Amides; Wiley: New York, 1979. 90) Fryzuk, M . D.; Haddad, T. S.; Berg, D. J. Coord. Chem. Rev. 1990, 99, 137. 91) Fryzuk, M . D.; Carter, A. ; Westerhaus, A. Inorg. Chem. 1985, 24, 642. 92) Fryzuk, M . D.; Carter, A. ; Rettig, S. J. Organometallics 1992,11, 469. 93) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Chem. Soc, Chem. Commun. 1996, 2783. 33 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 94) Lindoy, L . F. The Chemistry of Macrocyclic Ligand Complexes; Cambridge University Press: Cambridge, 1989. 95) Kyba, E. P.; Davis, R. E.; Hudson, C. W.; John, A. M . ; Brown, S. B.; McPhaul, M . J.; Liu, L . -K . ; Glover, A. C. J. Am. Chem. Soc. 1981,103, 3868. 96) Ansell, C. W. G.; Cooper, M . K.; Dancey, K. P.; Duckworth, P. A. ; Henrick, K.; McPartlin, M . ; Tasker, P. A . J. Chem. Soc, Chem. Commun. 1985, 439. 97) Caminade, A . - M . ; Majoral, J. P. Chem. Rev. 1994, 94, 1183. 98) Coles, S. J.; Edwards, P. G.; Fleming, J. S.; Hursthouse, M . B.; Liyanange, S. S. Chem. Commun. 1996, 293. 99) Fryzuk, M . D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445. 100) Manriquez, J. M . ; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1978,700,2716. 101) Hidai, M . ; Tominari, K.; Uchida, Y . J. Am. Chem. Soc 1972, 94, 110. 102) George, T. A. ; Tisdale, R. C. Inorg. Chem. 1988, 27, 2909. 103) Basch, H. ; Musaev, D. G.; Morokuma, K.; Fryzuk, M . D.; Love, J. B.; Seidel, W. W.; Albinati, A. ; Koetzle, T. F.; Klooster, W. T.; Mason, S. A.; Eckert, J. J. Am. Chem. Soc. 1999,121, 523. 104) Fryzuk, M . D.; Giesbrecht, G. R.; Rettig, S. J.; Yap, G. P. A. J. Organomet. Chem. 1999, 591, 63. 105) Fryzuk, M . D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928. 106) Giesbrecht, G. R. Amidophosphine Complexes of Electron-Poor Metals; Ph. D. thesis, University of British Columbia: Vancouver, 1998. 107) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,119, 9071. 34 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 108) Bowdridge, M . R. Niobium Phosphine Macrocyclic Complexes; M . Sc. thesis, University of British Columbia: Vancouver, 1998. 109) Fryzuk, M . D.; Haddad, T. S.; Mylvaganam, M . ; McConville, D. H. ; Rettig, S. J. /. Am. Chem. Soc. 1993,115, 2782. 110) Goldberg, K. I.; Hoffman, D. M . ; Hoffmann, R. Inorg. Chem. 1982, 21, 3863. 111) Rosi, M . ; Sgamellotti, A. ; Tarantelli, F.; Floriani, C ; Cederbaum, L. S. J. Chem. Soc, Dalton Trans 1989, 33. 112) Powell, C. B.; Hall, M . B. Inorg. Chem. 1984, 23, 4619. 113) Treitel, I. M . ; Flood, M . T.; Marsh, R. E.; Gray, H. B. J. Am. Chem. Soc. 1969, 91, 6512. 114) Chatt, J.; Fay, R. C ; Richards, R. L. J. Chem. Soc. A 1971, 2399. 115) Treitel, J. M . ; Flood, M . T.; March, R. E.; Gray, H. B. / . Angew. Chem. Int. Ed. Engl. 1974, 639. 116) Wigley, D. E.; Gray, S. D. Comprehensive Organometallic Chemistry II; Wilkinson, G., Stone, F. G. A . and Abel, E., Ed.; Permagon Press: Oxford, 1995; Vol. 5, pp 557. 117) Labinger, J. A. Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A. and Abel, E., Ed.; Permagon Press: Oxford, 1982; Vol . 3, pp 705. 118) Datta, S.; Wreford, S. S. Inorg. Chem. 1977,16, 1134. 119) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9. 120) Freundlich, J. S.; Schrock, R. R.; Davis, W. M . J. Am. Chem. Soc. 1994,118, 3643. 121) Freundlich, J. S.; Schrock, R. R.; Davis, W. M . J. Am. Chem. Soc. 1996,118, 3643. 122) Rocklage, S. M . ; Schrock, R. R. J. Am. Chem. Soc. 1980,102, 7808. 123) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31, 123. 35 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 124) Schrock, R. R. Acc. Chem. Res. 1979,12, 98. 125) Luo, L . ; Liting, L. ; Marks, T. J. J. Am. Chem. Soc. 1997,119, 8574. 126) Juvinall, G. L. J. Am. Chem. Soc. 1964, 86, 4202. 127) Schrock, R. R.; Meakin, P. J.Am. Chem. Soc. 1974, 96, 5288. 128) Schrock, R. R. J. Organomet. Chem. 1976,122, 209. 129) Schrock, R. R.; Parshall, G. W. Chem. Rev. 1976, 76, 243. 130) Schrock, R. R. / . Am. Chem. Soc. 1974, 96, 6796. 131) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978,100, 3359. 132) Rupprecht, G. A. ; Messerle, L. W.; Fellmann, J. D.; Schrock, R. R. / . Am. Chem. Soc. 1980,102, 6236. 133) Ivin, K. J.; Mol , J. C. Olefin Metathesis and Metathesis Polymerization; Academic Press: New York, 1997. 134) Rocklage, S. M . ; Fellmann, J. D.; Rupprecht, G. A. ; Messerle, L . W.; Schrock, R. R. J. Am. Chem. Soc. 1981,103, 1440. 135) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577. 136) Guggenberger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6578. 137) Schrock, R. R.; Sharp, P. R. / . Am. Chem. Soc. 1978,100, 2389. 138) Vahrenkamp, H. Angew. Chem. Int. Ed. Engl. 1978,17, 379. 139) Cotton, F. A. ; Walton, R. A. Multiple Bonds Between Metal Atoms; 2nd ed.; Oxford University Press: New York, 1993. 140) Messerle, L. Chem. Rev. 1988, 88, 1229. 36 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 141) Fryzuk, M . D.; McConville, D. H. Inorg. Chem. 1989, 28, 1613. 142) Belmonte, P. A. ; Schrock, R. R.; Day, C. S. J. Am. Chem. Soc. 1982,104, 3082. 143) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992,11, 1559. 144) Sattelberger, A. P.; Wilson, R. B. J.; Huffman, J. C. Inorg. Chem. 1982, 21, 2392. 145) Chisholm, M . H. ; Huffman, J. C ; Tan, L. Inorg. Chem. 1981, 20, 1859. 146) Canich, J. A. M . ; Cotton, F. A. Inorg. Chem. 1987, 26, 4236. 147) LaPointe, R. E.; Wolczanski, P. T. / . Am. Chem. Soc. 1986,108, 3535. 148) Templeton, J. L. ; McCarley, R. E. Inorg. Chem 1978, 20, 2716. 149) Cotton, F. A. ; Roth, W. J. Inorg. Chem. 1983, 22, 868. 150) Cotton, F. A. ; Diebold, M . P.; Roth, W. J. / . Am. Chem. Soc. 1986,108, 3538. 151) Cotton, F. A. ; Diebold, M . P.; Roth, W. J. / . Am. Chem. Soc. 1987,109, 5506. 152) Babaian-Kibala, E.; Cotton, F. A. ; Kibala, P. A . Inorg. Chem. 1990, 29, 4002. 153) Cotton, F. A. ; Daniels, L. M . ; Murillo, C. A. ; Wang, X . Inorg. Chem. 1997, 36, 896. 154) Sattelberger, A. P.; Wilson, R. B. J.; Huffman, J. C. Inorg. Chem. 1982, 21, 4179. 155) Wilson, R. B. J.; Sattelberger, A. P. I. Am. Chem. Soc. 1982,104, 858. 156) Cotton, F. A. ; Hall, W. T. J. Am. Chem. Soc. 1979,101, 5094. 157) Cotton, F. A. ; Hall, W. T. Inorg. Chem. 1980,19, 2352. 158) Cotton, F. A. ; Duraj, S. A. ; Roth, W. J. 1. Am. Chem. Soc. 1984,106, 6987. 159) Canich, J. A . M . ; Cotton, F. A.; Falvello, L. R. Inorg. Chim. Acta 1988,143, 185. 37 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 160) Campbell, G. C.; Canich, J. A. M . ; Cotton, F. A. ; Duraj, S. A. ; Haw, J. F. Inorg. Chem. 1986, 25, 287. 161) Scioly, A. J.; Luetkens, M . L . J.; Wilson, R. B.; Huffman, J. C ; Sattelberger, A . P. Polyhedron 1987, 6, 741. 162) Churchill, M . R.; Wasserman, H. J.; Belmonte, P. A. ; Schrock, R. R. Organometallics 1982,1, 559. 163) Miller, R. L. ; Toreki, R.; Lapointe, R. E.; Wolczanski, P. T.; Van Duyne, G. D.; Roe, D. C. I. Am. Chem. Soc. 1993,115, 5570. 164) Ting, C ; Messerle, L . J. Am. Chem. Soc. 1987,109, 6506. 165) Ting, C ; Baenziger, N . C ; Messerle, L. J. Chem. Soc, Chem. Commun. 1988, 1133. 166) Antifiolo, A. ; Carillo-Hermosilla, F.; Fajardo, M . ; Fernandez-Baeza, J.; Garcia-Yuste, S.; Otero, A . Coord. Chem. Rev. 1999,193-195, 43. 167) Barefield, E. K.; Parshall, G. W.; Tebbe, F. N . / . Am. Chem. Soc. 1970, 92, 5234. 168) Tebbe, F. N . ; Parshall, G. W. /. Am. Chem. Soc. 1971, 93, 3793. 169) Parshall, G. W. Acc. Chem. Res. 1975, 8, 113. 170) Mulford, D. R.; Clark, J. R.; Schweiger, S. W.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1999,18, 4448. 171) Profilet, R. D.; Rothwell, A. P.; Rothwell, I. P. J. Chem. Soc, Chem. Commun. 1993, 42. 172) Rothwell, I. P. / . Chem. Soc, Chem. Commun. 1997, 1331. 173) Ankianiec, B. C ; Fanwick, P. E.; Rothwell, I. P. / . Am. Chem. Soc. 1991,113, 4710. 174) Parkin, B. C ; Clark, J. R.; Visciglio, V. M . ; Fanwick, P. E.; Rothwell, I. P. Organometallics 1995,14, 3002. 38 References begin on page 28. Chapter One: The Activation of Dinitrogen and Ligand Design 175) Bochmann, M . J. Chem. Soc., Dalton Trans. 1996, 255. 176) Sperry, C. K.; Rodriguez, G.; Bazan, G. C. J. Organomet. Chem. 1997, 548, 1. 177) Dawson, D. Y. ; Arnold, J. Organometallics 1997,16, 1111. 178) Dawson, D. Y. ; Brand, H.; Arnold, J. / . Am. Chem. Soc. 1994,116, 9797. 179) Mullins, S. M . ; Bergman, R. G.; Arnold, J. Organometallics 1999,18, 4465. 180) Boring, E.; Sabat, M . ; Finn, M . G.; Grimes, R. N . Organometallics 1998,17, 3865. 181) Sperry, C. K.; Cotter, W. D.; Lee, R. A. ; Lachicotte, R. J.; Bazan, G. C. /. Am. Chem. Soc. 1998,120, 7791. 182) Rodriguez, G.; Bazan, G. C. J. Am. Chem. Soc. 1995,117, 10155. 183) Rodriguez, G.; Graham, J. P.; Cotter, W. D.; Sperry, C. K.; Bazan, G. C ; Bursten, B. E. J. Am. Chem. Soc. 1998,120, 12512. 184) Sperry, C. K.; Bazan, G. C ; Cotter, W. D. /. Am. Chem. Soc. 1999,121, 1513. 185) Weller, K. J.; Filippov, I.; Briggs, P. M . ; Wigley, D. E. Organometallics 1998,17, 322. 186) Clark, J. R.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1996, 15, 3232. 187) Toreki, R. T.; LaPointe, R. E.; Wolczanski, P. T. J. Am. Chem. Soc. 1987,109, 7558. 39 References begin on page 28. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2.1 Introduction In order to study the reactivity of tantalum complexes bearing the macrocyclic [P2N2] ancillary ligand (where [P 2N 2] = PhP(CH 2 SiMe 2 NSiMe2CH2)2PPh) , a means of introducing the ligand to the metal centre was required. The synthesis of transition metal halide complexes of the [P2N2] ligand for the majority of the transition metals studied in this laboratory has proven facile. The typical procedure involves adding a solvent, such as toluene or diethyl ether, to a mixture of the appropriate metal halide and [P2N2]Li2-(C 4H80 2) at low temperatures, and allowing the solution to warm. The formation of lithium chloride provides a thermodynamic driving force for the reaction. The [P2N2] metal chloride complex can be isolated by extraction into an appropriate solvent and removal of insoluble L i C l by filtration. This approach is illustrated in equation 2.1. Chapter Two 40 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand toluene, diethyl ether or tetrahydrofuran M r . . -78°C to 25°C MCIxLn • [P2N2]MCI(X.2) [2.1] - 1,4-dioxane - 2 LiCI - n L [P 2N 2]Li 2.(C 4H 80 2) L d = 1,4-dioxane MCIxLn = TiCI4, TiCI3(THF)3, ZrCI4(THT)2, ZrCI4(THF)2, HfCI4(THT)2, VCI3(THF)3, NbCI3(DME), NbCI4(THF)2, CrCI3(THF)3 The resulting [P2N2] metal halide complexes are versatile starting materials for the further study of both the organometallic chemistry and inorganic chemistry of these elements. For example, such derivatives allow for the preparation of alkyl and other organometallic and inorganic complexes via metathesis reactions,1 as well as the generation of lower-valent complexes and dinitrogen complexes via reduction,2. A list of the well-characterized early transition metal halide complexes of the [P2N2] ligand synthesized to date is shown in Table 2.1. Table 2.1. Early transition metal halide complexes of the [P2N2] ligand prepared to date in the Fryzuk laboratory. Group 4 Group 5 Group 6 1s t Row [P 2N 2]TiCl 23 [P 2 N 2 ]VC1 4 [P 2N 2]CrCl5 2 n d Row [P 2 N 2 ]ZrCl 2 1 [P 2 N 2 ]NbCl, 6 [P 2 N 2 ]NbCl 2 7 3 r d Row [P 2 N 2 ]HfCl 2 , [P 2 N 2 ]HfI 2 8 41 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Unlike the other group 5 metals vanadium and niobium, there is no convenient low-valent tantalum starting material. Although the vanadium complex [P 2 N2]VC1 4 is available via VC1 3(THF) 3 , and the niobium complex [P 2 N 2 ]NbCl 6 is readily prepared from NbCl 3 (DME) , 9 there is no simple route to [P 2N 2]TaCl. The only readily available tantalum halide starting materials are the Ta(V) halides. The most commonly used is TaCl 5 , which is prepared from the reaction of tantalum metal with chlorine gas and is available commercially. A search of the literature to examine how other ancillary ligands have been attached to Ta(V) reveals that there are several difficulties associated with using TaCls as a starting material. As shown in equation 2.2, the reaction of the trilithium salt of the triamidoamine ligand N(CH 2 CH 2 NSiEt 3 ) 3 with TaCl 5 generates the desired product [N(CH 2 CH 2 NSiEt 3 ) 3 ]TaCl 2 , but only in 20% yield. 1 0 This low yield was attributed to the reduction of the Ta(V) starting material to form intractable materials. The accessibility of the Ta(IV) oxidation state is a problem in the reaction of TaCl 5 with alkali metal salts of ligand precursors, where reduction competes with nucleophilic displacement of chloride ligands. Et,Si Et20 Li3[N3N]* + TaCI5 — • \ + intractable materials [2.2] [N3N]* = N(CH2CH2NSiEt3)3 [N3N]*TaCI2 20% yield A common approach to avoiding the reduction of Ta(V) caused by alkali metal salts of ligand precursors is to use silylated or stannylated ligand precursors in their place. The resulting byproduct of the reaction of these precursors with TaCls is a trialkylsilylchloride (R 3SiCl) or trialkylstannylchloride (R 3SnCl) respectively. This approach has proven to be a 42 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand reliable route to monocyclopentadienyl Ta(V) chlorides such as (r | 5 -C 5 H 5 )TaCl 4 , 1 M 3 related l igands, 1 4 - 1 6 and has also been adapted to attach a diamidopyridine ligand to Ta(V) without metal reduction, as shown in equation 2.3. 1 7 Ar / , * x S i M e s TaCL \ " "SiMe 3 Ar 80°C - 2 CISiMe3 Ar = 2,6-Pr' 2C 6H 3 Ar •N , N *Ta Cl -N Ar 84% yield [2.3] Aside from the accessibility of lower oxidation states, another problem associated with attaching ligands to TaCls is the inherent Lewis acidity of this species. In some cases, this Lewis acidic reactivity occurs in a controlled manner and generates ionic reaction products. One example of this is the reaction of TaCls with the tri-n-butylstannyl substituted indene, l-CBu'^SnjCgFL, shown in equation 2.4. 1 6 Although the anticipated product was the monosubstituted species (r^-CgFLOTaCL, the only product observed was the salt, [(r|5-C9H 7 )2TaCl2 ] + [TaCl6] _ . A similar reaction has been reported for an organotin complex of the sterically bulky Tp* ligand (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate); the reaction of Tp*SnClBu' ! 2 with TaCl 5 produced the salt [Tp*TaCl 3] +[TaCl 6]". 1 5 43 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2 H SnBu n 3 + 2 TaCI5 - 2 CISnBu"3 CH2CI2 [TaCI6] [2.4] Although in the previously mentioned cases the formation of salts of the [TaClg] -anion did not deter the isolation of the ligand-bearing tantalum chloride complexes, with other ligand systems the generation of salts leads to difficulties in separating and isolating compounds, often due to insolubility in hydrocarbon solvents. A method of dealing with the dual synthetic difficulties of tantalum reduction as well as salt formation due to the Lewis acidity of TaCls is therefore required; however, no widely applicable procedure has been described. There is one interesting example of a procedure that has proven to be the only reliable route to tantalum halides for the borollide ligand.' 8 The reaction of the borollide ligand lithium salt [C4H4B-N(CHMe2)2][Li2<C4H80)] with two equivalents of AICI3 generates a heterogeneous mixture that can be reacted with TaCls to produce [C4H 4B-N(CHMe 2) 2]TaCl3, as shown in equation 2.5. The exact nature of the ligated aluminum species is unknown; however, it is interesting to speculate that the success of this procedure may be dependent on the Lewis acidity of A1C13, as well as the less reducing nature of the aluminum derivative compared to that of the lithium salt. 2-2+ B Li2(C4H80) 2)TaCI5 C l 1)AICI3 Cl Cl [2.5] 44 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand The Lewis acidity of five coordinate TaCfs can also interfere in mechanisms other than salt formation. For example, TaCl 5 binds ethers, and can then cleave the C-0 bonds. This renders commonly used ethereal solvents such as tetrahydrofuran of little utility, and as the lithium salts of ligand precursors commonly contain bound ether donors, they can also interfere. On the other hand, this reactivity with ethers has been used to advantage in attaching the calix[4]arene ligand to TaCl.5, where a methylated version of the ligand precursor is reacted with TaCl 5 to generate one of the desired O-Ta linkages, presumably with the loss of M e C l . 1 9 The fourth phenoxide ligand could be generated by photolysis or reaction of the dialkyl with FL. This reaction sequence is shown in Scheme 2.1. Scheme 2.1. Cl Cl Mep HO _ OMe Ta 45 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand These examples all outline how various ancillary ligands have been successfully attached to TaCIs, but it is clear that these syntheses were not facile, and each suffers from unique synthetic difficulties. No single starting material or procedure has provided a reliable route to attaching ancillary ligands to TaCl 5 , and other starting materials have been sought after. This chapter describes attempts to synthesize a tantalum complex of the [P2N2] ligand; the ideal complex should provide a versatile entry to study a range of tantalum chemistry with this ancillary ligand. 2.2 Attempted Syntheses of [P2N2]TaCl3 (i) Reaction of [P 2N 2]Li 2 (C 4H 80 2) and [P2N2]Li2 (C 4H 80) with TaCl 5 Reactions of TaCIs with the 1,4-dioxane adduct of the lithiated ligand precursor, [P 2N 2]Li 2-(C4Hs0 2), were performed under a variety of conditions in an attempt to form [P 2 N 2 ]TaCl3. Conditions varied in both solvent, including benzene, toluene and diethyl ether, and temperature, from -78°C to H0°C. In each instance a colour change from the pale yellow TaCl 5 solution to a bright orange occurred, accompanied by the deposition of a considerable amount of precipitate. While a reaction had occurred, no product with the properties anticipated for [P 2N2]TaCl3 was isolated. The products that were observed by ' H and 3 1 P{ 'H} N M R spectroscopy varied with subtle changes in reaction conditions. The 3 1 P{ 'H} N M R spectrum of these reaction mixtures sometimes indicated that a clean reaction may have occurred; for example, in one case when the reaction of [P2N2]Li2-(C 4H 802) with TaCIs was carried out in benzene, two doublets were observed in the 3 I P{ 'H} spectrum, at 8 33.1 and 8 1.6, with a coupling constant of 47.9 Hz. The coupling constants seems reasonable for the complex [P 2N 2]TaCl3 in the case where the two phosphine ligands are bound, but in different chemical environments. However, the ' H N M R spectrum indicated that impurities were present, and the low yield of sample isolated from the reaction decomposed over the course of a day and precipitated an intractable solid. Under a variety of conditions, similar colour changes were noted for the reaction of [P 2N2]Li2(C 4Hg02) with TaCIs. The solution instantly changed from pale yellow to a darker yellow-orange as 46 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand previously noted, but over extended periods of time turned dark brown. Occasionally, small amounts of crystalline materials were isolated. In one case small amounts of yellow, red and green crystals formed from the reaction of [P2N2]Li 2 (C 4 H80 2 ) with TaCl 5 in diethyl ether. These crystals were only present in a few percent yield, and could not be separated from each other or identified as [P2N 2 ]TaCl3. The crystals did not redissolve into diethyl ether, the same solvent from which they had been crystallized, which perhaps is an indication that these crystals were decomposition products. Frequently, the reaction products included considerable amounts of unreacted [P 2 N2]Li 2 , as evidenced by a peak near -39 ppm in the 3 1 P{'H} N M R spectrum that is a 1:1:1:1 quartet due to coupling to 7 L i (I = 3/2, 92.6% abundance). Without the usual route to [P2N2] metal complexes available for TaCl 5 , we sought out other methods to attach the ligand to this metal chloride. As noted in the introduction, the literature provides many examples of failed attempts to use TaCl 5 as a metal starting material, as well as insights into what the problems are with its use. One obvious problem arises from the Lewis acidic behaviour of TaCfs, which could lead to difficulty in using the 1,4-dioxane adduct of the ligand precursor [P 2 N 2 ]Li 2 . The formation of the complexes TaCl5(C4Hg02) or (TaCl 5)2(C 4H 802), in which one or both of the oxygen donors in 1,4-dioxane could be coordinated to a tantalum metal centre, could lead to unwanted side reactions. The 1,4-dioxane adducts of TaCls are probably less soluble, and complexes such as (TaCl5)2(C4H802) could precipitate from solution. 2 0 Ether adducts of TaCl 5 are also known to undergo further reactions that cleave a C-0 bond to form alkoxide complexes,2 0 as previously shown in Scheme 2.1. 1 9 The reaction of the tetrahydrofuran adduct [P2N2]Li2-(THF) with TaCIs was examined in an attempt to avoid the synthetic difficulties encountered with the 1,4-dioxane adduct of [P 2 N 2 ]Li 2 . The 3 I P{ 'H} N M R spectrum of the products of the reaction of [P 2 N 2 ]Li2 - (C 4 H 8 0) with TaCl 5 contained resonances which indicated the minor product of this reaction was the compound previously described, with coupled 3 I P resonances present as doublets at 8 33.1 and 8 1.6. The major product in the 3 I P{ ! H} N M R spectrum, which was not observed before, appeared as two broad peaks of similar line width and intensity at 8 8.5 and 8 -15.5. 47 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand While clearly still a mixture, the ' H N M R spectrum of this reaction product also appeared to be largely composed of one compound. Unfortunately, attempts to isolate this material led to difficulties similar to those previously described. After several days of storage under a nitrogen atmosphere, the material decomposed into an intractable solid. Due to the apparently large effect in changing from the 1,4-dioxane adduct ligand precursor [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) to the THF adduct [P 2 N 2 ]Li 2 -(C 4 H 8 0), an attempt was made to prepare a ligand precursor that did not contain an ether donor. (ii) Synthesis of the ether-free ligand precursor [P2N2]Li2 The isolation of the lithium salt of the [P 2N 2] ligand as the 1,4-dioxane adduct [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) is a procedure that continues to be used due to its simplicity and convenience. The addition of 1,4-dioxane to the crude product [P 2N 2]Li? in toluene precipitates the 1,4-dioxane adduct in high purity and 80% yield, whereas previous attempts to isolate [P 2 N 2 ]L i 2 without adding an ethereal donor provided only an oily product. The discovery of this route to the isolation of the [P 2 N 2 ]Li 2 moiety was serendipitous. A trace amount of L i C l , which occurs as a very fine precipitate with slight solubility in diethyl ether, was not being removed by filtration through Celite. The addition of 1,4-dioxane was initially done in an attempt to precipitate the L i C l produced in the synthesis of [P 2 N 2 ]Li 2 , but instead resulted in the precipitation of the dioxane adduct [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) . Other ether adducts of [P 2 N 2 ]L i 2 can be made from the [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) adduct. For example, the addition of tetrahydrofuran to [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) displaces the 1,4-dioxane and produces the tetrahydrofuran adduct [P 2 N 2 ]Li 2 (C 4 HsO). However, in the case of the 1,4-dioxane and the tetrahydrofuran adducts of [P 2 N 2 ]Li 2 , it is not possible to remove the ether donor to produce the ether-free lithium salt [P 2 N 2 ]Li 2 . The isolation of ether-free [P 2 N 2 ]L i 2 was performed by slight modification of the current method for the synthesis of [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) shown in Chapter 1, Scheme 1.6. After the reaction of a diethyl ether solution of [PhP(H)CH 2Si(Me) 2] 2NH and [ClCH 2 Si(Me) 2 ] 2 NH with four equivalents of Bu"Li at 0°C, the diethyl ether was removed 48 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand under vacuum. This resulted in a bubbly paste that was then heated at 50°C for a further three hours to yield a dry powdery solid. This step is essential to prepare the adduct-free ligand precursor. Heating under vacuum removes the last remaining diethyl ether that otherwise remains coordinated to the [ P 2 N 2 ] L i 2 moiety, making its isolation troublesome. This solid was then dissolved in toluene and filtered to remove L i C l . The majority of the toluene was removed from the filtrate, followed by addition of hexanes. This resulted in the precipitation of [P 2 N 2 ]L i 2 (1) as a white crystalline solid in 80% yield. Both the ' H and 3 1P{ l H) N M R spectra give no reason to believe that the structure or bonding of the ether-free [ P 2 N 2 ] L i 2 ligand precursor varies significantly from the previously prepared ether adducts [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) or [P 2 N 2 ]Li 2 -(C 4 H 8 0). As with the ether adducts, the 3 I P{ 'H} N M R spectrum of 1 displays a quartet due to coupling to 7 L i (I =3/2, 92.6% abundance). A triplet in the 3 1 P{'H} N M R spectrum due to coupling in the isotopomer containing the less abundant 6 L i nuclei (1=1, 7.4% abundance) is also evident, although these resonances are largely obscured by the larger signals of the L i containing isotopomer. 1 7 The / U P coupling constant to L i of 56 Hz observed for 1 is slightly larger than the corresponding coupling constants reported for either the 1,4-dioxane or tetrahydrofuran adducts of 54 Hz and 52 Hz respectively. The larger coupling constant is consistent with a slightly stronger dative bond between the phosphine donor and the lithium atom. Unfortunately, the outcome of the reaction of [P 2 N 2 ]Li 2 with TaCls in benzene was no more favourable than the reaction of [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) with TaCls, and no pure product was isolated. These results indicate that the presence of 1,4-dioxane or THF is not the central problem in the synthesis of [P2N2]TaCl3. (iii) Synthesis of the less strongly reducing l igand precursor [P2N2JMg(C4H80) There are cases where salt metathesis reactions used to attach ancillary ligands to TaCIs were believed to result largely in metal reduction. 1 0 ' 2 1 To avoid the problem of metal reduction, which frequently accompanies metathesis reactions that use lithium reagents, other ligand precursors were examined. In the case of the reaction of [P 2 N 2 ]L i 2 with TaCIs a 49 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand reaction may ensue that produces a Ta(IV) species, LiCI, and oxidized ligand precursor instead of salt metathesis products. The darkening of the reaction mixture to a dark brown over time could be taken as an indication that reduction is taking place. Additionally, in some trials of the reaction of [P 2 N 2 ]Li 2 -(C4H 8 0 2 ) it was possible to obtain small amount of dark green crystalline material, a colour that is probably indicative of a reduced tantalum complex. Grignard reagents are less reducing than their corresponding lithium reagents; therefore, by analogy, magnesium salts of amides should be less reducing than lithium amides. The tetrahydrofuran adduct of the magnesium salt [P 2N 2]Mg-(C4H 80) (2) was prepared from the reaction of [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) with MgCl 2-(OEt 2) using tetrahydrofuran as the solvent, as shown in equation 2.6. Although the ' H N M R spectrum of the product differs little from that of the lithium salt [P 2 N2]Li2-(C 4 H 8 0), the 3 I P { ' H } N M R spectrum clearly demonstrates that the lithium cations have been replaced by magnesium; the 3 I P chemical shift of the ligand is not significantly changed, but the coupling to 7 L i observed in the lithium salts is no longer present. MgCI2.(OEt2) THF - 2 LiCI - C 4 H 8 0 2 [2.6] [P 2N 2]Li 2.(C 4H 80 2) [P2N 2]Mg.(C4H 80) L = 1,4-dioxane 2 The reaction of [P 2 N 2 ]Mg(C 4 H 8 0) with TaCl 5 failed to produce a material that could be identified as [P 2 N 2 ]TaCl 3 . The ' H and 3 I P{'H} N M R spectra indicate that a mixture of products was present. Although no evidence for metal reduction was observed in this reaction, it could not be definitively concluded that the magnesium salt 2 was weakly reducing enough to react cleanly with TaCl 5 to form [P 2N 2]TaCl3. 50 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand (iv) Synthesis of the less strongly reducing ligand precursor [P2N2]Zn With the expectation that it would be even less reducing than L i or Mg salts, a Zn complex of the [P2N2] ligand precursor was also investigated as a potential precursor to [P 2 N 2 ]TaCl 3 . The anticipated side-product of the reaction of [P 2N 2]Zn with TaCl 5 is ZnCl 2 , rather than LiCI, which would avoid any synthetic difficulties that might result from TaCIs forming an adduct with LiCI. The reaction of [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) with ZnCl 2 in THF provided a route to the ether-free species [P2N2]Zn (3), as shown in equation 2.7. The yield of [P 2N 2]Zn obtained is highly sensitive to the purity of the ZnCl 2 used. Fresh bottles of 0.5 M anhydrous Z n C l 2 in THF purchased from Aldrich appeared to contain considerable water, as evidenced by the formation of two additional products, identified as ([P 2N 2]H)ZnCl and ( [ P 2 N 2 ] H 2 ) Z n C l 2 in which one or both amide donors were protonated. Identical troubles with moisture contamination occurred with solid anhydrous ZnCl 2 that was purchased from Aldrich. Fortunately, [P2N2]Zn can be isolated from the unwanted side-products by means of its greater solubility in hexanes. A high-yield route to [P 2N 2]Zn that eliminates the side-product of water contamination involves drying solid Z n C l 2 by refluxing in S O C I 2 for twelve hours and removing excess S O C I 2 under vacuum. 2 2 This truly anhydrous Z n C l 2 reacts with [P 2 N 2 ]Li 2 - (C4H 8 0 2 ) to provide [P 2N 2]Zn in nearly quantitative yield with no detectable side-products or impurities, as determined by " P{ H} and H N M R spectroscopy as well as elemental analysis. [2.7] Me 2 Me 2 Me 2 Me 2 [P 2N 2]Li 2.(C 4H 80 2) [P2N2]Zn L = 1,4-dioxane 3 51 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand The solid-state molecular structure of [P2N2]Zn as determined by X-ray crystallography is shown in Figure 2.1. Only one of two independent molecules in the asymmetric unit is shown. Selected bond lengths, angles and dihedral angles for one of the two independent [P2N2]Zn molecules are given in Table 2.2. The structure clearly shows that the Zn centre is four-coordinate, with no additional donor atoms. Although four-coordinate Zn is not uncommon, the geometry around the Zn centre is somewhat unusual; it is best described as a slightly distorted disphenoid.23 A structurally intriguing feature of this geometry is that the Zn centre sits slightly nested in rather than perched on the [P2N2] macrocycle; the P(l)-Zn(l)-P(2) angle is 158.31(3)°, but it is better described as 201.69(3)° for comparison with other complexes of [P2N2] that contain the metal perched on the [P2N2] macrocycle. This nested in Zn centre likely arises from the relatively small ionic radius of the Zn(II) ion of 0.69 A. The absence of vacant d orbitals that could facilitate a non-VSEPR geometry renders the tetrahedral geometry more common for four-coordinate Zn(JJ), and the macrocyclic nature of the [P2N2] ligand undoubtedly plays a role in this considerable distortion from a regular tetrahedron. Considering the disphenoidal geometry enforced by the macrocyclic [P2N2] ligand, what seems unusual in the structure of 3 is the absence of an additional donor such as THF to fill the empty coordination site and form a five-coordinate trigonal bipyramidal structure that is more common for Zn(JJ). Another structural feature worth noting is the relatively small twist in the [P2N2] backbone. A measure of the twist in the [P2N2] macrocycle is given by the difference between the P(l)-Zn(l)-N(l)-Si(l) dihedral angle of 9.48(14)° and the P(2)-Zn(l)-N(l)-Si(4) dihedral angle of -18.32(13)°. This difference of 27.8(3)° is relatively small in comparison to the twist observed in other transition metal complexes; this aspect of the [P2N2] macrocycle will be expanded upon later. Although the small twist in the [P2N2] framework in this case may be due to the unusual geometry enforced by the small metal centre, it may also reflect the lack of it-bonding of the amido lone-pairs electron to the metal centre, as no d-orbitals are vacant in Zn(U). 52 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Table 2.2 Selected bond lengths, angles and dihedral angles in [P 2N 2]Zn (3). Atom Atom Distance (A) Atom Atom Distance (A) Zn(l) P(l) 2.3637(7) Zn(l) N(l) 2.018(2) Zn(l) P(2) 2.3520(7) Zn(l) N(2) 2.011(2) Atom Atom Atom Angle (°) Atom Atom Atom Angle O P(l) Zn(l) P(2) 201.69(3) P(l) Zn(l) N(2) 95.65(6) N(l) Zn(l) N(2) 125.95(10) P(2) Zn(l) N(l) 96.78(7) P(l) Zn(l) N(l) 93.01(7) P(2) Zn(l) N(2) 94.18(6) Atom Atom Atom Atom Dihedral Angle (°) P(l) Zn(l) N(l) Si(l) 9.48(14) P(2) Zn(l) N(l) Si(4) -18.32(13) P(l) Zn(l) N(2) Si(2) -24.25(13) P(2) Zn(l) N(2) Si(3) 4.74(13) C(3) Figure 2.1. ORTEP depiction of the solid-state molecular structure of complex 3, [P 2N 2]Zn. 53 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand The reaction of [P 2N 2]Zn with TaCl 5 failed to yield [P 2 N 2 ]TaCl 3 . The solution did darken from pale yellow to orange upon addition of [P 2N 2]Zn to TaCIs in toluene, and a solid did precipitate over the course of several hours. Unfortunately, neither the solution or solid precipitate contained any species identifiable as [P2N2]TaCl3, despite the fact that no darkly coloured species associated with metal reduction appeared to form. (v) Reaction of [P 2N 2]H 2 with TaCl 5 Another route to tantalum halide complexes that has proven successful for many alcohols and secondary amines is the direct reaction of these ligands with TaCIs, to generate metal alkoxide or amide complexes via elimination of HCI. In some cases it is necessary to add a weak base, such as NEt 3 , to react with the HCI byproduct and drive the reaction to completion. The synthesis of [P 2 N 2 ]H 2 , in which the two amido donors have both been protonated, is readily performed by reacting [P 2 N 2 ]Li 2 -(C4H 8 0 2 ) with two equivalents of {Et 3 NH} + CF. The reaction of [P 2 N 2 ]H 2 with TaCIs in toluene produced a dark orange product that exhibits a single resonance at 8 11.4 in the 3 1P{ 'HJ N M R spectrum. The ' H N M R spectrum was also indicative of a species of high symmetry, with only two silyl methyl environments. The ligand N - H signals are still apparent at 8 0.73. These data indicate that the reaction of [P 2 N 2 ]H 2 with TaCl 5 produces the complex ([P 2N 2]H 2)TaCl5 (4), where the Ta centre is coordinated to the two phosphine donors of the [P 2N 2] ligand and five chloride ligands, while the amine donors have not reacted to eliminate HCI and remain protonated. Attempts to crystallize complex 4 have failed, because solutions of 4 decompose to produce a mixture of both soluble products, and intractable products. The reaction of ([P 2 N 2 ]H 2 )TaCl 5 with NEt 3 failed to produce a species identifiable as [P 2N 2]TaCl 3 . The attempted reduction of a solution of ([P 2N 2]H 2)TaCl5 with two equivalents of Na in the form of a Na/Hg amalgam produced an intractable green solid. It is not clear how the complex ([P 2N 2]H 2)TaCls could be treated to provide [P 2 N 2 ]TaCl 3 . 54 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2.3 Attempted Synthesis of [P2N2] Complexes of Low-Valent Tantalum Halides With the lack of success in synthesizing [P 2 N 2 ]TaCl 3 despite many attempts by a variety of routes it became apparent that the preparation of [P2N2]TaCl3 might not be possible. Like the complex [P 2 N 2 ]H 2 TaCl 3 , the species [P 2 N 2 ]TaCl 3 might decompose, if it is formed at all. The literature does provide insight into the possible decomposition pathways available to the [P 2 N 2 ]TaCl 3 complex. Particularly susceptible to decomposition are the Si-N bonds in the [P 2N 2] ligand. Cleavage of one of these bonds can occur with the formation of a strong Si-Cl bond and a strong imido N=Ta linkage. In fact, the ability of tantalum chlorides to react with Si-N bonds has been used as a route to generating tantalum imide complexes.2 4 This reaction has also resulted in the unexpected generation of imido complexes from amido ligand precursors via loss of Me 3 SiCl , as shown in Scheme 2.2. 