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Synthesis, characterization, and properties of some molybdenum diene complexes and related compounds Christensen, Nancy J. 1989

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SYNTHESIS, CHARACTERIZATION, AND PROPERTIES OF SOME M O L Y B D E N U M DIENE COMPLEXES AND R E L A T E D COMPOUNDS by NANCY J. CHRISTENSEN B. S., University of Wisconsin at Stevens Point A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR T H E D E G R E E OF DOCTOR OF PHILOSOPHY in T H E FACULTY OF G R A D U A T E STUDIES (Department of Chemistry) We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH COLUMBIA September 1989 ©Nancy Jean Christensen, 1989 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 DE-6 (2/88) ii Abstract Reduction of [Cp ,Mo(NO)I2]2 (Cp* = r / 5 - C 5 H 5 (Cp) orr>5-C5Me5 (Cp*)) by sodium amalgam in T H F at -20°C in the presence of acyclic, conjugated dienes affords novel Cp'Mo(NO)(r/4-.y-/raAi5-diene) complexes in isolated yields of -10 - 60%. When the diene is 2,3-dimethylbutadiene, CpMo(NO)(rj4-cw-2,3-dimethylbutadiene) product complexes are also isolable in addition to the trans-diene isomers. However, these cis-diene compounds are the kinetic products and convert in solutions irreversibly to the isomeric trans-diene complexes. A prelirriinary kinetic study of this isomerization indicates that the isomerization is first order. Reactivity studies of the -trans-diene complexes show that they react reluctantly with Lewis bases such as carbon monoxide and phosphines to form the r?2-diene complexes, CpMo(NO)(L)(r?2-diene) (L = CO, PMePh2, PMe3). The ^-trans-diene complexes react with alkynes and acetone in ligand coupling reactions to form products in which the diene ligands have coupled with the unsaturated organic molecules. Three of these complexes have been structurally characterized using X-ray crystallography, those being CpMo(NO)[a,r; J-e/ido-C(Me)2-CH-CH-C(Me)2C(Me)C(Ph)], CpMo(NO)[<r, r,3-exo-C(Me)2-CH-CH-C(Me)2C(Me)26], and Cp*Mo(NO)[cr,n3-eAuio-CH2-CH-CH-CH2C(Me)20]. These complexes and some of their analogues have also been characterized extensively using * H and ^ C NMR spectroscopy. The product of the reaction between CpMo(NO)(r?4-fra«5-2,5-dimethyI-2,4-hexadiene) and acetone, CpMo(NO)[a,n3-ca:o-C(Me)2-CH-CH-C(Me)2C(Me)26], reacts further with acetone to form the trimer, [CpMo(NO)]3(/i2: n 2 : n 1 -OMe 2 ) 3 which has also been characterized by X-ray crystallography. In a preliminary study of the reactivity of the r;4-frans-diene complexes with electrophiles, it has been found that they react with protonic acids (HX: X = I, 0 3 S C 6 H 4 C H 3 , 0 2 C C F 3 ) to form CpMo(NO)(r/3-allyl)X complexes in which the proton iii has added to the diene ligand and the anion of the acid has coordinated to the metal center. The diiodo dimers [CpM(NO)I2]2 (M = Mo, W) react with 10 equivalents of PMe 3 to produce the new electron-rich nitrosyl complexes M(NO)I(PMe3)4, the other product being [(C5H5)PMe3]I. It has been shown that the formation of the tetrakis(phosphine) complexes proceeds sequentially via the isolable intermediate species (r/ 5-C 5H 5)M(NO)I 2(PMe 3) and [(rj5-C5H5)M(NO)I(PMe3)2]I. Treatment ofthe diiodo dimers with 4 equivalents of PMe 3 in the presence of sodium amalgam affords excellent yields ofthe related complexes (r?5-C5H5)M(NO)(PMe3)2. IV TABLE OF CONTENTS Abstract ii List of Figures viii List of Tables xi List of Schemes xiii List of Abbreviations xiv Acknowledgements xvi Chapter 1 General Introduction 1 References 25 Chapter 2 Novel Physical and Chemical Properties of Acyclic, Conjugated Dienes Coordinated to Cp'Mo(NO) [Cp' = r>5-C5H5 (Cp) or q 5 -C 5 Me 5 (Cp*)]. 30 Introduction 31 Experimental Section 33 NMR Experiments 33 Preparation of Cp'Mo(NO)(»j4-fran5-diene) [Cp' = »7 5 -C 5 H 5 (Cp) orr, 5-C 5Me 5 (Cp*); diene = acyclic, conjugated diene] 34 Reduction of [Cp*Mo(NO)I2]2 in the Presence of 1,3-Pentadiene 36 Preparation of Cp'Mo(NO)(»/4-cw-2,3-dimethylbutadiene) 37 Reaction of [CpMo(NO)I2]2 with 2/3 Equivalents of Na in the Presence of 2,3-Dimethylbutadiene 38 Electrochemical Studies of CpMo(NO)(r> 4-fro/is-2,5-dimethyl-2,4-hexadiene) and CpMo(NO)(r/4-cw-2,3-dimethylbutadiene) in CH 2 C1 2 38 Results and Discussion 40 Cp'Mo(NO)(r;4-frara-diene) Complexes, 1-9 40 Preparation of Cp*Mo(NO)(»j4-/rflra-l,3-pentadiene) Alternate Synthetic Approaches 54 Cp'Mo(NO)(r/4-cw-2,3-dimethylbutadiene) Complexes 56 The -^H and W C NMR Spectroscopic Properties of the rj4-Diene-containing Complexes, 1-11 59 Electrochemical Studies of CpMo(NC% 4-diene) Complexes 65 References 67 Chapter 3 A Preliminary Kinetic Study of the Isomerization of CpMo(NO)(r?4-c£s-2,3-dimethylbutadiene) to CpMo(NO) (r> 4-fra/w-2,3-dimethylbutadiene). 69 Introduction 70 Experimental Section 74 Kinetic Study 85 Results 85 Discussion 87 References 93 Chapter 4 Reactivity Studies of the Cp'Mo(NO)(r?4-fra/w-diene) Complexes. 94 Introduction 95 Experimental Section 96 Reaction of CpMo(NO)(»?4-frfln5-2,5-dimethyl-2,4-hexadiene) with CO 96 Reaction of CpMo(NO)(r;4-rrawj-2,5-dimethyl-2,4-hexadiene) with PMePh2 and PMe 3 97 vi Reactions of CpMo(NO)(r;4-fr-flw-2,5-diniethyl-2,4-hexadiene) with 1-Phenylpropyne and 1,7-Octadiyne 98 Reaction of CpMo(NO)(»?4-/ran^-2,5-dimethyl-2,4-hexadiene) with Acetone 100 Reactions of Cp'Mo(NO)(r;4-fra«5-butadiene) Complexes with Acetone 102 Reaction of Complex 8B with Acetone 102 Reaction of Complex 8B with l,3-bis(diphenylphosphino)propane 102 Reaction of CpMo(NO)(»?4-fram-2,5-dimethyl-2,4-hexadiene) with HX, X = I, 0 2 C C F 3 , 0 3S-/>-C 6H 4CH 3 103 Results and Discussion 104 Reactions of CpMo(NO)(rj4-fram-2,5-dimethyl-2,4-hexadiene) with Carbon Monoxide and Phosphines 104 Reactions of CpMo(NO)(r74-fra«j'-2,5-dimethyl-2,4-hexadiene) with Alkynes .106 Reactions ofthe Cp'Mo(NO)(r?4-fra>w-diene) Complexes with Acetone 120 A Preliminary Investigation into the Reactivity of some CpMo(NO)(r)4-{ra/is-diene) Complexes with Electrophiles: Reactions of CpMo(NO)(r?4-fra/w-diene) with Acids, H X 155 References 161 vii Chapter 5 Reduction of [(r;5-C5H5)M(NO)I2]2 by PMe^ Synthesis and Physical Properties of the Novel Complexes M(NO)(PMe3)4I (M = Mo,W). 163 Introduction 164 Experimental Section 165 Synthesis of (r? 5-C 5H 5)M(NO)I 2(PMe 3) (M = Mo or W) 165 Synthesis of [(IJ 5-C5H5)M(NO)I(PMe3)2]I (M = Mo or W) 165 Synthesis of M(NO)(PMe3)4I (M = Mo or W) 166 Results and Discussion 170 References 175 Epilogue 177 viii List of Figures Figure 1.1. Ground-State Energy Profile for Butadiene 5 Figure 2.1. Solid-State Molecular Structure of 7A 50 Figure 2.2. 300 MHz *H NMR Spectra of 7A (a) and 7B (b) in CDC13 53 Figure 2.3. 300 MHz J H NMR Spectrum of 8 in C 6 D 6 61 Figure 2.4. 300 MHz X H NMR Spectrum of 10 in C 6 D 6 62 Figure 2.5. Cyclic Voltammograms of 3 (a) and 11 (b) in C H 2 Q 2 66 Figure 3.1. Plots of Concentration of CpMo(NO)(rj4-rra/M-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances. (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 8°C 75 Figure 3.2. Plots of Concentration of CpMo(NO)(r>4-/ra/u-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances. (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 8°C 76 Figure 3.3. Plots of Concentration of CpMo(NO)(rj4-rron$-2,3-dimethylbutadiene)versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances. (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 20°C 77 Figure 3.4. Plots of Concentration of Q)Mo(NO)(r/4-fraro-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances. ix (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 20°C 78 Figure 3.5. Plots of the Natural Logarithm (ln) of the Concentration of of CpMo(NO)(»?4-fram-2,3-cIimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 8°C Figure 3.6. Plots of the Natural Logarithm (ln) of the Concentration of CpMo(NO)(r/4-?ra/w-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 8°C 80 Figure 3.7. Plots of the Natural Logarithm (ln) of the Concentration of CpMo(NO)(>74-fra/w-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 20°C 81 Figure 3.8. Plots of the Natural Logarithm (ln) of the Concentration of CpMo(NO)(r?4-fra/ts-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 20°C 82 Figure 4.1. 