2 5 Scheme 2.2. It has already been noted that, unlike vanadium and niobium, there are few low-valent halide complex precursors available for tantalum. A possible advantage of using a 55 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand low-valent halide is that with fewer halide ligands present, the complexes may be less sterically crowded relative to the seven-coordinate species [P 2 N 2 ]TaCl 3 , and therefore less likely to decompose by the mechanism in Scheme 2.2. Likewise, in low-valent complexes it is likely that the formation of the tantalum-nitrogen multiply bonded imido linkage would not provide such a driving force, because these so-called "hard" ligands 2 6 favour higher oxidation states. Two lower-valent tantalum halide complexes that could be utilized as starting materials for halide complexes of the [P 2N 2] ligand are the tetrahydrothiophene complex Ta 2Cl6(SC4H 8) 3 and the pyridine complex TaCi4(NC5H<;)2. (i) Reaction of [P 2N 2]Li 2 with Ta 2 Cl 6 (SC 4 H 8 )3 The Ta(III) tetrahydrothiophene complex Ta 2Cl6(SC4H 8) 3 has been known for some time, although it has found little use as a Ta(III) complex precursor despite the fact that it is straightforward to synthesize. The reaction of Na/Hg amalgam with TaCls in the presence of an excess of tetrahydrothiophene generates Ta 2Cl6(SC 4H 8) 3 in good yield. 2 7 The reaction of this complex with [P 2 N 2 ]L i 2 on a small scale in a N M R tube initially appeared promising. Over the course of an hour the solution went from red to brown and a fine solid, presumed to be L i C l , precipitated. The 3 I P{'H} N M R spectrum contained resonances at 5 4.0 and 2.1, with a coupling constant of 81.0 Hz, indicative that the two environments are due to two phosphorus donors bound to the same metal centre. Attempts to isolate this product on a large scale failed, however, as over the course of eight hours it decomposed to give a variety of products by ' H and 3 1 P{'H} NMR. Insufficient evidence is available to speculate on the bonding in the initial complex formed, though the diamagnetism of this complex is indicative that metal-metal bonding interactions are still initially present. (ii) Reaction of [P 2N 2]Li 2 with TaCl4(NC 5H 5) 2 Another low-valent tantalum complex which could be considered a useful starting point for the synthesis of a tantalum halide complex is the Ta(IV) pyridine complex TaCl4(NC 5H<0 2. 2 8 Few reports exist of this complex being used as a starting point for 56 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand tantalum chemistry; the reduction of Ta(V) precursors is a more common route to Ta(IV) amides.2 9 It is not clear if the unpopularity of TaCl 4 (NC5H 5 ) 2 is due to difficulties in controlling its reactivity, or difficulties associated with the characterization of N M R inactive d 1 complexes. The reported synthesis of TaCLCNCsHs^ is facile, and involves simply heating a solution of TaCIs in pyridine at 50°C for five weeks. The product precipitates as a purple solid that is insoluble in most solvents. Despite the insolubility of TaCl4(NC 5 H 5 )2 in tetrahydrofuran, a solution of [P 2 N2]Li2-(C4H80 2) in tetrahydrofuran did react with solid TaCUfNCsHs^ over two days, to give a brown solution, and no remaining insoluble species. Unfortunately it proved impossible to isolate or characterize any species that had the anticipated properties of [P 2 N2]TaCl2. Removal of the tetrahydrofuran solvent and extraction of the remaining residues into toluene resulted in the isolation of oily materials that resisted crystallization. 2.4 Synthesis and Characterization of [P2N2]TaMe3 (i) Synthesis and Structure of [P2N2]TaMe3 As described above, the synthesis of [P 2N2]TaCl3 from the reaction of [ P 2 N 2 ] L i 2 or other [P 2N 2] ligand precursors with TaCIs has proven unsuccessful. We therefore turned to an alternative starting material, TaMe 3 Cl2. The reaction of the colourless ligand salt [P2N2]Li2-C 4 H 8 02 with the pale-yellow starting material TaMe 3 Ci2 3 0 ' 3 1 generates a dark solution from which [P 2N 2]TaMe 3, (5), can be isolated in 80% yield (equation 2.8). Because the yellow trimethyl species 5 is only moderately soluble in hexanes, it can be separated from the more soluble dark-coloured impurities by rinsing the crude materials with hexanes. Cooling a saturated hexanes solution of 5 gives yellow crystals of 5. 57 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand c/s-[P2N2]Li2.clioxane + TaMe 3CI 2 Et 2 0 Ph [2.8] The solid-state molecular structure is shown in Figure 2.2; crystals of 5 contain 0.5 equivalents of cocrystallized hexanes per [P2N2]TaMe3 molecule. Selected bond angles and distances are given in Table 2.3. The complex can be regarded as a capped trigonal prism, where P(l) caps a rectangular face of the prism defined by C(26), C(27), N(l) , and N(2). One triangular face of the prism is composed of C(25), C(26), and C(27). The second triangular face is composed of N(l) , N(2), and P(2). The compound approaches C s symmetry, with an approximate mirror plane of symmetry defined by P(l), Ta(l), C(25), and P(2). A twist in the [P 2N 2] ligand, however, reduces the symmetry and directs the phenyl ring attached to P(l) in the direction of C(26); thus, all the metal-bound methyl groups are inequivalent, with Ta-C bond lengths of 2.239(3), 2.272(3), and 2.252(4) A. A measurable degree of the twist in the ligand backbone is available by comparing the P-Ta-N-Si dihedral angles. The P(l)-Ta(l)-N(l)-Si(4) dihedral angle is 163.32(14)°, whereas the P(l)-Ta(l)-N(2)-Si(3) dihedral angle is 130.7(2)°, a difference of 32.62(34)°. In the absence of the twist in the [P 2N 2] ligand backbone, these two dihedral angles should be equal. 58 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Figure 2.2. ORTEP representation of the solid-state molecular structure of [P2N2]TaMe3, 5. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. Table 2.3 Selected bond lengths, angles and dihedral angles in [P 2N 2]TaMe 3 (5). Atom Atom o Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.6180(8) Ta(l) C(25) 2.239(3) Ta(l) P(2) 2.6088(9) Ta(l) C(26) 2.272(3) Tad) N(l) 2.141(3) Tad) C(27) 2.252(4) Ta(l) N(2) 2.210(2) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) P(l) Ta(l) P(2) 143.00(3) P(l) Ta(l) N(2) 73.86(7) N(l) Ta(l) N(2) 96.39(9) P(l) Tad) C(26) 75.77(9) Pd) Ta(l) N(l) 84.58(7) Pd) Ta(l) C(27) 76.50(9) Atom Atom Atom Atom Dihedral Angle (°) Pd) Ta(l) N(l) Si(4) 163.32(14) P(2) Tad) N(l) Sid) 146.4(2) Pd) Tad) N(2) Si(3) 130.7(2) P(2) Ta(l) N(2) Si(2) 160.7(2) 59 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand As has been found for other [P2N2] complexes of the early transition elements, the metal is perched on, rather than nested in, the macrocycle. 1 ' 3 2 The larger P-Ta-P bite angle of 143.00(3)° compared to the smaller N-Ta-N bite angle of 96.39(9)° is consistent with the previously observed binding of the [P2N2] ligand in which the amide nitrogens are typically cis and the phosphines closer to a trans disposition. The capping phosphine Ta-P(l) distance of 2.6180(8) A is only marginally longer than the Ta-P(2) distance of 2.6088(9) A . The Ta-N(l) bond length of 2.141(3) A is shorter than the Ta-N(2) distance of 2.210(2) A . Both of these Ta-N distances are longer than those previously reported 2 9 ' 3 3 - 3 6 with the electronically similar amide N(SiMe 3 ) 2 , where Ta-N distances have been found in the range of 1.899-2.045 A; however, none of these compounds contain seven-coordinate tantalum. Therefore, it is uncertain whether these somewhat longer bonds are due to steric crowding or to electronic effects, such as a lack of metal orbitals suitable for bonding with the amido 7t-electrons. (ii) Variable-Temperature NMR Spectroscopy of [P2N2]TaMe3 (5) 1 3 1 1 The room-temperature H and P{ H} N M R spectra of trimethyl species 5 are not indicative of the lack of symmetry observed in the solid state structure. At room temperature a singlet is observed in the 3 1 P{ 'H} N M R spectrum at 8 30.4. As the temperature is lowered, this 3 1 P{'r i} N M R signal broadens and then decoalesces at 220 K. The low-temperature limiting spectrum at 180 K consists of two doublets at 8 41.1 and 19.6, with 2 J P P = 71.8 Hz, which is consistent with two different phosphorus-31 environments with both phosphines bound to the metal centre, as observed in the solid state. The effect of temperature on the 3 I P{ 'H} spectrum of [P2N2]TaMe3 is shown in Figure 2.3. 60 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 ' I 1 ' 1 1 I 1 1 1 1 I 1 1 60 50 40 30 , 20 (PPm) 10 0 2 9 5 K -10 Figure 2.3. Variable-temperature 3 I P{ !H} N M R spectra of [P 2N 2]TaMe 3 (5). A line-shape analysis3 7 of the variable-temperature 3 I P{ 'H} N M R spectra from 180 to 230 K combined with an Eyring plot of the resulting rate constants provided the activation parameters of AH* = 49.3 ± 1.5 kJ mol"1 and AS* = 57.7 + 3.6 J mol"1 K" 1 for the exchange of the phosphorus environments. The Eyring plot is shown in Figure 2.4. 61 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 6 i 5 -4 -3 -cf 1 -0 --1 --2 --3 -0.004 0.0045 0.005 0.0055 0.006 1/T Figure 2.4. Eyring plot for the exchange of 3 I P environments in [ P 2 N 2 ] T a M e 3 (5) (R 2 = 0.9914). A variable-temperature ' H N M R study of 5 was also undertaken. At room temperature there is only one signal for the tantalum-bound methyl groups. The chemical shift of this signal is solvent-dependent; it appears at 5 1.14 in C^D(, and at 8 0.45 in C D 2 C E . The [ P 2 N 2 ] ligand in 5 gives rise to two resonances for the silyl methyl groups at room temperature, corresponding to the "top" and "bottom" of the ligand (top refers to the side of the ligand to which the metal is coordinated), and similarly only two signals for the di as tereo topic ligand C H 2 protons are observed. The o-protons of the phenyl rings give rise to only one resonance, consistent with the single phosphorus environment seen in the room temperature 3 1 P{'H} N M R spectrum, as well as free rotation of these phenyl groups. The first peak to broaden significantly upon cooling a C D 2 C I 2 solution is that due to the TaCH 3 groups; decoalescence occurs near 210 K, though the broad peak is difficult to observe, as it overlaps with the silyl methyl peaks. At 200 K, the lowest frequency silyl methyl peak is significantly broadened, and the ligand C H 2 region is composed of a relatively sharp multiplet integrating to four protons, and a broad peak centred about the same region, due to the remaining four ligand C H 2 protons. By 180 K many of the features of the spectrum are beginning to sharpen, but are still broad. There are four distinct silyl methyl signals, as well 62 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand as four ligand C H 2 environments. There are also two ortho protons, as anticipated from two 3 I P environments in the low-temperature 3 I P{'H} N M R spectrum. The tantalum-bound methyl groups appear as two broad peaks, a peak at 8 0.81 integrating to one methyl group and a second at 8 0.17 integrating for the remaining two methyl groups. The absence of a low-temperature limiting spectrum makes the determination of activation parameters for the fluxional processes of 5 from ' H N M R measurements less reliable than those obtained from 3 I P{ 'H} N M R data. It is clear, however, that the low-temperature ' H N M R spectrum is consistent with a molecule of C s symmetry, with one metal-bound methyl, two phosphorus nuclei, and the tantalum centre defining a mirror plane, as would be expected for a capped trigonal-prismatic geometry. Nevertheless, the low-temperature solution structure, as determined by N M R spectroscopy to be of C s symmetry, and the solid-state structure do not quite match; the twist in the ligand framework observed in the solid state is not evident from the solution data even at 180 K, which suggests that the macrocyclic [P 2N 2] ligand framework must be conformational^ flexible. The mechanism of the exchange between the two phosphorus environments and the exchange between the two TaCFfj environments can be rationalized via a pseudo "turnstile" mechanism, as shown in Figure 2.5. The tantalum-methyl groups exchange sites in a trigonal prism by rotating in a turnstile manner while the ligand simultaneously pivots, exchanging phosphine environments. Only Me a is in a chemically identical environment after one rotation in Figure 2.5, and remains on a phosphine-capped face; one further rotation of the TaMe 3 fragment in the same direction exchanges the Me a environment. That 1 is fluxional is expected, considering the small energy differences between the various seven-coordinate metal geometries, and a similar AG* has been found in related seven-coordinate systems.38 The [P 2N 2] ligand must be quite flexible to accommodate such changes in geometry. 63 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Figure 2.5. Depiction of the fluxional behaviour of capped trigonal-prismatic 5 in solution. 2.5 Synthesis and Characterization of [P2N2]Ta=CH2(Me) (6) (i) Photolysis of [P2N2]TaMe3 and Isolation of [P2N2]Ta=CH2(Me) The trimethyl complex 5 is more thermally stable than its precursor, TaMe3Cl2. Solutions of 5 are light-sensitive, however, and convert to the orange methyl methylidene [P2N2]Ta=CH2(Me) ( 6 ) with the loss of methane (detected by GC-MS) upon photolysis (equation 2.9). Indicative of the formation of a methylidene is a resonance integrating to two protons at 8 9.18 in the *H N M R spectrum, as well as a signal in the l 3 C N M R spectrum at 8 244.8, typical of tantalum methylidene complexes. The aforementioned l 3 C N M R signal shows a 118 Hz 'JHC value which is lower than those observed for (r| 5-C5H5) 2Ta=CH 2(Me) ('JHC = 132 Hz) and (Ti5-C5Me5)2Ta=CH2(Me) ( '/ Hc = 127 Hz) but very similar to that observed for [PhC(NSiMe 3) 2] 2Ta=CH 2(Me) ( ' / C H =118 Hz). 64 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand [2.9] This transformation occurs under ordinary fluorescent room lighting over several weeks or, more conveniently, by exposure of solutions of 5 to direct sunlight or a sunlamp. Ordinary incandescent light results in no conversion, and stronger U V sources result in decomposition. As has been suggested by others for a similar reaction, 3 9- 4 0 the mechanism of this reaction presumably involves charge transfer from a methyl group to the metal centre, to produce a methyl radical that then readily abstracts a hydrogen atom from an adjacent methyl group. This can then eliminate methane and produce a methylidene moiety. Photochemical a-hydrogen abstraction has been noted before for both the early and late transition metals. 3 9" 4 2 UV-visible spectroscopy of 5 in hexanes shows two absorptions in the region expected for a ligand-to-metal charge transfer band corresponding to an alkyl-to-metal charge-transfer, at wavelengths of 262 and 322 nm 4 0 As was noted in Chapter 1, the formation of alkylidenes in one of the most studied and common reactivities of tantalum alkyl complexes. Despite this, the number of stable tantalum methylidene complexes is quite limited. The synthesis of (r) 5 -C5H 5 ) 2 Ta=CH2(Me), the first stable methylidene complex, 3 1- 4 3" 4 5 served as one of the starting points for the preparation of a new family of metal-carbon multiple bonds 4 6 This compound has since found applications as a precursor to catalytically active early-late heterobimetallics 4 7 as well as in the study of the Fischer-Tropsch process48 and in methylidene-transfer processes.49 While a number of other stable methylidene complexes have been prepared, most, like the aforementioned tantalum complex, are coordinatively saturated species, 5 0- 5 1 and the majority involve the late transition metals. 5 2" 5 9 Two recent group 5 examples are 65 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand [PhC(NSiMe 3 ) 2 ]2Ta=CH 2(Me) 3 8 and the thermally unstable derivative (ArO)2Ta=CH2(Me).39,40 Although N M R spectroscopy indicates that methylidene 6 is the major product of the reaction with only trace NMR-active impurities, performing the photolysis on an N M R scale in CgD6 with ferrocene as an internal concentration standard demonstrated that side-products are produced well before conversion to the methylidene is complete. For example, after 11 minutes of photolysis the solution turned from yellow to light orange and a 54% conversion to methylidene was observed, and nearly 90% of the total tantalum content was accounted for as compounds 5 and 6 by Integration of ' H N M R spectra. A maximum conversion of 60% was observed after 33 minutes, with only 72% of the total tantalum content being accounted for by NMR-active products. At this point the solution is dark brown. After 85 minutes the solution was dark green and the yield of 6 had dropped to 48%, and only 64% of the total tantalum was observed in the ! H N M R spectrum. The EPR spectrum of a solution of 5 after 60 minutes of photolysis under a sun lamp indicates that a tantalum species in oxidation state +4 was present; an eight-line pattern is observed in the EPR spectrum, due to an electron with hyperfine coupling to l 8 l T a (I = 7/2, 99.99% abundance). Superhyperfine coupling to 31 P is also observed, though poorly resolved. This EPR-active product likely resulted from the photolysis of the methylidene 6, and still contains the [P 2N 2] ligand. The nature of this compound remains uncertain. Like methylidene 6, the paramagnetic impurities are highly soluble in relatively nonpolar solvents such as pentane and hexamethyldisiloxane. To generate 6 with a minimum amount of these impurities, short photolysis times and correspondingly low conversion of the tantalum trimethyl 5 are required. Solubility differences allow 6 to be separated from 5 by washing with cold pentane, and the much less soluble 5 can be recovered and recycled. (ii) Structure of [P2N2]Ta=CH2(]VIe) (6) Once purified, methylidene 6 is quite thermally stable; solutions of 6 decompose only slowly at room temperature, and as a solid 6 can be stored at -40°C for months without 66 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand noticeable decomposition. Compound 6 is much more soluble in nonpolar solvents than trimethyl 5; for example, methylidene 6 is extremely soluble in hexanes, pentane, and even hexamethyldisiloxane. Due to this high solubility, crystals of 6 suitable for X-ray analysis could only be obtained by slow evaporation of a pentane solution at -40°C, as higher boiling solvents gave only waxy products. The solid state structure of 6 is shown in Figure 2.6, and selected bond distances and angles are given in Table 2.4. Figure 2.6. ORTEP representation of the solid-state molecular structure of [P2N 2]Ta=CH2(Me) (6). Hydrogen atoms and silyl methyl groups are omitted and only the ipso carbon of the phenyl rings attached to the phosphine donors are shown. The unshaded thermal ellipsoids of the disordered methyl and methylene group indicate that these atoms were refined with only isotropic thermal parameters. 67 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Table 2.4. Selected bond lengths, angles and dihedral angles in [P 2N 2]Ta=CH2(Me) (6). Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.550(2) Ta(l) N(2) 2.142(6) Ta(l) P(2) 2.542(2) Ta(l) C(25) 2.09(2) Ta(l) N(l) 2.146(6) Ta(l) C(26) 2.21(2) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) P(l) N(l) C(25) Ta(l) Ta(l) Ta(l) P(2) N(2) C(26) 165.11(7) 99.5(2) 86.1(7) N(l) N(2) Ta(l) C(25) Ta(l) C(26) 96.0(6) 83.7(6) Atom Atom Atom Atom Dihedral Angle (°) PQ) P(2) P(l) P(2) Ta(l) Ta(l) Ta(l) Ta(l) N(l) N(l) N(2) N(2) Si(4) Si(l) Si(3) Si(2) 160.7(4) 166.7(4) 166.4(4) 160.9(4) The solid-state structure verifies that 6 is monomeric. Similar to a tantalum methyl methylidene recently reported,38 the structure of 6 contains disorder. The metal-bound methyl and methylidene ligands were modeled as 1:1 2-fold disordered; as a result, the metal-methylidene Ta-C(25) distance of 2.09(2) A and metal-methyl Ta-C(26) distance of 2.21(2) A have large errors associated with them. The two geometries previously reported for six-coordinate species containing the [P 2N 2] ligand are exemplified by [P 2 N 2 ]ZrCl 2 and [P 2N 2]Zr(CH 2Ph) 2 . 1 In the former the chloride ligands, the zirconium centre and the two amido ligands lie in a plane, and thus the complex is approximately octahedral. In the bis(benzyl)zirconium species the benzyl ligands and the zirconium centre define a plane that approximately bisects the two planes defined by N-Zr-N and P-Zr-P; this geometry is best described as a distorted-trigonal-prismatic species. As the geometry of methylidene 6 is intermediate between these two cases, it can therefore be considered intermediate between octahedral and trigonal prismatic. While the trigonal-prismatic geometry is electronically preferred by early-metal alkyl compounds over the octahedral geometry,60-64 m e intermediate geometry observed here may be rationalized by the preference of the methylidene fragment for a more octahedral geometry, to maximize K-68 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand bonding of the methylidene /^-orbital with the tantalum rf-orbitals. This is discussed in more detail in Section 2.5(iii). The ligand geometry once again contains approximately trans phosphines with a P-Ta-P angle of 165.11(7)° and cis amides with a N-Ta-N angle of 114.1(5)°. The Ta-P(l) distance of 2.550(2) A and Ta-P(2) distance of 2.542(2) A are shorter than those for seven-coordinate 5. The Ta-N(l) and Ta-N(2) distances of 2.146(6) and 2.142(6) A are comparable to those seen in 5. (iii) Fluxional Processes in [P2N2]Ta=CH2(Me) (6) The 3 I P{'H} N M R spectrum of methylidene 6 is a singlet at 300 K and remains unchanged over the temperature range of 160-330 K; the distorted-trigonal-prismatic geometry observed in the solid state is therefore not rigid in solution. For this equivalence to occur both the Ta=CH 2 and Ta-CH 3 groups must readily pass through the plane that contains the amido ligands and the tantalum centre with an inconsequential energy barrier resulting from the approximately octahedral intermediate. This motion renders the 3 1 P environments identical, and thus we expect the ' H N M R spectrum to reflect a species with apparent C s symmetry. This fluxional process is depicted by the horizontal equilibria in Scheme 2.3. 69 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Scheme 2.3. Me 2 .Si. Me 2 Si -Ph—P—Ta—R.—Ph = .CH 2 Si ^Si Me 2 Me 2 higher energy barrier Me 2 ' Me 2 .SL .S i Ph—P-»Ta-«-R-—Ph Si Si Me 2 Me 2 — r lower energy barrier < Me 2 Me 2 .Si. / S i Ph—P -^Ta^R-—Ph > Si S i Me 2 Me 2 & lower energy barrier higher energy barrier Me 2 Me 2 .SL .Si v t > P h — P — T a ^ R — Ph iX \ .Nvjyie/ 'Si' Si Me 2 Me 2 — r < The second fluxional process, depicted as the vertical equilibria in Scheme 2.3, is observed in the ' H N M R spectra close to room temperature. At low temperatures (180-260 K), there are four sharp resonances observed for the silyl methyl groups of the ligand, and four ligand C H 2 environments, as expected from a [P2N2] complex with apparent C s symmetry in solution. When the temperature is raised, however, the silyl methyl and ligand methylene resonances begin to broaden and separately coalesce, until two silyl methyl and two ligand methylene peaks remain at -340 K; decomposition becomes significant at higher temperatures and impurities obscure relevant peaks. These observations indicate that there is some mechanism by which the position of the methyl and methylidene ligands can exchange. An E X S Y spectrum of 6 shows no positively phased cross-peak between the tantalum-bound methyl group protons and methylidene protons. Likewise, 1-D saturation transfer experiments show only nuclear Overhauser effects between the tantalum methyl group and methylidene protons, implying that the mechanism of exchange does not involve hydrogen atom transfer from the methyl group to the methylene group. Dimerization would 70 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand allow for exchange of methylidene fragments, but because coupling from the methylidene protons to the phosphines and methyl protons is maintained at higher temperature, this is not a feasible mechanism. Likewise, the line shape of the silyl methyl resonances is not affected by concentration, as would be anticipated if a second-order dimerization mechanism were active. The most probable mechanism, therefore, must involve rearrangement of the ligand to allow rotation of the Ta=CH 2 and TaCFL groups with respect to the [P2N2] ligand. From line-shape analysis 3 7 of the silyl methyl resonances in the ' H N M R spectrum from 280 to 320 K, it was possible to obtain kinetic parameters for this exchange process of ALL = 58.6 ± 1.2 kJ mol"1 and AS* = -16.7 ± 2.1 J mol"1 K" 1 . An Eyring plot of this data is shown in Figure 2.7. The small entropy of activation is consistent with an intramolecular mechanism, without any prior ligand dissociation. For rotation to occur, the phosphine ligand must become more cw-disposed, and the amido ligands must assume a more trans disposition. This indicates that the bound [P2N2] fragment must be quite flexible; the rearrangement from cz's-disposed amido ligands to trans amido ligands must be assisted by the distortion from ideal octahedral angles imposed by the macrocyclic nature of the ligand. 0.003 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 1/T Figure 2.7. Eyring plot for the exchange of silyl methyl environments in [P 2N 2]Ta=CH 2(Me) (6). 71 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand A third fluxional process observed by variable-temperature ' H N M R spectroscopy involves rotation of the methylidene unit. In the ! H N M R spectrum of 6 at 300 K the methylidene and methyl fragments are broad multiplets. However, using Lorentz-Gaussian enhancement it was possible to resolve the 1.2 Hz coupling between the methylidene and methyl fragment, as well as coupling of both signals to two identical " P environments, as shown in Figure 2.8. The uppermost two traces in Figure 2.8 are portions of the 500 M H z ! H N M R spectrum of [P2N2]Ta=CH2(Me) treated with an exponential decay function of 0.2 Hz. The left trace illustrates the peak assigned to the tantalum bound methylidene protons, and the right trace is assignable to the tantalum methyl moiety. In the upper traces, no coupling can be adequately resolved. In the two lower traces the Fourier induction decay was processed using Gaussian multiplication, rather than the usual exponential decay function, and the larger coupling of both fragments to 3 I P as well as the 1.2 Hz coupling to each other is clearly resolved. From this treatment of the data it is clear the two methylidene protons are chemically equivalent on the N M R time scale at this temperature. 72 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Figure 2.8. The effect of Gaussian multiplication on the Ta=CH 2 (left) and Ta-CH 3 (right) resonances in the 500 MHz ' H N M R spectrum of 6 at 300 K. The uppermost traces were processed using an exponential decay function. The lower traces were processed using Lorentz-Gaussian enhancement and clearly show the small 1.2 Hz coupling between these signals, as well as larger coupling to 3 1 P. The presence of only one methylidene resonance in the room temperature 500 MHz *H N M R spectrum is due to the rapid rotation of the methylidene unit. Variable temperature ' H N M R spectroscopy demonstrated that this rotation occurred, and selected spectra are shown in Figure 2.9. Cooling of the sample initially causes some loss of resolution of both the methyl and methylidene resonances, and obvious broadening of the methylidene signal. Decoalescence of the methylidene resonance occurs at approximately 190 K, and further cooling to 170 K leads to two resonances for the methylidene protons, separated by 1.08 ppm; this separation does not change upon additional cooling to 160 K. The resolution at 73 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand this temperature was not adequate to distinguish the anticipated couplings in these resonances. J V_ , 3 0 0 K 1 0 . 2 1 0 . 0 9 . 8 9 . 6 9.4 9 . 2 9 . 0 8 .8 8 . 6 (PPm) Figure 2 . 9 . The effect of temperature on the methylidene resonance in the 500 M H z ' H N M R spectrum of [P 2N 2]Ta=CH 2(Me) (6). The calculated AG* value for the rotation of the methylidene group from this N M R data is 33.5 ± 0.6 kJ mol"1 at 190 K ; this value is significantly lower than that previously observed for (r|5-C5H5)2Ta=CH2(Me), for which AG* for rotation of the methylidene group was estimated to be larger than 89.5 kJ mol" 1 . 4 4 This large difference is due to the availability of two perpendicular d-orbitals for 7t-bonding. Whereas (r|5-C 5 H 5 ) 2 Ta=CH 2 (CH 3 ) is an 18-electron complex, with only one orbital of correct symmetry for Tt-bonding,45 6 may be considered a 14-electron species, excluding Tt-donation from the two amido nitrogens. The two extreme arrangements of the Ta=CH 2 unit in an approximately octahedral complex are shown in Figure 2.10, along with the d-orbitals available for Ti-bonding. In conformation A, the methylidene hydrogens are in the same plane as the amido 74 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand nitrogens; thus, the p-orbital on the methylidene carbon overlaps with the dxz orbital. In A , two methylidene proton environments exist, as observed in the low-temperature *H N M R spectrum. Another possible conformation, B, has the methylidene unit rotated 90°; the dxy orbital now has the correct symmetry for overlap with the methylidene carbon p-orbital. The macrocyclic ligand restricts the possible orientations of the nitrogen-based p-orbitals, which contain the lone pairs of the amido nitrogens, and therefore the dxy orbital is also the only orbital of correct symmetry for reasonable overlap with these orbitals. As a result, in conformation B the methylidene 7t-electrons must compete with the two amido donors for the dxy orbital, and this is therefore predicted to be a less favourable orientation of the methylidene unit. This is in agreement with the experimental data, since in the second conformation the two methylidene protons are identical, contrary to what is observed in the low-temperature-limit ' H N M R spectrum. It is interesting to note that because the two amido p-orbitals compete for one orbital for 71-donation to tantalum, 6 can be no more than a 16-electron species. A B Figure 2.10. Representations of two possible orientations of the Ta=CH 2 double bond in [P 2N 2]Ta=CH 2(Me) (6). Competition with the 7t-donor amido ligands for the J x v orbital disfavours B. 2.6 Synthesis and Deprotonation of {[P2N2]TaMe2}+X~(X = BF4", B(C 6F 5) 41 The precedented chemical route to tantalum methylidenes involves the deprotonation of cationic methyl compounds, as exemplified by the synthesis of (r | 5 -C 5 H 5 )2Ta=CH2(Me) by treatment of { C p 2 T a M e 2 } + B F 4 ~ with the base Me 3 P=CH 2 , as shown in Scheme 2 . 4 . 3 1 ' 3 8 ' 4 3 A 75 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand similar route to 6 would be desirable, to avoid the paramagnetic side-products that are generated by photolysis of 5 at high conversion. Scheme 2.4. ) Q [ P h 3 C ] + [ B F 4 r T a r M e / ^ M e ~ " P h 3 C M e - M e 3 P = C H 2 ....»> M e [ M e 4 P ] + [ B F 4 ] - / ^ C H 2 (i) Synthesis of {[P2N2]TaMe2}+X- (X" = BF4", B(C6F5)4~) The reaction of 5 with Ph 3 C + BF 4 ~ in CH 2 C1 2 produces {[P 2N 2]TaMe 2} +BF 4~ (7), which is soluble in both CH 2 C1 2 and THF (equation 2.10). Solutions of 7 are not thermally stable; [2.10] decomposition in CH 2 C1 2 is complete after 3 days at room temperature and results in a colourless crystalline precipitate that is discussed in Section 2.6 iv). The stability of the cation in 7 is improved by using a less reactive anion. The reaction of {PhNMe 2 H} + B(C 6 F 5 ) 4 ~ with 5 produces {[P 2 N 2 ]TaMe 2 } + B(C 6 F 5 ) 4 -phNMe 2 (8) (equation 2.11), which does not decompose after several days in solution. 76 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Ph. Me v / M e M e / P h \ J/ / {PhNMe2H}+ B(C 6 F 5 ) 4 A ~ \ » « < CH 2CI 2 R Si M e 2 Me 2 a M e 2 M s * Me 2 Me 2 SL / S i P h — P - ^ t a - ^ F ^ — Ph Si' "Si Me 2 Me 2 p-^T —R— [2.11] B(C 6 F 5 ) 4 8 The ' H and 3 l P{ 'H} N M R spectra of the {[P 2N 2]TaMe 2} + ion in 7 and 8 are nearly identical, though the PhNMe 2 byproduct proved impossible to remove from 8 even under high vacuum. The similarity between the ' H N M R spectra of 7 and 8 is evidence that the PhNMe 2 molecule in 7 is not coordinated to the metal centre. The low-temperature 3 I P{ 'H} N M R spectra also provide evidence that the PhNMe 2 byproduct is not coordinated. A single phosphorus environment is observed for {[P 2N 2]TaMe 2} + from room temperature to 185 K, unlike the seven-coordinate trimethyl 5 which had two coupled signals at low temperature. The low-temperature (200 K) : H N M R spectrum of 8 contains a single tantalum methyl resonance, and four silyl methyl environments, consistent with a trigonal-prismatic geometry at tantalum; an octahedral geometry would be expected to have only two silyl methyl signals. However, at room temperature the ' H N M R spectrum of 8 shows only two silyl methyl environments (apparent C 2 v symmetry), indicating a relatively small energy difference between the trigonal-prismatic geometry and the octahedral transition state that exchanges the silyl methyl environments. 77 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand (ii) Attempted Deprotonation of {[P2N2]TaMe2}+ BF4~ with LiNPr'2 and NaN(SiMe3)2 Deprotonation of 7 or 8 did not prove to be a successful alternative route to the methylidene methyl 6. The reaction of the bulky amides LiNPr 2 ' and NaN(SiMe3) 2THF with the BF 4 ~ salt 7 instead generated [P 2N 2]TaMe 2F (9) as the major product (equation 2.12). 31 1 The room-temperature P{ H} N M R spectrum of 5 is indicative of the formation of a tantalum fluoride; a single doublet is observed due to coupling to one l 9 F nucleus. The methyl groups are equivalent in the ' H N M R spectrum and show coupling to two 3 I P nuclei and a 1 9 F nucleus. 7 9 The fluxional process seen in seven-coordinate 9 is similar to that observed for 5; at 31 1 the low-temperature limit the " P{ H} N M R spectrum of 9 consists of two coupled signals, one at 8 26.5, the other at 8 43.9, consistent with the capped-trigonal-prismatic geometry observed for 5. The variable-temperature ' H N M R spectra are more complicated. When the sample is warmed to 360 K produces a species with two silyl methyl environments and one tantalum methyl group, as was observed in the room-temperature spectrum of 5, and can be explained by the rotation of the methyl groups with simultaneous pivoting of the [P 2N 2] ligand. Four environments are observed for the silyl methyl groups of 9 at 240 K, and further cooling shows that they begin to decoalesce again, to give eight silyl methyl environments. This is consistent with a low-temperature structure lacking a mirror plane of symmetry, and therefore the fluorine atom must lie in one of the two equivalent sites eclipsing the nitrogen atoms in the trigonal prism and does not occupy the site in the trigonal prism which lies in a plane with the two phosphorus atoms and tantalum centre. This lowest energy structure is depicted as the top left and top right structures in Figure 2.11. The variable-temperature ' H 78 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand N M R spectra observed are consistent with exchange of the fluoride between these two equivalent sites on the N M R time scale in the ' H spectra at 240 K, where four silyl methyl environments are observed, but the fluoride does not exchange with the third site. This corresponds to equilibrium C in Figure 2.11. The complete rotation of the two methyl and fluoride groups is only observed at higher temperature, and leads to the two silyl methyl groups observed at 360K, depicted as the fluxional process D in Figure 2.11. Figure 2.11. Fluxional processes in [P 2N 2]TaMe 2F (9). 