300 MHz J H NMR Spectrum of 4 in C 6 D 6 107 Figure 4.2. 75 MHz B C NMR Spectrum of 4 in C 6 D 6 108 Figure 4.3. 2D-HETCOR NMR Spectrum of 4 111 Figure 4.4. 300 MHz SINEPT NMR Spectrum of 4: Irradiation at 5.41 ppm 112 Figure 4.5. 300 MHz SINEPT NMR Spectrum of 4: Irradiation at 4.62 ppm 113 X Figure 4.6. 300 MHz SINEPT NMR Spectrum of 4 : Irradiation at 1.87 ppm 115 Figure 4.7. Solid-State Molecular Structure of 4 117 Figure 4.8. 300 MHz X H NMR Spectra of 6A (a) and 6B (b) in C 6 D 6 127 Figure 4.9. (a - f). 300 MHz ^ ^ H } NMR Spectra of 6A 128 Figure 4.10. 300 MHz 2D-COSY Spectrum of 6A 130 Figure 4.11. 300 MHz 2D-COSY Spectrum of 6B 132 Figure 4.12. 75 MHz B C {/H} (a) and Gated Decoupled B C (b) NMR Spectrum of 6A 133 Figure 4.13. 75 MHz B C {*H} (a) and Gated Decoupled D C (b) NMR Spectrum of 6B 134 Figure 4.14. 2D-HETCOR Spectrum of 6A 136 Figure 4.15. Solid-State Molecular Structure of 7 137 Figure 4.16. 300 MHz *H NMR Spectra of 8A and 8B in C 6 D 6 141 Figure 4.17. 75 MHz nC{xYi} (a) and Gated Decoupled B C (b) NMR Spectra of 8B 143 Figure 4.18. Solid-State Molecular Structure of 8B 144 Figure 4.19. Molecular Structure of 9 151 Figure 4.20. Labelling Scheme for the Allyl Complexes, 10 -13 159 xi List of Tables Table 2.1. Elemental Analyses, Mass Spectral and Infrared Data for the r/4-Diene Complexes, 1-10 43 Table 2.2. Number Scheme for the Complexes, 1-11 44 Table 2.3. XH NMR Chemical Shifts for the Complexes, 1-10 45 Table 2.4. 1 H NMR Coupling Constants for the Complexes, 1-10 46 Table 2.5. B C NMR Chemical Shifts for the Complexes, 1 -10 47 Table 2.6. B C Coupling Constants for the Complexes, 1-10 48 Table 2.7. Selected Bond Lengths (A) and Angles (deg) in the Molecular Structure of 7A 51 Table 3.1. Kinetic Data for the Zero-Order Isomerizations from the Graphs in Figures 3.1 - 3.4 83 Table 3.2. Kinetic Data for the First-Order Isomerizations from the Graphs in Figures 3.5 - 3.8 84 Table 4.1 Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 4 118 Table 4.2. Elemental Analyses, Mass Spectral and Infrared Data for the Acetone Coupled Products, 6 -8 122 Table 4.3. J H NMR Chemical Shifts for the Complexes, 6 - 8 123 Table 4.4. B C NMR Chemical Shifts for the Complexes, 6 - 8 124 Table 4.5. B C NMR Coupling Constants for the Complexes, 6 - 8 125 Table 4.6. Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 7 138 Table 4.7. Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 8B 145 xii Table 4.8. Selected Bond Lengths (A) and Angles (deg) in the Molecular Structure of 9 152 Table 4.9. Elemental Analyses, Mass Spectral and Infrared Data for the »?3-Allyl Complexes, 10 -13. 156 Table 4.10. *H NMR Chemical Shifts for the Allyl Complexes, 10 -13 157 Table 5.1. Analytical, IR and Mass Spectral Data for the Phosphine Complexes 168 Table 5.2. ^ NMR Data for the Phosphine Complexes 169 List of Schemes Scheme 1.1 7 Scheme 1.2 11 Scheme 3.1 88 Scheme 3.2 89 xiv List of Abbreviations (C 5 H 4 N) 2 , bipyridine C H 3 C H 2 C H 2 C H 2 , butyl calculated benzene-d6 chloroform-^ dichloromethane-<22 acetone-J6 wavenumbers cyclooctadiene r , 5 - C 5 H 5 r/ 5 -C 5 Me 5 Cp or Cp* cyclic voltammogram carbon-13 proton-decoupled carbon-13 ( C 6 H l l ) 2 P C H 2 C H 2 P ( C 6 H l l ) 2 ' l,2-bis(dicyclohexylphosphino)ethane degrees E^PCr^CrL^PEt^ l,2-bis(diethylphosphino)ethane Ph 2 PCH 2 CH 2 PPh 2 , l,2-bis(diphenylphosphino)ethane 'pr 2 PCH 2 CH 2 CH 2 P , pr 2 , l,3-bis(diisopropylphosphino)propane Me 2 PCH 2 CH 2 PMe 2 , bis(diraethylphosphino)ethane Ph 2 PCH 2 CH 2 CH 2 PPh 2 , l,3-bis(diphenylphosphino)propane energy of activation C H 3 C H 2 , ethyl - (CH 3CH2) 20, diethyl ether eV - electron volts (1 eV = 1.60 x IO - 1 9 J) A E - energy difference H O M O - highest occupied molecular orbital *H - proton IR infrared / coupling constant (in the NMR spectrum) kcal - kilocalories (1 kcal = 4184 J) L U M O - lowest unoccupied molecular orbital Me - C H 3 , methyl mmol - millimole M O molecular orbital mol mole m/z mass-to-charge ratio in the mass spectrum NMR nuclear magnetic resonance P + molecular ion (in the mass spectrum) Pcy3 - P(C 6 H 1 1 ) 3 > tricyclohexylphosphine Ph - C 6 H 5 , phenyl PMe 3 - P(CH 3) 3, trimethylphosphine PMePh 2 - P(CH 3)(C 6H 5) 2 , methyldiphenylphosphine P(OMe) 3 - P(OCH3)3,trimethylphosphite PPh3 - P(C 6 H 5 ) 3 , triphenylphosphine Resoln - resolution SCE - saturated calomel electrode T H F - tetrahydrofuran UV-vis - ultraviolet-visible V - volts xvi Acknowledgements I would like to express my gratitude to Professor Peter Legzdins whose friendship and support, both in and out of the lab, made the completion of this work possible. His high standards and committment to the group helped me to achieve all that I have. I also want to thank the members of my research group, both past and present, for all their help and humor. Special thanks go to Allen, George, Neil and, T C for their humor and friendship. I am sure that the atmosphere in a lab makes or breaks the experience, and for me, you have all helped to make it. I'd like to thank all the people in the Department who have assisted me. In particular, I thank Steve Rak, Peter Borda, Bev Gray, Marietta Austria and the Mechanics and Electronics Shop guys. Last, but certainly not least, I thank Everett. As my nearly constant companion over the last five years, I owe everything to him. IVe always said that if we can make it through this, we can make it through anything. I guess we're ready for anything! To my parents, Dave and Lois Christensen. Thanks for everything. The following sentence is true. The preceding sentence is false. 1 CHAPTER 1 A General Introduction to Transition-Metal Diene Complexes 2 Diene complexes are known for almost every transition metal and have been the subjects of extensive investigations by several research groups around the world. The result has been the development of several systems which have proven to have applications to both organic synthesis and the study of homo- and heterogeneous catalytic systems. Dienes may bind to transition-metal centers in a number of different fashions depending on the electronic and steric requirements of the transition metal. These differences are important in determining the reactivity of the diene ligand. Until recently, the predominant stereochemistry reported for the coordination of a 1,3-diene in an r?4-fashion to a single metal center was found to be limited to the s-cis form,1 i.e., wherein the two double bonds of the diene are cis to each other across the formal single bond between them, and the diene is classically *• -bonded to the metal center in what is referred to as the bonding mode.2 These T,T diene complexes are characterized by the short-long-short carbon-carbon bond length alternation in the diene ligand as exhibited by the solid-state molecular structures. Also characteristic of this type of linkage are equivalent metal-carbon distances between the metal and the four coordinated carbons of the diene ligand. Particularly useful characterization techniques are and B C NMR spectroscopy, from which the proton-proton coupling constants CH-H) a n c * carbon-proton coupling constants (^C-H ) m a y D e measured and used to get an indication of the hybridization of the diene carbons (ideally sp2 for the JT, T bonding form). M 3 Recently, a different type of n4-s-cis-diene coordination has been reported, namely one resulting in a metallacyclopentene, also referred to as the a2, T mode, i.e. This results from increased back-bonding into the diene * 3 * molecular orbital which imparts more metallacyclopentene character