79 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand (iii) Attempted Deprotonation of {[P2N2]TaMe2}+ X" with Me 3P=CH 2 (X" = BF 4~, B(C 6F 5) 41 The use of Me3P=CH 2 as a base for the deprotonation of BF 4 ~ salt 7 resulted in a new major product, 10, with two singlets in the 3 1P{ 'H} N M R spectrum: one in the region typical of the bound [P 2N 2] and a second signal closer to the unbound ligand (equation 2.13). Compound 9 was observed as a minor impurity. The ' H N M R spectrum of 10 is indicative of a highly unsymmetrical species; there are eight silyl methyl resonances, two of which show coupling to phosphorus. While eight signals are expected for the ligand methylene groups, only seven are observed. One is at unusually high frequency, and shows only coupling to the two 3 1 P nuclei and not to a geminal proton. These data indicate that 10 is the result of the deprotonation of 7 not at a tantalum methyl group but rather at a methylene group of the [P 2N 2] ligand. The reaction of LiNPr' 2 , NaN(SiMe 3) 2-THF, or Me 3 P=CH 2 with 8 also produced 10 as the major product. The exact stereochemistry of the product is unknown; however, because there is only one product, the base appears to selectively deprotonate only one of the two diastereotopic methylene protons. - | + Me 2 Base S i Si B(C 6 F 5 ) 4 [2.13] Me 2 Me 2 e.g., Pr' 2NLi, (THF)NaN(SiMe3)2, Me 3P=CH 2 t Me 2 8 Me 2 Me 2 10 80 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand (iv) Decomposition of {[P2N2]TaMe2}+ BF4~ As was previously noted, the species {[P 2N 2]TaMe 2} + BF 4 ~ is not stable in CH 2 C1 2 solution. Over the course of two days at room temperature a colourless crystalline solid (11) precipitates that has no solubility in CH 2 C1 2 . In an attempt to better understand the instability of this complex, this decomposition was performed without disturbing the solution, and the product precipitated-as X-ray quality crystals. Unfortunately, disorder rendered the X-ray structure difficult to completely solve. Furthermore, the lack of solubility of complex 11 made it difficult to characterize by other means. Nevertheless, from the X-ray data a number of key observations can be made concerning the connectivity in the resulting product, and its relevance to the stability of [P 2N 2] complexes of tantalum. In particular, the X-ray structure demonstrates that in the final product, a Si-N bond has been cleaved to generate a tantalum imido double bond. The decomposition product is depicted in equation 2.14. It should be noted, however, that the exact distribution of the fluorine atoms between the boron centre and the dangling silicon atom that has been cleaved from the nitrogen atom that is now doubly bonded to tantalum is not clear. While equation 2.14 shows the product of the reaction to contain one fluorine bound to boron and one fluorine bound to silicon the X-ray data was not of sufficient quality to determine if these substituents were fluorine atoms or methyl groups, or a disordered combination of both. 81 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand - i + Me2 Me2 Si. .Si Ph* — p -»Ta^R—Ph S i SI Me? Me9 {[P2N2]TaMe2}+BF4" 7 BF, CH 2 CI 2 [2.14] Me2FSi Me2 Me 2Me 2 11 This decomposition reaction of {[P 2 N 2 ]TaMe 2 } + BF 4 - involves multiple steps; however, no reaction intermediates could be detected in solution. Lacking any evidence to indicate the order in which the various steps in this reaction sequence occur, a speculative reaction mechanism is shown in Scheme 2.4. The most likely initial step in the reaction is that BF 4 ~ fails to act as a weakly-coordinating anion, and interaction between the tantalum centre and a fluoride ligand of BF 4 ~ results in fluoride abstraction, to generate [P 2N 2]TaMe 2F and B F 3 . It is not clear if the B F 3 produced immediately coordinates to one of the [P 2N 2] phosphine donors or reacts with the methyl substituents on the tantalum centre to generate [P 2N 2]TaF 3 and BFMe 2 . Regardless of the order of these reactions, the key step in this unusual decomposition product is the cleavage of one of the [P 2N 2] Si-N linkages, to generate a Si-F bond and a Ta=N imido linkage. From the initially seven-coordinate [P 2N 2]TaMe 2F, quaternization of a phosphine donor with BFMe 2 and generation of an imido linkage through Si-F bond formation results in a five-coordinate tantalum centre. This five-82 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand coordinate tantalum centre can then undergo dimerization, to form a dimer with two six-coordinate tantalum centres bridged by two fluorine ligands. This decomposition product provides an opportunity to reexamine the reasons for the apparent difficulty in isolating the initially desired starting material [P2N2]TaCl3 This byproduct suggests that [P2N 2 ]TaF 3 , and by analogy also [P 2 N2]TaCl 3 , are not stable, decomposing by the formation of strong Si-F and Si-Cl bonds respectively. This decomposition pathway, previously noted in Section 2.3, also generates a strong Ta=N imido bond in the process of destroying the [P2N2] macrocycle, and results in a less sterically crowded metal centre. The overall strength of these bonds and the diminished steric crowding of the metal centre provide a thermodynamic driving force that renders the complex [P2N 2 ]TaCl 3 unstable. 83 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Scheme 2.4. 84 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2.7 Attempted In Situ Synthesis and Reaction of [P2N2]TaCl3 With a series of [P 2N 2]Ta alkyl complexes prepared, and a better understanding of the relative stability of these complexes, a new approach to determining the feasibility of [P2N2]TaCi3 as an entry to the chemistry of tantalum using the [P 2N 2] ancillary ligand was tested. If [P2N2]TaCl3 is prepared in reasonable yield from the reaction of [P 2 N 2 ]L i 2 and TaCl 5 at low temperatures, but lacks sufficient thermal stability due to Si-Cl bond and tantalum imide bond formation or other decomposition pathways, it might be possible to access compounds such as dinitrogen complexes by the in situ preparation of [P 2N 2]TaCl3, and immediate use of these solutions at low temperature. A solution of [P2N2]TaCl3 could then be reduced to form the dinitrogen complex before thermal decomposition occurred. To demonstrate that [P2N2]TaCl3 was at least the initial product of the reaction of [P 2 N 2 ]L i 2 and TaCls, a mixture of these components at -78°C was allowed to warm to 0°C. This in situ preparation of [P2N2]TaCl3 was then cooled back to -78°C and three equivalents of MeLi were added. After workup, the 3 I P{ 'H} N M R spectrum revealed the presence of many products that have been identified previously. One of the major products of the reaction was 5, [P 2N 2]TaMe 3, as desired; however, considerable impurities were present. Minor products included methylidene complex 6, [P 2N 2]Ta=CH 2(Me), and complex 10, in which the methylene linkage of the [P 2N 2] ligand has become metalated. An unknown product with a resonance at 8 -12.0 in the 3 1P{ 'H} N M R spectrum was also a major product of the reaction, present in only a slightly lower concentration than [P 2N 2]TaMe 3. In addition to these impurities, considerable [P 2 N 2 ]L i 2 remained unreacted according to the 3 I P{ 'H} N M R spectrum. While this reaction provides the insight that [P 2 N 2 ]TaCl 3 is probably initially present in the reaction of [P 2 N 2 ]L i 2 with TaCl 5 , the number of impurities in the final product is not promising in terms of using [P2N2]TaCl3 in situ. Not surprisingly, considering the poor conversion and low purity of [P 2N 2]TaMe 3 prepared by this method, attempts to reduce [P2N2]TaCl3 prepared in situ with three equivalents of KCg did not prove to be a successful route to isolating a dinitrogen complex of tantalum; no product could be isolated from the resulting mixture. 85 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2.8 Summary and Conclusions The desired halide complex [P 2N 2]TaCl3 has so far proved impossible to isolate, despite numerous attempted syntheses. However, TaMe3Cl 2 reacts with [P 2 N 2 ]Li 2 (C 4 H80 2 ) to produce [P 2N 2]TaMe3 in 80% yield. The seven-coordinate trimethyl derivative [P 2N 2]TaMe3 is fluxional and can be used as a starting material for other organotantalum complexes. An example of such a reaction is the photolysis of [P2N 2 ]TaMe3, which leads to the methylidene derivative [P 2N 2]Ta=CH 2(Me); the discovery of this chemistry was serendipitous, as the storage of solutions of [P 2N 2]TaMe3 under such weak light sources as fluorescent bulbs resulted into its conversion to [P 2N 2]Ta=CH 2(Me). The methylidene is also fluxional, although in this case there are at least three processes that are operating. In an effort to improve the yields of the methylidene complex, a nonphotochemical route to its synthesis was attempted. First, the trimethyl species was converted to the dimethyl cation {[P 2N 2]TaMe 2} + by reaction with Ph3C + . Attempts to form the methylidene complex from the dimethyl cation by deprotonation failed in different ways, depending on the anion used to stabilize the starting cation and the base used. When the counteranion was BF 4 ~, reaction with strong bases led to incorporation of fluoride and the formation of [P 2N 2]TaMe 2F. When the anion used was B(CeF5)4~, reaction with strong bases did involve deprotonation but not at the Ta-Me groups; instead, deprotonation occurred at the methylene protons of the macrocyclic framework. This latter reaction path indicates one of the drawbacks of this ligand system; the relatively acidic methylene protons can be deprotonated even in the presence of the sterically more accessible Ta-Me group. 2.9 Experimental 2.9.1 General Procedures Unless otherwise stated, all manipulations were performed under an atmosphere of dry oxygen-free dinitrogen by means of standard Schlenk or glovebox techniques (Vacuum Atmospheres HE-553-2 glovebox equipped with a MO-40-2H purification system and a -40°C freezer). Hexanes were predried by refluxing over CaH 2 and then distilled under 86 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand argon from sodium benzophenone ketyl with tetraglyme added to solubilize the ketyl. Anhydrous diethyl ether was stored over sieves and distilled from sodium benzophenone ketyl under argon. Toluene was predried by refluxing over CaH 2 and then distilled from sodium under argon. Nitrogen was dried and deoxygenated by passage through a column containing activated molecular sieves and MnO. Deuterated benzene and toluene were dried by refluxing with molten potassium metal and molten sodium metal, respectively, in a sealed vessel under partial pressure, then trap-to-trap-distilled, and freeze-pump-thaw-degassed three times. Deuterated methylene chloride was dried over CaH 2 prior to use. Unless otherwise stated, ' H , 3 1 P , 'H{ 3 I P}, 1 3C{ 'H}, l 3 C , and variable-temperature N M R spectra were recorded on a Bruker AMX-500 instrument operating at 500.1 MHz for ' H spectra. ' H N M R spectra were referenced to internal CeD 5H (7.15 ppm), CDHC1? (5.32 ppm), and C7D7H (2.09 ppm), 3 I P{ 'H} N M R spectra to external P(OMe) 3 (141.0 ppm with respect to 85% H3PO4 at 0.0 ppm), and 1 3 C N M R spectra to 1 3 C C 5 D 6 (128.4 ppm) and I 3 C D 2 C 1 2 (54.0 ppm). Simulation of variable-temperature N M R spectra were performed using the program DNMR-SJJVI.37 Simulated and actual spectra were compared by peak widths at half-height. U V -visible spectra were obtained using a Hewlett-Packard 8453 UV-visible spectrophotometer and quartz cuvettes with Teflon valves. EPR spectra were recorded on a Bruker ECS 106. Elemental analyses were performed by Mr. P. Borda of this department. 2.9.2 Materials The compounds [ P 2 N 2 ] L i 2 - C 4 H 8 0 2 , 6 5 TaCl 6 (SC 4 H 8 )3 , 2 7 T a C l 4 ( N C 5 H 5 ) 2 , 2 8 CH2=PMe3,66 and TaMe3Cl 2 3 1 were prepared according to literature procedures. Lithium diisopropylamide, 0.5 M anhydrous ZnCl 2 in THF and solid ZnCl 2 were purchased from Aldrich, and sublimed TaCls was purchased from STREM. 87 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 2.9.3 Synthesis and Reactivity of Complexes Synthesis of [P 2N 2]Li 2 (1) A solution of 1.6M Bu"Li in hexanes (50.0 ml, 0.0800 mol) was added dropwise to a stirred solution of (PhPHCH 2 SiMe 2 )NH (7.55 g, 0.0200 mol) and l,3-bis(chloromethyl)-1,1,3,3-tetramethyldisilazane (4.6lg, 0.0200 mol) in 200 ml ether. The solution was stirred for 3 hours then evaporated to dryness, to afford an oily solid. The oily solid was further dried by warming to 40°C under vacuum for an additional 3 hours, and resulted in a pale yellow powder. The solid was extracted into toluene, filtered, and the toluene was removed. The addition of 15 mL of hexanes and cooling to -40°C precipitated a microcrystalline white solid. The solid was collected by filtration, rinsed with a minimum amount of cold hexanes, and dried in vacuo to produce 9.1 g of [P 2 N 2 ]Li 2 . Yield 80%. ' H N M R (C 6 D 6 , 30°C): 5 0.21 and 0.35 (s, 24H total, SiCff 3), 1-00 and 1.11 (ABX, 2 7 H H = 14.3 Hz, 2 7 P H = 8.1 Hz and 6.0 Hz, 8H, CH2 ring), 7.10 (m, 2H, p-H), 7.15 (m, 4H, m-H), 7.57 (m, 4H, o-H). 3 I P{ !H} N M R (C 6 D 6 , 30°C): 8 -38.0 (q, ]JLip = 56 Hz). Anal. Calcd for C 2 4 H 4 2 L i 2 N 2 P 2 S i 4 : C, 52.72; H, 7.74; N , 5.12. Found: C, 53.06; H, 7.96; N , 5.16. Synthesis of [P2N2]Mg (OC 4H 8) (2) To an intimate mixture of [P 2 N 2 ]Li 2 - (C 4 H 8 0 2 ) (1.00 g, 1.58 mmol) and an excess of MgCl 2 -(OEt 2 ) 2 (1.00 g, 4.11 mmol) was added 30 mL of tetrahydrofuran at room temperature. The solution was stirred for one hour, after which the solvent was removed under vacuum. The remaining solid was extracted into toluene and filtered. The filtrate was evaporated to dryness, leaving a white powder that was then rinsed with hexanes and dried under high vacuum, to afford [P 2 N 2 ]Mg(OC 4 H 8 ) in 95% yield. ' H N M R (C 6 D 6 , 30°C): 8 0.19 and 0.41 (s, 24H total, SiC/7 3), 0.98 and 1.21 (ABX, 8H total, CH2 ring), 1.08 (m, 4H, C 4 H 8 0 ) , 3.73 (br m, 4H, C 4 H s O), 7.06 (overlapping m, 6H, PPh m-H and p-H), 7.42 (m, 4H, PPh o-H). 3 1 P{'H} N M R (C 6 D 6 , 30°C): 8 -43.6 (s). Anal. Calcd for C 2 8 H 5 oMgN 2 OP 2 Si4: C, 53.44; H, 8.01; N , 4.45. Found: C, 53.59; H, 8.06; N , 4.39. 88 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Synthesis of [P2N2]Zn (3) A solution of [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) (2.009 g, 3.164 mmol) in 20 mL THF was cooled to -78°C and added to a 0.5 M solution of ZnCl 2 (6.32 mL, 3.16 mmol) in THF. The mixture was stirred and allowed to warm to room temperature, then evaporated to dryness to provide a white powder. The white solid was extracted with hexanes. The hexanes were then removed under vacuum, and [P 2N 2]Zn was obtained as a white powder (80 % yield). Single crystals of sufficient quality for an X-ray diffraction study were grown by slow evaporation of a saturated hexanes solution. The synthesis of [P 2N 2]Zn could also be performed with solid anhydrous ZnCl 2 dried by reflux in SOCl 2 prior to use. ' H N M R (C 6 D 6 , 20°C, 200.13 MHz): 5 0.32 and 0.41 (s, 24H total, SiC# 3), 1.01 and 1.17 (ABX, 2 / P H = 3.8 Hz and 5.5 Hz, 8H total, SiCH 2 P), 7.00 (overlapping m, 6H, PPh m-H and p-H), 7.41 (m, 4H, o-H). 3 I P{ 'H] N M R (C 6 D 6 , 30°C): 5 -29.0 (s). Anal. Calcd for C 2 4H 4 2 N 2 P 2 Si 4 Zn: C, 48.18; H , 7.08; N , 4.68. Found: C, 48.35; H, 7.27; N , 4.75. Reaction of [P2N2]Zn with TaCl s A solution of [P 2N 2]Zn (1.00 g, 1.67 mmol) in 10 mL toluene was added to a solution of TaCls (0.599 g, 1.67 mmol) in 20 mL of toluene. The solution gradually became darker yellow, and a solid began to precipitate after several hours. After 24 hours the solid was removed by filtration, and the solvent was removed under vacuum. ' H and 3 I P{ 'H} spectroscopy of the toluene soluble products in C(D$ and the toluene insoluble solid in C 4DgO revealed that a plethora of products had been produced. Preparation of [P 2N 2]H 2 To a solution of [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) (3.09 g, 4.87 mmol) in 50 mL of toluene was added solid {HNEt 3 } + CF (1.34 g, 9.73 mmol). The solution was stirred at room temperature 89 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [PiN2] Ancillary Ligand for 12 hours. After this period of time the initially insoluble {HNEt 3 } + Cl~ was replaced by a much finer fresh precipitate. The solution was filtered through Celite to remove LiCI and excess {HNEt 3 } + CF, and the toluene was removed under vacuum to afford [P2N2]H2 as an oil (2.55 g, 98% yield). If stored for an extended period of time the oil solidified, however, the 'Hi and 3 I P{ 'H} N M R spectroscopy indicate that no chemical change had occurred. ' H N M R (C 6 D 6 , 20°C, 200.13 MHz): 5 0.03 and 0.11 (s, 24H total, SiC773), 0.79 (br s, 2H, NH), 0.93 and 1.26 (ABX, 8H total, SiCi7 2P), 7.08 (overlapping m, 6H, PPh m-H and p-H), 7.61 (m, 4H, o-H). 3 I P{ 'H} N M R (C 6 D 6 , 30°C): 5 -36.9 (s). Anal. Calcd for C24H44N2P2Si4: C, 53.89; H, 8.29; N , 5.24. Found: C, 53.97; H , 8.25; N , 5.45. Synthesis of [P 2N 2]H 2 TaCI5 (4) Solid TaCl 5 (0.337, 0.940 mmol) was added to a stirred solution of [P 2 N 2 ]H 2 (0.503 g, 0.940 mmol) in 20 mL of toluene at room temperature. The solution instantly turned bright orange. The solution was evaporated to dryness to afford [P 2 N 2 ]H 2 TaCl 5 as an orange solid. This solid decomposes over the course of a few days to produce a mixture of soluble products and an intractable solid, and its instability prevented characterization by elemental analysis. ' H N M R (C 6 D 6 , 20°C, 200.13 MHz): 5 -0.24 and 0.23 (s, 24H total, S iC/ / 3 ) , 0.73 (br s, 2H, NH), 1.90 and 2.02 (ABX, 8H total, SiC// 2 P), 6.99 (overlapping m, 6H, PPh m-H and p-H), 7.36 (m, 4H, o-H). 3 I P{ *H} N M R (C 6 D 6 , 30°C): 5 12.2 (s). Preparation of [P2N2]TaMe3 (5) A solution of TaMe 3 Cl 2 (5.214 g, 17.56 mmol) in 40 mL ether was added to a stirring solution of [P 2N2]Li 2-C 4H 802 (11.147 g, 17.56 mmol) in 1 L of ether at -78°C. The solution was shielded from light because the product is light-sensitive. The solution was warmed slowly to 0°C, over which time a white precipitate formed. The solution was evaporated to dryness. The resulting solid was extracted into 50 mL of toluene, filtered, and the solvent removed. The remaining solids were rinsed with a minimal amount of hexanes and 90 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand thoroughly dried under vacuum, to afford [P 2N 2]TaMe3 as a yellow powder in 80% yield. X -ray quality crystals that contained 0.5 equivalents of co-crystallized hexanes were obtained by cooling a saturated hexanes solution to -40°C; drying under vacuum generated solvent free material. *H N M R (500 MHz, C 6 D 6 , 25°C): 8 0.33 and 0.34 (s, 24H total, SiC# 3), 1.14 (t, V p H = 6.5 Hz, 9H, TaC// 3), 1.48 and 1.52 ( A M X , 2 J H H = 14.3 Hz, 8H, CH2 ring), 7.07 (m, 6H, m/p-H), 7.65 (m, 4H, o-H); 3 I P{ 'H} N M R (81.0 MHz, C 6 D 6 , 25°C): 5 30.4 (s). 3 I P{ 'H} N M R (81.0 MHz, C 7 D 8 , -93°C): 8 19.6 (d, 2 / P P = 71.8 Hz), 41.1 (d, 2 / P P = 71.8 Hz). 1 3 C{ 'H} N M R (50.3 MHz, C 6 D 6 , 25°C): 8 5.7 (s, SiCH 3 ) , 8.37 (t, SiCH 3 ) , 20.3 (s, PCH 2 Si) , 64.8 (s, TaCH 3), 129.7, 131.8 and 137.5 (Ph-C). UV-VIS: Xm a x(nm), efNr '-cm" 1) 262, 12500; 322, 6220; 399 (sh). Anal. Calcd for C 2 7 H5iN 2 P 2 Si 4 Ta: C, 42.73; H , 6.77; N , 3.69. Found: C, 42.61; H , 6.87; N , 3.73. Preparation of [P2N2]Ta=CH2(Me) (6) A yellow solution of [P 2N 2]TaMe 3 (0.75 g, 0.99 mmol) in 200 mL of hexanes was irradiated for 15 minutes using an Osram Ultra-Vitalux sunlamp, resulting in an orange solution. The solvent was removed and the remaining waxy solid was extracted into 10 mL of pentane. The pentane solution was cooled to -40°C, and then filtered. The solvent was removed and the sample dried thoroughly in vacuo, to afford [P 2 N 2 ]Ta(CH 2 )CH 3 (0.33 g, 45%) as an orange solid. The filtered solid was mostly unconverted [P 2N 2]TaMe 3. X-ray quality single crystals were obtained by slow evaporation of a pentane solution at -40°C. ' H N M R (500 MHz, C 7 D 8 , 260 K): 8 0.14, 0.16, 0.20 and 0.27 (s, 24H total, SiCH 3 ) , 0.75 (tt, 3JHp=3.3 Hz, 4 / H H=L2Hz , 3H, TaC# 3), 1.11 (dd, 2/HH=13.7 H Z , JHp=5.8Hz, 2H, CH2 ring), 1.32 (dd, 2JHH=13.7 Hz, JHp=5.2Hz, 2H, CH2 ring), 1.36 and 1.36 (t, /Hp=5.8Hz, 4H, CH2 ring), 7.15 (m, 2H, p-H), 7.22 (m, 4H, m-H), 8.03 (m, 4H, o-H), 9.18 (tq, l/Hp=3.0Hz, V -H H = L 2 H Z , 2H, TaC// 2). 3 1 P{ L H} N M R (81.0 MHz, C 6 D 6 , 25°C): 8 27.3 (s). I 3 C{'H} N M R (125.8 MHz, C 6 D 6 , 25°C): 8 4.9, 5.1, 5.8, 6.0 (br, SiCH 3 ) , 20.9 and 20.7 (br, SiCH 2 P), 84.4 (t, 2 J P C = 9.3 Hz, TaCH 3), 128.7 (t, JPC = 5.0 Hz, PPh-C), 130.1 (s, PPh-C), 132.9 (t, J?c = 6.5 Hz, PPh-C), 138.2 (t, 'ypc = 17.7 Hz, o-C), 244.8 (t, 2 / P C = 8.9 Hz, TaCH 2); l 3 C N M R (125.8 91 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand MHz, C 6 D 6 , 25°C): 5 244.8 (m, ' j H c = 119 Hz, TaCH 2). Anal. Calcd for C 26H47N 2P2Si4Ta: C, 42.04; H , 6.38; N , 3.77. Found: C, 42.34; H, 6.44; N , 3.65. {[P2N2]TaMe2}+BF4" (7) To a stirred solution of 1 (5.087 g, 6.702 mmol) in 80 mL CH 2 C1 2 cooled to -78°C was added a solution of Ph 3 C + BF 4 ~ (2.213 g, 6.702 mmol) in 30 mL of CH 2 C1 2 . The mixture was allowed to warm to room temperature, and the solution was filtered to remove a trace precipitate. The solvent was removed providing a white solid, which was thoroughly rinsed with toluene. The solid was dried under vacuum, to afford {[P 2N 2]TaMe 2} +BF 4~ as a white powder (4.8 g, 86% yield). Solutions of 3 in CH 2 C1 2 decompose over several days at room temperature; therefore, the solid is best stored at -40°C. ' H N M R (CD 2C1 2 , 25°C, 500 MHz): 5 0.40 and 0.54 (s, 24H total, SiC# 3), L28 (t, 2 J P H = 5.8 Hz, 6H, TaCH 3), 1.66 (dt, 2 7 H H = 14.9 Hz, 2J Hp = 6.6 Hz, 4H, CH2 ring), 1.84 (dt, 2JHH = 14.9 Hz, 2 / H P = 5.4 Hz, 4H, CH2 ring), 7.54 (m, 10H, VPh-H); 3 I P{ 'HJ N M R (CD 2C1 2 , 25°C, 81.0 MHz): 8 24.2 (s); l 3 C{ *H} N M R (CD 2C1 2 , 25°C, 125.8 MHz): 5 4.7 (s, SiCH 3 ) , 5.2 (br, SiCH 3 ) , 17.9 (br, SiCH 2 P), 80.2 (s, TaCH 3), 129.8 (t, 7 P C = 5.0 Hz, PPh-C), 131.8 (s, PPh-C), 131.9 (t, 7 P C = 5.3 Hz, PPh-C). Anal. Calcd for C 2 6 H4 8 BF 4 N 2 P 2 Si4Ta: C, 37.59; H, 5.82; N , 3.37. Found: C, 37.91; H, 5.99; N , 3.27. {[P2N2]TaMe2}+B(C6F5)4-(C6H5NMe2) (8) To a stirred solution of 1 (0.689 g, 0.908 mmol) in 20 mL CH 2 C1 2 cooled to -78°C was added PhNMe 2 H + B(C 6 F 5 ) 4 ~ (0.728 g, 0.908 mmol) in 20 mL CH 2 C1 2 . The solution was warmed to room temperature, and the solvent removed. The solid product was rinsed with toluene to produce an insoluble oil, which was separated from the toluene layer via pipette and dried under vacuum to afford 4 as a creamy white powder (1.102 g, 79% yield). ' H N M R (CD 2C1 2 , 25°C, 500 MHz): 5 0.40 and 0.54 (s, 24H total, SiC# 3), 1.28 (t, 2 7 P H = 5.8 Hz, 6H, TaC7/3), 1.66 (dt, 2 J H H = 14.9 Hz, 27Hp = 6.6 Hz, 4H, CH2 ring), 1.84 (dt, 2 / H H = 14.9 92 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Hz, 2 / H P = 5.4 Hz, 4H, CH2 ring), 2.92 (s, 6H, PhN(GfY3)2), 6.95 (m, 2H, NPh-o-H), 7.11 (m, 2H, NPh-m-H) 7.54 (m, 11H, PPh-H and NPh-p-H); 3 I P{ 'H} N M R (CD 2C1 2 , 25°C, 81.0 MHz): 8 24.2 (s). ' H N M R (500 MHz, CD 2 C1 2 , 200 K): 8 0.25, 0.33, 0.40 and 0.47 (s, 24H total, S iC/ / 3 ) , 1-08 (m, 6H, TaC7/3) 1.16, 1.46, 2.03 and 2.12 (m, 8H total, CH2 ring), 2.87 (s, 6H, PhN(C7/ 3) 2), 6.62 (t, 1H, NPh-p-H), 6.66 (m, 2H, NPh-o-H), 7.17 (m, 2H, NPh-m-H), 7.44 (m, 4H, PPh-o-H), 7.54 (m, 6H, PPh-o/m-H). Anal. Calcd for C 58H59BF 2oN 3P 2Si 4Ta: C, 45.11; H , 3.85; N , 2.72. Found: C, 45.46; H, 3.79; N , 2.41. [P2N2]TaMe2F (9) To a slurry of {[P 2N 2]TaMe 2} +BF 4~ (0.985 g, 1.19 mmol) in 15 mL of toluene was added a solution of NaN(SiMe 3) 2(THF) (0.288 g, 1.13 mmol, 0.95 equivalents) in 10 mL of toluene. The solution became yellow, and a fresh precipitate formed. The mixture was evaporated to dryness and the remaining solid extracted with toluene. The toluene solution was filtered through Celite then allowed to slowly evaporate, to afford yellow crystals of [P 2N 2]TaMe 2F. The crystals were crushed to a yellow powder then thoroughly dried under high vacuum, to afford 6 with one half of an equivalent of co-crystallized toluene that could not be removed (0.634 g, 70% yield). Solutions of 6 are thermally sensitive. The same product was formed using lithium diisopropylamide as a base. ' H N M R (C 6 D 6 , 340 K, 500 MHz): 8 0.28 and 0.33 (s, 24H total, SiCtf 3), 1.27 (dt, 2 7 P H = 7.5 Hz, 2 J P F = 6.2 Hz, 6H, TaC# 3), 1.55 and 1.55 (m, 8H, CH2 ring), 7.10 and 7.15 (m, 6H total, m/p-H), 7.63 (m, 4H, o-H); 3 1 P{ 'H} N M R (81.0 MHz, C 6 D 6 , 25°C): 8 30.2 (s). ' H N M R (500 MHz, C 7 H 8 , 240 K): 8 0.18, 0.22, 0.47 and 0.49 (s, 24H total, SiCH 3 ) , 1.3, 1.3, 1.52, and 1.58 (m, 8H total, SiCH 2 P), 1.30 (m, 6H, TaC// 3 ), 7.09 (m, 2H, p-H), 7.10 (m, 4H, m-H), 7.54 (m, 4H, o-H). 3 1 P{'H} N M R (81.0 MHz, C 7 D 8 , 185 K): 8 26.5 (dd, 2 7 P P = 74.8 Hz, 2 H P F ~ 70 Hz), 43.9 (d, 2 / P P = 74.8 Hz). Anal. Calcd for C 2 6 H 4 8 FN 2 P 2 Si 4 Ta(C 7 H 8 ) 0 . 5 : C, 43.80; H, 6.48; N , 3.46. Found: C, 43.42; H, 6.69; N , 3.61. 93 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand TaMe2(CHSiMe2NSiMe2CH2P(Ph)CH2SiMe2NSiMe2CH2PPh) (10) To a slurry of {[P 2 N 2 ]TaMe2} + B(C 6 F 5 )4~ (0.713 g, 0.501 mmol) in 15 mL of toluene was added a solution of NaN(SiMe 3) 2-(THF) (0.128 g , 0.501 mmol) in 10 mL of toluene. The colour changed to yellow, and a light-brown oil separated. The mixture was evaporated to dryness and the remaining solid extracted with toluene. The toluene solution was then evaporated to dryness to afford 10 (0.348g, 93% yield). The use of lithium diisopropylamide or Me 3 P=CH 2 as a base also produced 10. ' H N M R (C 6 D 6 , 295 K, 500 MHz): 5 -0.12, 0.08, 0.13, 0.27, 0.39, and 0.55 (s, 3H each, SiCH 3 ) , 0.49 (d, 4 / H P = 3.9, 3H, SiC/73), 0.67 (d, 4 / H P = 1.3 , 3H,SiC/73), 0.75 (ABX, 2JHH = 14.7, 2JHP = 7.0, 1H, SiCflHP), 1.12 (ABX, 2 7 H H = 14.7, 2J Hp = 17.0, 1H, SiCHT/P), 0.89 (ABX, 2 / H H = 14.1, 27Hp = 7.3, 1H, SiCflHP), 1.23 (ABX, 2 / H H = 14.1, 27Hp = 12.0, 1H, SiCHflP), 1.15 (ABX, 2 7 H H = 12.9, 2 / H p = 8.8, 1H, SiCflHP), 1.37 (ABX, 2 7 H H = 12.9, 27Hp = 3.8, 1H, SiCHHP), 1.37 (d, 37Hp = 6.1, 6H, TaC/73), 2.39 (dd, 2 / H p = 2.8, 3/Hp = 2.5, 1H, TaCflP), 7.08 (m, 1H, p-H), 7.12 (m, 2H, m-H), 7.14 (m, 1H, p-H), 7.31 (m, 2H, m-H), 7.48 (dd, 3 / H H = 6.8, 2H, o-H), 7.79 (dd, 3 / H H = 7.0, 2H, o-H). 3 1 P (C 6 D 6 , 295 K): 5 17.9 (s, IP, TaP), -17.1 (s, IP, TaCHP). 1 3 C (C 6 D 6 , 295 K): 5 1.9 (d, 3 / C p = 3.8, SiCH 3 ) , 2.7 (d, 37Cp = 20.0, SiCH 3 ) , 3.9 (d, 3 / C p = 4.3, SiCH 3 ) , 4.0 (s, SiCH 3 ) , 5.1 (s, SiCH 3 ) , 5.5 (s, SiCH 3 ) , 6.0 (s, SiCH 3 ) , 6.4 (dd, 7Cp = 7.1, 7Cp = 9.1, SiCH 3 ) , 16.8 (d, 27Cp = 2.9, SiCH 2 P), 18.9 (d, 2 / C p = 2.9, SiCH 2P), 20.7 (d, 27Cp = 29.1, SiCH 2 P), 58.8 (dd, JCp = 13.4, 7Cp = 39.6, TaCHP), 66.8 (s, TaCH 3), 127.2 (s, PPh), 128.7 (d, 7Cp = 4.8, PPh), 129.0 (d, 7Cp = 8.6, PPh), 130.6 (d, 7Cp = 1.9, PPh), 131.6 (d, JCP = 17.2, PPh), 133.0 (d, 7Cp = 10.4, PPh). Anal. Calcd for C26H47N2P2Si4Ta: C, 42.04; H , 6.38; N , 3.77. Found: C, 42.41; H , 6.46; N , 3.61. Decomposition product of {[P2N2]TaMe2}+BF4~ (11) Solid {[P2N 2]TaMe 2} +BF4~ (0.040 g, 0.048 mmol) was dissolved in 5 mL of CH 2 C1 2 . Over the course of two days a colourless crystalline solid precipitated. The solid was collected and dried under vacuum. Dissolving this colourless solid in C 4DgO resulted in a yellow solution, apparently from the decomposition of 11. 94 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand Effect of photolysis time on conversion of 5 to 6 A solution of 5 (32.6 mM) and ferrocene (42.55 mM) in C 6 D 6 was sealed in a N M R tube. The tube was photolysed for a measured period of time, and then the ' H N M R spectrum was collected. The concentrations of 5 and 6 were obtained by integrating the o-phenyl protons for the two species relative to the internal ferrocene standard. The photolysis was then continued, with spectra collected at 5, 11, 18, 24, 33, 44, 65, and 85 minutes. Photolysis time(minutes), yield of 6(%), percent N M R active Ta species remaining(%): 5, 27, 89; 11, 54, 89; 18, 58, 75; 24, 72, 59; 33, 61, 72; 44, 57, 69; 65, 53, 67; 85, 48, 64. EPR of products of excessive photolysis of 5 A solution of 5 in hexanes was sealed in an EPR tube equipped with a Teflon valve. The solution was photolysed for 50 minutes, resulting in a dark green solution. EPR (hexanes): g = 1.93; a( 3 lP) = 35 G, 2 P; a( l 8 1Ta) = 171 G, ITa. X-ray Crystallographic Analyses of Complexes 3, 5 and 6 Selected crystallographic data, fractional coordinates and thermal parameters are provided in Appendix 1. 2.10 References 1) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. Organometallics 1998,17, 846. 2) Fryzuk, M . D.; Love, J. B.; Rettig, S. J.; Young, V . G. Science 1997, 275, 1445. 3) Fryzuk, M . D.; Love, J. B., unpublished results. 95 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 4) Giesbrecht, G. R. Amidophosphine Complexes of Electron-Poor Metals; Ph. D. thesis, University of British Columbia: Vancouver, 1998. 5) Leznoff, D. B. Paramagnetic Organometallic Complexes; Ph. D. thesis, University of British Columbia: Vancouver, 1997. 6) Bowdridge, M . R. Niobium Phosphine Macrocyclic Complexes; M . Sc. thesis, University of British Columbia: Vancouver, 1998. 7) Fryzuk, M . D.; Kozak, C , unpublished results. 8) Corkin, J. R. Hafnium Complexes Stabilized by a Macrocyclic Ligand; M . Sc. thesis, University of British Columbia: Vancouver, 2000. 9) Roskamp, E. J.; Pederseon, S. F. J. Am. Chem. Soc. 1987,109, 6551. 10) Freundlich, J. S.; Schrock, R. R.; Davis, W. M . Organometallics 1996,15, 2777. 11) Bunker, M . J.; De Cian, A. ; Green, M . L. H. J. Chem. Soc, Chem. Commun. 1977, 59. 12) Bunker, M . J.; De Cian, A. ; Green, M . L. H.; Moreau, J. E.; Siganporia, N . J. Chem. Soc, Dalton Trans. 1980, 2155. 13) Burt, R. J.; Chatt, J.; Leigh, G. L ; Teuben, J. J.; Westerhof, A. J. Organomet. Chem. 1977,129, C33. 14) Fox, P. A.; Gray, S. D.; Bruck, M . A. ; Wigley, D. E. Inorg. Chem. 1996, 35, 6027. 15) Mashima, K. ; Oshiki, T.; Kazuhide, T. Organometallics 1997,16, 2760. 16) Jefferis, J. M . ; Morris, R. J.; Huffman, J. C. Inorg. Chem. 1997, 36, 3379. 17) Guerin, F.; McConville, D. H.; Vittal, J. J.; Yap, G. A. P. Organometallics 1998, 17, 1290. 18) Sperry, C. K.; Cotter, W. D.; Lee, R. A. ; Lachicotte, R. J.; Bazan, G. C. I. Am. Chem. Soc. 1998,120, 7791. 96 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 19) Castellano, B.; Zanotti-Gerosa, A.; Solari, E.; Floriani, C. Organometallics 1996, 15, 4894. 20) Fairbrother, F. The Chemistry of Niobium and Tantalum; Elsevier: New York, 1967. 21) Freundlich, J. S.; Schrock, R. R.; Davis, W. M . J. Am. Chem. Soc. 1996,118, 3643. 22) Pray, A . R. Inorg. Synth. 1990, 28, 321. 23) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry; Allyn and Bacon: Needham Heights, Massachusetss, 1991. 24) Chao, Y.-W.; Wexler, P. A. ; Wigley, D. E. Inorg. Chem. 1989, 28, 3860. 25) Gountchev, T. I.; Tilley, T. D. J. Am. Chem. Soc. 1997,119, 12831. 26) Pearson, R. G. /. Chem. Ed. 1968, 45, 581. 27) Cotton, F. A. ; Falvello, L. R.; Najjar, R. C. Inorg. Chem. 1983, 22, 375. . 28) Allbutt, M . ; Feenam, K.; Fowles, G. W. A. J. Less-Common Metals 1964, 6, 299. 29) Suh, S.; Hoffman, D. M . Inorg. Chem. 1996, 35, 5015. 30) Juvinall, G. L. J. Am. Chem. Soc. 1964, 86, 4202. 31) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978,100, 2389. 32) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,119, 9071. 33) Hoffman, D. M . ; Suh, S. J. Chem. Soc, Chem. Commun. 1993, 714. 34) Bradley, D. C.; Hursthouse, M . B.; Howes, A. L ; Jelfs, A. N . d. M . ; Runnacles, J. D.; Thornton-Pett, M . J. Chem. Soc, Dalton Trans. 1991, 841. 35) Bradley, D. C.; Hursthouse, M . B.; Malik, K. M . A. ; Nielson, A . J.; Vuru, G. B. C. J. Chem. Soc, Dalton Trans. 1984, 1069. 97 References begin on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 36) Bradley, D. C ; Hursthouse, M . B. ; Malik, K. M . A. ; Vuru, G. B . C. Inorg. Chim. Acta 1980, 44, L5. 37) Hagele, G.; Fuhler, R. DNMR-SIM; 1.00 ed., 1993. 38) Dawson, D. Y. ; Arnold, J. Organometallics 1997,16, 1111. 39) Chamberlain, L. R.; Rothwell, A. P.; Rothwell, I. P. J. Am. Chem. Soc. 1984,106, 1847. 40) Chamberlain, L . R.; Rothwell, I. P. J. Chem. Soc, Dalton. Trans. 1987, 163. 41) Edwards, D. S.; Blondi, L. V . ; Ziller, J. W.; Churchill, M . R.; Schrock, R. R. Organometallics 1983, 2, 1505. 42) Fryzuk, M . D.; MacNeil, P. A. ; Rettig, S. J. J. Am. Chem. Soc. 1985,107, 6708. 43) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577. 44) Guggenberger, L . J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6578. 45) Takusagawa, F.; Koetzle, T. F.; Sharp, P. R.; Schrock, R. R. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1988, C44, 439. 46) Schrock, R. R. Acc. Chem. Res. 1979,12, 98. 47) Hostetler, M . J.; Bergman, R. G. /. Am. Chem. Soc. 1990,112, 8621. 48) Proulx, G.; Bergman, R. G. Science 1993, 259, 661. 49) Berry, D. H. ; Koloski, T. S.; Carroll, P. J. Organometallics 1990, 9, 2952. 50) Antonelli, D. M . ; Schaefer, W. P.; Parkin, G.; Bercaw, J. E. J. Organomet. Chem. 1993, 462,213. 51) van Asselt, A. ; Burger, B. J.; Gibson, V. C ; Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 5347. 52) Heinekey, D. M . ; Radzewich, C. E. Organometallics 1998, 17, 51. 98 References begin:on page 95. Chapter 2: Synthesis and Reactivity of a Ta(V) Trimethyl Complex of the Macrocyclic [P2N2] Ancillary Ligand 53) Gunnoe, T. B. ; White, P. S.; Templeton, J. L.; Casarrubios, L . J. Am. Chem. Soc. 1997, 119, 3171. 54) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,118, 100. 55) Fryzuk, M . D.; Gao, X . ; Joshi, K.; MacNeil, P. A. ; Massey, R. L. J. Am. Chem. Soc. 1993, 775, 10581. 56) Burrell, A . K.; Clark, G. R.; Rickard, C. E. F.; Roper, W. R.; Wright, A. H. J. Chem. Soc, Dalton Trans. 1991, 609. 57) Roger, C ; Lapinte, C. J. Chem. Soc, Chem. Commun. 1989, 1598. 58) Hi l l , A . F.; Roper, W. R.; Waters, J. M . ; Wright, A. H. J. Am. Chem. Soc. 1983, 105, 5939. 59) Patton, A . T.; Strouse, C. E.; Knobler, C. B.; Gladysz, J. A. J. Am. Chem. Soc. 1983, 105, 5804. 60) Kang, S. K.; Tang, H.; Albright, T. A. / . Am. Chem. Soc. 1993,115, 1971. 61) Rodriguez, G.; Bazan, G. C. J. Am. Chem. Soc. 1995,117, 10155. 62) Haaland, A. ; Hammel, A. ; Rypal, K.; Volden, H. V. J. Am. Chem. Soc. 1990,112, 4547. 63) Morse, P. M . ; Girolami, G. S. / . Am. Chem. Soc. 1993,115, 1971. 64) Gillespie, R. J.; Robinson, E. A. Angew. Chem. Int. Ed. Engl. 1996, 35, 495. 65) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. /. Chem. Soc, Chem. Commun. 1996, 2783. 66) Koster, R.; Simic, D.; Grassberger, M . A. Justus Liebigs Ann. Chem. 1970, 739, 211. 99 References begin on page 95. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Chapter Three Agostic Interactions in a Tantalum Ethyl Complex 3.1 Introduction In chapter two the synthesis of the methyl methylidene complex [P2N 2]Ta=CH2(Me) (6) from the photolysis of [P 2N 2]TaMe 3 (5) was described. In general, methylidene complexes of the early transition metals are not stable, and complex 6 is no exception. Over the course of several months solutions of 6 decompose at room temperature. At 70 °C this decomposition occurs within hours. Labeling studies and kinetic data have previously shown that the decomposition pathway of the archetypal methylidene complex, (r|5-C5H 5)2Ta=CH 2(Me) 1 could be determined by trapping the reaction products with ethylene gas. As shown in Scheme 3.1, the decomposition of (r| 5-C5H 5) 2Ta=CH 2(Me) occurs by dimerization. Coupling of the methylidene units provides the ethylene complex (r)5-C 5 H 5 ) 2 T a ( C 2 H 4 )Me and presumably (r| 5-C 5H 5) 2TaMe, which can be trapped in the presence of ethylene to provide ( r | 5 -C 5 H 5 )2Ta(C2H 4 )Me. 100 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Scheme 3.1. By analogy, it was anticipated that the decomposition of the less electronically saturated [P 2N 2]Ta(=CH 2)Me in the presence of ethylene gas would yield [P 2N 2]Ta(C 2H 4)Me; however, the reaction of 6 with ethylene does not proceed by this mechanism. The major product of the reaction of 6 with ethylene gas is [P2N2]Ta(C2H-4)Et (12). This chapter describes the reaction of [P 2N 2]Ta(=CH 2)Me with ethylene and a description of the (3-agostic interaction and associated fluxional processes observed in [P2N2]Ta(C2H4)Et. 3.1.1 Introduction to Agostic Interactions Agostic interactions involving C-H bonds are more than just a structural curiosity; they have been shown to be important in C-H activation processes2'3 as well as in olefin polymerization. With respect to polymerization, p-agostic interactions have been implicated in the resting state of many early and late metal polymerization catalysts,4"7 and thus may be relevant to catalyst activity and polymer molecular weights.8"1 5 Similarly, oc-agostic interactions have been examined for their ability to direct olefin insertion.1 2 101 References begin on page 135. Chapter 3: Agostic Interactions In a Tantalum Ethyl Complex There are numerous examples of (3-agostic interactions, where a |3-C-H bond of an alkyl ligand interacts with the metal centre providing a 3-centre, 2-electron bonding interaction. 5' 1 6 A depiction of this three orbital overlap is shown in Figure 3.1, along with the half-arrow convention typically used to depict such an interaction. For the simplest alkyl group in which a (3-agostic interaction is possible, the ethyl group, (3-agostic interactions are usually associated with coordinatively unsaturated and cationic complexes. 1 7 - 2 0 While a few (3-agostic ethyl groups have been postulated on the basis of NMR, EPR and IR data , 2 1 - 2 3 the only structurally characterized example of a (3-agostic ethyl group in a neutral compound is the 12-electron species TiCl3(dmpe)Et, 2 4 ' 2 5 and the bonding in this species continues to be the subject of theoretical studies. 2 6 - 3 0 There are also a number of reports of a-agostic interactions,5'1 2 although for the ethyl group this interaction is believed to be preferred only when steric bulk disfavors a (3-agostic interaction.31 Figure 3.1. Depiction of the three-centre two-electron bonding model for an ethyl group with a (3-agostic interaction (left) and the half-arrow convention used to depict such an interaction (right). 3.2 Reactivity of [P2N2]Ta=CH2(Me) (6) with Ethylene As noted in the introduction, the decomposition of [P 2N 2]Ta=CH 2(Me) (6) in the presence of ethylene was studied to gain insight into the method by which this complex decomposes. A solution of [P2N2]TaMe3 (5) gradually darkens as it is photolyzed and uncharacterized paramagnetic impurities are formed, likely from the further photolysis of the 102 References begin on page 135. Chapter 3: Agostlc Interactions in a Tantalum Ethyl Complex major product, [P 2N 2]Ta=CH 2(Me). The photolysis of [P 2N 2]TaMe 3 in the presence of ethylene is visibly different, because the solution does not darken even with long photolysis times, and a light orange/red solution results. The initial product of the photolysis in the presence of ethylene remains the methylidene [P 2N 2]Ta=CH 2(Me), and both the ! H and 31 1 P{ H} N M R spectra show it to be the major product; however, performing this reaction in a N M R tube containing an internal concentration standard demonstrates that a considerable amount of paramagnetic, NMR-inactive side products must also exist. When the photolysis time is sufficiently long for all of the [P 2N 2]TaMe 3 starting material to react, only a -50% yield of [P 2N 2]Ta=CH 2(Me) is obtained (relative to internal ferrocene by ' H N M R spectroscopy), although this value varies from 40-60% depending on the exact conditions employed. Variables that appear to affect this value could include the concentration of the [P 2N 2]TaMe 3 solution, the temperature at which the photolysis is performed, and the source and intensity of the ultraviolet light source. Although ethylene appears to react immediately with the dark paramagnetic impurities produced in the photolysis of [P 2N 2]TaMe 3, as evidenced by the difference in the color of the solution after photolysis, ethylene also reacts with the methylidene complex [P 2N 2]Ta=CH 2(Me). This latter reaction is slow, comparable in rate to the thermal decomposition of [P 2N 2]Ta=CH 2(Me). After monitoring the reaction by ' H and 3 I P{ 1 H} N M R spectroscopies over the course of two weeks, there were two new products, and no [P 2N 2]Ta=CH 2(Me) remained. The major product was identified by a combination of ' H , l 3 C{'H} and 3 1 P{'H} N M R spectroscopies as [P 2N 2]Ta(C 2H 4)Et, 12, and the minor product as [P 2N 2]Ta(C 2H4)Me, 13. The total concentration of complexes 12 and 13 was equal to that of the original [P 2N 2]Ta=CH 2(Me) by comparison to an internal standard, which indicates that the source of complexes 12 and 13 was [P 2N 2]Ta-CH 2(Me) and not the NMR-inactive products. The dark red NMR-inactive products resulting from the reaction with ethylene have different solubilities than complexes 12 and 13 and could be removed by washing with hexanes. Analysis of N M R spectra ('H and l 3 C{'H}) and GC mass spectra identified propylene and 1-butene as the organic products of this reaction. The tantalum-containing reaction products are depicted in Scheme 3.2. 103 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Scheme 3.2. [P2N2]Ta(C2H4)Et [P2N2]Ta(C2H4)Me 12 13 (i) NMR Spectral Data for [P2N2]Ta(C2H4)Et The resonances for the ethyl group in 12 are prominent in the 'H{ 3 I P} N M R spectrum, with a quartet for the a-hydrogens of the ethyl group at 8 -1.27 and a triplet for the (3-protons at 8 -0.51. The ' H N M R signals for of the tantalum ethyl group provide coupling constants for the a and (3 hydrogens to 3 I P of /PH = 3.1 Hz and /PH = 4.4 Hz, respectively. The larger coupling to the (3-hydrogens of the ethyl group provides strong evidence that there is an agostic interaction of a (3-carbon C-H bond with the tantalum metal centre in solution, 104 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex as does its appearance at low frequency. In comparison, for the related 18-electron species (r i 5 -C5H5)2Nb(C2H 4 )Et , 3 2 in which there is no agostic interaction, the P-CH3 resonance appears at 8 1.83; in a less electronically saturated ethylniobium species with an a-agostic interaction, the (3-CH3 group appears at 8 1.15.31 Both of these groups have chemical shifts in the same region as ethyl groups in organic compounds. The single chemical environment for the (3-hydrogens of the ethyl group in the ' H N M R spectrum of 12 indicates that there is rapid rotation of the terminal C H 3 group, so that the observed 4.4 Hz coupling is an average of the P-H couplings arising from the single agostic hydrogen (quasi two-bond coupling) and the two terminal hydrogens (four-bond coupling). If the coupling of the phosphorus nuclei to the more distant terminal (3-hydrogens is assumed to be much smaller, the actual two-bond coupling to the (3-agostic hydrogen is approximately three times this value, or 13.2 Hz. Likewise, the chemical shift for the (3-CH3 group (8 -0.51) is an average of the chemical shifts of the two terminal and one agostic (3-hydrogens; however, even at 180 K only one chemical environment is observed for the P-CH3 group in the ! H N M R spectrum. On the N M R time-scale complex 12 has a mirror plane of symmetry such that the two phosphorus donors are equivalent but the two amido donors are not. Thus, two signals are evident in the ' H N M R spectrum for the tantalum-bound ethylene group. Only one signal was observed for the ethylene group in the l 3 C{ 'H} N M R spectrum, indicating that the C-C vector of the ethylene group must be arranged perpendicular to the mirror plane of symmetry. The [ P 2 N 2 ] ligand resonances in the ' H N M R spectrum are as expected for a complex with apparent C s symmetry, with four silyl methyl resonances and four ligand methylene proton environments. The resonances associated with the chemically equivalent phenyl substituents of the phosphine ligands are also consistent with this symmetry. Variable-temperature ' H N M R spectroscopy demonstrates that the chemical shift of the (3-hydrogens of the ethyl group is significantly temperature dependent, as is one of the resonances of the tantalum bound ethylene group, presumably the hydrogens directed towards the ethyl ligand. The ' H N M R signal of the (3-hydrogens of the ethyl group shifts to lower frequency on cooling to 200K. These data indicate that the (3-agostic interaction in 12 is fluxional, with the entropically disfavored (3-agostic r\2-ethyl group being increasingly favoured at lower temperature. This fluxional process, shown in Figure 3.2, accounts for the observed C s 105 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex symmetry of 12 in the ' H and 3 1P{ 'H} N M R spectra. An in-place rotation of the ethyl group around the C«-Cp bond would not account for the C s symmetry of species 12 in solution. 3 3 The intermediate species in Figure 3.2 is depicted as having an a-agostic interaction, despite the fact that no data have yet been provided to support such an a-agostic-(3-agostic equilibrium. The study of the effect of partial deuteration of the ethyl group provided evidence for this process in the solution structure of 12. Figure 3.2. Depiction of the fluxional nature of the (3-agostic interaction that results in the C s symmetry of complex 12 on the N M R timescale. The intermediate species shown here is drawn with an oc-agostic interaction. (ii) Isotopic Perturbation of Resonance It has been noted that at temperatures as low as 180 K the P-CH3 group of the agostic ethyl moiety in 12 shows only a single resonance in the 500 MHz ' H N M R spectrum. Thus, the only evidence for a (3-agostic interaction in the solution structure of 12 is an unusually low frequency chemical shift of the (3-protons and a large coupling of these protons to the phosphorus nuclei of the [P2N2] ligand. Evidence for an oc-agostic interaction in equilibrium with the P-agostic interaction is more difficult to obtain. The chemical shift of the a-protons are anticipated to be affected by their proximity to the metal centre regardless of whether or not an a-agostic interaction is present. Likewise, a large coupling of the resonance of the oc-Ph 106 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex hydrogens to 3 I P is anticipated. A valuable technique for examining agostic interactions is isotopic perturbation of resonance ( IPR) . 3 4 ' 3 5 For species with (3-agostic interactions, this technique involves partially deuterating the |3-CH 3 group, so that the (3-CH 2D and (3-CHD 2 isomers are also present in solution. The difference in the zero-point energies for terminal C-D bonds versus agostic C-D bonds compared to terminal C-H bonds versus agostic C-H bonds causes the protons to accumulate in the (3-agostic position, whereas the deuterons will accumulate in the terminal sites. Therefore, if an agostic interaction is present, this experiment should result in an IPR that produces a noticeable chemical shift separation between the isotopomers present. The preparation of a variety of deuterated isotopomers of 12 proved to be facile. The addition of ^-ethylene to a solution of 12 provided a mixture of isotopomers. Attempts to monitor this exchange by ' H N M R spectroscopy were unsuccessful, because considerable exchange had already occurred by the time the solution could be transferred to the N M R probe. It is not clear whether the C 2 D 4 exchanges with the ethyl group or the bound ethylene or both. Regardless, a mechanism that exchanges proton environments from the ethyl to the ethylene group exists and will be discussed later. Figure 3.3 shows the ethyl region of the ' H N M R spectrum of complex 12 before and after a small amount of C 2 D 4 has been added. After adding C 2 D 4 , the resonances for the C H 2 C H 3 isotopomer are still the most intense, with the (3-CH 3 group at 5 -0.52. The (3-CH 2D resonance occurs at 8 -0.64 ppm, a shift of -0.12 ppm from the undeuterated resonance. The third isotopomer, (3-CHD 2, occurs at two apparent resonances, with an average chemical shift of 8 -0.75; the appearance of two resonances for this signal was unexpected, and it appears to imply that the degree of deuteration of the adjacent methylene group also affects the ! H chemical shift of the (3-CHD 2 resonance. The appearance of a large IPR for the oc-CH2 group was also unanticipated. The resonance shifts from 8 -1.28 for the a - C H 2 isotopomer to 8 -1.52 for the a-CHD isotopomer. The resonance at -1.52 is also comprised of more than one actual chemical shift, so that the C H D C H 3 and C H D C H 2 D isotopomers, for example, have slightly different ' H chemical shifts for their a-CHD resonances. The effect of deuteration of 107 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex the bound ethylene moiety has a negligible affect on its apparent chemical shift in the ' H N M R spectrum, as was expected. i 1 1 1 1 J-—I 1 1 1 1 1 1 1 r -0.4 -0.8 -1J2 -1.6 Figure 3.3. The ethyl region of the ' H N M R (500 MHz, 295 K) spectrum of [P2N2]Ta(C2H4)Et, 12, before (top) and after (bottom) C 2 D 4 was added. In order to understand the effect of deuteration of the a - C H 2 group on the chemical shift of the P-CH3 resonance in the *H N M R spectrum of 12, a more highly deuterated sample was prepared which consisted of -75% deuterium in the ethyl group. At room temperature, the many signals for the (3-C-H group overlap; however, after the sample is cooled to 255 K these signals separate sufficiently so that at least six distinct resonances can be identified. By taking into account the degree of deuteration, it was possible to determine the likelihood of all the possible isotopomers of the ethyl group, and this assisted with the assignment of the resonances. The ' H spectrum of the ethyl group at 255 K is shown in 108 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Figure 3.4. For the p-C ' H environments the lowest frequency resonance at 8 -1.16 is assigned to the CD2CHD2 isotopomer, the resonance at 8 -1.10 to the C H D C H D 2 isotopomer, and the higher frequency resonances to CD2CH2D, C H D C H 2 D , C D 2 C H 3 , and C H D C H 3 , respectively. The resonances of the p-C protons in isotopomers containing two protons in the a-C position were too low in intensity to assign. The difference in chemical shifts of the P-C-H protons compared to the spectrum shown in Figure 3.3 is due to the temperature dependence of the chemical shifts of these protons, as noted previously. That the lowest frequency corresponds to the most deuterated sites at the p-C is as expected, because this causes the protons to preferentially accumulate in the agostic position. As mentioned above, the appearance of extra signals in the P -C-H region for isotopomers containing different degrees of deuteration in the oc-C position was not expected. The effect of increased deuteration at the oc-C position is a decrease in the frequency of the P~ C-H resonances. If only the p-agostic interaction was present in the structure of 12 in solution, then only three p-C-H resonances should be observed, and no significant effect should be observed when the oc-C is deuterated. However, if an oc-agostic interaction is in equilibrium with the p-agostic interaction, as shown in Figure 3.2, then the observation of six p-C-H resonances and the large IPR for the oc-C-H protons can be rationalized. The origin of this effect is similar to that noted previously, where partial deuteration of the p-C-H group causes the protons to accumulate in the p-agostic position, due to the slightly higher energy of the deuteron in the P-agostic position. The effect of increased deuteration of the a -C would provide a slight destabilization of the a-agostic interaction versus the P-agostic interaction. If the energy difference between the P-agostic interaction and the a-agostic interaction is very small, this small destabilization should favor the p-agostic interaction sufficiently to cause the slight shift of the P-C-H resonance to lower frequency, as is observed. The rapid rate of the process shown in Figure 3.2 is consistent with a small energy difference between the P-agostic and a-agostic ethyl group for 12 in solution. Unlike the P-C-H resonances, the a -C-H resonances are not greatly affected by the degree of deuteration at the p-C. This is consistent with the observation that the chemical 109 References begin on page 135. Chapter 3: Agostic Interactions In a Tantalum Ethyl Complex shift of the a-C protons is not greatly temperature dependent, unlike the (3-C protons, so that a shift in the equilibrium between the a-agostic and (3-agostic structure does not result in a significant change in the chemical shift of the a-C-H resonances. The relatively small temperature dependence of the a-C proton resonance eliminates the possibility that the large IPR observed for 12 is due simply to an equilibrium between a [3-agostic and an nonagostic structure. For an IPR to be observed both a difference in chemical shift and a difference in C-H bond length is necessary, and so the significant IPR observed necessitates a structure in which the two a-C-H environments and C-H bond lengths are vastly different in the species lacking a (3-agostic interaction. The most probable structure that fits these requirements is one containing an a-agostic interaction, as shown in the center of Figure 3.2. CHDCH2D CHDCHD2 and CHDCD3 — i — i — i — i — i — i — i — i — i — i — i -0.8 -1.0 -1.2 -1.4 -1.6 (ppm) Figure 3.4. The ethyl region of the *H N M R (500 MHz, 255 K) spectrum of [P2N2]Ta(C2H4)Et (12) in which the ethyl group is -75% deuterated. The presence of an a-agostic interaction in a tantalum ethyl species such as 12 has precedence; a-agostic interactions are implicated in the formation of tantalum alkylidene complexes from sterically crowded tantalum alkyls. 3 6 Additionally, examples of niobium ethyl complexes with a-agostic interactions but no (3-agostic interactions have been reported, 3 1 ' 3 7 and the zwitterionic tantalocene derivative (r| 5-C5H5)2Ta(=CH2B(C6F5)3)(Me) has a strong a-agostic interaction with a methylidene hydrogen. One example exists where D,CHD, CHDCHD CD2CH2 CHDCH CHDCH, 110 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex both a-agostic and p-agostic interactions were identified to be in equilibrium in the ' H N M R spectrum;38 two distinct species corresponding to an a-agostic and a P-agostic niobium bound isopropyl group were observed by ' H N M R spectroscopy, a result that is unique even in that study. For the early transition metals, where both a-agostic and p-agostic interactions are quite common, it is possible that many complexes undergo equilibria of this type; however, such processes may be too fast to be observed on the N M R timescale. In such cases, thorough labeling studies could provide additional information about the solution structures of these complexes. Complex 12 provides the first example where an equilibrium between a-agostic and P-agostic interactions is identified by a study of the isotopic perturbation of resonance of both the a and P positions. This could be used to study isolable polymerization catalyst resting states, such as those noted in section 3.1.2. The absence of other reports of equilibria of this type may be due to the exclusive labeling of the p-C of the ethyl group in other studies, 1 7 ' 2 4 ' 2 5 which prevented the detection of such an equilibrium in solution. On the other hand, for this effect to be observed the a-agostic and P-agostic structures must be similar in energy, and this may be uncommon. It has been suggested that the competition between a-agostic and p-agostic interactions is likely governed by steric factors, where increasing steric bulk disfavors p-agostic interactions.31 (iii) NMR Spectral Data for Carbon-13 Labeled [P2N2]Ta(13C2H4)13Et The synthesis of an isotopomer of 12 with the ethyl group carbon-13 labeled was performed to determine the effect of the agostic interactions in species 12 on the ' / C H values. The reaction of [P 2N 2]Ta=CH 2(Me) with labeled 1 3 C 2 H 4 yields the fully labeled species [P 2N 2]Ta( l 3C 2FL;) 1 3Et, where the four carbons of the ethyl and ethylene groups of complex 12 are 1 3 C labeled. The l 3 C{'H} N M R spectrum of this reaction mixture illustrates a ' / C c coupling constant in the ethyl moiety of 30.0 Hz, as expected for a C-C single bond. From the ' H N M R spectrum of this 13C-labeled species, it was possible to determine the ' /CH coupling constants of 126.5 Hz and 123.1 Hz for the a-carbon and P-carbon of the ethyl group respectively. Both are within the values expected for sp' hybridized carbons. In some cases a large ' /CH of -150 Hz to the a-carbon has been used as evidence for an agostic 111 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex interaction; 1 7 ' 2 1 ' 2 5 however, there are other complexes in which p-agostic ethyl groups are implicated where a value similar to that found here was observed.22 The 1 JCH values for the ethylene fragment are somewhat obscured in the ' H N M R spectrum, because these signals are in the same region as the ligand methylene groups. From the l 3 C N M R data these coupling constants were -145 Hz. These values are larger than those observed for the ethyl group, but smaller than the -156 Hz coupling constant expected for ethylene complexes with minimal backbonding. The variable-temperature l 3 C N M R spectra of [P 2 N2]Ta( l 3C 2H4) l 3Et was also investigated to compliment the variable-temperature ' H N M R data obtained for 12. The resonance of the a-C shifts from 8 75.9 at 350 K to 8 53.5 at 180 K, and the corresponding 'JCH value increases from 123.4 Hz at 350 K to 133.6 Hz at 233 K. These data are consistent with an increased contribution from an a-agostic structure versus a P-agostic structure at higher temperatures.16 The resonance of the P-C is less strongly affected and shifts from 8 7.3 at 350 K to 8 2.0 at 180 K and the corresponding ' J C H value changes from 123.8 Hz at 350 K to 122.8 Hz at 233 K. In comparison, the resonance of the tantalum bound ethylene group is 8 45.5 at 350 K and 8 43.3 at 180 K, which indicates that this resonance is not greatly affected by temperature. (iv) NMR Spectral Data for [P2N2]Ta(C2H4)Me (13) The ethylene methyl complex, 13, is produced as a minor product in the reaction of [P 2N 2]Ta=CH 2(Me) with ethylene. The ' H N M R spectrum of complex 13 contains a low frequency resonance for the tantalum-bound methyl group at 8 -1.49 coupled to phosphorus ("/PH = 3.1 Hz). In complex 13 four silyl methyl peaks are present in the H N M R spectrum, indicating that again, as for species 1, this complex appears to have C s symmetry. Unfortunately, other peaks in the same region, particularly the [P 2N 2] ligand methylene signals, obscure the ' H N M R signals for the ethylene protons. An a-agostic interaction may be present in this species, however, this possibility has not been probed by IPR. For complex 13, a single isotopomer [P 2 N 2 ]Ta(' 3 C 2 H 4 )Me results from the reaction of [P 2N 2]Ta=CH 2(Me) with l 3 C 2 H 4 . The 1 3 C{'H} N M R spectrum contains a signal for the 112 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex tantalum bound ethylene group at 5 49.1, near that observed for the ethylene group in complex 12. The tantalum methyl group of 13 is not 1 3 C labeled, and appears as a triplet at 5 51.4. 3.3 Alternate Synthesis and Structure of [P2N2]Ta(C2H4)Et (12) Complexes 12 and 13 proved difficult to separate by fractional crystallization. Both are soluble in aromatic solvents and have poor solubility in hexanes. Attempts to isolate complex 12 by exposing a benzene solution of these two complexes to air with the intention that the less sterically crowded methyl complex 13 might be more reactive led to the surprising revelation that both these complexes are air-stable. The benzene solution of 12 and 13 showed no signs of decomposition of either complex after 4 hours of exposure to air. Due to the lack of success in separating complex 12 from complex 13, an alternate route to 12 was sought, in order to determine its structure in the solid state by X-ray crystallography. A viable route to pure 12 might be through the reaction of ethylene with a Ta(V) hydride, such as [P2N 2 ]TaH 3 . Attempts to prepare this hydride via the hydrogenation of [P 2N 2]TaMe3 led only to a dinuclear hydride, ([P 2N 2]Ta) 2(u-H 4) (14), which does not react with ethylene. The hydrogenation of [P 2N 2]TaMe 3 in the presence of ethylene resulted in catalytic hydrogenation of the ethylene to ethane and a species characterized as the monohydride [P 2 N 2 ]TaMe 2 H (15). It proved possible to generate the mononuclear trihydride [P 2N 2]TaH 3(PMe 3) (16), by the hydrogenation of [P 2N 2]TaMe 3 in the presence of excess PMe 3 . The ethylene ethyl complex 12 was then readily synthesized in an overall yield of 95% by the reaction of [P 2N 2]TaH 3(PMe 3) with ethylene, as shown in Scheme 3.3. The characterization and reactivity of hydrides 14 and 15 will be addressed in detail in Chapter 4. The characterization and reactivity of trihydride 16 will be expanded upon in Chapter 6. 113 References begin on page 755. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Scheme 3.3. Ph toluene H 2 PMe3 [P2N2]TaMe3 [P2N2]TaH3(PMe3) [P2N2]Ta(C2H4)Et 5 16 12 An ORTEP depiction of the solid-state molecular structure of 12 as determined by X-ray crystallography is shown in Figure 3.5, and crystallographic data are given in Table 3.1. The nine hydrogen atoms associated with the ethylene and ethyl ligands were identified in an electron density difference map, and their locations were refined. Immediately evident from this structure is the (3-agostic interaction that was implied from the already discussed N M R data. The agostic Ta(l)-C(26) distance of 2.498(4) A is only -0.25 A longer than the G-bound Ta(l)-C(25) distance of 2.251(3) A. The ethylene moiety also has two significantly different Ta-C bond lengths, where the longer Ta-C bond is with the carbon diagonally opposing the agostically bound (3-carbon of the ethyl group; the Ta(l)-C(27) distance is 2.251(3) A , whereas the Ta(l)-C(28) is 2.362(4) A . A single hydrogen atom attached to the (3-carbon of the ethyl group is directed towards the metal with a Ta(l)-H(45) distance of 2.07(4) A . The backbonding to the ethylene moiety is significant enough that it is probably best described as a metallacyclopropane;39 the C(25)-C(26) and C(27)-C(28) bond lengths are identical at 1.449(5) A . These distances are both slightly shorter than typical carbon-carbon single bonds, but much longer than a carbon-carbon double bond. 3 9 If one considers the T|2-ethylene and r]2-ethyl group to each occupy a single coordination site, the overall geometry of complex 12 is best described as distorted octahedral. As is common for complexes of the [P2N2] l igand, 4 0 - 4 5 the phosphorus donors are approximately trans-disposed with a P(l)-Ta(l)-P(2) angle of 160.18(3)°, and the nitrogen donors are closer to a 114 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex cis disposition with a N(l)-Ta(l)-N(2) angle of 97.02(9)°. The "twist" of the amide donors of the [P2N2] ligand, which was described previously in Chapter 2, is quite small in 12. The twist can be quantified by the difference between the P(l)-Ta(l)-N(l)-Si(4) and P(2)-Ta(l)-N(l)-Si(l) dihedral angles, which is 19.2(3)°. Compound 12 is a 16-electron complex if the donation of an electron pair from the agostic interaction is included, but this does not consider possible 7t-donation from the amide lone-pairs. As has been shown previously, 4 4 ' 4 5 without a twist in the [P 2N 2] ligand framework only one linear combination of the amide lone pairs has overlap with an available metal orbital of appropriate symmetry, and therefore compound 12 can be considered an 18-electron species. C13 Figure 3.5. ORTEP diagram of the solid-state molecular structure of [P2N 2 ]Ta(C2H 4 )Et, 12, as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. 115 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Table 3.1. Selected bond lengths, angles and dihedral angles for [P2N2]Ta(C2H4)Et (12). Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.5701(8) Ta(l) C(27) 2.251(3) Ta(l) P(2) 2.5525(7) Ta(l) C(28) 2.362(4) Ta(l) N(l) 2.153(2) C(25) C(26) 1.449(5) Ta(l) N(2) 2.182(2) C(27) C(28) 1.449(5) Ta(l) C(25) 2.251(3) Ta(l) H(45) 2.07(4) Ta(l) C(26) 2.498(4) Atom Atom Atom Angle O Atom Atom Atom Angle O PQ) N(l) Ta(l) Ta(l) Ta(l) C(25) P(2) N(2) C(26) 160.18(3) 97.02(9) 81.8(2) Ta(l) C(25) Ta(l) H(45) C(26) C(26) H(45) C(26) C(25) 102.0(20) 116.5(22) 63.1(2) Atom Atom Atom Atom Dihedral Angle (°) PQ) P(2) P(l) P(2) Ta(l) Ta(l) Ta(l) Ta(l) NQ) NQ) N(2) N(2) Si(4) Si(l) Si(3) Si(2) -179.87(14) 160.93(13) 163.12(13) -178.12(14) 3.4.Mechanism of the Reaction of Ethylene with [P2N2]Ta=CH2(Me) (6) Previous studies on the thermal decomposition of the 18-electron complex (r|5-C 5H 5) 2Ta=CH 2(Me) demonstrated that it decomposed via dimerization and coupling of the two bridging methylidene moieties. 1 ' 4 6 The species (r) 5-C 5H 5) 2Ta(r| 2-C 2H4)Me resulted, where the ethylene unit is derived from the coupled methylidene units. The remaining 16-electron "(Ti 5-C 5H 5) 2TaMe" moiety could be trapped using ethylene (Scheme 3.1). This same mechanism could be responsible for the formation of methyl complex 13, and is illustrated in Figure 3.6 as mechanism A. The starred carbon atoms indicate where the 1 3 C labels would be expected if the reaction was performed with 1 3 C 2 H4. An alternative mechanism to the formation of complex 13 would involve initial metallocyclobutane formation upon addition of ethylene to the tantalum methylidene, and subsequent decomposition of this intermediate by p-hydrogen elimination. Reinsertion of the resulting olefin into the hydride bond in the opposite manner would generate a propylene 116 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex complex, [P 2N 2]Ta(CH2=CHMe)Me, which could then further react with ethylene, eliminating propylene and generating complex 13, as shown in Figure 3.6 as mechanism B. As for mechanism A, the starred carbon atoms indicate where the 1 3 C labels would be expected in the products if the reaction was performed with 1 3 C 2 H 4 . Mechanism A Me 2 [P2N2]Ta Me H 2 [P2N2]Ta' Ta[P2N2] C \ H 2 Me 2 n 4 Me [P2N2]Ta + Me [P2N2]Ta 13 Mechanism B Me [P2N2]Ta C o H 2 n 4 H 2C=CH CH, H 2C=CH *CH, Me [P 2N 2]Ta—CH 2 H ? C—CH? B-Hydrogen Elimination Me + [P2N2]Ta C o H 2 n 4 13 Me ,H [P2N2]Ta- C H / / H2C—CH Alkene Insertion Into Metal-Hydride Bond Me [P2N2]T<L Figure 3.6. Two possible mechanisms for the formation of [P 2N 2]Ta(C 2H 4)Me, 13. The starred carbon atoms in the products illustrate where carbon-13 labels would appear in the products if the reaction was performed with l 3 C 2 H 4 . 117 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Inspection of the 1 3 C{'H} N M R spectrum of the products of the reaction of 13 [P2N 2]Ta=CH 2(Me) with C 2 H 4 reveals that indeed two isotopomers of propylene are produced, 1 3 C H 2 = I 3 C H C H 3 and H 2 C = I 3 C H 1 3 C H 3 , which supports mechanism B. The tantalum-bound ethylene moiety is fully labeled, and the tantalum methyl group remains unlabeled. The metallocyclobutane product once formed appears to decompose rapidly, as it does not accumulate in solution, and no productive metathesis, to form [P 2N 2]Ta= l 3CH 2(Me) and 1 3 CH 2 =CH 2 , is observed. The mechanism of formation of the ethyl complex 12 could involve a similar mechanism to that proposed for complex 13. Initial metallocyclobutane formation could be followed by p-hydrogen elimination; however, if instead of reinserting into the double bond to form a propylene complex, ethylene inserts into the hydride bond, then [P 2N 2]Ta(CH 2CH=CH 2)MeEt would form. If insertion of the double bond into the tantalum methyl bond then occurs a butene complex would be formed, which could exchange with ethylene to form the ethyl complex 12 and one equivalent of butene. This mechanism is labeled mechanism C in Figure 3.7. A second mechanism is also shown in Figure 3.7, mechanism D. In this case the methylidene inserts into the tantalum-methyl bond, forming an ethyl complex, which is then trapped by ethylene. The migratory insertion of an alkylidene into a els tantalum alkyl linkage has been demonstrated to occur with cationic tungsten complexes, 4 7 - 4 9 late transition metal carbene complexes, 5 0 ' 5 1 and a niobium alkylidene bearing electron withdrawing substituents,52 presumably because in these compounds the alkylidene is more electrophilic. However, this insertion is not restricted to electrophilic alkylidenes, as insertion of an alkylidene into a tantalum alkyl has been proposed as a reaction step in the decomposition of alkylidenes only slightly more sterically crowded than (r| 5-C5H 5) 2Ta=CH 2(Me), such as the ethylidene complex (T | 5-C 5H 5) 2Ta=CHMe(Me). 5 3 118 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Mechanism C Me [P2N2]Ta C o H 2 n 4 H .Me [P 2N 2 ]Ta^ C H , / / ' H 2C— CH Ethylene Insertion Into Metal-Hydride Bond Et C o H H 2 C=CH H 2C-.CH, CH, H o C = Q H H 2C [P2N2]Ta H 2C C H 3 12 2 n 4 [P 2 N 2 ]Ta^ C H / / H 2C—CH t Et C o H [P2N2]Ta, 2 n 4 Mechanism D Me-[P2N2]Ta CH 5 [P2N2]Ta Et .CH, C o H 2 n 4 H 2C [P 2N 2 ]Ta x 12 Figure 3.7. Two possible reaction mechanisms for the formation of [P2N 2]Ta(C 2H4)Et (12) from the reaction of ethylene with [P 2N 2]Ta=CH 2(Me). A labeling study should reveal if mechanism C or mechanism D occurs in the formation of ethyl complex 12. In Figure 3.7, the starred carbon atoms indicate where the labeled atoms are anticipated to be located if the reaction of [P 2N 2]Ta=CH 2(Me) is performed with l 3 C 2 H 4 . Although the tantalum product contains completely labeled [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 , as is predicted for mechanism C, the 1-butene observed in the product mixture is fully labeled l 3 C H 2 = 1 3 C H l 3 C H 2 1 3 C H 3 , rather than the partially labeled 1-butene that is anticipated from mechanism C. The 1-butene observed is therefore simply 119 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex produced from the dimerization of ethylene. Examples of tantalum complexes that catalyze the dimerization of ethylene and also produce higher oligomers are known; however, neither complex 12 nor 13 catalyzes these reactions, so intermediates or trace impurities must be involved in 1-butene production. Mechanism D is therefore most consistent with the data. The formation of the fully labeled product can be rationalized by the exchange of the bound ethyl group with the carbon-13 labeled ethylene. This exchange has been noted before in the reaction of C2D4 with [P2N2]Ta(C2H4)Et to produce partially deuterated isotopomers. The reaction of [P2N2]Ta=CH2(Me) with C2D4 in a sealed N M R tube results in the appearance of a resonance in the *H N M R spectrum in the region anticipated for ethylene. This resonance is slightly broadened, presumably because isotopomers such as C 2 D3H are the end products. This provides further evidence that despite forming the fully labeled product, mechanism D is the likely mechanism for the formation of 12. The ability of complex 12 to react with labeled ethylene such as 1 3 C 2 H 4 to exchange both the ethylene and ethyl groups can be explained by two possible mechanisms. The most obvious mechanism would involve (3-elimination of ethylene from the ethyl group to form an ethylene hydride, [P2N 2 ]Ta(C2H 4 )H, which could then react with 1 3 C 2 H 4 to generate the labeled product. The stability of 12 in air and the observation that the loss of ethylene is not observed from solutions of 12 over several days indicates that the mechanism is probably not this straightforward. The bound ethylene moiety could also be involved in an exchange mechanism by which the insertion of ethylene into the bound ethylene forms a metallocyclopentane, which can then eliminate ethylene and result in exchange. This olefin exchange mechanism is believed to occur in the formation of 13, where propylene is displaced by ethylene, and therefore seems a likely mechanism for the exchange of free ethylene with bound ethylene in 12. If mechanisms B and D from Figure 3.6 and Figure 3.7 are correct, then it should be possible to increase the amount of 13 produced from this reaction by increasing the ethylene concentration. Attempts to study the kinetics of the formation of 12 and 13 with various ethylene concentrations proved difficult. A significant amount of complex 13 forms during or shortly after photolysis, perhaps an indication that the reaction of 6 with ethylene to form a metallacyclobutane is faster photochemically than thermally. Also, the decomposition of 6 in 120 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex the absence of ethylene takes about twice as long as the decomposition of 6 in the presence of ethylene, a result that is not expected for mechanism D as it is depicted in Figure 3.7. Attempts to perform reproducible kinetics studies on this reaction were unsuccessful. Analysis of the data obtained indicates that the reaction is very sensitive to photolysis time and photolysis conditions, such as the intensity of the U V light source used. 3.5 Exchange of Proton Environments in Ethyl Complex 12 The rapid rate at which deuterium is scrambled into both the tantalum-ethyl and tantalum-ethylene groups when 12 is exposed to C2D4 strongly suggests the existence of a fluxional process that interconverts the tantalum ethyl and tantalum ethylene groups. Such a process is known to occur in the cationic late transition metal complex [(TV-CsFDCoCq -C 2 H4 ) (T | 2 -E t ) ] + , where the intermediate is the bis(ethylene) hydride [(r| 5-C 5Me5)Co(r| 2-C 2 H 4 ) 2 H ] + . 