to the metal-diene linkage. Several examples of this type of diene coordination have been reported, and a common denominator has been that the metal involved has been an early transition metal or a member of the actinide series. The a2, TT diene complexes are characterized by long-short-long bond length alternation among the diene carbons. The compounds of this type which have been structurally characterized exhibit structures in which the terminal carbons of the diene ligand are closer to the metal atom than the inner carbons. In addition, J H and 1 3 C NMR spectral data available for representative examples of this class of compounds show the terminal carbons to be substantially sp3-hybridized, while the inner carbons are still sp2-hybridized. The bonding in diene complexes may only be accurately described by delocalized molecular orbital methods. The results of such calculations3 indicate that the structure of the metal-diene linkage is primarily determined by the degree of back-bonding from the filled metal centered <i-orbitals to the diene * 3 * antibonding orbital. In valence bond terms, this is represented as a contribution from the two resonance forms shown below, with more back-bonding increasing the contribution from the a , *• form. For reasons of M 4 descriptive convenience, s-as-diene complexes are often described as being either * or a2, 7r; however, these labels are merely descriptions of the predominant bonding interactions and should not be interpreted too literally. At approximately the same time that the a 2, ir coordination mode of a 1,3-diene was reported, there were reports of an even more interesting mode of 1,3-diene coordination to a single metal center, namely as an rj4-s-fra«s-diene ligand, i.e., in which the double bonds of the 1,3-diene are oriented trans to each other across the single bond between them. Examples of s-trans-diene coordination had previously been seen only in dimeric complexes where each double bond of the diene was coordinated to one of the metal centers, i.e. Characterization of these trans-diene complexes is analogous to that described above for the s-cis TC, TC and a2, TC diene complexes, the short-long-short and long-short-long carbon-carbon bond length alternation being indicative of the type of metal-diene bonding extant in the examples of this class. Metal diene complexes exhibiting each type of coordination discussed above, as well as some other types of metal-diene complexes which appear to be "one-of-a-kind" and are therefore difficult to categorize, will be discussed below. First though, it will be helpful to consider the conformations of the 1,3-dienes before they are complexed to the metal M 5 centers, looking first at the simplest (i.e., 1,3-butadiene) and then moving to substituted 1,3-dienes. Acyclic molecules which contain two (or more) conjugated double bonds display a marked electronic preference for planar geometries. They may exist in either of the s<is or the s-trans conformations, i.e. It has long been known that butadiene exists as an equilibrium mixture of the two conformers. 4 Numerous experiments 5 have been conducted to determine the energy difference (A E ) between the two conformers. The results vary depending on the method used, but the estimates fall in the range of 2.5 - 3.1 kcal mol" 1 with the 5-fra/w-butadiene being the conformer of lower energy. In addition, the energy of activation ( E a ) for the thermal conversion of 5-cis-butadiene to s-trans butadiene has been found experimentally to fall in the range 3.9 - 4.7 kcal mol' 1. 4 b Force field calculations by T a i and A l l i n g e r 4 a predict E a to be 4.7 kcal mol" 1. These data may be used to construct a qualitative ground-state energy profile for butadiene which is shown in Figure 1.1. s-cis s-trans reaction coordinate Figure 1.1. Ground-State Energy Profile for Butadiene. 6 Tai and Allinger have also calculated that the ground-state energy surface changes very slightly with methyl substitution at the end of the diene (e.g. for 1,3-pentadiene, A E = 2.58 kcal mol"1), but more dramatically with substitutions at the inner carbons. For example, the decrease in energy in going from s-cw-isoprene to s-fra/u-isoprene was calculated to be 2.2 kcal mol"1 with a rotational barrier of 3.8 kcal mol'1 while the difference in energy between j-cw-2,3-dimethylbutadiene to 5-rra/t5-2,3-dimethyIbutadiene was calculated to be only 0.78 kcal mol"1 (no rotational barrier given). In addition it was found that the s-cis conformer of 2,3-dimethylbutadiene is not planar, but rather twisted by 43°, presumably due to steric effects. butadiene 1,3-pentadiene isoprene 2,3-dimethylbutadiene r\/^^ r v / ^ h\/J^ 2.5-3.1 2.58 2.2 0.78 A E (cis -* trans) (kcal mol"1) In the light of these results, then, it is very interesting that the s<is conformer is the predominant form of the diene ligand in metal-diene complexes. That fact is illustrative of the changes in the character of the 1,3-diene that occur upon coordination to a metal. Having this basic understanding of dienes and transition metal diene complexes, it is now possible to traverse the transition-metal series from left to right on the periodic table to look at and compare specific examples. It will be seen that the major differences in metal-diene linkages become apparent when diene complexes of the early transition metals are compared to those of the late transition metals. Cyclobutadiene-metal complexes will not be discussed and, where possible, the discussion will be limited to acyclic, 1,3-dienes. Complexes containing r?2-l,3-dienes are not discussed here but are included in Chapter 4 where they are more relevant. 7 GROUP 4: A wide variety of diene complexes of Group 4 have recently been reported. These species are of interest because of the unusual preponderance of the a2, TT bonding mode for the s-cis species and because the first s-trans-diene complexes reported were of this group. The most common diene complexes of the Group 4 metals are those having the general formula Cp'2M(r;4-diene)2 (Cp' = Cp or Cp*, M = Zr or Hf). These types of complexes are of great interest since they were the first examples of 5-fra/ts-diene coordination to a single metal center. Interestingly, the rj4-.s-cw-diene complexes of these same systems are also isolable and the two conformers are interconvertible. Scheme 1.1 summarizes the methods of preparation of these complexes. Cp' 2MCl 2 Cp'2M(CH=CH2)2 Cp2ZrPh2 \ Mg(C 4H 6)-2THF hv / hv / ' -Ph-Ph ^ hi/, A —• Cp' 2 M—J Scheme 1.1. The s-cis and the s-trans conformers for most of these 18-electron diene complexes of Zr and Hf exist in solutions in thermodynamic equilibrium. These equilibria favor the s-cis diene compound at room temperature. The thermodynamically favored s-cis-diene complexes may be converted to their s-trans conformers by either thermal or photolytic methods. 8 Structural studies of examples of the two conformers show the 5-cis-diene complexes to exist as the bent metallacyclopentene structure (a2, it) which are fluxional and undergo a rapid ring flipping process in which a planar metallacyclopentene is the proposed intermediate,2 i.e. Cp'2M<_Vs Cp'2 M The s-trans diene complexes of these metallocenes, on the other hand, exist as the it, it r?4-diene compounds, i.e., and do not exhibit fluxional behavior other than the cis-to-trans diene isomerization. Interesting analogues to these Cp'2M(rj4-diene) complexes are the Cp*M(r/4-diene)Cl compounds in which a Cp' ligand is formally replaced by a halide Iigand. These types of electron-deficient complexes have very recently been prepared for Ti, Zr, and Hf via eq 2 and 3.6 Cp*MCl 3 + 2 Na/Hg + diene Cp»M(r/4-5-c£s-diene)Cl + 2 NaCl + Hg (2) C p ' M ^ ^ - c i s - C ^ X f j ^ r l y ) + Cp*MCl 3 Cp*M(r,4-5-cu-C4H6)Cl + <yM(r,3-C3H7)C12 (3) 9 Representative examples of these product 14-electron diene complexes have been structurally characterized. They possess the a2, r structures seen in the related metallocene systems, but they exhibit no fluxional behavior. The allyl-diene starting complexes of eq 3 (M = Ti, Zr, Hf) have been prepared by thermolysis of Cp*M(l-methallyl)3,7 i.e., Cp*M(l-methallyl)3 »• Cp*M(r/3-C4H7)(r;4-C4H6) + fra/u-2-butene (4) Furthermore, Gambarotta and co-workers8 have also prepared some related 18-electron complexes, CpM(dmpe)(r;4-C4H6)Cl, (M = Zr or Hf) (eq 5), which they have subsequently converted to the corresponding hydride complexes by treatment with Red-Al (eq 6). C 4 H 6 , 1 atm CpM(L)2(dmpe)Cl • CpM(dmpe)(r>4-C4H6)Cl + 1^ (5) (L) 2 = (CO)2, dmpe NaH,Al(OCILCH,OCH-), CpM(dmpe)(rj4-butadiene)Cl — — ^•CpM(dmpe)(r/4-butadiene)H (6) toluene X-ray crystallographic analysis of CpZr(dmpe)(r/4-s-cw-C4H6)H indicates the diene ligand is coordinated in an s-cis fashion with long-short-long carbon-carbon bond length alternation. However, all four diene carbons are approximately equidistant from the zirconium. Thus, the exact nature of the metal-diene interaction is somewhat ambiguous, even though a classically * -bonded diene coordination mode has been proposed rather than the metallacyclopentene alternative. These hydrido-diene complexes have been shown to insert olefins into the metal-hydride bond to form new alkyl-diene complexes, CpM(dmpe)(r?4-5-a5-C4H6)R , a reaction which has been proposed to proceed via dissociation of one of the two phosphorus atoms of the chelating phosphine ligand from the coordination sphere of the metal, allowing coordination of the olefin and subsequent 10 insertion into the M-H bond (eq 7). Another class of diene complexes of Group 4 is the (n -C 4H 6) 2M(P-P) species (P-P = dmpe, depe, diphos; M = Ti, Zr, Hf) prepared according to eq 89 and 9.10 THF MC14 + 2 Mg(C 4H 6)-2THF + P-P • (r?4-*-cw-C4H6)2M(P-P) + 2 MgCl 2 (8) ZrCl4(dmpe)2 + C 4 H 6 + 4 Na/Hg » • (r/4-5-ci5-C4H6)2Zr(dmpe) + 4 NaCl (9) The solid-state molecular structure of (C 4H 6) 2Hf(dmpe) 1 0 has an "approximately octahedral" coordination of the ligands about the central metal with the diene ligands exhibiting the s-cis conformation and the bonding mode being a2, IT. GROUP 5: Diene complexes of the Group 5 metals are much less numerous than those of Group 4. This is most likely due to the difficulty of preparation of the requisite starting materials, a problem which has only been solved in the recent past.11 The only 1,3-diene complex of a metal of Group 5 known until 1980 was a vanadium-diene complex described in 1960 by Fischer and co-workers.12 They reported that the photolysis of Cp V(CO) 4 in the presence of butadiene led to the diene complex, C p V ( C O) 2(rj 4 - C 4 H 6 ) . Although the conformation of the diene ligand was proposed to be r)4-s-cis (TT , *•), the only characterization data reported for this compound were infrared spectral and elemental analysis. By analogy to other early transition metal complexes, it seems reasonable to suggest that a structural analysis of this complex using characterization 11 techniques now available would probably show the diene ligand to be bound to the vanadium in an n4-s-cis (a2, *•) fashion. With the development of some potential starting materials came the preparation of a more extensive selection of diene complexes of the Group 5 metals, although no more diene complexes of vanadium have been reported. In 1981, Wreford9 prepared NbCl(dmpe)2(»?4-C4H6) according to eq 10. NbCl4(dmpe)2 + 2Mg(C 4H 6)2THF • NbCl(dmpe)2(r;4-C4H6) + 2 MgCl 2 (10) However, no structural analysis of the product complex was reported. Yasuda, Nakamura, and co-workers13 have published an extensive report on the preparation and subsequent chemistry of a series of niobium diene complexes, the essential details of which are summarized in Scheme 1.2 below. Cp'NbCl4 Cp'NbCl4 Cp'NDCl4 Mg(C4H 6)-2THF R 5 0.7Mg(C4H6)-2THF -Nb-Cl -Nb 'A R 3 2Mg(C4H6)-2THF •A N b — i y Scheme \2. 12 The diene ligand in these species is coordinated to the niobium center in an s<is a , x fashion in almost every case. The exception is C^Nb(r/4-5^£s-C4H6)(r;4-5-franj-C4H6) which contains an s-trans-dient ligand. An analogous compound, ( T p N b ^ t ^ - C ^ H ^ has now been structurally characterized by Melendez, et. al . 1 4 The complex was prepared in a rather unusual manner through the coupling of the organic reagents, i.e. eq 11. CpNbCl 4 + 4 K(2A-OjHn) • C p N b ^ ^ ^ - C ^ H ^ (11) The product has the rj4: r?4-bis diene ligand bonded to the metal center in the very interesting fashion shown below. Nb = CpNb As shown above, the end of the bis-diene ligand which is oriented in an s-cis manner is bound to the metal center as a bent metallacyclopentene (a2, JT) while the end which is s-trans is bonded in a x, TC fashion. There are tantalum analogues for some of the niobium diene compounds, namely, CpTa(f?4-C4H6)Cl2 and CpTa(r/ 4-C 4H 6) 2 1 5 in which the diene ligands are bonded to the metal in the familiar j-cw-metallacyclopentene, a2, * fashion. GROUP 6: There have been several reports of diene complexes of the Group 6 metals (Cr, Mo and W). A thermally unstable compound of chromium may be prepared by metal atom vapor synthesis, e.g. eq 12. 1 6 13 1) butadiene Cr(g) - (r , 4-C 4H 6)Cr(CO) 4 (12) 2) CO This complex decomposes above 0 °C, but low temperature spectroscopic studies (41 NMR, IR) as well as elemental analysis and mass spectral data led the authors to propose a x, x rj4-s-c£s-butadiene conformation of the diene ligand. Later, stable analogues of this complex, namely (r/4-diene)Cr(CO)3L (L = PMe 3, P(OMe)3) were prepared and characterized,17 thereby confirming the above prediction. Interestingly, Kreiter, et al. have also found that if the diene is "sufficiently bulky" (i.e., 2,4-dimethyl-l,3-pentadiene,18 or 1,3-cycloheptadiene19), a C-H bond of the diene ligand will displace a carbon monoxide ligand to form a formally coordinatively unsaturated Cr complex which is stabilized by an agostic interaction, e.g., for the 2,4-dimethyl-l,3-pentadiene complex, Cr = Cr(CO)2{P(OMe)3} These complexes have been structurally characterized in the solid state by X-ray crystallography, and the results of these studies confirm the presence of an agostic interaction as the hydrogens were located and successfully refined. In solution, the compounds undergo fluxional processes, which a labelling study indicated was a 1,5-hydride shift.18 Cr = Cr(CO)2{P(OMe)3} 14 Interesting diene complexes of molybdenum and tungsten include the M(r;4-diene)320 species (M = Mo, W) whose solid state molecular structures have been determined and which contain three n4-s-cis- 1,3-diene ligands bonded to each metal center in a x, JT bonding mode. It was subsequently found21 that in the 2,3-dimethylbutadiene analogue, the bonding mode of the diene ligands is ri4-s<is again but now has substantially greater a2, TT character. An intermediate case was found for the tris-o-xylylidene tungsten complex which has been shown22 to have approximately equal carbon-carbon bond lengths among the coordinated carbons in the solid state. It therefore seems to possess a structure intermediate between the * , * and a 2 * forms, i.e. The most interesting molybdenum diene compounds to date are those having the composition, Cp'Mo(NO)(r/4-diene),23 where diene = acyclic, conjugated diene. In most cases, the conformation of the diene ligand in these species is n*-s-trans with the diene ligand exhibiting approximately equal carbon-carbon bond lengths and the metal-diene linkage considered to be intermediate between a2, JT and J T , J T , i.e. When the diene ligand is 2,3-dimethylbutadiene, an intermediate product may be isolated in which the diene ligand is n4-s<is, again with no localization of the bonds in the diene ligand, i.e. Mo—, Mo = Cp'Mo(NO) 15 Mo = Cp'Mo(NO) These complexes are the subjects of more discussion as to their preparation and characterization (Chapter 2), cis-to-trans isomerization (Chapter 3) and characteristic reactivity (Chapter 4) later in this thesis. GROUP 7: In Group 7, manganese-diene complexes are the most numerous. Two dimeric complexes were reported in which the diene was coordinated to the manganese in an n2:T)2-s-trans manner, [CpMn(CO)2]20*2: n2:n2-C4H6)24 and [(OC)4Mn]22:v2'-v2-C^Hfr)25 wherein each double bond of the diene ligand is coordinated in an r/2-fashion to each metal, i.e. In the case when Mn = CpMn(CO) 2 the carbon-carbon bond lengths of the diene ligand indicate the diene ligand is T J 2 - J T -bound to each ofthe metal centers. For the other case (Mn = Mn(CO)4), however, the bond lengths in the butadiene ligand are approximately equal, and the double bonds, therefore, are not believed to be localized, i.e. Structural analyses of the paramagnetic bis-diene complexes of manganese, (r ? 