5 4 ' 5 5 Despite this precedent, this mechanism seems unlikely in complex 12. Although the metal centre in species 12 could formally be considered Ta(III), with a d2 configuration, extensive back-bonding to the tantalum ethylene moiety suggests the complex is more accurately described as a Ta(V) metallacyclopropane, with no remaining metal-based electrons, and no available higher oxidation states. Because it lacks the electrons necessary to allow backbonding to two ethylene moieties to form [P2N 2]Ta(C2H4)2H, complex 12 would be more likely to eliminate ethylene to form [P 2N2]Ta(C 2H4)H. Although solutions of 12 turn green over the course of several months in the absence of ethylene due to a trace amount of a new unidentified complex, the species [P 2 N 2 ]Ta(C2H 4 )H could not be detected by 'PI NMR, and the slow rate at which this decomposition reaction occurs prohibits it from being of importance in the more rapid exchange of proton environments described here. Interestingly, many ethylene ethyl compounds have been prepared, and (with the exception of the previously mentioned cobalt example) none exhibit (3-agostic interactions or fluxional processes that exchange ethyl and ethylene proton environments.56-59 Examining the crystal structure of 12, it is notable that the bond lengths in the tantalum ethyl ligand and the tantalum ethylene ligand are remarkably similar. The C-C 121 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex bond lengths are identical, and the Ta-C a distance is identical to one of the Ta-C bond lengths of the bound ethylene. The Ta-Cp interaction is only 0.136 A longer than the longer Ta-C bond length to the bound ethylene. From consideration of the structural similarities between the ethylene and ethyl ligands in species 12, a valid mechanism for the exchange of proton environments in complex 12 could simply involve direct transfer of a hydrogen atom from the ethyl group to the ethylene group, as shown in Figure 3.8. The (3-agostic interaction will weaken the C-H bond that is interacting with the metal centre in the ground state, and in the postulated intermediate the hydrogen atom that is transferred has bonding interactions with two carbon atoms and the tantalum centre. Figure 3.8. Depiction of the mechanism believed to exchange the proton environments of the ethyl and ethylene ligands of species 12. Unfortunately, the rate at which this exchange reaction occurs is too slow to be monitored by variable-temperature ' H NMR. In an attempt to verify that this process does occur even in the absence of labeled ethylene, a phase-sensitive E X S Y (exchange spectroscopy)6 0'6 1 spectrum of complex 12 was obtained at 300 K, and a portion of this spectrum is shown in Figure 3.9. This 2-D spectrum displays four cross-signals of the correct sign and intensity to be due to chemical exchange between both resonances of the ethyl unit and both resonances of the tantalum ethylene, thus indicating that exchange between both ethyl group environments and both ethylene ligand environments is possible, as depicted in Figure 3.8. This process occurs in conjunction with the more rapid process illustrated in Figure 3.2. Another possible mechanism,21 involving transfer of an a-agostic 122 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex hydrogen atom from the ethyl group to generate the intermediate [P2N2]Ta=CHMe(Et), would not allow for exchange of all the observed chemical environments. Furthermore, cross-signals occur between pairs of the ligand silyl methyl environments, as would be expected if a fluxional process occurred which exchanged the location of the ethyl and ethylene ligands in complex 12. Cross-signals between pairs of ligand methylene environments should also be observed; however, it appears that the pairs of signals are too close together (8 1.34,1.41; 1.70,1.70) for these cross-signals to be distinguishable from the large diagonal signals. Ta(C2H4) SiCH,P a-CHf® P-CH 3 SiCR SiCR Ta(C2H4) SiCH2P 1.6 0.8 0.0 (PPM) -0.8 -0.8 0.0 (PPM) 0.8 1.6 Figure 3.9. A portion of the E X S Y spectrum (x = 0.4 seconds) of [P2N2]Ta(C2H4)Et, illustrating four cross-signals between the ethyl and ethylene ligands, and cross-signals between pairs of silyl methyl environments. 3.6 Summary and Conclusions The reaction of [P 2N 2]Ta=CH 2(Me) with ethylene produces the complex [P2N2]Ta(C2H4)Et (12) as the main product along with [P2N2]Ta(C2H4)Me (13). Extensive 123 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex evidence exists for a (3-agostic interaction in 12, and this same interaction was also present in the solid-state X-ray structure. By careful examination of the l H N M R spectrum of a mixture of partially deuterated isotopomers of 12, evidence was collected that supports an equilibrium between the (3-agostic structure and an a-agostic interaction of the ethyl ligand in solution. Such interactions are of great importance in early transition metal olefin polymerization catalysts, and this complex is particularly interesting because it contains an ethyl ligand arranged cis to an ethylene ligand. However, it is worth noting at this point that no polymerization activity has been observed with complex 12. The elongation of the C-C bond of the ethylene unit from this backbonding interaction renders the geometry of the bound ethylene quite similar to that of the (3-agostic ethyl ligand. This similarity in structure undoubtedly assists in an exchange process whereby the (3-agostic hydrogen atom from the ethyl group is transferred to the ethylene moiety. This process exchanges these groups without the intermediacy of an ethylene hydride species, and is a process that is believed to be important in olefin polymerization chain termination.62 3 . 7 Future Work The agostic interactions in complex 12 are well characterized both in the solid-state by an X-ray diffraction study and in solution by isotopic perturbation of resonance experiments. The methyl complex 13 could be further characterized by an isotopic perturbation of resonance labeling study to determine if an a-agostic interaction is present with the methyl ligand. This would require the synthesis of [P2N 2]Ta(C2H4)CH 2D and [P2N 2 ]Ta(C 2 H4)CHD2 from Ta(CDH 2 ) 3 Cl 2 and Ta(CD 2 H) 3 Cl 2 . The least understood aspect of the photolysis of trimethyl complex 5 in the presence of ethylene is the identity of the N M R inactive products, which constitutes upwards of 40% of the tantalum content the crude reaction mixture, and their mechanisms of formation. The N M R inactive products of the photolysis of 5 in the presence of ethylene are soluble in hexanes, and are readily separated from 12 and 13, which have poor solubility in hexanes. A crystalline orange-red solid was obtained from evaporation of the hexanes soluble products. 124 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex From the solid-state molecular structure of this complex it is possible to tentatively assign this complex as [P2N2]Ta(C2H4)2 (17); an ORTEP depiction of this complex is shown in Figure 3.10. Selected bond lengths and angles for 17 are given in Table 3.2. Figure 3.10. ORTEP depiction of the solid-state molecular structure of 17 as determined by X-ray crystallography. 125 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Table 3.2. Selected bond lengths, angles and dihedral angles for complex 17. Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.5615(11) Ta(l) C(26) 2.469(5) Ta(l) P(2) 2.6039(11) Ta(l) C(27) 2.331(5) Ta(l) N(l) 2.197(3) Ta(l) C(28) 2.485(4) Ta(l) N(2) 2.240(3) C(25) C(26) 1.370(7) Ta(l) C(25) 2.319(4) C(27) C(28) 1.379(7) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) P(l) N(l) Ta(l) Ta(l) Ta(l) C(25) P(2) N(2) C(26) 160.43(4) 93.91(12) 79.5(3) Ta(l) Ta(l) C(27) C(28) C(28) C(27) 79.6(3) 67.3(3) Atom Atom Atom Atom Dihedral Angle (°) P(l) P(2) P(l) P(2) Ta(l) Ta(l) Ta(l) Ta(l) N(l) N(l) N(2) N(2) Si(4) Si(l) Si(3) Si(2) 162.1(2) -15.6(2) 163.12(13) 162.7(2) The eight hydrogen atoms associated with the ethylene ligands of 17 were located in an electron density difference map generated from the X-ray data. The C(25)-C(26) and C(27)-C(28) bond lengths are 1.370(7) A and 1.379(7) A respectively, which are much shorter than the ethyl C-C bond length in 12 of 1.449(5) A as well as the ethylene C-C bond length in 12, also 1.449(5) A . The shorter bond lengths for C(25)-C(26) and C(27)-C(28) implies significant double-bond character in these bonds, and is consistent with the formulation of species 17 as a bis-ethylene complex. Compared to complex 12, in which two electrons are available to back-donate to the single ethylene unit to generate what can formally be considered a metallocyclopropane, in 17, only three electrons are available to back-donate to both ethylene units. This could explain the shorter C-C bond lengths observed in complex 17. The Ta-C bond lengths in complex 17 are unsymmetrical. Each ethylene ligand has one Ta-C distance that is significantly longer than the other does. Thus, the Ta(l)-C(25) distance is 2.319(4) A and the Ta(l)-C(26) distance is 2.469(5) A. The longer Ta-C distance on the C(27)-C(28) ethylene unit is diagonally disposed relative to the longer Ta-C distance 126 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex of the C(25)-C(26) ethylene unit; the Ta(l)-C(28) bond length is 2.485(6) A and the Ta(l)-C(27) bond length is 2.331(5) A . It is notable that the longer Ta(l)-C(26) and Ta(l)-C(28) distances in 17 are similar to the Ta(l)-C(26) distance of 2.498(4) A in 12; C(26) is the (3-agostic carbon of the ethyl group. Therefore, complex 17 could conceivably be the bis-ethyl complex [P2N2]TaEt2 in which both ethyl groups exhibit a p-agostic interaction; however, the short C(25)-C(26) and C(27)-C(28) distances in 17 are difficult to reconcile with this model. For complex 17, the P(l)-Ta(l)-P(2) bond angle of 160.43(4)° and the N(l)-Ta(l)-N(2) bond angle of 93.91(12)° are very similar to those observed in complex 12. The Ta-N bond lengths are slightly longer, and the although the Ta(l)-P(l) distance of 2.5615(11) A is similar to the Ta-P distances observed in complex 12, the Ta(l)-P(2) distance in complex 17 is longer, at 2.6039(11) A . Although the X-ray data could be interpreted to identify 17 as [P 2N2]Ta(C 2H 4)2, this assignment must be treated with some skepticism, as there is no precedent for this type of species in the chemistry of tantalum. If complex 17 is a bis-ethylene species, it differs from complex 12 by only one hydrogen atom. Unfortunately, hydrogen atoms are difficult to locate by X-ray crystallography, so it is not impossible that complex 17 could contain additional hydrogen atoms. As suggested earlier, complex 17 could be a Ta(IV) bis-ethyl complex, [P 2N 2]TaEt 2, with two P-agostic ethyl groups. Further characterization of complex 17 is required to ascertain its identity. A magnetism study of species 17 should clarify the number of unpaired electrons associated with the tantalum centre. A solution of 17 exhibits an EPR spectrum consistent with a Ta(IV) complex with one unpaired electron. An eight line pattern is observed in the EPR spectrum, due to an electron with hyperfine coupling to l 8 l T a (7 = 7/2, 99.99% abundance); 31 Superhyperfine coupling to P is also observed in the EPR spectrum, but poorly resolved. Further work is necessary to insure that this EPR spectrum is indeed due to 17 and not a trace impurity. Currently, complex 17 lacks a reliable high yield synthesis, which would allow for its complete characterization. Complex 17 is probably thermally sensitive; the X-ray structure of 17 had to be obtained at low temperature because crystals of 17 immediately decomposed 127 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex in the X-ray beam at room temperature. If complex 17 is indeed [P2N 2]Ta(C 2H4)2, then it should be possible to synthesize 17 by the reaction of 12 with a reagent capable of abstracting a hydrogen atom. Unfortunately, the reaction of 12 with a source of the trityl radical resulted in no reaction. A convenient entry into the study of this N M R inactive reduced tantalum species will probably require the synthesis of a convenient starting material, such as [P 2 N 2 ]TaCl2. 3.8 Experimental 3.8.1 General Procedures Unless otherwise stated, general procedures were performed according to Section 2.9.1. 3.8.2 Materials Complex 5 was prepared as described in chapter 1. Ethylene was purchased from Praxair and condensed onto P2O5 prior to use to remove any trace water. Extra dry grade hydrogen gas was purchased from Praxair and was dried and deoxygenated by passing the 1 o gases through a column containing molecular sieves and MnO. The labeled olefins " C2H4 and C2D4 were purchased from Cambridge Isotope Laboratories, and PMe3 was purchased from Aldrich and used as received. 3.8.3 Synthesis, Characterization and Reactivity of Complexes Photolysis of [P2N2]TaMe3 under C 2 H 4 A sample of [P 2N 2]TaMe 3 (1.05 g, 1.38 mmol) in 100 mL hexanes was sealed in a glass vessel equipped with a Teflon valve under an atmosphere of ethylene. The vessel was 128 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex exposed to a Vitalux sunlamp for 30 minutes and then stirred for two weeks at room temperature. The ethylene was then removed under vacuum, and an orange solid (0.55 g) crystallized that had poor solubility in hexanes but was soluble in toluene and benzene. The solid was identified as a mixture of [P2N2]Ta(C2H4)Et (12) and [P 2N 2]Ta(C 2H 4)Me (13). Exact ratios and yields of these products depended considerably on the conditions used, such as photolysis duration and intensity, but [P 2N 2]Ta(C 2H 4)Et was always the major product. Product ratios of compounds 12 to 13 as high as 10:1 and as low as 2:1 were observed. Both compounds 12 and 13 are air stable but moisture sensitive. ' H N M R of selected peaks (C6D6, 295 K, 500 MHz): 8-1.49 (t, 3 / P H = 3.1 Hz, [P 2N 2]Ta(C 2H 4)C// 3), -1.28 ( A 2 M 3 X 2 , 3 / H H = 7.8 Hz, 3 / H p = 3.1 Hz, [P 2 N 2 ]Ta(C 2 H 4 )C// 2 CH 3 ) , -0.53 ( A 3 M 2 X 2 , 3 7 H H = 7.8 Hz, 7Hp = 4.4 Hz, [P 2 N 2 ]Ta(C 2 H 4 )CH 2 C#i), 0.13, 0.23, 0.37 and 0.51 (s, [P 2N 2]Ta(C 2H 4)Et ligand SiC7/3), -0.01, 0.30, 0.46, 0.58 (s, [P 2N 2]Ta(C 2H 4)Me ligand SiC# 3). Photolysis of [P2N2]TaMe3 under 1 3 C 2 H 4 A sample of [P 2N 2]TaMe 3 (20 mg, 0.026 mmol) in 1 mL of C 6 D 6 was sealed in a N M R tube under 1 atmosphere of l 3 C 2 H 4 . The tube was exposed to a Vitalux sunlamp for 30 minutes and then stirred for two weeks at room temperature. The two N M R active tantalum-containing products were identified as [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et and [P 2 N 2 ]Ta( l 3 C 2 H 4 )Me and 13 were present in approximately a 2:1 ratio. The C N M R spectrum identified the 1-butene isotopomer as the fully l 3C-labeled l 3 C H 2 = l 3 C H 2 l 3 C H 2 l 3 C H 3 and the two propylene isotopomers as C H 2 = I 3 C H 2 I 3 C H 3 and l 3 C H 2 = l 3 C H 2 C H 3 . ' H NMR, selected peaks (C 6 D 5 , 299 K, 500 MHz): 8 -1.51 (t, [P 2N 2]Ta(' 3C 2H 4)C# 3), -1.29 (m, 7 C H = 126.8 Hz, [P 2 N 2 ]Ta ' 3 C/ / 2 l 3 CH 3 ) , -0.52 (m, 7 C H = 123.2 Hz, [P 2N 2]TaCH 2C7/j). 3 I P ( C 6 D 6 , 299 K): 8 21.3 (vt, 27Cp = 4.2 Hz, [P 2 N 2 ]Ta( l 3 C 2 H 4 )Me), 24.0 (m, [P 2 N 2 ]Ta( ' 3 C 2 H 4 ) l 3 Et). 1 3 C{ ! H} N M R (C 6 D 6 , 299 K, 129.76 MHz): 8 6.3 (dt, '7Cc = 30.0 Hz, 2 J P C = 4.5 Hz, [P 2 N 2 ]Ta ( 1 3 C 2 H 4 ) 1 3 CH 2 l 3 CH 3 ) , 13.6 (ddd, ' / C c = 34.5 Hz, 2JCC = 2.2, 3JCC = 4.2, 1 3 C H 2 = I 3 C H 2 I 3 C H 2 1 3 C H 3 ) , 19.7 (d, 2JCC = 42.2 Hz, C H 2 = ' 3 C H 2 I 3 C H 3 ) , 27.3 (dd, 7 C c = 34.3 Hz, ' J c c = 41.5 Hz, l 3 C H 2 = 1 3 C H 2 l 3 C H 2 1 3 C H 3 ) , 45.1 (vt, 2 7 P C = 4.8 Hz, [P 2 N 2 ]Ta( l 3 C 2 H 4 ) l 3 Et) , 49.1 (vt, 2 7 P C = 4.2 Hz, [P 2 N 2 ]Ta( l 3 C 2 H 4 )Me), 70.6 (dt, ' i C c = 30.0 129 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Hz, VPC = 9 .0 Hz, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 CTl 2 1 3 CH 3 ) , 113 .8 (dd, lJcc = 6 9 . 9 Hz, 3JCC = 4 . 2 Hz, l 3 C H 2 = ' 3 C H 2 ' 3 C H 2 1 3 C H 3 ) , 1 1 6 . 2 (d, ' 7 C c = 6 9 . 5 Hz, l 3 C H 2 = l 3 C H 2 C H 3 ) , 1 3 4 . 0 (d, 7 C C = 6 9 . 9 Hz, l 3 C H 2 = l 3 C H 2 C H 3 ) , 1 3 4 . 0 (d, ' j C c = 4 2 . 2 Hz, C H 2 = 1 3 C H 2 1 3 C H 3 ) , 140 .8 (ddd, lJcc = 6 9 . 9 Hz, 7 Cc = 4 1 . 5 Hz, V C C =2 .2 Hz, 1 3 C H 2 = I 3 C H 2 1 3 C H 2 I 3 C H 3 ) . I 3 C N M R (C 6 D 6 , 2 9 9 K, 1 2 9 . 7 6 MHz): 5 6.3 (dt, [JCC = 3 0 . 0 Hz, ' 7 C H = 1 2 3 . 2 Hz, [P 2 N 2 ]Ta ( l 3 C 2 H 4 ) 1 3 CH 2 1 3 CH 3 ) , 4 5 . 1 (m, '7CH ~ 1 4 0 Hz, [P 2 N 2 ]Ta( l 3 C 2 H 4 ) 1 3 Et), 4 9 . 1 (m, 'JCH ~ 1 4 0 Hz, [P 2N 2]Ta(' 3C 2H 4)Me), 7 0 . 6 (dt, ]JCC = 3 0 . 0 Hz, ' 7 C H = 1 2 6 . 8 Hz, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 ) . Variable-Temperature 1 J C NMR Spectra of [P 2N 2]Ta( uC 2H 4) 1 3Et l 3 C NMR, selected peaks ( C 7 D 8 , 350 K, 129.76 MHz): 5 7.3 (m, ' j C H = 123.8 Hz, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) l 3 C H 2 l 3 C H 3 ) , 45.5 (m, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et , 75.9 (m, ' j C H = 123.4 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 ) . I 3 C NMR, selected peaks ( C 7 D 8 , 330 K, 129.76 MHz): 5 6.7 (m, ' J C H = 123.6 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) l 3 C H 2 l 3 C H 3 ) , 45.2 (m, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et) , 73.4 (m, ' J C H = 125.3 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C T 2 l 3 C H 3 ) . I 3 C NMR, selected peaks ( C 7 D 8 , 300 K, 129.76 MHz): 5 5.7 (m, ' j C H = 123.7 Hz, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) l 3 C H 2 l 3 C H 3 ) , 44.7 (m, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et) , 69.3 (m, ' j C H = 127.5 Hz, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) l 3 C H 2 l 3 C H 3 ) . 1 3 C NMR, selected peaks ( C 7 D 8 , 273 K, 129.76 MHz): 8 4.8 (m, ' j C H = 123.1 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) l 3 C H 2 l 3 C H 3 ) , 44.2 (m, [P 2 N 2 ]Ta ( l 3 C 2 H4) 1 3 Et ) , 65.4 (m, ' j C H = 128.5 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 l 3 C H 3 ) . 1 3 C NMR, selected peaks ( C 7 D 8 , 253 K, 129.76 MHz): 5 4.1 (m, 'JCH = 123.1 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 ) , 44.0 (m, [P 2 N 2 ]Ta( l 3 C 2 H 4 ) 1 3 Et , 62.6 (m, 'JCH = 131.1 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 l 3 C H 3 ) . 1 3 C NMR, selected peaks ( C 7 D 8 , 233 K , 129.76 MHz): 8 3.5 (m, ' j C H = 122.8 Hz, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) 1 3 C H 2 l 3 C H 3 ) , 43.7 (m, [P 2 N 2 ]Ta( l 3 C 2 H 4 ) l 3 Et) , 59.9 (m, 'JCH = 133.6 Hz, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 l 3 C H 3 ) . 1 3 C NMR, selected peaks ( C 7 D 8 , 213 K, 129.76 MHz): 5 2.8 (m, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) l 3 C H 2 1 3 C H 3 ) , 43.5 (m, [P 2 N2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et) , 57.2 (m, [ P 2 N 2 ] T a ( l 3 C 2 H 4 ) l 3 O E 1 3 C H 3 ) . I 3 C NMR, selected peaks ( C 7 D 8 , 193 K, 129.76 MHz): 5 2.3 (m, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 l 3 C H 3 ) , 43.3 (m, [P2N 2 ]Ta( l 3 C 2 H 4 ) l 3 Et) , 55.0 (m, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 ) . I 3 C NMR,. selected peaks 130 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex (C 7 D 8 , 180 K, 129.76 MHz): 5 2.0 (m, [ P 2 N 2 ] T a ( 1 3 C 2 H 4 ) 1 3 C H 2 1 3 C H 3 ) , 43.2 (m, [P 2 N 2 ]Ta( 1 3 C 2 H 4 ) 1 3 Et), 53.5 (m, [P 2 N 2 ]Ta( , 3 C 2 H 4 ) 1 3 CH 2 1 3 CH 3 ) . Photolysis of [P2N 2]TaMe 3 under C 2 D 4 A sample of [P 2N 2]TaMe 3 (20 mg, 0.026 mmol) in 1 mL of C^D^ was sealed in a N M R tube under 1 atmosphere of C 2 D 4 . The tube was exposed to a Vitalux sunlamp for 30 minutes and then stirred for two weeks at room temperature. Synthesis of [P2N2]Ta(PMe3)H3 (16) A yellow solution of [P 2N 2]TaMe 3 (1.00 g, 1.32 mmol) in 120 ml ether was transferred to a 500 ml thick wall glass vessel equipped with a Teflon valve and stir bar. The mixture was degassed, and a five-fold excess of PMe 3 was vacuum transferred into the reaction vessel, which was then sealed under 4 atmospheres of hydrogen gas. The mixture was stirred for 3 days in the absence of light. The solution was then evaporated to dryness and the resulting solid was rinsed with minimal pentanes, filtered, and dried, to afford red [P 2N 2]Ta(PMe 3)H 3 in 95% yield. Some ' H N M R coupling constants were determined with the assistance of Lorentz-Gaussian resolution enhancement.63 ' H N M R (500 MHz, C^Df,, 30°C): 8 0.34, 0.38, 0.38, 0.39, 0.40, 0.40, 0.53, and 0.65 (s, 24H total, SiCH 3 ) , 0.67 (d, 2 J H P = 7.2 Hz, 9H, P(C# 3) 3), 0.87 and 1.27 ( A M X , 2JHH = 13.9 Hz, 2H total, CH2 ring), 1.40 (ABX, 2 / H H = 14.3 Hz, 1H, CH2 ring), 1.44 ( A B M X , 2JHH = 14.3 Hz, 4 7 H H = 1.4 Hz, 1H, CH2 ring), 1.54 and 2.10 ( A M X , 2JHH =14.4 Hz, 2H total, CH2 ring), 1.56 and 1.73 ( A M X , 2 / H H = 13.4 Hz, 2H total, CH2 ring), 7.08 and 7.10 (m, 2H total, PPh p-H), 7.17 and 7.22 (m, 4H total, PPh m-H), 7.80 ( A M N X Y Z , JHP = 110 Hz, 7Hp = 104 Hz, 7 HP (B) = 18.6 Hz, 2 / H H ( B ) = 7.1 Hz, VHHCO = 7.1Hz, 1H, TatfA), 8.17 ( A M N X Y Z , 7Hp(o = 61.9 Hz, / H P ( A ) = 24.1 Hz, J H P ( B) = 16.4 Hz, 27HH(o = 9.3 Hz, 2JHHW = 7.1Hz, 1H, Taif B), 7.98 and 8.03 (m, 4H total, PPh o-H), 9.87 ( A M N O X Y Z , 7 H P(B) = 60.2 Hz, J m A ) = 37.0 Hz, J m Q = 9 Hz, 2 J H H(B ) = 9.3 Hz, 2 / H H ( A ) = 7.1Hz, VHH = L4 Hz, 1H, TaHc). 3 1 P { 1 H } N M R (C 6 D 6 , 30°C): 5 -21.5 (dd, 2 / P P = 31.7, 2 7 P P 131 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex = 78.2 , TaPMe 3 , P A ) , 20.2 (dd, 2JPP = 31.7, 2JPP = 86.6, [P 2N 2] ligand P B), 37.2 (dd, 2JPP = 86.6, l/pp = 78.2, [P 2N 2] ligand P c ). Anal. Calcd for C 2 7 H 5 4 N 2 P 3 S i 4 T a : C, 40.90; H, 6.86; N , 3.54. Found: C, 41.03; H, 6.80; N , 3.39. Synthesis of [ P 2 N 2 ] T a ( C 2 H 4 ) E t (12) A stirred red solution of [P 2N 2]TaH 3(PMe 3) (1.3 g, 1.64 mmol) in 20 mL hexanes was sealed in a glass vessel equipped with a Teflon valve under an atmosphere of ethylene. After 5 minutes the gases were evacuated and the vessel was again charged with ethylene. This was repeated and the light orange solution was allowed to stir for 6 hours. The gases were evacuated again, and the remaining liquid was transferred to an Erlenmeyer flask in a glove box. Over 30 minutes a microcrystalline pale orange solid precipitated from solution. The solid was collected by filtration and dried in vacuo, to produce [P 2N 2]Ta(C 2H 4)Et in 95% yield. X-ray quality single crystals were obtained by slow evaporation of a benzene and hexamethyldisiloxane solution. The species [P 2N 2]Ta(C 2H 4)Et is soluble in aromatic solvents but insoluble in hexanes. ! H N M R (C 6 D 6 , 25°C, 500 MHz): 5-1.27 ( A 2 M 3 X 2 , 3 7 H H =. 7.8 Hz, 3 J H P = 3.1 Hz, 2H, TaC7/ 2CH 3), -0 .51(A 3 M 2 X 2 , 3 J H H = 7.8 Hz, 7Hp = 4.4 Hz, 3H, TaCH2CH3), 0.13, 0.23, 0.37 and 0.51 (s, 24H total, SiC7/ 3), 0.55 and 1.47 (m, 4H total, Ta(C 2// 4), 1.34, 1.41, 1.70, 1.70 ( A M X , 8H total, CH2 ring), 7.07 (m, 2H, p-H), 7.19 (m, 4H, m-H), 7.66 (m, 4H, o-H). 3 I P{'H} N M R (C 6 D 6 , 25°C): 8 24.0 (s). I 3 C N M R (C 6 D 6 , 25°C, 125.76 MHz): 8 5.4 (vt, 7 P C = 3.2 Hz, SiCH 3 ) , 5.8 (vt, 7P C= 1.6 Hz, SiCH 3 ) , 6.0 (vt, 7 P C = 1.9 Hz, SiCH 3 ) , 6.2 (t, J P C = 4.5 Hz, TaCH 2 CH 3 ) , 7.0 (vt, JPC = 3.2 Hz, SiCH 3 ) , 20.4 and 21.2 (s, C H 2 ring), 45.0 (vt, 7 P C = 4.8 Hz, Ta(C 2H 4)), 69.9 (t, 7 P C = 9.0 Hz, TaCH 2 CH 3 ) , 128.6 (vt, 7 P C= 4.3 Hz, m-C), 129.2 (s, p-C), 131.0 (vt, 7 P C = 4.8 Hz, o-C), 140.9 (dd, 7 P C =15.3, 16.2 Hz, ipso-C). Anal. Calcd for C 2 8 H 5 , N 2 P 2 S i 4 T a : C, 43.62; H, 6.67; N , 3.63. Found: C, 43.93; H, 6.79; N , 3.54. 132 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Reaction of [P2N2]Ta(C2H4)Et (12) with C 2 D 4 A solution of [P 2N 2]Ta(C 2H 4)Et (40 mg, 0.052 mmol) in 1 mL of d8-toluene in a N M R tube was placed under 1 atmosphere of C 2D 4 then frozen in liquid N 2 and sealed. A ' H N M R spectrum was obtained immediately after thawing. ' H N M R spectrum of selected peaks (C 6 D 6 , 295 K, 500 MHz): 5-1.52 (br m, [P 2N 2]Ta(C 2(D/H) 4)(C//(D)Me)), -1.28 (m, [P 2N 2]Ta(C 2(D/H) 4)(CH 2Me)), -0.75 (br m, [P2N2]Ta(C2(D/H)4)(CH2C77(D)2)), -0.64 (br m, [P 2N 2]Ta(C 2(D/H) 4)(CH 2C// 2(D)), -0.52 (br m, [P 2N 2]Ta(C 2(D/H) 4)(CH 2C// 3). Variable-Temperature X H NMR Study of [P2N2]Ta(C2H4)Et (12) A solution of [P 2N 2]Ta(C 2H 4)Et (20 mg, 0.026 mmol) in 1 mL of d8-toluene was sealed under vacuum in a N M R tube. ' H N M R (C 6 D 6 , 299.9 K, 500 MHz): 5 -1.33 (m, 2H, TaC/ / 2 CH 3 ) , -0.58 (m, TaCH 2GrY 5), 0.11, 0.22, 0.34 and 0.49 (s, 24H total, SiC773), 0.44 and 1.38 (m, 4H total, Ta(C 2# 4), 1.32, 1.38, 1.68, 1.68 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). ' H N M R (C 6 D 6 , 279.9 K, 500 MHz): 8-1.31 (m, 2H, TaC/ / 2 CH 3 ) , -0.68 (m, TaCH 2 C// i ) , 0.14, 0.24, 0.36 and 0.50 (s, 24H total, SiC# 3), 0.33 and 1.36 (m, 4H total, Ta(C 2# 4), 1.30, 1.36, 1.66, 1.67 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). ' H N M R (C 6 D 6 , 259.9 K, 500 MHz): 5 -1.28 (m, 2H, TaCH 2 CH 3 ) , -0.79 (m, TaCH 2 C// 5 ) , 0.17, 0.26, 0.36 and 0.50 (s, 24H total, SiC7/ 3), 0.27 and 1.34 (m, 4H total, Ta(C 2H 4), 1.27, 1.34, 1.64, 1.65 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). *H N M R (C 6 D 6 , 239.9 K, 500 MHz): 5 -1.26 (m, 2H, TaCtf 2 CH 3 ) , -0.87 (m, TaCH 2 C// 5 ) , 0.20, 0.28, 0.38 and 0.53 (s, 24H total, SiC# 3), 0.21 and 1.34 (m, 4H total, Ta(C 2H 4), 1.24, 1.34, 1.64, 1.64 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). ' H N M R (C 6 D 6 , 200.0 K, 500 MHz): 8 -1.21 (m, 2H, TaCH 2 CH 3 ) , -1.03 (m, TaCH 2Gr7 5), 0.28, 0.33, 0.41 and 0.59 (s, 24H total, SiC/f 3), 0.11 and 1.34 (m, 4H total, Ta(C 27/ 4), 1.19, 1.34, 1.61, 1.61 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). ' H N M R (C 6 D 6 , 183.0 K, 500 MHz): 8 -1.17 (m, 2H, TaC/ / 2 CH 3 ) , -1.10 (m, T a C H 2 0 / 5 ) , 0.32, 0.37, 0.43 and 0.62 (s, 24H total, SiCtf 3), 0.08 and 1.35 (m, 4H total, 133 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex Ta(C2H4), 1.16, 1.29, 1.62, 1.62 ( A M X , 8H total, CH2 ring), 6.98-7.63 ppm (aromatic region). Variable Temperature Isotopic Perturbation of Resonance Experiment To a sample of [P2N2]Ta(C2H4)Et (30 mg, 0.0389 mmol) in benzene was condensed C 2 D 4 gas (-6.4 mL, 0.26 mmol) to produce a mixture of isotopomers containing -75% deuterium labels in the ethyl group. After reacting for several months, this solution was evaporated to dryness and dissolved in (/-toluene, and sealed in a N M R tube. The calculated relative intensities of the N M R signals for the various possible isotopomers with 75% deuterium labels is as follows: a-position C # 2 C H 3 , 0.2; C7/DCH 3 , 0.6; C D 2 C H 3 , 0.0; C / / 2 C H 2 D , 1.8; C / 7 D C H 2 D , 5.3; C D 2 C H 2 D , 0.0; C7/ 2 CHD 2 , 5.3; C / 7 D C H D 2 , 15.8; C D 2 C H D 2 , 0.0; C77 2CD 3, 5.3; C77DCD 3 , 15.8; C D 2 C D 3 , 0.0. fj-position CH 2 C/7 3 , 0.3; CHDC/7 3 , 1.8; CD2CH3, 2.6; CH 2 C/7 2 D, 1.8; C H D C / M ) , 10.5; CD 2 C# 2 D, 15.8; CH 2 C/7D 2 , 2.6; C H D C M ) 2 , 15.8; CD2C/YD2, 23.7; C H 2 C D 3 , 0.0; C H D C D 3 , 0.0; C D 2 C D 3 , 0.0. ' H NMR, ethyl region ( C 7 H 8 , 295 K) -0.65 (br overlapping, CDHC/7 3 and CD 2 C7/ 3 ) , -0.75 (br overlapping, CDHC/7 2 D and CD 2 C/7 2 D), -0.81 (br, C D H C 7 / D 2 ) , -0.85 (br, C D 2 C / / D 2 ) , -1-37 (br overlapping, C77 2CHD 2 and C/7 2CD 3), -1.57 (overlapping, C7/DCHD 2 and C/YDCD 3). *H N M R , ethyl region ( C 7 H 8 , 274.6 K) -0.81 (br overlapping, CDHCi f 3 and CD2CH3), -0.85 (br, CDHC/7 2 D), -0.90 (br, CD 2 C/7 2 D), -0.96 (br, CDHC/7D 2 ), -1.00 (br, CD 2 Cy7D 2 ), -L34 (br overlapping, C/7 2 CHD 2 and C/7 2CD 3), -1.53 (overlapping, C7/DCHD 2 and C#DCD 3 ) . ' H NMR, ethyl region ( C 7 H 8 , 254.6 K) -0.90 (br, CDHC77 3 ), -0.95 (br, CD 2 Ci7 3 ), -1.00 (br, CDHCH 2 D), -1.05 (br, CD 2 C7/ 2 D), -1.10 (br, CDHCT/LV), -L16 (br, CD 2 C/7D 2 ) , -1.31 (overlapping multiplets, C / / 2 C H D 2 and C/7 2CD 3), -1.49 (overlapping, C / / D C H D 2 and Gf/DCD 3 ) . ' H NMR, ethyl region (C 7 H 8 , 234.6 K) -0.99 (br, CDHC/ / 3 ) , -1.05 (br, C D 2 C / / 3 ) , -1.10 (br, CDHC/7 2D), -1.17 (br, CD 2 C/7 2 D), -1.19 (br, CDHC/7D 2 ) , -1.28 (br, CD 2 C#D 2 ) , -1-28 (overlapping, CH2CKD2 and C7/ 2 CD 3 ) , -1.46 (overlapping, C7/DCHD 2 and C/ /DCD 3 ) . 134 References begin on page 735. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex E X S Y Spectrum of [P2N2]Ta(C2H4)Et (12) ' H E X S Y (C 6 D 6 , 500.13 MHz, x = 0.4 s, 300 K) Cross-signals 8 -0.51, 0.55 and -0.51, 1.47 (TaC/7 2CH 3 , Ta(C2#4)); -1.28, 0.55 and -1.28, 1.47 (TaCH 2 CH 3 , Ta(C 2// 4)); 0.13, 0.37 and 0.23, 0.51 (ligand SiCtf 3, ligand SiCH 3 ). Isolation of Complex 17 The hexanes soluble fraction from the procedure described the section "Photolysis of [P 2N 2]TaMe 3 under C 2 H 4 " was cooled to -40°C immediately after the removal of ethylene and filtration of the hexanes insoluble products. After 24 hours this red solution was filtered again, and then allowed to evaporate over the course of 24 hours, to afford crystals of 17 in up to 30% yield (calculated for [P 2N 2]Ta(C 2H 4) 2). Yields are highly variable, and complex 17 may be thermally sensitive. Complex 17 has not been thoroughly characterized, and it is possible that it the bis-ethylene complex [P 2N 2]Ta(C 2H 4) 2 or the bis-ethyl complex [P 2N 2]Ta(Et) 2. EPR (hexanes): g = 1.98; a( 3 lP) = 35 G, 2 P; a ( m Ta) = 131 G, ITa. X-ray Crystallographic Analyses of Complexes 12 and 17 Selected crystallographic data, fractional coordinates and thermal parameters are provided in Appendix 1. 3.9 References 1) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577. 2) Ginzburg, A. G. Russ. Chem. Rev. 1988, 57, 1175. 3) Gleiter, R.; Hyla-Kryspin, I.; Niu, S. Q.; Erker, G. Organometallics 1993,12, 3828. 4) Brookhart, M . ; Green, M . L. H. J. Organomet. Chem. 1983, 250, 395. 5) Brookhart, M . ; Green, M . L. H. ; Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1. 135 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex 6) Brintzinger, H. H. ; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M . Angew. Chem. Int. Ed. 1995, 34, 1143. 7) Woo, T. K.; Margl, P. M . ; Lohrenz, J. C. W.; Blochl, P. E.; Ziegler, T. J. Am. Chem. Soc. 1996,118, 13021. 8) Schmidt, G. F.; Brookhart, M . J. Am. Chem. Soc. 1985,107, 1443. 9) Burger, B. J.; Thompson, M . E.; Cotter, W. D.; Bercaw, J. E. 1. Am. Chem. Soc. 1990,112, 1566. 10) Resconi, L . ; Piemontesi, F.; Franciscono, G.; Abis, L. ; Fiorani, T. J. Am. Chem. Soc. 1992,114, 1025. 11) Guo, Z.; Swenson, D. C ; Jordan, R. F. Organometallics 1994,13, 1424. 12) Grubbs, R. H. ; Coates, G. W. Acc. Chem. Res. 1996, 29, 85. 13) Prosenc, M . H. ; Janiak, C ; Brintzinger, H. H. Organometallics 1992,11, 4036. 14) Roll, W.; Brintzinger, H. H.; Rieger, B.; Zolk, R. Angew. Chem. Int. Ed. 1990, 29, 279. 15) Cavallo, L. ; Guerra, G.; Vacatello, M . ; Corradini, P. Macromolecules 1991, 24, 1784. 16) Crabtree, R. H.; Hamilton, D. G. Adv. Organomet. Chem. 1988, 28, 299. 17) Jordan, R. F.; Bradley, P. K.; Baenziger, N . C ; LaPointe, R. E. I. Am. Chem. Soc. 1990, 112, 1289. 18) Mole, L . ; Spencer, J. L. ; Carr, N . ; Orpen, A. G. Organometallics 1991,10, 49. 19) Conroy-Lewis, F. M . ; Mole, L. ; Redhouse, A . D.; Lister, S. A. ; Spencer, J. L. 1. Chem. Soc. Chem. Commun. 1991, 1601. 20) Cracknell, R. B.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc. Chem. Commun. 1984, 326. 21) Fellmann, J. D.; Schrock, R. R.; Trificante, D. D. Organometallics 1982,1, 481. 136 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex 22) Thompson, M . E.; Baxter, S. M ; Bulls, A. R.; Burger, B. J.; Nolan, M . C ; Santasiero, B. D.; Shaefer, W. P.; Bercaw, J. E. /. Am. Chem. Soc. 1987,109, 203. 23) Lukens, W. W. J.; Smith, M . R. I.; Andersen, R. A. J. Am. Chem. Soc. 1996,118, 1719. 24) Dawoodi, Z.; Green, M . L . H.; Mtetwa, V . S. B.; Prout, K. J. Chem. Soc, Chem. Commun. 1982, 802. 25) Dawoodi, Z.; Green, M . L. H.; Mtetwa, V . S. B.; Prout, K.; Schultz, A. J.; Williams, J. M . ; Koetzle, T. F. / . Chem. Soc Dalton Trans. 1986, 1629. 26) Haaland, A. ; Scherer, W.; Ruud, K.; McGrady, G. S.; Downs, A. J.; Swang, O. / . Am. Chem. Soc. 1998,120, 3762. 27) Scherer, W.; Priermeier, T.; Haaland, A. ; Volden, H. V. ; McGrady, G. S.; Downs, A . J.; Boese, R.; Blaser, D. Organometallics 1998,17, 4406. 28) McGrady, G. S.; Downs, A. J. Coord. Chem. Rev. 2000,197, 95. 29) Cotton, F. A. ; Petrukhina, M . A. Inorg. Chem. Commun. 1998,1, 195. 30) Munakata, H. ; Ebisawa, Y. ; Takashima, Y. ; Wrinn, M . C ; Scheiner, A . C.; Newsam, J. M . Catal. Today 1995, 23, 403. 31) Ettiene, M . ; Mathieu, R.; Donnadieu, B. / . Am. Chem. Soc. 1997,119, 3218. 32) Guggenberger, L . G.; Meakin, P.; Tebbe, F. N . I. Am. Chem. Soc. 1974, 96, 5420. 33) Tempel, D. J.; Brookhart, M . Organometallics 1998,17, 2290. 34) Calvert, R. B.; Shapley, J. R. I. Am. Chem. Soc. 1978,100, 7726. 35) Green, M . L . H. ; Hughes, A. K.; Popham, N . A.; Stephens, A. H. H.; Wong, L . -L . J. Chem. Soc. Dalton Trans. 1992, 3077. 36) Schrock, R. R. Acc. Chem. Res. 1979,12, 98. 137 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex 37) Hierso, J. C ; Etienne, M . Eur. J. Inorg. Chem. 2000, 5, 839. 38) Jaffart, J.; Mathieu, R.; Etienne, M . ; McGrady, J. E.; Eisenstein, O.; Maseras, F. Chem. Commun. 1998, 2011. 39) Mingos, D. M . P. Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A. and Abel, E. W., Ed.; Pergamon: New York, 1982; Vol. 3, pp 43. 40) Fryzuk, M . D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,119, 9071. 41) Fryzuk, M . D.; Love, J. B.; Rettig, S. J.; Young, V . G. Science 1997, 275, 1445. 42) Fryzuk, M . D.; Love, J. B. ; Rettig, S. J. Organometallics 1998, 17, 846. 43) Fryzuk, M . D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928. 44) Fryzuk, M . D.; Johnson, S. A. ; Rettig, S. J. Organometallics 1999,18, 4059. 45) Fryzuk, M . D.; Johnson, S. A. ; Rettig, S. J. Organometallics 2000,19, xx. 46) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978,100, 2389. 47) Hayes, J. C.; Pearson, J. D. N ; Cooper, N . J. J. Am. Chem. Soc. 1981,106, 3026. 48) Hayes, J. C ; Cooper, N . J. J. Am. Chem. Soc. 1982,104, 5570. 49) Jernakoff, P.; Cooper, N . J. J. Am. Chem. Soc. 1984,106, 3026. 50) Thorn, D. L. ; Tulip, T. H. J. Am. Chem. Soc. 1981,103, 5984. 51) Kleitzein, H.; Werner, H.; Serhadli, P.; Ziegler, M . L. Angew. Chem. Int. Ed. 1983, 22, 46. 52) Threlkel, R. S.; Bercaw, J. E. J. Am. Chem. Soc. 1981,103, 2650. 53) Sharp, P. R.; Scrock, R. R. J. Organomet. Chem. 1979,171, 43. 138 References begin on page 135. Chapter 3: Agostic Interactions in a Tantalum Ethyl Complex 54) Brookhart, M . ; Lincoln, D. M . ; Bennett, M . A. ; Pelling, S. J. Am. Chem. Soc. 1990,112, 2691. 55) Brookhart, M . ; Green, M . L. FL; Pardy, R. B. A. J. Chem. Soc. Chem. Commun. 1983, 691. 56) Basickes, N . ; Hutson, A. C.; Sen, A. ; Yap, G. P. A.; Rheingold, A. L . Organometallics 1996, 75,4116. 57) Spencer, M . D.; Morse, P. M . ; Wilson, S. R.; Girolami, G. S. / . Am. Chem. Soc. 1993, 115, 2057. 58) Fernandez, F. J.; Gomez-Sal, P.; Manzanero, A. ; Royo, P.; Jacobsen, FL; Berke, H . Organometallics 1997,16, 1553. 59) Barry, J. T.; Chacon, S. T.; Chisholm, M . FL; Huffman, J. C ; Streib, W. E. / . Am. Chem. Soc. 1995,117, 191A. 60) Jeener, J.; Meier, B. H. ; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. 61) Perrin, C. L. ; Dwyer, T. J. Chem. Rev. 1990, 90, 935. 62) Margl, P.; Deng, L. ; Ziegler, T. J. Am. Chem. Soc. 1999,121, 154. 63) Braun, S.; Kalinowski, H.-O.; Berger, S. 750 and More Basic NMR Experiments; Wiley-V C H : Toronto, 1998. 139 References begin on page J35. Chapter 4: Dinuclear Tantalum Hydride Complexes Chapter Four Dinuclear Tantalum Hydrides Complexes 4.1 Introduction The reactivity of monomeric tantalum hydrides has been found to include catalytic H/D exchange of aromatic protons and hydrogenation of aromatic rings.1"8 Due to the intriguing reactivities that have been observed for tantalum hydrides, the species [P 2N2]TaH 3 was of interest. A possible route to this species from the available starting materials would involve the hydrogenation of [P2N 2 ]TaMe3; however, as noted in Chapter 3 this species does not appear to be accessible, and instead a dinuclear Ta(IV) hydride is produced. This chapter details the reaction of [P2N2]TaMe3 with dihydrogen, and the formation not of the monomeric hydride [P 2N 2]TaH3, but of a binuclear tantalum hydride with unexpected reactivity towards strong electrophiles. 140 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 4.2 Hydrogenation of [P2N2]TaMe3 A solution of [P2N2]TaMe3 under 4 atmospheres of dihydrogen gradually darkens from light yellow to a dark brick red. This reaction takes 3 days to go to completion, monitored by 3 1 P{'H} N M R spectroscopy. The only gaseous product of this reaction is methane, as determined by GC-MS; no ethane is observed. When this reaction is performed in hexanes, the product precipitates as a dark red-brown crystalline solid. The dark colour of the product (toluene solution, A-max = 455 nm, e = 6200 M~'-cm"') is not consistent with a Ta(V) species; likewise, the ' H N M R spectrum is not consistent with the anticipated Ta(V) hydrogenation product [P2N2]TaH3. The only non-[P2N2] ligand resonance in the ' H N M R spectrum occurs at 8 5.86. This signal integrates to two protons per metal centre and is a quintet due to coupling to four equivalent phosphorus-31 nuclei, and is assigned to the hydride ligands. From these data, the hydrogenation product can be assigned as the dimeric Ta(IV) hydride ([P2N2]Ta)2(p>H)4 (14), as illustrated in equation 4.1. The diamagnetism of hydride 14 can be explained by the formation of a tantalum-tantalum a-bond. silyl methyls omitted for clarity 141 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes The solid-state molecular structure of 14 as determined by X-ray crystallography is shown in Figure 4.1. Pertinent bond lengths and angles are given in Table 4.1. Although there is no crystallographically imposed symmetry to the molecule in the solid-state structure, the molecule has approximate D2 symmetry, with a C2 axis running through the Ta-Ta bond and two perpendicular C2 axes that are oriented through the centre of the Ta(l)-Ta(2) bond that pass between, rather than through, the tantalum hydrides. Si(l) Figure 4.1. ORTEP diagram of the solid-state molecular structure of ([P 2N 2]Ta)2(|i-H)4 (14) as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. 142 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Table 4.1. Bond lengths, angles, and dihedral angles for complex 14. Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.612(3) Ta(l) H(3) 1.853(11) Ta(l) P(2) 2.617(3) Ta(l) H(4) 1.856(11) Ta(l) N(l) 2.199(6) Ta(2) H(l) 1.91(6) Ta(l) N(2) 2.219(6) Ta(2) H(2) 1.93(6) Ta(l) Ta(2) 2.6165(5) Ta(2) H(3) 1.847(11) Ta(l) H(l) 1.80(6) Ta(2) H(4) 1.851(11) Ta(l) H(2) 1.87(6) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) P(l) N(l) Ta(l) Ta(l) P(2) N(2) 139.87(9) 101.7(2) Ta(2) Ta(2) Ta(l) N(l) Ta(l) P(l) 129.3(2) 110.02(7) Atom Atom Atom Atom Dihedral Angle (°) P(l) N(l) Ta(2) Ta(l) Ta(l) Ta(l) Ta(2) Ta(2) N(l) P(4) N(4) Si(l) -3.00(9) 10.0(3) 129.6(4) Additional symmetry elements are lacking, due to a twist in the [P2N2] ligand backbone such that the Ta(2)-Ta(l)-N(l)-Si(l) torsion angle is 129.6(4)°, rather than near 90°. This twist has been noted before in other structurally characterized [P 2N 2] complexes;9 however, unlike in other cases, the twist in this case does not appear to be fluxional in solution. The room-temperature ' H N M R spectrum contains four silyl methyl environments. If the twist in the ligand backbone were not rigid, only two silyl methyl signals would be anticipated, corresponding to silyl methyl groups directed toward the front and the back of the [P2N2] ligand on each [P2N2] unit. Although this locked conformation of the [P2N2] ligand may arise from electronic effects due to the interaction of the amide lone pairs with metal-based orbitals, it may also be explained by steric interactions between the [P2N2] ligands imposed by other electronic factors. The opposing [P2N2] ligands on Ta(l) and Ta(2) are eclipsed, with a negligible P(l)-Ta(l)-Ta(2)-P(4) torsion angle of -3.00(9)°. This eclipsed arrangement of the [P 2N 2]Ta units is likely electronic in origin, as sterically it appears less favourable; in this orientation the 143 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes phenyl rings attached to phosphorus point away from each other only because of the twist in the [P2N2] framework. For this twist in the conformation of the [P2N2] framework to be fluxional, the [P 2N 2]Ta units would have to be able to rotate with respect to each other, as steric interactions between phenyl groups on opposing [P2N2] ligands prevent this twisting motion with the [P2N2] ligands eclipsed. The solution ' H N M R spectrum therefore confirms that the solution structure is identical with the solid-state structure, and that the [P2N2] conformation and orientation with respect to the opposing [P2N2] ligand is rigid in solution. As expected, the [P2N2] ligand has an influence on the geometry at each tantalum centre. The amide nitrogens are approximately cw-disposed, with a N(l)-Ta(l)-N(2) angle of 101.7(2)°. The phosphine donors are closer to a trans disposition, with a P(l)-Ta(l)-P(2) angle of 139.87°. The Ta-Ta distance of 2.6165(5) A is within that expected for metal-metal bonding interaction but longer than in structurally related tantalum complexes; shorter Ta-Ta distances of 2.511(2) A and 2.5359(4) were reported for the complexes [Cl2(PMe3)2Ta]2(u-H)4,1 0 and [(cb)(PMe2Ph)2(H)Ta]2(lt-H)4,11 where cbH is 9H-carbazole. Longer Ta-Ta interactions of 2.854(1) A and 2.8280(4)A have also been observed in the compounds (C 5Me 4Et) 2ClTa(u:-H)2TaCl2(C 5Me 4Et) 12 and [(CyN) 2ClTa] 2(|l-H)2, 1 3' 1 4 where the tantalum centres are bridged by only two hydrides, as well as in the compound [(Bu'3SiO)2(H)2Ta]2,1 5 where an unbridged Ta-Ta distance of 2.720(4) A was reported. The long Ta-Ta distance in 14 compared to those in the closely related [Cl 2(PMe 3)2Ta]2(fi-H)4, 1 0 and [(cb)(PMe2Ph)2(H)Ta]2((i-H)411 may be due to the steric repulsion between the ligands on the metal centres, or perhaps due to the competition of the 71-electrons of the amido donors for metal orbitals utilized in bridging the metal-metal bond, as will be discussed later in the section on bonding considerations. The four hydridic hydrogen atoms in 14 were identified in electron density difference maps, and their locations were refined, though with distance constraints for H(3) and H(4). The bridging hydrides are staggered 45° with respect to the ligand donors, so that the hydride atoms lie between the planes described by P(l)-Ta(l)-P(2) and N(l)-Ta(l)-N(2). Not considering the Ta-Ta bond, the geometry at each tantalum centre is approximately square-144 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes antiprismatic. The tantalum hydride distances determined from the X-ray data range from 1.80(6) A to 1.93(6) A , similar to those in [Cl 2(PMe3)2Ta]2(p-H)4, where the Ta-H bond length is 1.81(21) A as determined from X-ray crystallography,10 and the related rhenium complex Re2Hs(PEt2Ph)4, where the Re-H distance is 1.878(7) A as determined from neutron diffraction data. 1 6 Not surprisingly, the structure of species 14 is similar to that of the aforementioned compound [Cl2(PMe3) 2Ta]2(p-H) 4; 1 0 however, the structural differences are intriguing. While the end groups in [Cl2(PMe3)2Ta]2(p-H)4 are eclipsed, as in 14, the phosphine ligands on opposing tantalum centres are staggered by 90°. Eclipsed and staggered by 90° could have various meanings in the context of these complexes; it is intended here to describe the eclipsed and staggered by 90° geometries that are depicted in Figure 4.2. Similar staggered by 90° ligand arrangements have been observed in the group 6 quadruple metal-metal bonded dimers Mo 2 Cl 4 (PMe 3)4 and W 2 Cl 4 (PMe 3)4, 1 7 although examples of the eclipsed ligand conformation are known both in metal-metal bonded complexes as well as in a dinuclear tantalum hydride featuring the 9H-carbazole amido ligand, [(cb)(PMe2Ph)2(H)Ta]2(p-H)4.'1 The [P2N2] ligand clearly has an influence on the bonding in 14, either via electronic effects caused by the difference in the bonding of the amides versus chlorides, slightly modified phosphines, or the geometrical restrictions of the macrocyclic ligand, or via the steric interactions of the [P2N2] ligand. The theoretical aspects of the bonding in 14 will be addressed in greater detail in the section on bonding considerations and density functional theory calculations. 145 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes X | X | X - | - T a — x P-|-T.a •Ta-j-p p—-TaJ -p | P | X X X staggered by 90° eclipsed P = phosphine donor X = halide or amide Figure 4.2. Depiction of "staggered by 90°" and "eclipsed" ligand arrangements. Species 14 is not particularly reactive as evidenced by the fact that exposure of solutions of 14 to ethylene or carbon monoxide resulted in no reaction. This lack of reactivity is quite different from that reported for the less coordinately saturated hydride complex ([NPN]Ta)2(u\-H)4 which reacts directly with dinitrogen and eliminates 1 equivalent of hydrogen to form a dinitrogen complex. 1 8 Likewise, complex 14 did not react directly with D 2 gas to form the deuterated analogue ([P2N2]Ta)2(u.-D)4; this latter complex was synthesized from the reaction of [P 2N 2]TaMe 3 with D 2 gas. A comparison of the IR spectra of the hydride complex 14 and the deuterated species 14-<£? did not definitively identify the bridging hydride stretches, as the region in which they were expected was obscured by other peaks associated with the [P 2N 2] ligand. There were no stretches in the IR spectra of 14 or \A-d4 in the region normally associated with terminal Ta-H bonds. 4.3 Partial Hydrogenation of [P2N2]TaMe3 The failure of [P2N2]TaMe3 to hydrogenate to form the mononuclear Ta(V) hydride [P 2 N 2 ]TaH 3 raises the question of when during the hydrogenation does the reduction to Ta(IV) take place. Monitoring the hydrogenation of [P 2N 2]TaMe 3 by ' H N M R spectroscopy in a sealed tube under 1 atmosphere of hydrogen revealed that an intermediate hydrogenation product, 15, is formed, which is converted entirely to 14 over 3 days. Performing the hydrogenation of [P 2N 2]TaMe 3 in a sealed N M R tube under 0.5 atmospheres of hydrogen 146 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes resulted in a pale orange solution after 1 week. Unlike the hydrogenation under 1 31 1 atmosphere of H2, the P{ H} N M R spectrum indicates that under these conditions very little 14 is formed, and the solution is mostly a mixture of the starting material and the hydrogenation intermediate 15. Unfortunately, attempts to isolate and purify this intermediate have failed. Logical candidates for this presumed Ta(V) species are the partially hydrogenated species [P2N 2 ]TaMe2 (H) and [P2N2]TaMe(H) 2, although hydride-bridged dimers of these compounds are also a possibility. 1 9 The 3 1 P{'H} N M R spectrum displays two phosphorus-31 environments for 15, coupled to each other with a coupling constant of 125 Hz. Prominent in the ' H N M R spectrum of the hydrogenation intermediate is a peak at 8 5.68, the same region in which the bridging hydrides in 14 were observed; no coupling to phosphorus could be resolved. Attempts to obtain an accurate integration of this peak proved difficult. An inversion-recovery measurement of the T\ relaxation times for the ' H spectrum indicates that the hydridic proton relaxed more slowly than the ligand protons. From inversion-recovery experiments, the T\ value of this hydride signal is estimated to be -20 seconds at room temperature in a 500 MHz spectrometer. The integration of this peak from a ' H N M R spectrum obtained with an increased delay time, compared with the integration of the peaks associated with the ortho protons of the PPh groups and the new tantalum methyl group at 8 0.88, supports an empirical formula of [P2N 2]TaMe 2(H). The combination of the location of the hydride peak at a similar chemical shift as that observed for the hydride ligands in the ' H N M R spectrum of 14, the large T\ value, and the lack of coupling to phosphorus-31 indicate that a dimeric formulation is most likely. Coupling constants between phosphine ligands and terminal hydrides tend to be larger than their bridging counterparts20 because bridging hydrides generally contain longer metal-hydride bonds.2 1 The longer metal-hydride bond length may be partly responsible for the unusually long T\ relaxation time of this hydride,2 2 although the correlation between structure and T\ relaxation times of metal hydrides is fraught with uncertainty.2 3-2 4 The dimeric formulation is also consistent with the observed ~ P{ H} N M R spectrum, as it is likely that an eight-coordinate tantalum complex would display a square anti-prismatic geometry with two strongly coupled phosphorus environments. The seven-coordinate monomeric complex is possible, but less likely, because the seven-coordinate tantalum 147 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes complexes bearing the [P2N2] ligand synthesized in Chapter 2 were all fluxional on the N M R timescale, and exhibit only one phosphorus chemical environment in the room-temperature 3 I P{ 'H} N M R spectrum.9 The suggested bonding in 15 is shown in Scheme 4.1. The formation of a dimer may explain the increased stability of this hydride towards further hydrogenation, which allows it to be observed as an intermediate. Further a-bond metathesis reactivity with H2 should be more difficult at the more coordinatively saturated metal centre of the eight-coordinate hydride dimer than the seven-coordinate [P2N2]TaMe3. Unfortunately none of these data provides definitive proof for the dimeric structure of 15, and the possibility that 15 is monomeric in solution cannot be conclusively disproven. Scheme 4.1. 15 ([P2N2]TaMe2)2(u-H)2 silyl methyls omitted for clarity This a-bond metathesis reaction could also be performed with silanes. The reaction of [P2N2]TaMe3 with Bu"SiH 3 also produced 14 and intermediate 15, but not as cleanly as the reactions of [P 2N 2]TaMe 3 with hydrogen. During the reaction of [P2N2]TaMe3 with Bu"SiH 3 , intermediate 15 accumulates in a larger amount relative to the concentration of 14 than in the analogous hydrogenation reaction, consistent with the much slower reaction of the bulkier Bu"SiH 3 with intermediate 15 than with hydrogen. 148 References begin on page J 72. Chapter 4: Dinuclear Tantalum Hydride Complexes 4 . 4 Reactivity of ([P2N2]Ta)2(|i-H4) as a Reducing Agent (i) Synthesis and Identification of {([P2N2]Ta)2(u-H)4}+r The reaction of alkyl halides with transition-metal hydrides is a well-known reaction; the formation of a new C-H bond and a metal halide complex has been used as a diagnostic and quantitative test for metal hydride complexes. 2 5" 2 7 It was anticipated that this reaction could also be used in a synthetic manner to produce a [P 2 N 2 ]Ta l v halide starting material, such as ([P 2N 2]TaI 2)A. The reaction of a solution of 14 in toluene with excess M e l resulted in the precipitation of an insoluble crystalline paramagnetic green solid, 18. This solid dissolved in CH 2 C1 2 to give a dichroic solution from which transmitted light appears blue-green and reflected light appears red. In contrast to our expectations, X-ray crystallography identified the compound as {([P 2N 2]Ta) 2(p-H)3} +r, a cationic hydride, with three hydride ligands bridging the tantalum metal centres. The paramagnetic behaviour as this compound could be explained if there were no metal-metal bond between the two Ta(IV) centres. The overall reaction would then be the result of the electrophilic abstraction of a hydride ligand by CH3I, with 1 equivalent of CH4 being produced in the reaction. However, X-ray crystallography is not always a reliable method for determining hydride ligand locations, and as one of the hydrides in this solution to the X-ray data lies on a symmetry element, where errors in electron density often accumulate, the structure could not be accepted without further study. A second possible product is the result of the one-electron oxidation of 14 to produce {([P 2N 2]Ta) 2(p-H) 4} +r, a cationic tetrahydride in which one of the electrons in the tantalum-tantalum bond has reduced Mel by one electron, to produce I~ and a methyl radical, which is likely rapidly converted to methane by reaction with the solvent. This compound should be paramagnetic regardless of whether or not the electron resides in a tantalum-tantalum bonding orbital, because the formal oxidation state of the two metal centres is now intermediate between Ta(IV) and Ta(V). A variety of approaches were taken to ascertain the nature of 18. Protonation of a hydride is a common route to cationic hydrides and proceeds via hydride abstraction/Similar 149 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes to the reaction of CH 3 I with 14, the reaction of 14 with {H(OEt 2 )} + {B[3,5-(CF 3 ) 2 C 6 H3] 4 r also produced a paramagnetic green solid; this solid dissolves in THF to give a solution that exhibits the same dichroic behavior observed for 18. Conversely, the reaction of 18 with the hydride source K+{BEt3H}~ regenerates the neutral tetrahydride 14. Although both of these observed reactivities would be expected if 18 were the cationic trihydride {([P2N2]Ta)2(p-H) 3 } + r, they do not exclude the possibility that 18 is the cationic tetrahydride {([P2N2]Ta)2(p-H)4}"T~. A more definitive study was performed using deuterium-labeled 14-^4, ([P 2N 2]Ta) 2(p-D) 4, prepared from the reaction of [P2N2]TaMe3 with D 2 . Reaction of 14-d4 with CH 3 I did not produce C H 3 D , as determined by GS-MS; rather, C H 4 was produced. As additional evidence, the reaction of deuterated 18 with K + {BEt 3 H}~ did not produce the partially deuterated 14-rf3, ([P2N2]Ta)2(p-H)(p-D)3, but rather the fully deuterated 14-rf4 ([P 2N 2]Ta) 2(p-D) 4. The reaction of 14 with CH 3 I or {H(OEt 2)} +{B[3,5-(CF 3) 2C 6H3]4} _ is therefore not a hydride abstraction reaction, and instead, we can assign compound 18 as the cationic tetrahydride {([P2N2]Ta)2(p-H)4}"T. The role of K + {BEt 3 H}"in the reaction with 18 is therefore not as a hydride source but simply as a reducing agent. These two reactions are depicted in Scheme 4.2. Dibenzyl was observed as a product in both reactions by ! H and 1 3 C{'H} N M R spectroscopy. 150 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Scheme 4.2. 1 4 1 8 ([P2N2]Ta)2(u-H)4 {([P2N2]Ta)2(u-H4)rT 90% yield silyl methyls omitted for clarity To add further data regarding the nature of compound 18 and to assess the reducing ability of the tantalum-tantalum bond in 14, cyclic voltammetry was performed on both the neutral and the cationic species. The cyclic voltammograms were performed in C H 2 C I 2 containing {NBu n 4}+{PF6}~ as the electrolyte and were referenced with respect to (CsMes^Fe as an internal standard. Both compounds 14 and 18 exhibit two identical one-electron oxidations; one occurs at —1.18 V and the second at -0.33 V relative to the SCE, providing further proof as to the identity of 18. The cyclic voltammogram for 14 is shown in Figure 4.3. Species 18 exhibits additional oxidations at 0.23 and 0.61 V , due to oxidation of the counteranion. A l l the oxidations observed were reversible. The first oxidation of 14 is expected to produce the cation observed in 18; the second oxidation indicates that the species {([P2N 2]Ta)2((i-H) 4} 2 + , which contains two Ta(V) metal centres and no electrons in a Ta-Ta bonding orbital, is also stable. The large separation between the two oxidation potentials is consistent with the derealization of the single unpaired electron in a metal-metal bond between the two tantalum atoms, rather than a structure that contains localized Ta(IV) and Ta(V) metal centres. 151 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes c <D i_ i_ U O i I I I I I 1 1 I 1 1 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 Voltage (V vs . SCE) Figure 4.3. Cyclic voltammogram of ([P2N2]Ta)2(|i-H)4 (14). Compound 18 is moderately air stable; dilute solutions in C H 2 C I 2 only lose colour after several hours when exposed to atmospheric oxygen. In fact, upon exposure of C H 2 C I 2 solutions of 14 to air, a distinctive dichroic solution results that is indicative of the cation in complex 18. Solutions of 18 do not react with simple small molecules such as ethylene or carbon monoxide; this lack of reactivity provides indirect evidence that the dinuclear nature of 18 is maintained in solution. The solid-state molecular structure of 18 as determined by X-ray crystallography is shown in Figure 4.4. The locations of the bridging hydride ligands could not be adequately determined, and are omitted from the model. Pertinent bond lengths and angles are given in Table 4.2. The molecule has crystallographic C 2 symmetry, with the C2 axis passing through the centre of the molecule and parallel to the plane containing the four phosphine ligands. The arrangement of the ancillary [P2N2] ligands in the cationic portion of species 18 is remarkably similar to that in. 14. The phosphine ligands on opposing metal centres are eclipsed with a P(l)-Ta(l)-Ta(l)*-P(l)* torsion angle of -2.81(10)°. The P(l)-Ta(l)-P(2) angle of 143.58(5)° and the N(l)-Ta(l)-N(2) angle of 102.4(2)° are also nearly identical with 152 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes those observed in 14. The Ta(l)-P(l) distance of 2.602(2) A is only about 0.01 A shorter than the Ta-P distances in 14, whereas the Ta(l)-N(l) distance of 2.145(5) A is shorter by 0.054 A than the Ta(l)-N(l) distance in 14, perhaps due to the increased positive charge on the tantalum centres strengthening the bonds to these anionic ligands, or perhaps from the ' availability of an extra orbital of correct symmetry to overlap with occupied amide lone-pair orbitals. The twist in the ligand conformation present in 14 is also observed in the solid-state structure of 15, with an identical Ta(l)-Ta(l)*-N(l)*-Si(l)* torsion angle of 129.6(4)°. The iodide anion has no close contacts with the metal centres. Despite the remarkable similarity between the arrangement, bond angles, and bond lengths in the [P2N2]Ta fragments of the solid-state structures of 14 and 18, a significantly longer Ta-Ta distance is observed. The Ta(l)-Ta(l)* distance of 2.7721(5) A in the tetrahydride cation 18 is 0.156 A longer than the Ta(l)-Ta(2) distance in the neutral tetrahydride 14. This difference in bond length is due to the removal of an electron in a Ta-Ta bonding orbital in the conversion of 14 to 18. 153 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Figure 4.4. ORTEP diagram of the solid-state molecular structure of the cationic fragment of {([P2N2]Ta)2(|l-H)4}+r (18) as determined by X-ray crystallography. Silyl methyls have been omitted for clarity and only the ipso carbons of the phenyl rings attached to phosphorus are shown. The bridging hydride ligands were not located. Table 4.2. Bond lengths, angles, and dihedral angles for complex 18. Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.602(2) Ta(l) N(2) 2.127(4) Ta(l) P(2) 2.602(2) Ta(l) Ta(2) 2.7721(5) Ta(l) N(l) 2.145(5) Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) P(l) Ta(l) P(2) 143.58(5) Ta(2) Ta(l) N(l) 127.25(12) N(l) Ta(l) N(2) 102.4(2) Ta(2) Ta(l) P(D 108.91(4) 154 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Atom Atom Atom Atom Dihedral Angle (°) P(l) N(l) Ta(l) Ta(l) Ta(l) Ta(2) Ta(2) Ta(2) N(l) P(l) N(l) Si(l) -2.81(10) 8.9(5) 129.6(4) The ' H N M R spectrum of species 18 in CD2CI2 contains only broad paramagnetically shifted peaks, though over several weeks CH2CI2 solutions of 18 decompose to yield colourless diamagnetic products. As has been noted, solutions of 18 in CH 2 C1 2 are dichroic; this behaviour has been noted before for other paramagnetic species, such as the ubiquitous ferrocenium ion, [(r|5-C5H 5) 2Fe] +, which also exhibits a blue-green/red dichroism. The solution and solid-state EPR spectra of complex 18 provide little structural data, as both contain a very broad signal, with no hyperfine or superhyperfine coupling to tantalum, phosphorus, or hydrogen resolved. The solid state EPR contains a signal at a g value of 1.89 with a very large half-height width of 800 G. In contrast, the monomeric Ta(IV) complex 181 TaH4(dmpe)2 reportedly exhibits hyperfine coupling to Ta (99.99 %, I - 1/2, a T a = 106.3 G) and 3 1 P (100.0 %, I = 1/2, aP = 32.9 G ) , 2 8 and the related dinuclear complex {[(PEt2Ph) 2 H 2 Re]2( | i -H)4} + contains a complex spectrum due to one unpaired electron that resides in a Re-Re bonding orbital and therefore couples to two rhenium nuclei. 2 9 (ii) Magnetism of { ( [ P 2 N 2 ] T a ) 2 ( u . - H ) 4 } + r Figure 4.5 shows a plot of %, the molar magnetic susceptibility corrected for diamagnetic contributions, and jieff, the effective magnetic moment, in the temperature range of 2-300 K. 155 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 0.24 0.19 0.14 0.09 0.04 •0.01 1 o 0 i r~ r 2 .7 2.5 =fi 1.9 1.7 CD 0 50 100 150 200 250 300 Figure 4.5. Measured molar magnetic susceptibilities, %, (A) and effective magnetic moments, peff, (o) versus temperature for {([P2N2]Ta)2(p-Ff)4}+r (18). Attempts to model the entire data set for 18 with the Curie-Weiss law, using g, © and a temperature-independent paramagnetism term (TIP) as parameters, did not provide a good fit. This is possibly due to spin-orbit coupling or perhaps due to the error in the approximation of the diamagnetic correction for species 18. For the third-row transition elements strong spin-orbit coupling is common, unlike for the first-row transition elements, and is not trivial to model. While there are no strictly degenerate orbitals on the individual tantalum metal centres that would allow for a large contribution from first-order spin-orbit coupling, low-lying orbitals that may be thermally populated could affect the magnetism, leading to a second-order spin-orbit coupling effect. Modeling only the low-temperature data from 2 to 50 K, which should be less affected by thermally dependent second-order spin-orbit coupling, as well as by errors in the diamagnetic correction, the parameters g = 1.86, TIP = 1670x10" emu- mol"1, and G =-0.11 K were determined. This model is shown in Figure 4.6. The g value is as anticipated from the EPR data, and is consistent with a dl metal centre where there are no degenerate orbitals present for a large first-order spin-orbit coupling effect. The negative 0 value indicates that small intermolecular interactions 156 References begin on page J 72. Chapter 4: Dinuclear Tantalum Hydride Complexes between unpaired electrons on adjacent molecules do in fact occur, although the exact nature of these interactions is unclear. 3 0 > 3 1 0 10 20 30 40 50 Figure 4.6. A plot of the effective magnetic moment (Ueff) versus temperature for compound 18 from 2-50 K. The circles represent the data points and the bold line a model using the Curie-Weiss law with the parameters g = 1.86, TIP = 1670xl0~6 emumol"1, and 0 = -0.11 K. 4.5 Bonding Considerations and Density Functional Theory Calculations The bonding of hydride-bridged dinuclear metal complexes has been treated at various levels of theory in the past. A general approach using fragment molecular orbital analysis and supported by extended Hiickel theory has been applied to a variety of metals and metal-ligand fragment geometries.32 A similar analysis supported by a multiple-scattering X a calculation was performed on [Cl2(PH3)2Ta]2(U/-H)4, a model complex for the species [Ci2(PMe3)2Ta]2((i-H)4, which is pertinent to the complexes described here. 1 0 Both methods relied on arbitrary geometries, chosen from relevant crystal structure data, rather than optimized geometries. Both analyses also present a similar approach to describing the bonding in these structures, where the available metal-ligand fragment orbitals are 157 References begin on page J 72. Chapter 4: Dinuclear Tantalum Hydride Complexes overlapped with the four possible linear combinations of the four bridging hydride ligands. This approach as applied to complexes 14 and 18 is illustrated in Figure 4.7. Figure 4.7. A depiction of the overlap of metal-based orbitals with the bridging hydride ligands to generate delocalized molecular orbitals for complex 14 along with the appropriate symmetry labels for its respective point group (D2). The orbital analysis for 14 in Figure 4.7 shows the four linear combinations of the hydride Is orbitals and the metal-based orbitals that are of the appropriate symmetry for overlap to form three-centre rj, %, or 8 bonding orbitals. It has been noted previously that the 158 References begin on page J 72. Chapter 4: Dinuclear Tantalum Hydride Complexes lowest energy fully bonding linear combination of the hydride orbitals is of much lower energy than the metal-based orbitals of appropriate symmetry to form a rj-bonding orbital. The combination of the two metal orbitals of rj-symmetry forms a metal-metal bonding orbital that includes only a small contribution by the bridging hydride ligands. Complex 14 contains 10 electrons that fill these 5 orbitals, and therefore, this complex is diamagnetic and the metal-metal bonding orbital is expected to be the highest in energy. It is worth noting that there is one remaining metal-based d orbital on each metal centre that is not considered in either of the two aforementioned bonding descriptions, as it has no significant overlap with the hydride orbitals in either complex. The utility of density functional theory (DFT) in modeling compounds of the second-and third-row transition metals that contain metal-metal bonds has recently been demonstrated, even utilizing relatively small basis sets. 3 3 - 3 5 While DFT does not strictly provide molecular orbitals and their respective energies in the same manner as Hartree-Fock calculations, most investigations to date have demonstrated that the resultant Kohn-Sham orbitals are very similar to those predicted by Hartree-Fock theory. 3 6 - 4 0 Therefore, it has been suggested that the Kohn-Sham orbitals of DFT are of equal utility to chemists in the qualitative analysis of reactivity and structure. Even the energies of the Kohn-Sham orbitals appear to be empirically related to the actual molecular orbital energies.36 Encouraged by these results, we decided to attempt to model complex 14 using ab initio DFT calculations in order to gain a better understanding of the bonding in this complex. It appeared unusual that the arrangement of the ligands in both 14 and 18 has the phosphine ligands on opposite metal centres eclipsed, rather than staggered by 90° as in the structure of the related complex [Cl2(PMe3)2Ta]2(p-H)4, and additionally the Ta-Ta bond length in these related species differ by 0.105 A. Also of interest was the interaction of the amide donors with the metal-metal bonding orbitals. The interactions of rt-bonding ligands with multiple bonds between metals are of current interest in generating one-dimensional conducting polymers,4 1 and these interactions are of potential importance in the reactivity of other [P2N2] complexes. Rather than using the [P2N2] fragment, complex 14 was modeled using the species [(H 2N)2(H3P)2Ta] 2(p-H)4 (14A) to simplify the calculations. Constraints were added to 159 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes ensure that this complex simulated the geometrically restricted [P2N2] macrocycle. The Ta-Ta-P angles were set at 110.0° and the Ta-Ta-N angle at 129.5°, as observed in the solid-state X-ray structure of 14. In an attempt to simulate the "twist" in the [P2N2] ligand noted in 14, the H-N-Ta-Ta dihedral angle was restrained to 120.0°. Failure to restrict this angle in calculations on 14A resulted in a H-N-Ta-Ta dihedral near 180°; however, the Si-N-Ta-Ta dihedral angle in the [P2N2] macrocycle cannot be that large, due to geometrical restraints in the macrocyclic ring. The geometry of model complex 14A was optimized (with the exception of the previously stated constraints) with idealized D2 symmetry using the Gaussian 98 program 4 2 and the hybrid functional B3LYP method.4 3 The basis functions and effective core potentials (ECP) used were those in the L A N L 2 D Z basis set 4 4 but with additional d-polarization functions added to P atoms with the exponent of the d-functions set at 0.37. This level of theory has recently been shown to be adequate for modeling complexes containing metal-metal bonds of various orders, although metal-metal bonds were consistently slightly longer than experimental values.3 3 Selected optimized bond lengths, bond angles, and dihedral angles for the model complex 14A are shown in Table 4.3. The general features of the optimized geometry of this model complex are in good agreement with the structure of complex 14. The arrangement of the ligands on the opposing metal centres as described by the P-Ta-Ta-P dihedral angle 2.37° is similar to that in 14; the phosphine ligands on opposing metal centres are essentially eclipsed. The calculated Ta-Ta distance of 2.644 A is only slightly longer than the actual Ta-Ta bond length in complex 14 of 2.6165(5) A. 160 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes T a b l e 4.3. Selected bond lengths and energies for the ab initio DFT geometry optimizations of 14A. Compound: Tetrahydride Model 14A Ta-Ta 2.6444 A Ta-H 1.8969 A, 2.0862 A Ta-N 2.0766 A Ta-P 2.6680 A Total Energy -375.1650764 Ha The Kohn-Sham orbitals obtained from the geometry optimization for 14A appear to qualitatively fit the bonding description for 14 shown in Figure 4.7; however, several other interesting features appear, such as the influence of the amido "lone pairs" (occupied p-orbitals on the amido donors) on the bonding in 14A. The HOMO of 14A is the Ta-Ta bonding orbital, as anticipated. An isosurface of this orbital is shown in Figure 4.8 and clearly shows good orbital overlap between the two metal orbitals. There is very little contribution from the bridging hydride Is orbitals in the HOMO. The ensemble of hydride orbitals of appropriate symmetry lies much lower in energy. 