4 -C 4 H 6 ) 2 Mn(CO) 2 6 and (r/4-C4H6)2Mn{P(OMe)3},27 have shown the diene ligands to be bonded to the Mn with the diene in an n4-s-cis conformation. However, the slight Mn = CpMn(CO) 2 or Mn(CO) 4 16 differences in the bonding have been found to be a function of the ancillary ligands. In (774-carbon-carbon bond lengths in the diene ligand are approximately equivalent and the bonding is proposed to be intermediate between x,x and a2, *,i.e. This is probably a consequence of increased back-donation from the Mn center to the butadiene LUMO, a reflection of the increased electron density available at manganese due to the substitution of P(OMe)3 for CO. 1,3-diene complexes of rhenium are much less numerous. In fact, to the best of my knowledge, only one such species has been structurally characterized, that being (PPh3)2ReH3(r;4-l,3-cyclohexadiene).28 The diene conformation is n4-s-cis, a restriction placed upon the diene by the fact that it is cyclic, with approximately equal carbon-carbon bond lengths among the coordinated carbons indicating again an intermediate bonding mode. This complex was, however, found to be fluxional in solution, rapidly exchanging via a mixture of agostic hydride species,29 i.e. C 4 H 6 ) 2 Mn(CO), the diene-metal bonding is T,T. In (C4H6)2Mn{P(OMe)3}, however, the Re = ReH 2(PPh 3) 3 17 GROUP 8: The diene complexes of the Group 8 metals, especially iron, are numerous, and several comprehensive works have been published which discuss the diene iron tricarbonyl systems.30 Interest in these species stems from their extensive application to regio-controlled organic synthesis. In (rj 4-C 4H 6)Fe(CO)3, the diene is in the r\A-s-cis conformation and the bonds are delocalized on the diene ligand, i.e.3 1 This is also the case for (n 4-5-cis-butadiene)2Fe(CO) , and some ruthenium33 and osmium complexes.34 Several trimetallic diene complexes of osmium are worthy of note here. These are (r)4-s-trans-C4H6)Os3(CO)10 and (r/ 4-s-cw-C 4H 6)Os 3(CO) 1 0 3 5 In the fra/is-diene compound, the diene is bonded to two metal centers in a manner similar to the dimeric Mn systems considered earlier. In the cis-diene compound, on the other hand, the diene is bound in an n4-s-cis fashion to only one metal. In both complexes, the carbon-carbon bond lengths on the diene ligands are approximately equivalent. GROUP 9: The first diene complex of Group 9 to be structurally characterized was Rh(r/ 4 -C 4 H 6 ) 2 Cl, which was reported in 1965.36 The diene ligands are bound to the Rh in an n4-s<is ( J T , J T ) fashion. In the case of analogous cobalt,37 and iridium38 1,3-diene complexes, structural analyses indicate the bonding between the metal and the diene varies between J T , J T and the intermediate structure (between i r ,*and a2, x), depending on the other ligands on the metal center and the substituents on the diene ligand. A novel form of metal-diene bonding was found in a rhodium system by Fryzuk, et. al . 3 9 who discovered that the treatment of [(dippp)Rh]2(/i-H)2 with butadiene leads to the isolation of a monomelic allyl product, (dippp)Rh(rj3-C3H7), and a dimeric diene complex, c c c o o o 18 [(dippp)Rh]20*2: ^ : f C4H$), in which the diene ligand (in the s-cis conformation) is partially sandwiched between the two rhodium centers, i.e., GROUP 10: Only one monomelic 1,3 diene complex of the Group 10 metals has been structurally characterized, that being Ni(dcpe)(r/ 4-CH 2=CHCH=CHC0 2Me) 4 0 prepared as outlined in eq 13. (dcpe)Ni(COD) + CH 2 =CHCH=CHC0 2 Me »• (dcpe)Ni(r? 4 -CH 2 =CHCH=CHC0 2 Me) (13) The diene ligand in the nickel complex is bound to the metal center in an »?4-s-cis (n, w) fashion, as determined by an X-ray structural analysis. The product complex is believed to be an intermediate in the co-dimerization and cyclo-dimerization of penta-2,4-dienoic acid methyl ester by Ni(0) catalysts. The scarcity of monomelic 1,3-diene complexes of the Group 10 metals is interesting in light of the fact that all three of the metals will coordinate molecules such as norbornadiene and 1,5-cyclooctadiene and will also coordinate two olefin molecules. It is noted that the Group 10 metals appear to have a propensity to coordinate non-conjugated diene ligands. This is probably due to the electronic properties of the metal centers. Also, Group 10 metal complexes are known to polymerize butadiene and so it may be that the 1,3-diene complexes are too reactive to be isolated 4 1 A dimeric diene complex of nickel is [(bipy)Ni(fj2-C4H6)]2vu2: r/2: n2-s-trans-C4H^)42 in which the bonds are localized on the Rh Rh = Rh(dippp) Rh 19 bridging diene ligand as they are on free butadiene and the two metals are not bonded directly to one another. In solution, this complex equilibrates as shown below. This emphasizes the reluctance of the nickel to coordinate a 1,3-diene to a single metal center in an r/4-fashion in preference to therj2-coordination mode. Ni = Ni(bipy) Similar complexes, (CpNi)2(^2: n2: r)2-s-cis-C4H6) and (CpNi) 2(/i 2: n2: n2-s-trans-C4H6)43 have also been prepared and characterized and exist with the s-cis conformer as the major isomer in solution, i.e., R' N i N i N i — N i N i = C p N i 20 GENERAL: In summary then, it has been shown that, with a few notable exceptions, the diene ligands of the early transition metal diene complexes are in the n4-s-cis conformation with the bonding between the metal and the diene being primarily a2, 7r in nature. The exceptions are Cp'2M(r;4-s-fra/is-diene) (M = Zr, Hf) and Cp'Nb(r;4-5-cis-diene)(r?4-5-fran5-diene) in which the trans-diene ligands are bound to the metal in the x, x form and the diene complexes of Cp'Mo(NO) which exhibit two conformations of the diene moieties, r/4-5-cw-diene and»?4-iy-fra/is-diene, bound to the metal in a form intermediate between the a2, K and the JT, JT forms. For the late transition metals, the diene ligand is strictly in the s-cis conformation in the monomeric complexes with the metal-diene bonding showing a stronger contribution from the form. CHARACTERISTIC REACTIVITY OF SOME TRANSITION METAL DIENE COMPLEXES Some remarkable organic transformations take place only in the presence of transition metal complexes. Homogeneous hydrogenation of olefins has been developed using Wilkinson's catalyst, (Ph 3P) 3RhCl, 4 4 and Crabtree's catalyst, [Ir(COD)(Pcy3)(py)]+PF6"45 Stereoselective polymerization of olefins has been achieved in the Ziegler-Natta type catalytic systems. In more recent years, the oxidative addition of carbon-hydrogen bonds of saturated hydrocarbons to metal centers has been extensively studied. Coordination of dienes to transition metals has also been studied in the hope that coordination of the diene to a metal will change its reactivity from that of the free diene, and this has indeed been found to be the case. The literature contains many accounts of the reactivity of transition-metal-diene complexes (e.g. refs 2,30,34,41). Since the accounts are too numerous and extensive to be outlined here, I will only describe some reactions which are pertinent to the remainder of the work contained in this thesis. 21 The reactivity of diene complexes of the transition metals generally falls into one of four classes, i.e., 1) metathetical displacement of ancillary ligands or the coordinated diene ligand by incoming ligands, 2) nucleophilic attack at the diene ligand, 3) electrophilic attack at the diene ligand, and 4) coupling reactions between the diene and other unsaturated molecules. Specific examples of each are outlined below. In 1960, F. G. A. Stone46 reported that (rj4-diene)Fe(CO)3 reacts with triphenyl phosphine to displace the diene ligand in a metathesis reaction (eq 14). Fe A / | \ +3 PPh3 • (OC)2Fe(PPh3)3 + C 4 H 6 + CO (14) C C C 0 0 0 Later studies47 showed that one CO ligand in these systems could be replaced thermally by an incoming ligand, but that the coordination of a second phosphorus donor ligand resulted in replacement of the hydrocarbon ligand, e.g. eq 15. / | \ - L A J ' I \ C C C C C L (15) 0 0 0 0 0 L, L' = phosphine or phosphite Chaudhari and Pauson,48 however, showed that under photolysis, two of the carbonyl ligands could be replaced by phosphites to form (rj 4 -C 4 H 6 )Fe(CO)(L) 2 . The organic chemistry of (»?4-diene)Fe(CO)3, specifically, nucleophilic and electrophilic addition to the diene ligands, has been extensively studied and applied to organic synthesis 3 2 and in many cases exhibits regio-specific addition to the diene ligand, e.g., eq 16.49 22 -78'C o*c + R-25 *C FT (16) IT Fe = Fe(CO)3 More recently, an analogue to the (rj4-diene)Fe(CO)3 compound, (rj 4-diene)Fe(PMe3)3, has been shown to react with C0 2 in a coupling reaction to formr/3-allylcarboxylates which may be removed from the metal center by acid methanolysis50 i.e. Fe- co, CC^Me C 0 2 M e COjMc Fe = Fe(PMe3)3 Reactivity of the diene ligand in systems other than the (r/4-diene)Fe(CO)3 species has been investigated, although not as extensively. This is most likely due to the fact that these diene complexes are generally less accessible to those using radimentary techniques for handling air-sensitive compounds and have only recently been characterized in detail. There has been a flurry of activity lately with early transition metal diene complexes, e.g., Cp*TaCl(y-() °°- - [Cp*TaCl(0)]n + {~\ ( 1 7 ) 5 i 23 C p * H f C l ( ) ^ ( ) - ^ -R 2CO CpjZr RC = C R since these display novel reactivity patterns. These reactions are examples of the coupling of organic molecules at a metal center to form new ligands which may then be removed from the metal center to produce new organic molecules. This seems to be the direction that most research in this area is headed. Very recently, Wink 5 5 has shown regio- and stereoselective addition of H" to (7?4-diene)Cr(CO)3{P(OMe)3} to form anionic r/3-allyl complexes which react with electrophiles to give free olefins, i.e., Cr(CO)3{P(OMe)3} (17 4 -CH 2 =CHCR' = CHR") i) K B H R 3 ii) NEt 4Br [NEt4][Cr(CO)3{P(OMe)3}(r;3-CH3CH-CR ,-CHR")] (21) 24 E + [NEt4][Cr(CO)3{P(OMe)3}(r; 3-CH 3CH-CR'-CHR")] ^ (£)-MeCH = CHCHER (22) E = C 0 2 , SiMe3, H The few reactions outlined here illustrate some of the immensely varied reactivity seen in these diene systems, and accounts for the impetus behind transition metal-diene chemical studies. The remainder of this thesis (excluding Chapter 5) involves the discussion of the synthesis, characterization and reactivity of Cp'Mo(NO)(r;4-diene) complexes. 25 REFERENCES For free dienes, the prefix s refers to the fact that the double bonds are oriented cis or trans across the single bond in the diene. 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Res. 1979,12,331. (46) Manuel, T. A.; Stone, F. G. A. /. Chem Soc, Dalton Trans. 1960,366. (47) Reckzeigel, A.; Bigorgne, M. /. Organomet. Chem 1965,3, 341. (48) Chaudhari, R. M.; Pauson, P. L. /. Organomet. Chem 1966,5,73, and references therein. (49) (a) Semmelhack, M. F.; Le, H . T. M. /. Am. Chem Soc. 1984,106,2715. (b) Semmelhack, M. F.; Herndon, T. W. Organometallics 1983,2,363. (50) Hoberg, H. ; Jenni, K. /. Organomet. Chem. 1987,322,193. (51) Blenkers, J.; DeLiefde Meijer, H. J.; Teuben, J. H . Organometallics 1983,2,1483. 29 (52) Hessen, B.; van Bolhuis, F.; Teuben, J. H . Organometallics 1987,6,1352. (53) Yasuda, H. ; Kajihara, Y.; Mashima, K.; Nagasuna, K.; Nakamura, A. Chem. Lett. 1981, 671. (54) (a) Skibbe, V.; Erker, G. /. Organomet. Chem. 1983,241,15. (b) ref 53. (55) Wink, D. J.; Wang, N. -F.; Springer, J. P. Organometallics 1989,8, 259. CHAPTER 2 Novel Physical and Chemical Properties of Acyclic, Conjugated Dienes Coordinated to Cp'Mo(NO) [Cp' = r? 5-C 5H 5 (Cp) orr?5-C5Me5 (Cp*)] 31 Introduction The first diene complex of CpMo(NO) was prepared in 19831 and the unique modes of coordination of the diene ligands to molybdenum in these species were communicated in 1985.2 These complexes are most generally prepared via the sodium amalgam reduction of [CpMo(NO)I2]2 in the presence of the acyclic, conjugated diene (eq 1). In most cases, the products of reactions 1 are then -trans-diene complexes, CpMo(NO)(r/4-trans-diene). Interestingly, when the diene used in reductions 1 is 2,3-dimethylbutadiene, in addition to the isolation of the r)4-fron5-2,3-dimethylbutadiene complex of molybdenum, one may also isolate an intermediate product, CpMo(NO)(rj4-ciy-2,3-dimethylbutadiene), i.e. [CpMo(NO)I2]2 +4 Na/Hg + 2(2,3-dimethylbutadiene) CpMo(NO)(r/ 4-fra/ts-2,3-dimethylbutadiene) + CpMo(NO)(r?4-cw-2,3-dimethylbutadiene) + 4NaI + Hg (2) Reactions 1 and 2 have now been extended to include an extensive series of diene ligands and to include some pentamethylcyclopentadienyl analogues, Cp*Mo(NO)(r/4-fra/iy-diene) and Cp * Mo(NO)(r; 4<w-2,3-dimethylbutadiene). In this chapter is presented the preparation and characterization of a series of these diene complexes followed by a discussion of their properties. Chapter 3 discusses a preliminary kinetic study of the isomerization of CpMo(NO)(T?4-cw-2,3-dimethylbutadiene) [CpMo(NO)I2]2 + 4 Na/Hg + 2 diene +2 CpMo(NO)(r/4-diene) + 4 Nal + Hg (1) 32 to CpMo(NO)(f?4-from-2,3-dimethylbutadiene). In Chapter 4, investigations of the characteristic reactivity of the r/4-fra/tt-diene complexes presented here are summarized. 33 Experimental Section All reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions using an atmosphere of dinitrogen. Conventional Schlenk-line techniques for the manipulation of air- and moisture-sensitive compounds3 or a Vacuum Atmospheres Corporation Dri-Lab model HE-43-2 drybox were employed. All reagents were purchased from commercial suppliers or prepared according to their published procedures. Florisil (mesh size 60 - 270) was used for preparing chromatography columns. Reagent purity was ascertained by elemental analysis and X H NMR spectroscopy. Solvents were dried according to conventional procedures,4 distilled and deaerated with dinitrogen just prior to use. Infrared spectra were recorded on a Nicolet 5DX F U R spectrometer which was internally calibrated with a He/Ne laser. Proton and carbon-13 nuclear magnetic resonance spectra were recorded on either a Varian X L 300 or Bruker W H 400 spectrometer with reference to the residual proton or ^ C signal of the solvent employed (usually C 6 D 6 ) . All chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane. Ms. Marietta Austria, Ms. Liane Darge and Mr. S. Orson Chan assisted in the collection of some of the NMR spectra. Mass spectra were recorded at 70 eV on an Atlas CH4B or a Kratos MS50 spectrometer using the direct insertion method by Dr. Gunter Eigendorf or Mr. Marshall Lapawa. Probe temperatures were between 100 and 150°C. Elemental analyses were performed by Mr. Peter Borda of this department. NMR Experiments: Some non-routine NMR experiments were conducted during this study. The specific experiments are described below. a) ^C^H} NMR Experiments: To avoid unnecessary heating