161 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Figure 4.8. Depiction of the metal-metal a-bond interaction in the H O M O of tetrahydride 14A. The P H 3 and N H 2 donors are labeled by P and N respectively. The frontmost N H 2 ligand on each tantalum centre obscures the view of the rear N H 2 ligand. The next lowest energy orbitals for 14A are different symmetry combinations of the nonbonding electrons on the amido ligands. The interaction of the amido lone pairs with metal-based orbitals was ignored in the model shown in Figure 4.7; however, these calculations demonstrate that they are not negligible. It was mentioned previously that attempts to optimize the geometry of 14A with no restriction on the H-N-Ta-Ta dihedral angle resulted in a dihedral angle of 180°. The amide lone pairs in this situation have good overlap with the 8 symmetry orbitals (labeled bj in Figure 4.7), as illustrated in Figure 4.9a). As mentioned, the backbone of the [P 2N 2] macrocycle renders this conformation impossible. If the H-N-Ta-Ta dihedral angle is near 90°, the best overlap now occurs with the metal-based a-bonding orbitals (the highest energy orbital of a symmetry in Figure 4.7; this overlap is illustrated in Figure 4.9b). 162 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes a) b) Figure 4.9. Illustration of the amide lone-pair orbital overlaps for model 14A when the H-N-Ta-Ta dihedral angle is 180° (a), and when the H-N-Ta-Ta dihedral angle is 90° (b), depicted in the N-Ta-N plane. The P H 3 donors and bridging hydrides are omitted. In the case of the actual model 14A, the H-N-Ta-Ta dihedral angle was restricted to 120° in an attempt to simulate the conformational restrictions of the macrocyclic [P2N2] ligand. Therefore, overlap with both the metal orbitals in Figure 4.9 is possible, although each overlap may be poorer than illustrated in Figure 4.9. There are four possible linear combinations of the amide TC-donor orbitals, and these are shown in Figure 4.10. These have a,bj,b2 and bi symmetry respectively. 163 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Figure 4.10. Four possible linear combinations of the nitrogen lone-pair orbitals in 14A, in D2 symmetry, depicted in the N-Ta-N plane, along with their symmetry labels. The P H 3 donors and bridging hydrides are not shown. There are orbitals of each of these symmetries in model complex 14A, as can be seen from Figure 4.7; however, the b2 and b3 symmetry metal-hydride rt-bonding orbitals are too low in energy for a large interaction with the amide lone-pair orbitals. Therefore, in model complex 14A the two highest energy amide lone-pair orbitals, HOMO-1 and HOMO-2, are of b2 and b3 symmetry and do not have a significant contribution from any metal-based orbital. Another example has been noted previously where the restricted conformation of the [P2N2] ligand does not allow for all the symmetry combinations of the amido lone-pair orbitals to overlap with the available metal orbitals.9 The next lowest energy symmetry combination of the amido lone pair is of a symmetry and has some overlap with the metal-metal orbital of a symmetry similar to the HOMO orbital. This metal orbital has already been used to form the metal-metal bond of the HOMO orbital. There is a slight antibonding contribution from the lone pair orbital to the metal centre in the HOMO, so that overall this interaction does not greatly strengthen metal-amide bonding. The first three symmetry combinations of the amido donors are shown in Figure 4.11. 164 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes HOMO-1 HOMO-2 HOMO-4 jfc>2 £>3 9 Figure 4.11. Depictions of the molecular orbitals containing three of the four possible linear combinations of the amido "lone-pair" />orbitals for tetrahydride 14A. The front N H 2 groups and their 7t-donor lone-pair orbitals obscure the view of the back N H 2 groups. The fourth symmetry combination of the amido lone pairs is of bj symmetry, which is the correct symmetry to overlap with the 5-bonding interaction between metal centres. This metal orbital is also occupied and involved in 5 bonding with the hydrides. This overlap results in two occupied orbitals delocalized over the nitrogen donors, the tantalum centres and the hydrides, as shown in Figure 4.12. 165 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes H O M O - 3 H O M O - 5 bi b 1 Figure 4.12. Depiction of the overlap of the fourth symmetry combination of the amido "lone-pair" p-orbitals with the metal orbitals involved in 5-bonding through the hydride ligands for tetrahydride 14A. Along with the rj-bonding and 8-bonding interactions in the Ta2(p-H)4 moiety, there are also two strong rc-bonding interactions in 14A, as anticipated. These are shown in Figure 4.13. These Tt-bonding interactions are likely responsible for the eclipsed phosphines on the opposite metal centres. In compound 14A it is not clear that the a - or 8-bonding interaction should lead to a preference in the arrangement of the ligands on the opposing metal centres (i.e. if the phosphines are eclipsed or staggered by 90°). If the energies of the two n-interactions are largely disparate, as for 14A, the eclipsed conformation should be favoured, as observed here. 166 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes An interesting aspect of the [P2N2] ligand is that the orientation in which amido lone-pair orbitals are directed is restricted to a very small range. The bonding of the amido lone-pair orbitals appears to destabilize the metal a-bonding orbital relative to the other remaining metal d-orbitals. This destabilization may be responsible for the longer Ta-Ta distance in 14 compared to [Cl2(PMe3)2Ta]2(u>H)4,10 because in 14 the bridging hydride, amide lone-pair and metal-based electrons must compete for bonding with the same metal orbitals. A further result of this competition of the amide donors with the metal-metal bonding orbital should be an increased reduction potential as compared to that in the absence of these strong 7t-donors. 4.6 Summary and Conclusions The hydrogenation of [P2N 2 ]TaMe 3 did not generate [P 2 N 2 ]TaH 3 , and instead produced the Ta(IV) dimer ([P2N2]Ta)2(u-H)4. The reactivity of ([P2N2]Ta)2((i-H)4 is limited, and it behaves primarily as a reducing agent; even when reacted with strong 167 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes electrophiles, such as Me l and {H(OEt)2}+{B[3,5-(CF3)2C6H3]4r, only oxidation of the tantalum centres is observed. The product of the reaction with M e l is the paramagnetic hydride {([P2N2]Ta)2(p-H)4}+r. The cyclic voltammagrams of both ([P2N2]Ta)2(p-H)4 and {([P2N2]Ta)2(p-H)4}+r contained reversible one-electron oxidations at -1.18 V and -0.33 V versus the SCE, corresponding to the conversion of ([P2N2]Ta)2(p-H)4 to ([P2N2]Ta)2(p-H)4+ and ([P2N2]Ta)2(p-H)42+ respectively, and allow us to quantify the reducing power of this tantalum-tantalum bond. Ab initio DFT studies on the model complex [(H2N)2(H3P)2Ta]2(p-H) 4 demonstrated that the lone pairs present on the amido donors can interact with the tantalum-tantalum a-bonding and 8-bonding orbitals, and this interaction increases the energy of the HOMO. The destabilization of metal-metal bonding electrons should increase the reduction potential of electrons in these orbitals, an effect that may have importance in other related systems where electrons stored in metal-metal bonds are used to reduce small molecules. In complex 14, however, the lack of reactivity with small molecules such as ethylene or carbon monoxide may be ascribed to the coordinative saturation at the metal centre, and the strong bridging tantalum-hydride bonds, which do not allow small molecules to access the metal centre. 4.7 Experimental 4.7.1 General Procedures Unless otherwise stated, general procedures were performed according to Section 2.9.1. Variable temperature magnetic susceptibility data were collected on a Quantum Design (MPMS) SQUID magnetometer. Magnetic susceptibilities were corrected for the background signal of the sample holder and for diamagnetic susceptibilities of all atoms (-870xl0 6 cm 3 G mol"1). Measurements were made from 2 K to 300 K and at a field strength of 10000 G on a microcrystalline sample that had been ground into a fine powder. 4.7.2 Materials 168 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes Complex 5 was prepared as described in Chapter 2. Extra dry hydrogen gas was purchased from Praxair and was dried and deoxygenated by passing the gases through a column containing molecular sieves and MnO. Methyl iodide was purchased from Aldrich, degassed via three freeze-pump-thaw cycles and stored over copper wire. Brookhart's acid, {H(OEt2)} +{B[3,5-(CF3)2C6H3]4}~, was prepared as described in the literature.45 4.7.2 Synthesis and Characterization of Complexes Synthesis of ([P2N2]Ta)2(u-H)4 (14) A yellow solution of [P 2N 2]TaMe 3 (1.0 g, 1.3 mmol) in 200 mL of hexanes was transferred to a 400 mL thick-walled glass reaction vessel equipped with a Teflon valve and a stir bar. The solution was degassed by two freeze-pump-thaw cycles. With the solution frozen in a liquid N 2 bath, 1 atmosphere of hydrogen gas was added and allowed to cool for 20 minutes. The Teflon valve was then sealed, and the solution was warmed to room temperature. Once at room temperature the solution was stirred rapidly, and within 30 minutes the solution turned dark red, and a microcrystalline precipitate formed. After two days the hydrogen gas was removed and the brick red precipitate was collected by filtration, rinsed with 10 mL hexanes, and dried under vacuum, to afford ([P2N 2 ] T a ) 2 ( ( i - H)4 in 80% 1 3 1 1 yield. The dark brown hexane rinse did not appear to contain any H o r " P{ H} N M R active species. Species 14 has only trace solubility in hexanes and is moderately soluble in aromatic solvents. Single crystals suitable for X-ray analysis were obtained by performing the reaction without stirring. ' H N M R (500 MHz, C 7 H 8 , 25°C): 8 0.230, 0.228, 0.46, and 1.17 (s, 48H total, SiCH 3 ) , 0.66 and 0.99 (ABX, V H H = 13.5 Hz, 8H total, C/7 2 ring), 1.04 and 1.43 (ABX, 2 J H H = 13.1 Hz, 8H total, C/7 2 ring), 5.86 (q, 2Jm = 9.9 Hz, 4H, \i-H), 7.13 (m, 12H, mlp-H), 7.42 (m, 8H, o-H). 3 I P{ ! H} N M R (C 7 H 8 , 25°C): 8 10.75. UV-vis (toluene: k(nm), e(M"1-cm"1) 455, 6200; 348, 8600; 313, 18000. Anal. Calcd for C 2 4H4 4 N 2 P2Si4Ta: C, 40.27; H, 6.19; N , 3.91. Found: C, 40.37; H, 6.34; N , 3.85. 169 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes ([P2N2]Ta)2(p-D)4 (14-rf,) The deuterated analogue of 14 was prepared in an identical manner using D 2 gas in lieu of H 2 gas. [P2N2]TaMe2(H) (15) A yellow solution of [P 2N 2]TaMe 3 (20 mg) in 1 mL of C 6 D 6 was sealed in a N M R tube under 0.5 atmospheres of hydrogen gas, and stirred in the absence of light for 1 week. 1 3 1 1 The resulting solution was identified by H and P{ H} N M R as having the empirical formula [P 2N 2]TaMe 2(H). *H N M R (500 MHz, C 6 H 6 , 25 °C): 5 0.06, 0.22, 0.39, and 0.46 (s, 24H total, SiC/ / 3 ) , 0.88 (dd, 2 J H P = 7 Hz, 2J Hp = 7 Hz, 6H, TaC# 3), 0.93, 1.34, 1.39 and 1.90 (dd, 8H total, SiCH 2 P), 5.68 (s, WU2 = 4.0 Hz, 1H, Ta2(p-H)), 7.11 (overlapping multiplets, m/p-H), 7.47 (m, 2H, PPh o-H), 7.79 (m, 2H, PPh o-H). 3 1P{ *H} N M R ( C 6 H 6 , 25 °C): 5 17.2 (d, 2 / P P = 125 Hz) 24.4 (d, 2 / P P = 125 Hz). Synthesis of {([P2N2]Ta)2(p>H)4}+r (18) A stirred solution of [P 2N 2]Ta(p-H) 4 (0.429 g, 0.300 mmol) in 60 mL of toluene was degassed, and then a large excess of Mel was added by vacuum transfer. Over 3 hours the dark solution became colourless and a green microcrystalline solid precipitated. The toluene and excess Mel were removed under vacuum. The solid was collected on a glass filter and rinsed with 20 mL of toluene and then 20 mL of hexanes. The remaining green solid was dried under vacuum, to afford {([P 2N 2]Ta) 2(p-H) 4}T (0.420 g, 90 %). This green solid is insoluble in aromatic solvents and tetrahydofuran, but soluble in methylene chloride, though solutions decompose over the course of 1 week to give a colourless solution. Methylene chloride solutions of 18 are dichroic; reflected light appears burgundy and transmitted light is blue-green. Single crystals suitable for X-ray analysis were obtained by performing the synthesis in an identical manner, but without stirring. Anal. Calcd for C 4 8 H 8 8 N 4 P 4 S i 8 T a 2 : C, 170 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 37.01; H , 5.63; N , 3.60. Found: C, 37.25; H, 5.45; N , 3.47. UV/Vis (CH 2C1 2) A,(nm), efM-'-cm"1) 576, 2900; 450, sh; 346, 16000; 298,15000. EPR (solid, 295 K) g = 1.88 Wm = 800 Gauss. EPR(solution, 295 K) g = 1.89, broad. The colourless toluene soluble reaction products were evaporated to dryness and identified by ' H and 1 3 C{'H} N M R as dibenzyl (PhCH 2CH 2Ph). *H N M R (C 6 D 6 , 400 MHz, 298 K): 5 2.73 (s, 4H, PhC/¥ 2C/7 2Ph), 6.98 (m, 4H, o-H), 7.05 (m, 2H p-H), 7.13 (m, 4H, m-H). 1 3 C{'H} (C 6 D 6 , 298 K): 5 38.1 (s, PhCH 2CH 2Ph), 126.2 (s, p-C), 128.5 (s, m-C), 128.8 (s, o-C), 142.0 (s, ipso-C). X-ray Crystallographic Analyses Selected crystallographic data, fractional coordinates and thermal parameters are provided in Appendix 1. 4.7.3 Calculations The ab initio DFT calculations on the model compound [(H 2N) 2(H 3P) 2Ta] 2(u,-H) 4 were performed with the hybrid functional B3LYP method4 3 using the Gaussian 98 package 4 2 The basis functions and effective core potentials (ECP) used were those in the L A N L 2 D Z basis set developed by Hay and Wadt 4 4 and provided with the Gaussian 98 program, but with an additional d polarization functions added to P atoms. The exponent of the d function was 0.37. This level of theory has been shown to be adequate for modeling complexes containing metal-metal bonds of various orders.33 Model complex 14A was optimized with D2 symmetry with the Ta-Ta-P and Ta-Ta-N angles frozen at 110° and 129.5° respectively, and the H-N-Ta-Ta dihedral angle frozen at 120°. Orbital depictions were created using the M O L D E N program.4 6 171 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 4.8 References 1) Antinolo, A. ; Carillo-Hermosilla, F.; Fajardo, M . ; Fernandez-Baeza, J.; Garcia-Yuste, S. Coord. Chem. Revs. 1999,193-195, 43-72. 2) Barefield, E. K.; Parshall, G. W.; Tebbe, F. N . /. Am. Chem. Soc. 1970, 92, 5234. 3) Tebbe, F. N . ; Parshall, G. W. J. Am. Chem. Soc 1971, 93, 3793. 4) Parshall, G. W. Acc. Chem. Res. 1975, 8, 113. 5) Mulford, D. R.; Clark, J. R.; Schweiger, S. W.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1999, 18, 4448-4458. 6) Profilet, R. D.; Rothwell, A . P.; Rothwell, I. P. J. Chem. Soc, Chem. Commun. 1993, 42-44. 7) Ankianiec, B. C ; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1991,113, 4710-4712. 8) Parkin, B. C ; Clark, J. R.; Visciglio, V. M . ; Fanwick, P. E.; Rothwell, I. P. Organometallics 1995,14, 3002-3013. 9) Fryzuk, M . D.; Johnson, S. A. ; Rettig, S. J. Organometallics 1999,18, 4059-4067. 10) Scioly, A. J.; Luetkens, M . L. J.; Wilson, R. B.; Huffman, J. C ; Sattelberger, A . P. Polyhedron 1987, 6, 741-757. 11) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992,11, 1559-1561. 12) Belmonte, P. A. ; Schrock, R. R.; Day, C. S. /. Am. Chem. Soc. 1982,104, 3082-3089. 13) Cotton, F. A. ; Daniels, L . M . ; Murillo, C. A. ; Wang, X . J. Am. Chem. Soc. 1996,118, 12449-12450. 14) Scoles, L. ; Ruppa, K. B. P.; Gambarotta, S. J. Am. Chem. Soc. 1996,118, 2529-2530. 172 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 15) Miller, R. L. ; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; Van Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993,115, 5570-5588. 16) Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. J. Am. Chem. Soc. 1977, 99, 3872-3874. 17) Cotton, F. A. ; Extine, M . W.; Felthouse, T. R.; Kolthammer, B.- W. S.; Lay, D. G. J. Am. Chem. Soc. 1981, 103, 4040. 18) Fryzuk, M . D.; Johnson, S. A. ; Rettig, S. J. / . Am. Chem. Soc. 1998,120, 11024-11025. 19) Mayer, J. M . ; Wolczanski, P. T.; Santarsiero, B. D.; Olson, W. A. ; Bercaw, J. E. Inorg. Chem. 1983, 22, 1149-1155. 20) Crabtree, R. H. Acc. Chem. Res. 1979,12, 331. 21) Bau, R.; Carroll, W. E.; Hart, D. W.; Teller, R. G.; Koetzle, T. F. Adv. Chem. Ser. 1978, 167, 73-92. 22) Bakhmutov, V . I.; Vorontsov, E. V. ; Nikonov, G. I.; Lemenovskii, D. A. Inorg. Chem. 1998, 37, 279-282. 23) Crabtree, R. H. ; Segmuller, B. E.; Uriarte, R. J. Inorg. Chem. 1985, 24, 1949-1950. 24) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. Rev. 1985, 65, 1-48. 25) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: M i l l Valley, 1987. 26) Cotton, F. A. ; Wilkinson, G. Advanced Inorganic Chemistry; 5th ed.; Wiley-Interscience: New York, 1988, pp 1105. 27) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; 2nd ed.; Wiley-Interscience: New York, 1994, pp 62. 28) Elson, I. H. ; Kochi, J. K.; Klabunde, U . ; Manzer, L . E.; Parshall, G. W.; Tebbe, F. N . J. Am. Chem. Soc. 1974, 96, 7374. 173 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes 29) Allison, J. D.; Walton, R. A. J. Am. Chem. Soc. 1984,106, 163-168. 30) O'Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203. 31) Carlin, R. L. Magnetochemistry; Springer-Verlag: Heidelberg, 1986. 32) Dedieu, A. ; Albright, T. A. ; Hoffmann, R. J. Am. Chem. Soc. 1979,101, 3141. 33) Cotton, F. A. ; Feng, X . J. Am. Chem. Soc. 1997,119, 7514-7520. 34) Cotton, F. A. ; Feng, X . J. Am. Chem. Soc. 1998,120, 3387-3397. 35) Alonso, E.; Casas, J. M . ; Cotton, F. A. ; Feng, X . ; Fornies, J.; Fortuno, C ; Milagros, T. Inorg. Chem. 1999, 38, 5034-5040. 36) Stowasser, R.; Hoffmann, R. / . Am. Chem. Soc 1999,121, 3414-3420. 37) Baerends, E. J.; Gritsenko, O. V. ; van Leeuwen, R. Chemical Application of Density Functional Theory; Laird, B. B., Ross, R. and Zeigler, T., Ed.; ACS Symposium Series 629; American Chemical Society: Washington, DC, 1996, pp 20. 38) Bickelhaupt, F. M . ; Baerends, E. J.; Ravenek, W. Inorg. Chem. 1990, 29, 350. 39) DeKoch, R. L. ; Baerends, E. J.; Hengelmolen, R. Organometallics 1984, 3, 289. 40) Sargent, A. L. ; Titus, E. P. Organometallics 1998,17, 65. 41) Chisholm, M . H. Acc. Chem. Res. 2000, 33, 53. 42) Frisch, M . L ; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M . A. ; Cheeseman, J. R.; Zakrzewski, V . G.; Montgomery, J. A. ; Stratmann, R. E.; Burant, J. C ; Dapprich, S.; Millam, J. M . ; Daniels, A . D.; Kudin, K. N . ; Strain, M . C ; Farkas, O.; Tomasi, J.; Petersson, V. A. ; Ayala, P. Y . ; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A . D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V. ; Stefanov, B. B.; Liu, G.; Liashenko, A . ; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M . A. ; Peng, C. Y . ; Nanayakkara, A. ; Gonzalez, C ; Challacombe, M . ; Gi l l , P. M . W.; Johnson, 174 References begin on page 172. Chapter 4: Dinuclear Tantalum Hydride Complexes B. G.; Chen, W.; Wong, M . W.; Andres, J. L.; Head-Gordon, M . ; Repogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburg PA, 1998. 43) Becke, A . D. J. Chem. Phys. 1993, 98, 5648. 44) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. 45) Brookhart, M . ; Grant, B.; Volpe, A. F. J. Organometallics 1992,11, 3920-3922. 46) Schaftenaar, G. Molden; 3.5 ed.; C A O S / C A M M Center, University of Nijmegen, The Netherlands, 1991. 175 References begin on page 172. Chapter 5: A New Bonding Mode for Dinitrogen Chapter Five A New Bonding Mode for Dinitrogen 5.1 Introduction The previous chapters have described the chemistry derived so far from the [P2N2]TaMe3 (5) starting material. In general the complexes are remarkably stable. The methylidene complex [P2N2]Ta=CH2(Me) (6) described in Chapter 2 is a rare example of an isolable methylidene complex. 1 - 7 The complexes [P2N2]Ta(C2H4)Et (12) and [P2N2]Ta(C2H4)Me (13) described in Chapter 3 are both stable in the presence of atmospheric oxygen, which is unusual for organometallic complexes of tantalum. The tetrahydride species ([P2N2]Ta)2(p-H)4 (14) described in Chapter 4 does undergo reactivity with electrophiles, but only as a reducing agent; coordinative saturation prevents the binding and activation of small molecules such as carbon monoxide or ethylene. The molecular orbital description of the bonding in tetrahydride 14 assists in understanding its stability. The frontier orbitals of the [P 2N 2]Ta fragment seem ideally suited to forming a bridging 176 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen tetrahydride species, and the L U M O of 14 is poorly suited to forming bonds with incoming molecules. The rationalization of the lack of reactivity of tetrahydride (14) from a frontier orbital analysis led to a rethinking of what type of ligand design might prove useful for generating reactive tantalum complexes. If a similar tantalum tetrahydride species could be obtained with a tridentate ligand design that consisted of two amido donors and one phosphine donor, a vacant orbital would be available at each metal centre, which would allow the metal centre to bind molecules. A simple depiction of this idea is shown in Figure 5.1, where one of the phosphine donors of each of the [P2N2] ligands in hydride 14 has been removed, to produce an unoccupied metal-based orbital at each tantalum centre. In addition, the presence of electrons in the tantalum-tantalum bond should allow the metal centre to reduce molecules that bind to these sites. It was anticipated that the combination of coordinative unsaturation at the metal centre and the reducing nature of the metal-metal bond should lead to increased reactivity in such a dinuclear complex. Figure 5.1. Depiction of the anticipated effect on the vacant orbitals available in the tetrahydride complex 14 upon removal a phosphine donor from the [P2N2] ligand. There is one additional feature that deserves mention; the diamido phosphine ligand shown in the complex in Figure 5.1 has a functional group represented as R. It should be possible to synthesize ligands containing different functional groups at the amido donors. 177 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen This provides an additional means to fine tune the steric and electronic environments provided by diamido phosphine ligands, which was not available with the macrocyclic [P2N2] ligand. Similar diamido ligands containing amine and ether donors have been reported 8 - 1 1 and this area of ligand design has been reviewed.1 2 The reported reactivity of complexes of these ligands is almost entirely confined to olefin polymerization catalysis using the group 4 metals. The functional groups on the amido donors of these ligands are typically sterically demanding alkyl or aryl groups, such as Bu' or 3,5-Me2C6H3. The trend towards steric bulk in ligand design is common in organometallic chemistry, and of particular importance in polymerization catalysts. 1 3 - 1 5 In choosing the functional groups on the amido donors for the initial diamido phosphine studied here, it was decided to avoid sterically demanding functional groups for two reasons: (i) dinuclear complexes are more likely to form using less sterically bulky ligands, and (ii) less steric shielding of the metal centres should lead to enhanced reactivity. The phenyl group was chosen as the functional group for the amido donor for its relatively small size. The target ligand design was therefore PhP(CH 2SiMe 2NPh)2, which will be abbreviated as [NPN] in this thesis. Aside from the investigation of dinuclear hydride chemistry using the [NPN] ligand, the potential of this ligand design for dinitrogen chemistry was also recognized. The reduction of species such as [NPN]VC1 or [NPN]NbCl would be expected to produce coordinatively unsaturated dinitrogen complexes of the type ([NPN]M) 2(|i-N 2) (where M= V or Nb). These ([NPN]M)2(ji-N2) fragments should be isolobal and isoelectronic with the complex [(Ar(Bu')N)3Mo]2(p>N2), which is known to cleave the dinitrogen bond. 1 6 - 1 7 This is illustrated in Scheme 5.1. 178 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Scheme 5.1 2 M = V, Nb, Ta While this study was underway, a closely related diamido amine complex [Me3SiN(CH2CH2NSiMe3)2V(p-Cl)]2 was reported, which when reduced by K C 8 results in the binding and cleavage of dinitrogen, to give [Me3SiN(CH2CH2NSiMe3)2V(p-N)]2. Further reduction led to the potassium salt K+{[Me3SiN(CH2CH2NSiMe3)2V(p-N)]2}_; these reactions are depicted in Scheme 5.2. 1 8 Theoretical investigations into the intermediate dinitrogen complex implicate a dinuclear side-on bound dinitrogen complex. 1 9 This aspect of the application of the [NPN] ligand design is currently under investigation by other members of the Fryzuk group. 179 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Scheme 5.2. This chapter describes the synthesis of the [NPN] ligand and focuses on its application to tantalum chemistry. The synthesis of a tetrahydride similar to that depicted in Figure 5.1 was performed, and this reactive species results in the formation of a dinitrogen complex where the dinitrogen moiety is bound in the side-on end-on dinuclear mode. A preliminary investigation of the reactivity of this dinitrogen complex is also described. 5.2 Synthesis and Characterization of the [NPN] Ligand Precursor One of the keys to the success of the R[PNP] and [P 2N 2] ligands was the ease of the syntheses of their lithiated ligand precursors R [PNP]Li and [ P 2 N 2 ] L i 2 ( C 4 H 8 0 2 ) , which 180 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen allowed large quantities of these ligands to be prepared in high yield and purity. 2 0 ' 2 1 A similar synthesis for the precursor [NPN]Li 2 was required. The synthetic strategy used is shown in Scheme 5.3. The silylated aniline ClCH 2 SiMe2NHPh is prepared by the reaction of LiNHPh with ClCH 2 SiMe2Cl. The reaction of 4 equivalents of B u T i with a mixture of 2 equivalents of ClCH 2 SiMe 2 NHPh and 1 equivalent of PhPH 2 in diethyl ether was expected to provide [NPN]Li 2 , after separation of the LiCI by filtration. The ' H N M R spectrum of the resulting solid, however, was complex and composed of a multitude of broad peaks; the P{ H J N M R spectrum of this solid was also composed of several broad peaks, which were in the region of -40 ppm. In an attempt to imitate the method by which the [P 2 N 2 ]L i 2 salt had been isolated, 1,4-dioxane was added to a toluene solution of this product and resulted in the precipitation of a species that analyzed as [NPN]Li 2(C4Hs0 2). The insolubility of this species in the common solvents hexanes, toluene, diethyl ether and methylene chloride, however, made this unsuitable as a starting material. A more soluble lithiated ligand precursor is prepared via the addition of approximately four equivalents of tetrahydrofuran to a slurry of the presumed [NPN]Li 2 crude product in hexanes. The solid immediately dissolves, and later precipitates a colourless crystalline solid identified as [NPN]Li 2(C4HgO) 2 (19) in 85% yield. Scheme 5.3. Me 2 Cl VCI + 2 LiHNPh Me 2 < N \ i v s P h — p ^ U Li / 1>S Si Ph S = THF 19 181 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen An ORTEP depiction of the solid-state molecular structure of 19 as determined by X-ray crystallography is shown in Figure 5.2. Similar to the structure of [ P 2 N 2 ] L i 2 - ( C 4 H g 0 2 ) , the nitrogen and lithium atoms form a four-membered N 2 L i 2 core. 2 1 One of the lithium atoms is coordinated to both the phosphine donor and a tetrahydrofuran donor, and is four-coordinate. The other lithium atom is coordinated to the second tetrahydrofuran donor, and is three-coordinate. The 3 I P{ 'H} and 7 Li{ 'H} N M R spectra of 19 in CeDg are consistent 31 1 with this structure being maintained in solution. The ' P{ H} N M R spectrum contains a single resonance that is a 1:1:1:1 quartet due to coupling to 7 L i (I = 3/2, 92.6% abundance); the couplings due to the less predominant isotopomer 6 L i (7=1, 7.4% abundance) are largely 7 7 1 obscured by the resonances associated with the L i isotopomer. The Li{ Ff} spectrum consists of two resonances, a singlet and a doublet due to coupling to ' P, as anticipated from the solid-state structure. The ' H N M R spectrum of the ligand framework is comprised of two silyl methyl resonances, two ligand methylene resonances, and two sets of ortho, meta and para phenyl proton resonances due to the phenyl groups attached to the nitrogen and phosphorus donors. This number of resonances is consistent with the maximum symmetry possible for the [NPN] ligand, which can only have a mirror plane of symmetry. 182 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Figure 5.2. ORTEP depiction of the solid-state molecular structure of [NPN]Li 2-(C4H 80)2 (19) as determined by X-ray crystallography. Table 5.1. Selected bond lengths and angles for [NPN]Li 2 - (C 4 H 8 0) 2 (19). Atom Atom Distance (A) Atom Atom Distance (A) N(l ) Li( l ) 2.058(3) P(l) Li ( l ) 2.569(4) N(2) Li( l ) 2.134(4) O(l) Li( l ) 1.908(3) N(l) Li(2) 2.010(4) 0(2) Li(2) 1.891(4) N(2) Li(2) 1.987(3) 183 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) N(l) L i ( l ) N(2) 98.85(15) 0(2) Li(2) N(2) 121.9(2) P(l) L i ( l ) 0(1) 116.7(2) Li( l ) N(l) Li(2) 76.86(15) P(l) L i ( l ) N(l) 95.25(13) Li( l ) N(2) Li(2) 75.60(14) P(l) L i ( l ) N(2) 94.46(12) Si(l) N(l) C(13) 122.68(13) N(l) Li(2) N(2) 105.6(2) Si(2) N(2) C(19) 124.01(13) 0(2) Li(2) N(l) 132.0(2) 5 . 3 Synthesis of [NPN]TaMe3 The synthesis of [NPN]TaMe 3 (20) was accomplished by the reaction of 19 with TaMe3Cl2, as shown in equation 5.1. Complex 20 is a pale yellow light-sensitive solid that was isolated in 80% yield. M e 2 p . Ph , S C / P h Ph -N Me / N. \ D . V / V r " S E t2° Me2Si^ ;N«A /.Me P h — P - ^ L i J - 1 + TaMe 3 CI 2 • M q r e m / )>s -2 LiCl M e 2 S ' l / a [5-1] V / N \ -2 THF V > P ^ M e I Ph Si p n M e 2 S = THF 19 20 Single crystals of trimethyl complex 20 were obtained by slow evaporation of a benzene/hexamethyldisiloxane solution. An ORTEP depiction of the solid-state molecular structure of 20 as determined by X-ray crystallography is shown in Figure 5.3. The [NPN] ligand in 20 is facially bound and the methyl groups are all cis disposed. The overall geometry at the tantalum centre is very distorted, and cannot be described adequately as either octahedral or trigonal prismatic. Compared to the complex [P2N2]TaMe3 (5) the Ta-P o distance of 2.7713(13) A in 20 stands out as unusually long. This bond length is difficult to reconcile with the increased coordinative and electronic unsaturation in complex 20 compared to seven-coordinate complex 5, where the two Ta-P distances were 2.6180(8) and 2.6088(9) A . In fact, this Ta-P bond is longer than in any of the [P 2N 2] complexes of 184 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen tantalum discussed in the previous chapters. In contrast, both the Ta-N and Ta-Me distances are slightly shorter than those found in complex 5. A selection of bond lengths, and bond angles for complex 20 are given in Table 5.2. C(16) C(15) Figure 5.3. ORTEP depiction of the solid-state molecular structure of [NPN]TaMe 3 (20) as determined by X-ray crystallography. Table 5.2. Selected bond lengths and bond angles for [NPN]TaMe 3 (20). Atom Atom Distance (A) Atom Atom Distance (A) Ta(l) P(l) 2.7713(13) Ta(l) C(25) 2.224(5) Ta(l) N(l) 2.078(4) Ta(l) C(26) 2.228(5) Ta(l) N(2) 2.025(4) Ta(l) C(27) 2.204(5) 185 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) N(l) Ta(l) N(2) 113.0(2) N(l) Ta(l) C(27) 89.6(2) P(l) Ta(l) N(l) 81.72(11) N(2) Ta(l) C(25) 106.0(2) P(l) Ta(l) N(2) 70.3(1) N(2) Ta(l) C(26) 92.0(2) P(l) Ta(l) C(25) 160.74(14) N(2) Ta(l) C(27) 137.7(2) P(l) Ta(l) C(26) 119.28(15) C(25) Ta(l) C(26) 79.3(2) P(l) Ta(l) C(27) 78.94(15) C(25) Ta(l) C(27) 112.2(2) N(l) Ta(l) C(25) 82.6(2) C(26) Ta(l) C(27) 78.2(2) N(l) Ta(l) C(26) 152.4(2) The room-temperature ' H N M R spectrum of 20 provides no evidence for the lack of symmetry observed in the solid-state molecular structure, which indicates that there is some fluxional process that occurs in 20. A single resonance is observed for the three tantalum-bound methyl groups, and there are two silyl methyl environments and two ligand methylene resonances. There is only one set of ortho, meta, and para protons for the nitrogen phenyl groups, and one set of ortho, meta and para protons for the phosphorus phenyl groups. At 185 K, two resonances are observed for the tantalum-bound methyl groups in the ' H N M R spectrum, in a 2:1 ratio, consistent with either an octahedral or trigonal prismatic geometry. Variable-temperature ' H N M R also reveals that the resonances due to the ortho and meta protons of the NPh groups broaden and decoalesce at low temperature, which is indicative of hindered rotation of these phenyl groups. The overlapping nature of these resonances has prevented the determination of kinetic parameters from these results. 5.4 Hydrogenation of [NPN]TaMe3: Synthesis of ([NPN]Ta)2(M-H)4 The hydrogenation of 20 was performed by adding 4 atmospheres of H2 gas to a diethyl ether solution of 20. This reaction is complete within 12 hours, which is much faster than the hydrogenation of [P2N2]TaMe3 (5), which took several days. The solution becomes dark purple, suggestive of a reduction from Ta(V) to Ta(IV). A single resonance in the 3 I P{ 'H} N M R spectrum of this species is consistent with a symmetrical structure; this singlet is unchanged even at 180 K. The corresponding ' H N M R spectrum is likewise unchanged by 186 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen variations in temperature, and the observation of only two silyl methyl resonances is also consistent with a highly symmetrical species. The room-temperature ' H N M R contains the expected ligand resonances, as well as a resonance at 8 10.62, which can be assigned to four bridging hydride ligands. This resonance is a broad singlet, with no coupling to 3 1 P resolved. Based on these data, a dinuclear structure of the formula ([NPN]Ta)2(n-H)4 (21), is proposed (equation 5.2). P h Ph Ph , 5 H 2 M e 2 S ! ^ N % ^ 2 [NPNjTaMe 3 2 » M e 2 S i f ^ T a < - — T a ^ | \ [5.2] -6 MeH \ l / S?„ *y Y'%, ^ | - S i M e 2 ^ T H N*E Ph J P h Ph 21 To date, the solid-state molecular structure of 21 has not been determined by X-ray crystallography. Although a structure with four bridging hydrides seems probable from the previous study with the [P2N2] ligand presented in Chapter 4, a structure with bridging and terminal hydrides in rapid exchange cannot be entirely ruled out from the data obtained from the 'IT N M R spectrum. Other techniques were employed to provide evidence for the structure shown in equation 5.2. The IR spectra of 21 and the deuterated analogue formed from the reaction of 20 with D2 contained no peaks in the region anticipated for a terminal tantalum hydride or tantalum deuteride respectively.22 Unfortunately, the region below 1400 cm"1, where bridging hydride or deuteride peaks would be expected, is obscured by ligand absorptions. It was possible to provide evidence that the structure of 21 contains 4 bridging hydrides using isotopic perturbation of resonance. The reaction of 20 with a mixture of H 2 and D 2 gas created a series of isotopomers, ([NPN]Ta)2(p>H)4, ([NPN]Ta)2(|i-H)3(u,-D), ([NPN]Ta) 2(|l-H) 2(u-D) 2, ([NPN]Ta)2(u-H)(u-D)3, and ([NPN]Ta)2(u-D)4. This mixture of isotopomers could also be generated by the addition of D2 gas to 21. The ' H N M R spectrum 187 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen of this mixture showed that the isotopic shift of the hydride signal was only very slightly different for each of these isotopomers; the difference in the chemical shift for each was only about 0.02-0.03 ppm. These data are consistent with the hydrides all occupying bridging positions, rather than there being two different chemical environments for the hydride ligands (bridging and terminal), which should produce a much larger isotopic perturbation of resonance. 2 3 ' 2 4 The reactivity of this species with a variety of small molecules has yet to be thoroughly investigated; however, initial indications are that the reactivity of hydride 21 is much greater than that of hydride 14. For example, as previously mentioned, 21 reacts with D 2 gas to exchange hydride ligands with deuterides. Furthermore, attempts to store solutions of 21 under a nitrogen atmosphere resulted in an interesting reaction that afforded an unusual product. The nature of this species and its reactivity are discussed below. 