of the samples, the low-power Waltz-165 broad-band decoupling technique was used. 34 b) Gated Decoupled 13C NMR Experiments: Gated proton decoupling with the proton decoupler off during data acquisition ("0.8 s) and on between the acquisitions("1.6 s) was used to produce proton-coupled B C NMR spectra with excellent signal-to-noise ratios and resolution in reasonable times (4 -12 h). c) Proton Nuclear Overhauser Effect (NOE) Difference Experiments: These spectra were collected using a 90° pulse (48ps) with gated, low-power homonuclear decoupling (i.e. {XH} off during data acquisition ("2 s) and on between acquisitions ("4s)). Usually, the resonance due to the cyclopentadienyl protons was selectively decoupled. An "undecoupled" spectrum (i.e. irradiated "1500 Hz downfield of the edge of the spectrum) was also collected and the FTD of the undecoupled spectrum was subtracted from that of the decoupled spectrum to produce the difference spectra. d) 2-Dimensional Heterocorrelation NMR Experiments (2D-HETCOR): Varian's2D-HETCOR pulse program was used in these experiments. The 90° B C pulse was 18 us, the 90° *H pulse from the decoupler was 46 us, the acquisition time was "0.6 s and presaturation was used. The number of incremental spectra was determined according to the concentration of the sample and spectral width used for collection of the FIDs. Zero-filling and a 2D-Fourier transformation resulted in a spectrum with a resolution of "6 Hz and " 100 Hz in the proton and carbon dimensions, respectively. Depending on the specific requirements of the experiments, spectra with adequate signal-to-noise ratios were obtained in 6 -16 h. Preparation of Cp'Mo(NO)(r/4-fro/w-diene) [Cp' = r?5-C5H5 (Cp) orr>5-C5Me5 (Cp*); diene = acyclic, conjugated diene]. The preparations of all the compounds of this class were similar and the preparation of CpMo(NO)(r?4-/ram-2,5-dimethyl-2,4-hexadiene) is presented as a representative example. In a 300-mL, 3-necked round bottom flask equipped with a magnetic stir bar and a gas inlet was mixed 25.0 g Na/Hg (1.3 mmol Na per gram, 32.5 mmol Na) and 5 - 7 mL Hg. When the mixture had liquefied, THF (100 mL) was added, and the system was cooled to 35 "-30°C using a saturated CaCl2(aq)/Dry Ice bath. Then, 2,5-dimethyl-2,4-hexadiene (2-3 mL) was added with a Pasteur pipette followed by the addition of [CpMo(NO)I2]26 (5.0 g, 6.3 mmol). The reaction mixture was stirred, and the progress of the reaction was followed by FTTR spectroscopy. During the reaction, the i> N O band at 1686 cm"1 diminished while a band at 1652 cm"1 grew in. Then, the band at 1652 cm"1 diminished as one grew in intensity at 1547 cm"1 and the color of the solution changed to green. Next, a band appeared at 1586 cm"1, and the solution turned to a dark brown color. Finally, the band at 1586 cm'1 diminished in intensity as a new one appeared at 1599 cm"1. After approximately 1 h, the IR spectrum was devoid of all y N O bands assignable to intermediate species in the reaction, and only the v^Q band at 1599 cm"1 remained in the spectrum. The supernatant solution was quickly cannulated away from the mercury residue to a new flask, and the solvent was removed in vacuo to leave a dark brown residue. The residue was extracted with Et^O (9 x 50 mL) and the yellow extracts were filter-cannulated into a 500-mL flask. The resulting EtoO solution was concentrated to "200 mL and then filtered through a column (3x6 cm) of Florisil supported on a medium-porosity frit. The Florisil was washed with Et 2 0 until the washings were colorless (3 x 10 mL). The washings were combined with the filtrate, and then the El^O solution was taken to dryness under reduced pressure to yield 0.50 g CpMo(NO)(r74-from-2,5-dimethyl-2,4-hexadiene) (60 % yield) as a yellow, crystalline powder. The use of other acyclic, conjugated dienes (except 2,3-dimethylbutadiene, vide infra) in place of 2,5-dimethyl-2,4-hexadiene afforded the analogous Cp'Mo(NO)(»74-frwu-diene)complexes in variable yields (Table 2.1). The analytical, mass spectral, IR, and X H and NMR data for all the new »74-diene complexes are collected in Tables 2.1 - 2.6. Included in Tables 2.2 - 2.6, for comparison, are data for some f?4-diene complexes prepared earlier in this study by A. D. Hunter7 of our research group. The butadiene complexes, Cp'Mo(NO)(r/4-fra/iy-butadiene), were prepared by the reaction ofthe [Cp'Mo(NO)X2]2 (Cp' = Cp, X = I, CI; Cp' = Cp*, X = CI) with 36 magnesium butene-diyl, Mg(C 4H 6)-2THF. 8 The example where Cp1 = Cp* and X = Cl is described as a representative example. A sample of pale yellow Mg(C4H6)-2THF (1.35 g, 6.1 mmol) was dissolved in THF (50 mL). The mixture was cooled to -78°C using an acetone/Dry Ice bath at which time the [Cp*Mo(NO)Cl2]2 (2.0 g, 6.1 mmol) was added. The mixture was stirred and allowed to warm to room temperature, during which time the color of the mixture changed from orange to green and finally to green-brown. The reaction mixture was stirred for 3 - 4 h until an IR spectrum of the solution exhibited a i > N O band at 1591 cm"1. The solvent was removed in vacuo, and the resulting green-brown residue was extracted with Et^O (4 x 25 mL). The yellow Et^O extracts were filter-cannulated into a new flask, concentrated to "75 mL and filtered through a column of Florisil (3x5 cm) supported on a medium-porosity frit. The Florisil was washed with EtjO (3 x 10 mL) until the washings were colorless. The washings were combined with the filtrate, and the entire volume as taken to dryness under reduced pressure to leave 0.77 g (40 %) of bright yellow Cp*Mo(NO)(r/4-trans-butadiene), 6, as a microcrystalline powder. The physical and analytical data for 6 are included in Tables 2.1 - 2.6. The analogous complex, CpMo(NO)(r?4-rra«5-butadiene),7 was prepared from the dichloro-nitrosyl starting material in 41 % yield. The reaction was complete in 12 h. Reduction of [Cp*Mo(NO)I2]2 in the Presence of 1,3-Pentadiene. As outlined above for the preparation of the fra/is-diene complexes, a mixture of Na/Hg (20 mmol Na), Hg, ("5 mL), THF (80 mL), and 1,3-pentadiene (piperylene, 1 mL) was cooled to "-30°C. Then [Cp*Mo(NO)I2]2 (5.15 g, 5 mmol) was added to the stirred solution. The usual reaction monitoring by IR spectroscopy indicated that the reaction was finished when the only v N O band evident in an IR spectrum of the reaction solution was observed at 1586 cm"1. The reaction solution was cannulated away from the mercury-containing residue and taken to dryness in vacuo. The resultant brown residue was extracted with E ^ O (4 x 50 mL) to give a dark yellow solution ( i / N O 1597 cm"1). The combined extracts were dried 37 under reduced pressure, and the resultant brown residue was extracted with hexanes (4 x 50 mL) to yield a yellow solution whose IR spectrum exhibited a i / N O at 1607 cm"1. The hexanes extractions were combined and concentrated to "30 mL. This solution was eluted through a Florisil column (3 x 12 cm) with hexanes. The first band to develop and be eluted was yellow. Collection of this band followed by concentration of the eluate to "10 mL gave a yellow solution (y^Q 1607 cm"1). This yellow solution was stored at "-20°C for one week to allow the growth of yellow crystals of Cp*Mo(NO)(774-frans-(£)-l,3-pentadiene), 7A. In this fashion, a crystal suitable for X-ray structural analysis was obtained. The analytical and physical data for 7A are included in Tables 2.1 - 2.6. The second band (y N O 1607 cm"1) to be eluted from the Florisil column above was red in color. Collection and concentration of this band, followed by cooling to -20°C for one week allowed the isolation of, Cp*Mo(NO)(r?4-fra/u-(Z)-l,3-pentadiene), 7B as an analytically pure, nncrociystalline red solid. Physical and analytical data for 7B are included in Tables 2.1 - 2.6. Preparation of Cp'Mo(NO)(r?4-cis-2r3-dimethylbutadiene). The synthetic methodologies used in the