5.5 Reaction of ([NPN]Ta)2(u-H)4 with Dinitrogen Attempts to crystallize complex 21 under a nitrogen gas atmosphere resulted in a visible colour change. Hydride 21 is dark purple; however, after storage of solutions of 21 under N 2 gas for 1 hour at 15°C, the solution is noticeably lighter coloured, and turns brown 1 3 1 1 within 24 hours. Both H and P{ H} N M R spectroscopies indicate clean conversion to a new compound. The room-temperature ' H N M R spectrum is indicative of a species of lower symmetry than hydride 21; four silyl methyl resonances are observed, as would be expected if the two metal centres of the bimetallic complex were not equivalent. The ' H N M R spectrum also indicates that the number of hydride ligands present has changed. Once again, a single hydride signal is present at 8 10.85; however, by integration this accounts for only 31 two hydrides, which are coupled, to two different " P environments. The chemical shift of the hydride resonance (similar to that observed in 21) and the coupling of this hydride resonance to both phosphine donors together provide evidence that the two hydride ligands bridge the metal centres. At 190 K, the ' H N M R spectrum is indicative of an even less 188 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen symmetrical species. There are eight silyl methyl resonances, and two bridging hydride resonances, consistent with a species of C\ symmetry. 31 1 The P{ H} N M R spectrum of this brown product shows two chemical environments (8 7.8 and 11.0) and it is unusual for two reasons. Firstly, the resonances are significantly broader than in hydride 21, and this broadness is not affected by temperature. Most unique 31 1 about the ~ P{ H} N M R spectrum of this product is that although the peak at 8 11.0 is a 31 doublet due to coupling to the second * P environment, the other peak is apparently a poorly resolved 1:2:2:1 quartet, as shown in Figure 5.4. ppm Figure 5.4. The 3 1 P{ 'H} N M R spectrum (202.46 MHz) of the product of the reaction of 21 under N 2 gas. The two unusual aspects of the 3 I P{ !H} N M R spectrum can both be explained by the presence of a quadrupolar nucleus bound to tantalum; quadrupolar nuclei are known to cause rapid nuclear spin relaxation, and therefore line broadening, as observed in the 3 I P{ 'H} N M R spectrum.25 The only source of quadrupolar nuclei under the reaction conditions under which this complex was formed is the N 2 gas that was intended to provide an inert atmosphere. The presence of an apparent 1:2:2:1 quartet in the 3 1 P{'H} N M R spectrum is 189 References begin on page 254, Chapter 5: A New Bonding Mode for Dinitrogen consistent with the coupling of this 3 1 P resonance to a single 1 4 N environment (I = 1, 99.6% abundance). The coupling constant is coincidentally nearly identical to the coupling constant between the two phosphorus atoms, and combination of a 1:1:1 triplet from coupling to l 4 N with a 1:1 doublet due to coupling to 3 1 P results in the unusual appearance of the multiplet. Note that the coupling of 1 4 N to 3 1 P is not commonly observed.26 Elemental analysis confirms the presence of one mole of dinitrogen, and this reaction product can be assigned as ([NPN]TaH)2(N2) (22), a dinuclear tantalum complex containing metal-bound dinitrogen. The nature of the bonding mode of dinitrogen in complex 22 was not obvious from the ' H or P{ H} N M R spectra. The two bridging hydride ligands in complex 22 should draw the two metals centres close enough together that the end-on bonding mode is impossible, and the side-on bridging mode would be more likely; however, the lack of symmetry evident from the ' H and 3 1 P{'H} N M R spectra is not what one would expect for a complex containing either end-on or side-on bound dinitrogen. 5.5.1 Solid-State Molecular Structure of ([NPN]Ta(p-H))2N2 (22) To obtain more information about the nature of complex 22, and the bonding mode of the dinitrogen moiety, the solid-state structure was determined by X-ray crystallography. An ORTEP depiction of the solid-state molecular structure of 22 is shown in Figure 5.5, and Table 5.3 provides selected bond lengths, bond angles and dihedral angles. The bridging hydrides could not be located from the difference map. The most intriguing aspect of this structure is the bonding mode of the dinitrogen moiety; it is end-on bound to one tantalum and side-on bound to the other tantalum, thus explaining the decrease in symmetry observed 1 3 1 1 1 2 in the H and P{ H} N M R spectra. This p-r| :r| bonding mode has never before been reported in a bimetallic dinitrogen complex. 190 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen C(l) C(25) S i(3) Figure 5.5. ORTEP depiction of the solid-state molecular structure of ([NPN]TaH)2N2 (22) as determined by X-ray crystallography. The two bridging hydrides were not located. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Table 5.3. Selected bond lengths, bond angles, and dihedral angles for ([NPN]TaH) 2N 2 (22). Atom Atom Distance (A) Atom Atom Distance (A) N(5) N(6) 1.319(6) Tad) Pd) 2.625(2) Ta(l) N(5) 2.141(4) Ta(2) P(2) 2.605(2) Ta(l) N(6) 1.975(5) Tad) N(l) 2.066(5) Ta(2) N(5) 1.888(5) Tad) N(2) 2.040(5) Ta(l) Ta(2) 2.8311(3) Ta(2) N(3) 2.072(5) Ta(2) N(4) 2.086(5) 191 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) Ta(2) N(5) N(6) 151.9(4) Ta(l) Ta(2) P(2) 114.02(4) Ta(l) N(5) Ta(2) 89.0(2) Ta(l) Ta(2) N(3) 127.24(14) Ta(l) N(6) N(5) 78.3(3) Ta(l) Ta(2) N(4) 123.37(13) P(l) Ta(l) N(l) 79.19(14) N(l) Ta(l) N(2) 105.7(2) P(l) Ta(l) N(2) 77.66(13) N(l) Ta(l) N(6) 132.9(2) P(D Ta(l) N(6) 75.78(15) N(2) Ta(l) N(6) 107.0(2) P(2) Ta(2) N(5) 161.91(14) P(2) Ta(2) N(3) 85.17(15) Ta(2) Ta(l) P(l) 154.45(4) P(2) Ta(2) N(4) 77.72(13) Ta(2) Ta(l) N(l) 120.01(14) N(3) Ta(2) N(4) 108.2(2) Ta(2) Ta(l) N(2) 109.72(13) N(3) Ta(2) N(5) 110.1(2) N(4) Ta(2) N(5) 105.3(2) Atom Atom Atom Atom Dihedral Angle (°) Ta(l) Ta(2) N(5) N(6) -19.5(8) P(l) Ta(l) Ta(2) P(2) -167.3(1) o The N(5)-N(6) distance is 1.319(6) A, consistent with a formal assignment of the bridging dinitrogen moiety as (N2)4~. The Ta(l)-Ta(2) distance of 2.8311(3) A is short enough for a metal-metal bonding interaction; however, because the two Ta centres are formally Ta(V), no electrons are available for a metal-metal bonding interaction. The shortest of the tantalum-dinitrogen interactions is the "end-on" Ta(2)-N(5) bond, at 1.888(5) A. This distance is consistent with the Ta(2)-N(5) bond having considerable double-bond character. The side-on bonding interactions are both longer; the Ta(l)-N(6) bond length is 1.975(5) A and the Ta(l)-N(5) bond is longer yet at 2.141(4) A. The difference between the Ta(l)-N(5) and Ta(2)-N(5) bond lengths is 0.253(9) A . A slightly smaller difference of 0.166(9) A is observed between the Ta(l)-N(6) and Ta(l)-N(5) bond lengths. The Ta(l)-P(l) and Ta(2)-P(2) distances are 2.625(2) A and 2.605(2) A respectively. These distances are significantly shorter than observed for [NPN]TaMe3 (20), but are similar to the Ta-P distances observed for the [P2N2] complexes of tantalum reported in previous chapters. The room-temperature ' H N M R spectrum is consistent with Cs symmetry for 22 in solution; however, no symmetry is evident in the solid-state structure. The Ta(l)-Ta(2)-N(5)-N(6) dihedral angle of-19.5(8)° clearly indicates that the N 2 moiety does not lie in the same plane as the Ta-Ta bond. The arrangement of the [NPN] ligands with respect to the 192 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen dinitrogen moiety and to each other is also worth mentioning, because the fluxionality in their arrangement is the cause of the C s symmetry observed in the structure of 22 in solution, as determined by ' H N M R spectroscopy. The P(2)-Ta(2)-N(5) angle of 161.91(14)° demonstrates that this phosphine is approximately trans disposed to the end-on nitrogen of the dinitrogen moiety. The P(l)-Ta(l)-N(6) bond angle of 75.78(15)° indicates that this phosphine is cis disposed relative to the dinitrogen moiety. The P(l)-Ta(l)-Ta(2)-P(2) dihedral angle is -167.3(1)°; the [NPN] ligands on the adjacent tantalum centres differ in orientation by almost a 180° rotation around the Ta-Ta vector. This asymmetry is consistent with the low-temperature ! H N M R spectrum, and the fluxional process observed in the *H N M R is likely simply a slight rocking of the [NPN] ligands, so that the P(l)-Ta(l)-Ta(2)-P(2) dihedral angle goes through 180°. 5.5.2 Characterization of the Side-on End-on Bonding Mode by 1 5 N NMR Spectroscopy Although l 5 N N M R spectroscopy has been applied to only a minority of dinitrogen complexes, 2 6" 2 8 it was hoped that this method would provide a definitive means of identifying the side-on end-on binding mode in the solution structure of 22. By reacting hydride 21 with 1 5 N 2 , it was possible to prepare the isotopically labeled complex ([NPN]TaH) 2(|l- l 5N 2) (22-15N2). The l 5 N N M R spectrum of this species contains two vastly different chemical shifts for the dinitrogen moiety. The terminal nitrogen resonance (where terminal refers to N(6) in Figure 5.5) is a multiplet at 8 163.6 (relative to nitromethane at 0 ppm) with a one-bond coupling to the adjacent l 5 N of the dinitrogen moiety of 21.5 Hz, a strong coupling to one ligand P nucleus (7NP = 21.2 Hz), and a weak coupling to the other ligand 3 I P nucleus (7NP = 3.5 Hz). The difference in the magnitudes of the couplings to 3 1 P are presumably the result of the former being a two-bond coupling while the latter coupling is through three-bonds. The end-on nitrogen resonance (where end-on refers to N(5) in Figure 5,5) occurs at 8 -20.4 and is a multiplet, with the previously mentioned coupling to the adjacent 1 5 N of 21.5 Hz, a strong coupling to one ligand 3 1 P nucleus (7Np = 24.6 Hz), and a weak coupling to the other ligand 3 I P nucleus (7NP = 6.6 Hz). The difference in magnitude of 1 5 N - 3 1 P couplings in this case cannot be ascribed to the distance between the nuclei, as these 193 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen 3 I are both two-bond couplings. The larger coupling is due to the " P nucleus that is trans disposed to the 1 5 N nucleus (P(2) in Figure 5.5). This l 5 N - 3 l P coupling constant of 24.6 Hz corresponds to a l 4 N - 3 l P coupling constant of 17.5 Hz, due the difference in magnetogyric ratios between these two nuclei, and is consistent with the overlapping multiplet observed in the 3 1P{ 'H} N M R spectrum of unlabeled species 22. It is worth noting that the two l 5 N{ 'H} resonances are separated by 184 ppm; this separation may prove to be indicative of the side-on end-on bonding mode of dinitrogen when other examples of this bonding mode are discovered. 5.5.3 Significance of the Reaction of Hydride 21 with N 2 The reaction of the hydride 21 with N 2 to form the dinitrogen complex 22 is illustrated in equation 5.3. This reaction is of interest for a number of reasons. Remarkably, no strong reducing agents, such as Na or K, were used to generate this dinitrogen complex. Despite this, dinitrogen has been incorporated into a dinuclear complex and is strongly activated; the dinitrogen moiety can be formally treated as (N2)4~. The only reducing agent used in producing hydride 21 was hydrogen gas, so it is most peculiar that hydride 21 has the ability to activate dinitrogen so strongly. The starting material was a d° Ta(V) complex [NPN]TaMe3, which was reduced to the dinuclear Ta(IV) hydride 21 when reacted with hydrogen gas. The two tantalum-based electrons were then presumably present in a tantalum-tantalum bond, and this accounts for two of the electrons that are used to reduce the dinitrogen moiety in the reaction that forms 22. The additional two electrons required are obtained by the loss of H 2 gas in this reaction. The loss of H 2 upon binding of dinitrogen is not uncommon for the late transition metals; 2 9 ' 3 0 however, these reactions do not result in strongly activated dinitrogen. With the early transition metals H 2 loss to generate a dinitrogen complex has been observed with titanium, and in this example the N 2 moiety is only weakly bound, and not amenable to further reactivity.31 No example of H 2 loss to give such a highly activated dinitrogen species has been reported. This reaction could prove important in the catalytic functionalization of dinitrogen, because it generates a strongly activated dinitrogen complex without the requirement of costly strong reducing agents. 194 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Interestingly, the nitrogenase enzyme is known to produce at least one equivalent of H2 gas in the conversion of N 2 to N H 3 , although there is some debate as to whether or not this arises from a necessary step in the reaction or from a side-reaction of this highly reducing enzyme with H + . 3 0 ' 3 2 > 3 3 The formation of complex 22 may provide insight into how the polynuclear iron-sulfide core of nitrogenase strongly activates dinitrogen by the loss of hydrogen from a hydride complex, because 21 contains hydrides bridging two metal centres that are reductively eliminated as hydrogen gas when dinitrogen is bound. It has also been suggested that the electrons that reduce dinitrogen in the iron-sulfide core of nitrogenase are initially stored in metal-metal bonds, 3 4 ' 3 5 much like the tantalum-tantalum bonding electrons in hydride 21 that are transferred to dinitrogen to form complex 22. Ph Ph \ a Ph Ph Ph 21 N 2 - H 2 T [5.3] Ph Ph \ 1 Ph Ph Ph 22 195 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen 5.5.4 Bonding i n the Side-On E n d - O n Mode . The bonding of dinitrogen in the side-on dinuclear mode and end-on dinuclear mode has been described in detail in the literature. 2 8 ' 3 6 - 4 4 The orbital overlaps in these two modes were illustrated in Chapter 1, Figure 1.2. A density functional theory (DFT) calculation was performed to gain similar insight into the bonding of the N 2 moiety in the new side-on end-on mode. To simplify the calculations the model complex [(H3P)(H2N)2Ta(|j,-H)]2(^-V:Tr-N2) (22A) was studied with restrictions placed on the Ta-Ta-N-H dihedral angles. These restrictions were intended to mimic the rigid geometry imposed by the [NPN] ligand, which prevents the amido donors from freely rotating. The geometry of model complex 22A was optimized (with the exception of the previously stated constraints) with C\ symmetry using the Gaussian 98 program 4 5 and the hybrid functional B 3 L Y P method 4 6 The basis functions and effective core potentials (ECP) used were those in the L A N L 2 D Z basis set 4 7 but with additional d-polarization functions added to P atoms with the exponent of the d-functions set at 0.37. This level of theory is identical to that used in Chapter 4, and has been used previously in the study of metal-metal bonded species.48 Selected optimized bond lengths, bond angles and dihedral angles for the model complex 22A are shown in Table 5.4. The numbering scheme used is the same as for the structure of complex 22 shown in Figure 5.5, to allow for easy comparison. The general features of the optimized geometry of this model complex are in good agreement with the structure of complex 22, with the exception of the slightly longer Ta-P bonds in model 22A, and a slightly larger Ta(l)-Ta(2) distance. The bond lengths and angles of the dinitrogen moiety are well reproduced, although Ta(l), Ta(2), N(5), N(6), P(l), and P(2) are closer to occupying the same plane in the model complex, which almost gives this model complex a mirror plane of symmetry. 196 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen T a b l e 5.4. Selected bond lengths, bond angles and dihedral angles for the ab initio DFT geometry optimization of the model complex [(H 3P)(H 2N) 2Ta(p-H)] 2(p-r|^r| 2-N 2 ) (22A). Atom Atom Distance (A) Atom Atom Distance (A) N(5) N(6) 1.371 Tad) P(l) 2.820 Ta(l) N(5) 2.193 Ta(2) P(2) 2.748 Ta(l) N(6) 1.988 Ta(l) N(l) 2.020 Ta(2) N(5) 1.873 Ta(l) N(2) 2.005 Tad) Ta(2) 2.891 Ta(2) N(3) 2.008 Ta(2) N(4) 2.023 Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) Ta(2) N(5) N(6) 152.91 Tad) Ta(2) P(2) 106.91 Ta(l) N(5) Ta(2) 90.29 Ta(l) Ta(2) N(3). 126.58 Ta(l) N(6) N(5) 79.19 Tad) Ta(2) N(4) 123.31 P(l) Ta(l) N(l) 78.78 N(l) Ta(l) N(2) 109.31 P(l) Ta(l) N(2) 79.00 N(l) Ta(l) N(6) 123.18 P(l) Ta(l) N(6) 70.65 N(2) Ta(l) N(6) 110.20 P(2) Ta(2) N(5) 156.23 P(2) Ta(2) N(3) 92.14 Ta(2) Ta(l) Pd) 148.84 P(2) Ta(2) N(4) 82.61 Ta(2) Ta(l) N(l) 120.14 N(3) Ta(2) N(4) 108.08 Ta(2) Ta(l) N(2) 112.77 N(3) Ta(2) N(5) 103.50 N(4) Ta(2) N(5) 108.66 Atom Atom Atom Atom Dihedral Angle (°) Ta(l) Ta(2) N(5) N(6) 8.29 P(l) Ta(l) Ta(2) P(2) 176.18 The HOMO of model complex 22A is shown in Figure 5.6. The HOMO of 22A can be described as the ft-overlap between two metal-based <i-orbitals and a Tt-antibonding orbital of the dinitrogen moiety. The rt-overlap of the rightmost tantalum with the end-on nitrogen (corresponding to Ta(2)-N(5) in Figure 5.5) is visibly larger than the 7t-overlap of the leftmost tantalum centre with the terminal nitrogen (corresponding to Ta(l)-N(6) in Figure 5.5). Both the larger contribution of the d-orbital on the rightmost tantalum as well as its orientation relative to the dinitrogen moiety contribute to the greater interaction of this tantalum centre with this dinitrogen rc-antibonding orbital compared to the leftmost tantalum centre. 197 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen N Figure 5.6. Depiction of an isosurface of the HOMO for model complex 22A (left), and an illustration clarifying the metal-dinitrogen overlaps in the HOMO (right). The HOMO-1 is shown in Figure 5.7. Similar to the HOMO, the orbital contribution from the dinitrogen moiety is 7i-antibonding with respect to the N - N bond in the HOMO-1; however, this 7i-antibonding contribution is perpendicular to the one present in the HOMO. The dinitrogen-based 7t-antibonding orbital interacts with a metal-based d-orbital on the leftmost tantalum. Although the rightmost metal centre makes the largest 71-bonding contribution to the dinitrogen moiety in the HOMO, in the HOMO-1 there is comparatively little contribution from the rightmost metal centre. In the HOMO-1 there are also small contributions from the amido-donor lone-pair orbitals, which are negligible in the HOMO. Figure 5.7. Depiction of an isosurface of the HOMO-1 for model complex 22A (left), and an illustration clarifying the metal-dinitrogen overlaps in the HOMO-1 (right). 198 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen The approximate metal-dinitrogen a-bonding interactions are depicted in Figure 5.8. The calculated molecular orbitals of model complex 22A that contain a-bonding interactions between the metal centre and the dinitrogen moiety are complicated by contributions from other orbitals and are much lower in energy than the HOMO and H O M O - l . A depiction of the metal-based d-orbital and dinitrogen-based orbitals in the side-on a-bonding interaction is shown on the left side of Figure 5.8, and involves a contribution from the dinitrogen moiety that is 7t-bonding in nature. The right side of Figure 5.8 depicts the end-on a-bonding interaction, which occurs with the dinitrogen-based molecular orbital that is approximately nonbonding in nature. Figure 5.8. Simplified depictions of the orbital overlaps in the two a-bonding interactions between the dinitrogen moiety and the tantalum centres in 22A. As might be expected, the bonding interactions in the side-on end-on mode contain features of both the side-on and end-on bonding modes of dinitrogen. Unexpectedly absent, is a 5-symmetry interaction with the side-on bound dinitrogen moiety, which was anticipated because of the presence of this interaction in side-on bound dinitrogen complexes,2 8 as was described in Chapter 1, Section 1.4. Instead, a TC-interaction occurs from the terminal nitrogen to the leftmost metal centre. These orbital interactions account for the range of Ta-N bond lengths observed for complex 22. o 199 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen 5.6 Why Does Complex 22 Contain the Side-On End-On Bonding Mode? The previous section described the orbital interactions that occur in the side-on end-on bonding mode of dinitrogen in 22. The absence of a 8-bonding interaction between the side-on bound dinitrogen and the tantalum centre in the calculated molecular orbitals of the model complex 22A raises the question as to why the side-on end-on bonding mode of dinitrogen is preferred over the end-on dinuclear bonding mode in 22. One possible explanation is that the bridging hydride ligands force the two tantalum centres too close together to allow the end-on dinuclear bonding mode. Presumably, the energetic advantage of maintaining bridging hydride ligands outweighs the energy difference between the end-on dinuclear bonding mode and the side-on end-on binding mode. Two complexes are described below that provide evidence that the bridging hydrides ligands are the cause of the side-on end-on bonding mode observed in complex 22. 5.6.1 Reaction with Propene The reaction of 22 with alkenes was performed in an attempt to observe an insertion reaction with the tantalum-bound dinitrogen moiety. The reaction of 22 with propene occurs over a period of two weeks to produce a crystalline red product. A similar reaction with ethylene was observed; however, several side-products were also produced, and the initial product could not be isolated. The product of the reaction of 22 with propene was not derived from an insertion reaction into the tantalum-dinitrogen bonds as originally desired. Instead, the bridging hydrides reacted with propene to yield ([NPN]Ta(CH2CH2CH3))2(|i-r| l:ri1-N2) (23). This reaction is illustrated in equation 5.4. 200 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen P h Ph H H, P^—, ^ C -Ph \ \ \ \ H P . ^ S i ^ N ^ . / ^ \ 2 H 3 C ^ C H 2 Me2Si | Ta .Ta | \ \ n v i [5.4] SiMe2 iMe2 P h P h P R CH 3 2 2 / C H 2 P f h N ^ M f 2 Ph H 2C V N ^ N A v / > / N ^ T a = N — N = T a ^ - P M e 2 S i / f | \ h Me2Si^ P v H 2 C. V \ h " C H 2 CH 3 2 3 The 3 1 P { 1 H } N M R spectrum of 2 3 contains two resonances, at 8 1 4 . 2 and 8 19 .7 that, in contrast to complex 2 2 , are not coupled. To confirm the bonding mode of dinitrogen, the l 5 N 2 labeled species ( [NPN]Ta(CH 2 CH 2 CH 3 ) ) 2 (p - r i l : r i 1 - 1 5 N2) was prepared from the reaction of 22-15N2 with propene. In the 3 I P{ * H } N M R spectrum of 23-15N2, the resonance at 8 19 .7 is a singlet, whereas the resonance at 8 14 .2 is coupled to two different l 5 N 2 environments, with coupling constants of 3 0 . 5 and 6 .6 Hz. The l 5 N N M R spectrum of 2 3 - / 5 / V 2 contains two doublets of doublets at 8 16 .2 and 2 8 . 0 . The resonance at 8 16 .2 is coupled to a single 3 I P 2 15 1 " environment, with a 7PN value of 3 0 . 5 Hz, as well as to "N with a J N N value 11 .3 Hz; the resonance at 8 2 8 . 0 also exhibits the identical one-bond coupling to l 5 N ( ' / N N = 11 .3 Hz), as well as a two-bond coupling to 3 I P ( 2 7 P P = 6.6 Hz). The ' / N N value of 11.3 Hz is much smaller than the analogous 2 1 . 5 Hz coupling observed for 2 2 . The ' H and L 3 C { ' H } N M R spectra are consistent with two ligand and two propyl environments and were assigned with the assistance of a ' H / H - C O S Y spectrum and a ' H , 1 3 C - H S Q C spectrum. The solution structure of 2 3 contains only a mirror plane of symmetry. The low symmetry of 2 3 was unexpected because both metal centres are coordinated to identical ligands. The bonding of the dinitrogen moiety in this species was investigated by X-ray crystallography. 2 0 1 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen The solid-state molecular structure of complex 23 as determined by X-ray crystallography is shown in Figure 5.9, along with selected bond lengths in Table 5.5. As in solution, the solid-state-structure of 23 contains two chemically unique tantalum centres, despite the fact that both tantalum centres are coordinated to a [NPN] ligand, a propyl ligand, and an end-on bound dinitrogen unit. The stereochemistry at Ta(l) is best described as trigonal bipyramidal, where the P(l) and N(5) atoms occupy the axial positions, and N(2), N(l) and C(25) occupy the equatorial positions. The stereochemistry at Ta(2) is best described as square-based pyramidal, where the square base is defined by P(2), N(3), C(52) and N(4), and is capped by N(6). The N(5)-Ta(l)-P(l) angle is 175.1(3)°, and it is presumably this phosphorus, approximately trans disposed to the dinitrogen moiety, that couples to both the N(5) and N(6) nuclei in the l 5 N and 3 I P{'H} spectra of 23-15N2. In contrast, the N(6)-Ta(2)-P(2) angle is 109.3(1) A. The Ta(l)-P(l) distance of 2.734(1) A is o significantly longer than the Ta(2)-P(2) distance of 2.640(1) A , probably due to the strong trans influence of the dinitrogen moiety for the former. Complex 23 provides an opportunity to compare bond distances in the end-on dinuclear bonding mode of dinitrogen versus the side-on end-on mode for electronically similar complexes. The N(5)-N(6) bond length of 1.289(6) A is not significantly shorter than o the N - N bond length in 22 of 1.319(6) A , which indicates the side-on end-on mode is activates dinitrogen to the same degree as the end-on dinuclear mode. On the other hand, the Ta(l)-N(5) and Ta(2)-N(6) distances in 23 are 1.818(4) A and 1.815(4) A respectively, which are significantly shorter than the end-on Ta(2)-N(5) distance of 1.888(5) A in 22, and much shorter than the side-on Ta(l)-N(5) and Ta(l)-N(6) interactions. 202 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Figure 5.9. ORTEP depiction of the solid-state molecular structure of ([NPN]Ta(CH2CH2CH3))2(Ll-ri l:ri1-N2) (23) as determined by X-ray crystallography. The silyl methyl groups are omitted for clarity, and only the ipso carbons of the PPh and NPh groups are shown. Table 5.5. Selected bond lengths, bond angles and dihedral angles for ([NPN]Ta(CH 2CH 2CH 3)) 2(|l-Ti 1 :T| 1 -N 2) (23). Atom Atom 0 Distance (A) Atom Atom Distance (A) N(5) N(6) 1.289(6) Ta(l) N(2) 2.061(4) Ta(l) N(5) 1.818(4) Ta(2) N(3) 2.092(4) Ta(2) N(6) 1.815(4) Ta(2) N(4) 2.061(4) Ta(l) P(l) 2.734(1) Ta(l) C(25) 2.194(5) Ta(2) P(2) 2.640(1) Ta(2) C(52) 2.193(5) Ta(l) N(l) 2.057(4) 203 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle O Ta(l) N(5) N(6) 174.6(3) Ta(2) N(6) N(5) 171.8(4) N(5) Ta(l) P(l) 173.5(1) N(6) Ta(2) P(2) 109.3(1) N(5) Ta(l) N(l) 98.2(2) N(6) Ta(2) N(3) 108.3(2) N(5) Ta(l) N(2) 101.5(2) N(6) Ta(2) N(4) 107.3(2) N(5) Ta(l) C(25) 98.4(2) N(6) Ta(2) C(52) 108.4(2) P(l) Ta(l) N(l) 78.2(1) P(2) Ta(2) N(3) 77.4(1) P(l) Ta(l) N(2) 77.3(1) P(2) Ta(2) N(4) 74.6(1) P(l) Ta(l) C(25) 88.0(1) P(2) Ta(2) C(52) 142.3(1) N(l) Ta(l) N(2) 130.9(2) N(3) Ta(2) N(4) 140.2(2) N(l) Ta(l) C(25) 111.5(2) N(3) Ta(2) C(52) 91.7(2) N(2) Ta(l) C(25) 109.5(2) N(4) Ta(2) C(52) 93.5(2) Ta(l) C(25) C(26) 119.8(4) Ta(2) C(52) C(53) 108.4(4) Atom Atom Atom Atom Dihedral Angle (°) C(52) Ta(2) Ta(l) C(25) -119.5(4) There is no difference in stereochemistry at the tantalum centres in the solid-state and solution structures of 23, which is evidence that these five-coordinate geometries are not fluxional. The orientation of the [NPN] ligands with respect to the bridging hydrides in 22 is maintained as the orientation of the [NPN] ligands with respect to the propyl ligands in 23. In Figure 5.9, P(l) is cis disposed to the propyl group, and trans disposed relative to the dinitrogen moiety, so this corresponds to P(2) of complex 22 in Figure 5.5, which is cis disposed to the bridging hydride ligands and trans disposed to the dinitrogen moiety. Similarly, in Figure 5.9 P(2) is trans disposed to the propyl group, and cis disposed relative to the dinitrogen moiety, and this corresponds to P(l) in Figure 5.5, which is trans disposed to the bridging hydride ligands and cis disposed relative to the dinitrogen moiety. Asymmetrical end-on dinuclear dinitrogen compounds of this nature might not be accessible by other routes, because the asymmetry in 23 is a direct result of the asymmetry in 22. More importantly, this compound demonstrates that the bridging hydrides in complex 22 are necessary for maintaining the side-on end-on dinitrogen bonding mode. 204 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen 5.6.2 Synthesis of ([NPN]NbCl)2(uJ-Ti1:r|1-N2) This thesis has so far focused on the chemistry of tantalum; however, the chemistry of both vanadium and niobium with the [NPN] ligand is also of interest. The reaction of [NPN]Li r (THF) 2 with VC1 3(THF) 3 generates the paramagnetic complex ([NPN]V(u-Cl)) 2. Attempts to reduce this vanadium compound have so far failed to yield a complex that incorporates N 2 . However, the product of the reaction of [NPN]Li 2 (THF) 2 with NbCl 3 (DME) is the diamagnetic dinitrogen complex ([NPN]Nb)2(u,-r) l:ri1-N2) (24), as shown in equation 5.5. N 2 2 [NPN]Li2(THF)2 + 2 NbCI3(DME) - > t o l u e n e 19 [5.5] 24 The *H, 1 3 C { ' H } and 3 l P{'H} N M R spectra of 24 are consistent with a species of 31 1 high symmetry. A single resonance is observed in the P{ H J N M R spectrum, and only two ligand silyl methyl environments are observed in the ' H N M R spectrum, which is unlike both species 22 and species 23. The elemental analysis is consistent with a dinitrogen complex. The solid-state molecular structure of 24 was determined by X-ray crystallography, and an ORTEP depiction is shown in Figure 5.10. Bond lengths and angles are given in Table 5.6. In the solid-state structure, the two halves of the molecule are related by an inversion centre that lies in the middle of the N - N bond. Unlike complex 23, which is unsymmetrical, the geometry at both Nb centres in 24 is best described as trigonal bipyramidal, with the P(l) phosphine donor and the N(3) atom of the dinitrogen moiety in 205 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen axial positions and the N(l) , N(2) and Cl(l) atoms in equatorial positions. The N(3)-N(3) bond distance of 1.237(4) A is slightly shorter than that observed for the dinitrogen moiety in 23 (1.289(6) A) . Although chloride ligands can behave as bridging ligands, as is observed in the structure of ([NPN]V(|i-Cl))2, they are terminal in 24; bridging chlorides would require the bonding mode of dinitrogen to change in 24. The difference in dinitrogen bonding mode between 22 and 24 indicates that not any ligand capable of bridging the metal centres will favour the side-on end-on mode; the side-on end-on bonding mode is presumably less thermodynamically favourable than the end-on dinuclear bonding mode of dinitrogen. Clearly the possible thermodynamic advantage of bridging versus terminal chloride ligands is not sufficient to overcome the thermodynamic disadvantage of the side-on end-on binding mode versus the end-on dinuclear binding mode of dinitrogen, and therefore the end-on dinuclear mode is observed in complex 24. This example confirms the necessity of a more thermodynamically favoured bridging ligand to induce the side-on end-on mode. The hydride ligands perform this role in complex 22. 206 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Figure 5.10. ORTEP depiction of the solid-state molecular structure of ([NPN]NbCl) 2(p-r | 1 : r | l- N2) (24) as determined by X-ray crystallography. Table 5.6. Selected bond lengths and bond angles for ([NPN]NbCl)2(p-r|1:ri l-N2) (24). Atom Atom Distance (A) Atom Atom Distance (A) N(3) N(3) 1.237(4) Nb(l) N(l) 2.023(2) Nb(l) N(3) 1.843(2) Nb(l) N(2) 2.053(2) Nb(l) P(l) 2.7189(8) Nb(l) Cl(l) 2.3922(9) 207 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) Nb(l) P(l) Cl( l ) N(l) N(2) P(D N(3) Nb(l) Nb(l) Nb(l) Nb(l) Nb(l) =F N(3) N(3) N(3) N(3) N(3) N(l) 176.4(2) 171.57(8) 99.03(8) 95.70(9) 99.44(9) 76.16(7) P(l) Cl(l) Cl(l) Cl(l) N(l) Nb(l) Nb(l) Nb(l) Nb(l) Nb(l) N(2) P(D N(l) N(2) N(2) 81.83(7) 87.07(3) 119.31(7) 123.18(7) 111.59(10) 5.7 Reactivity of ([NPN]Ta(|i-H))2(|i-r|1:r|2-N2) (22) The side-on end-on bonding mode of dinitrogen in complex 22 opens up new possibilities for dinitrogen-based reactivity. Lewis structure A in Figure 5.11 is drawn with a negative formal charge on the terminal nitrogen, and a positive formal charge on the end-on nitrogen. Therefore, the terminal nitrogen should be expected to react with electrophiles. Of course, this Lewis structure is only one possible representation of the bonding in 22, which fits reasonably well with the bond lengths observed in the solid-state structure of 22. The molecular orbital calculations on model complex 22A indicate that the terminal nitrogen-tantalum bond also has some double bond character, which is represented by a dashed bond in structure B in Figure 5.11. Structure B is also drawn with an occupied lone-pair orbital on the terminal nitrogen. Regardless of the degree of Tt-donation from the terminal nitrogen to the tantalum centre, there will always be an electron pair localized on the terminal nitrogen that is incapable of bonding to the tantalum centre due to symmetry restrictions. This pair of electrons is therefore nonbonding. This is promising in terms of reactivity, because, as speculated in the Chapter 1, the presence of a nonbonding pair of electrons should allow for greater reactivity at the dinitrogen moiety, as was observed in the side-on end-on dinitrogen complex ([P 2N 2]Zr) 2(ii-ri 2:ri 2-N2). 4 9 208 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen Ph Ph \ Me2Si-^.N Me2Si Ph 3, Ph T a 1 SiMep N^SiMe; N" '© N'T © |ph Ph Ph Ph \ Me 2ST^N v Me2Si Ph \ P •6 SiMe2 N^-SiMe2 N T | Ph Ph B Figure 5.11. Two simplified depictions of the bonding in 22. At this point, it is worth mentioning a related polynuclear dinitrogen complex that was reported previously in the literature,50 and characterized by X-ray crystallography. This complex contains four titanium centres and is shown in Figure 5.12 as A . Due to a cocrystallized molecule ((r | 5 -C 5 H5)2Ti(MeOCH2CH 2 OMe) or {(if-C 5 H 5 )2Ti(MeOCH 2 CH20Me)} + ) in the crystal lattice, and the lack of many other methods of characterization, the charge of this complex is ambiguous; it may be uninegative. The bonding in the complex can be simplified by considering it an adduct of the hypothetical complex shown in Figure 5.12 as B . 5 0 This hypothetical fragment contains dinitrogen bound in the side-on end-on mode. The common feature between complex 22 and the titanium complex shown in Figure 5.12 is that both contain a bridging ligand, which controls the distance between the two metals. In the case of complex 22, the hydride ligands bridge the tantalum centres, while in the titanium example, the linked fulvalene ligand holds the titanium centres in close proximity. As noted in the previous section, this restriction likely impedes the formation of the more frequently observed end-on binuclear binding mode. Of more importance to the potential reactivity of 22, the description of the polynuclear titanium complex A shown in Figure 5.12 as an adduct of fragment B supports the proposal that complex 22 will react with electrophiles at the terminal nitrogen of the side-on end-on dinitrogen moiety. 209 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen A Figure 5.12. A depiction of the polynuclear titanium dinitrogen complex reported by Pez et al. is labeled as A; the charge on this fragment is unknown and may be neutral or uninegative. Removal of a titanium fragment known to form a THF adduct produces the hypothetical fragment shown as B, in which the dinitrogen bonding mode resembles the bonding in complex 22.50 5.7.1 Reaction of ([NPNlTaH^Qi-V^-Ni) (22) with Lewis Acids i) Reaction with B(C 6F 5)3 To demonstrate that the terminal nitrogen of the dinitrogen moiety in complex 22 bears a non-bonding pair of electrons, the Lewis acid tris(pentafluorophenyl)borane51 was added to a benzene solution of 22. A colour change was instantly observed, to yield a darker 31 1 brown solution. By monitoring the reaction by ' P{ H} NMR, it was evident that the reaction occurred immediately at room temperature. Over the course of 48 hours, ([NPN]Ta(u-H)) 2N 2B(C 6F 5) 3 (25) crystallized from solution (equation 5.6). 210 References begin on page 254. Chapter 5: A New Bonding Mode for Dinitrogen |-SiMe2 v "'N^SiMe2 B(C 6F 5) 3 [5.6] 22 |-SiMe2 £L^SiMe2 25 The solid-state molecular structure of 25 was determined by X-ray crystallography, and is shown in Figure 5.13; relevant bond distances and angles are given in Table 5.7. The solid-state structure demonstrates that the bulky B(C6Fs)3 molecule is coordinated to the terminal dinitrogen moiety. Only a few donor adducts of B(CeF 5)3 have been structurally characteriz