syntheses of these two complexes are similar and the synthesis of Cp*Mo(NO)(r?4<£s-2,3-dimethylbutadiene) is presented as a representative example. In a manner analogous to the preparation of the Cp'Mo(NO)(r74-/raac-diene) complexes, Na/Hg (25.0 g, 32.5 mmol Na) was liquefied with Hg. THF (100 mL) was added, and the system was cooled to "-30°C. The 2,3-dimethylbutadiene (2-3 mL) was added to the mixture. Then, [Cp*Mo(NO)I2]2 (7.0 g, 6.8 mmol) was added, and the reaction was monitored by IR until the only bands in the IR spectrum attributable to nitrosyl ligand stretching were at 1589 (w) and 1549 (s) cm"1. The supernatant solution was quickly cannulated to a 300-mL flask which had been pre-cooled with an ice bath. Solvent was removed in vacuo, and the resulting brown residue was extracted with cold ("0°C) E ^ O (5 x 50 mL). The red-orange E ^ O extracts were filter-cannulated to a cold ("0°C) flask where they were concentrated to " 100 mL under reduced pressure. The resultant 38 dark red E ^ O solution (J/ n o 1597 and 1557 cm"1) was cannulated to the top of a water-jacketed (" 10°C) Florisil column (3x7 cm) which had been prepared with hexanes. Elution of the column with cold E ^ O resulted in the development of two bands, the first yellow and the second orange. Collection of these two bands followed by solvent removal in vacuo from the two fractions led to the isolation of red Cp*Mo(NO)(r?4-c£s-2,3-dimethylbutadiene) and yellow Cp*Mo(NO)(r/4-fron5-2,3-dimethylbutadiene) in 21% and 32% yields, respectively, as analytically pure solids. Reaction of [CpMo(NO)I2]2 with 2/3 Equivalents of Na in the Presence of 2,3-Dimethylbutadiene. To a reaction flask equipped with a magnetic stir bar and gas inlet was added Na/Hg (3.0 g, 3.9 mmol Na) and Hg ("5 mL). When the mixture had liquefied, T H F (50 mL) and 2,3-dimethylbutadiene (2-3 mL) were added. The mixture was cooled to -25°C, the [CpMo(NO)I2]2 (2.0 g, 2.3 mmol) was added, and the mixture was stirred. Infrared monitoring of the reaction indicated the formation of a product with a v N O 1653 cm"1 as well as the formation of a small amount of CpMo(NO)(r74-franj-2,3-dimethylbutadiene) ( v N O 1599 cm"1). The same work-up procedure as that outlined above for the isolation of the diene complexes, followed by fractional crystallization led to the isolation of CpMo(NO)I(r7 3 -C 6 H 1 1 ) as a orange-red solid in 24% isolated yield. Anal. Calcd for C N H 1 6 N O I M o : C, 32.95: H , 3.99; N, 3.49. Found: C, 33.26; H , 4.00; N, 3.40. IR (THF) vNO 1644 (s) cm"1; low-resolution mass spectrum (probe temperature 160°C), m/z 404 (P +); 4l NMR (C 6D 6) s Isomer A: 5.15 (s, 5H, Cgl5), 3.08 (d, IH, / = 4.0 Hz, C H A H B ) , 2.40 (d, IH, / = 4.0 Hz, C H ^ B R ) , 2.30 (s, 3H, CH 3 ) , 1.65 (s, 3H, CH 3 ) , 0.85 (s, 3H, CH 3 ) ; Isomer B: 5.08 (s, 5H, C ^ ) , 2.90 (d, IH, / = 3.8 Hz, C H A H B ) , 2.81 (d, I H , / = 3.8 Hz, C H A H B ) , 2.24 (s, 3H, CH 3 ) , 1.54 (s, 3H, CH 3 ) , 0.93 (s, 3H, CH 3 ) ; Isomer ratio A:B = 5:1. Electrochemical Studies of CpMo(NO)(»74-fra/w-2,5-dimetliyl-2,4-hexadiene) and CpMo(NO)(r74-cis-2,3-dimethylbutadiene) in CH2C12. Electrochemical measurements were accomplished with a PAR Model 173 potentiostat equipped with a Model 176 current-to-39 voltage converter and a Model 178 electrometry probe, using a 3-electrode cell and methods described previously.9 All potentials are reported versus the aqueous saturated calomel electrode (SCE). Compensation for i7? drop in potential measurement was not employed in this study. The [n-Bu4N]PF6 support electrolyte was prepared according to the published procedure.93 The CH 2 C1 2 was obtained from BDH Chemicals (spectral grade) and was stirred over alumina (Woelm neutral, activity 1) while simultaneously being purged with N 2 for 15 min just prior to use. The solutions employed were ca. (5 - 7) x 10"4 M in the organometallic complex and 0.1 M in [n-Bu4N]PF6 and were maintained under an atmosphere of N 2 . Under these conditions, the Cp 2 Fe/Cp 2 Fe + couple was measured at E°' = 0.46 V versus the SCE in CH 2C1 2 , and the ratio of the cathodic peak current to anodic peak current ( i p Ji c ) 1 0 was 1. The separation of the cathodic and anodic peak potentials (AE) has been found to increase with scan rate9b and consequently, redox couples exhibiting similar behavior to that of the Cp 2 Fe/Cp 2 Fe + couple (which is known to be highly reversible11) were considered to be reversible. 40 Results and Discussion Cp'Mo(NO)(774-fra/u-diene) Complexes, 1-9. The reduction of [Cp'Mo(NO)I2]2 [Cp1 = r j 5 -C 5 H 5 (Cp) or rj 5-C 5Me 5 (Cp*)] by sodium amalgam in the presence of acyclic, conjugated dienes results in the formation of Cp'Mo(NO)(r;4-/rcvw-diene) complexes, i.e. The transformations are straightforward, and an excess (but not deficiency) of sodium amalgam may be used with no deleterious effects. The reactions are, however, accompanied by extensive decomposition of the organometallic species present when they are effected at ambient temperatures and are thus performed at approximately -25°C, a temperature just above the melting point of the sodium amalgam. Optimum yields are achieved if the reaction solution is removed from the mercury-containing residues as soon as the reaction is deemed to be completed, as ascertained by FTIR spectroscopy. This appears to be due to the fact that the rate of decomposition of the product f?4-diene complexes is accelerated by the presence of trace impurities in the final reaction mixtures. The isolated yields of the T74-fra/zs-diene complexes prepared in this study are slightly higher than those originally reported due to an improved method of work-up. The increased yield is presumably due the the fact that the impure r;4-rra/is-diene complexes are not maintained in solution for such long periods of time, since they are somewhat thermally sensitive in solution. The change in the work-up procedure has been the filtration of the Et^O extracts of the reaction mixture through Florisil. In addition, the dried Et^O extracts are not extracted with hexanes, as the filtration through Florisil is sufficient to purify the solution and, thus, prevent further decomposition. [Cp'Mo(NO)I2]2 +• 4 Na/Hg + 2 diene -2 Cp'Mo(NO)(r;4-rran5-diene) + 4 Nal + Hg (3) 41 The reaction to form the rj4-diene complexes is still somewhat perplexing, however, as a similar procedure does not consistently yield the diene complexes in similar yield. For example, attempted syntheses of CpMo(NO)(r;4-/ranj-2,5-dimethyl-2,4-hexadiene) using seemingly identical reagents and solvents will yield 50 - 60 % product approximately 14 times out of 15 but yield 0 -10 % on the 15th time. The decomposition of an organometallic intermediate complex may be observed in the IR spectra of the reaction solution as the reaction is monitored and generally produces a solution devoid of all uN0 bands in the 1500 - 1700 cm"1 region of the spectrum. Attempts to isolate any products of this decomposition have thus far been futile and are exacerbated by the unpredictability (and therefore unreliability) of the decomposition reaction. Considerable effort was spent in the attempts to isolate and identify the organometallic products of the decomposition, to no avail. The reaction to produce the CpMo(NO)(r/4-/ranj-butadiene) compound has been significantly improved from the originally reported preparation procedure.715 Both butadiene complexes (Cp' = Cp or Cp*) are most easily prepared from the dichloro precursor complexes and the magnesium butene-diyl reagent, as outlined in the Experimental Section (eq 4). T H F [Cp'Mo(NO)Cl2]2 + 2 Mg(C4H6)-2THF 2 Cp*Mo(NO)(r,4-frort5-C4H6) + 2 MgCl 2 (4) Using the dichloro starting complexes results in shorter reaction times (i.e. hours rather than days) with comparable isolated yields of the desired products. A rationale for the difference in reactivity of the dihalo complexes, [Cp'Mo(NO)X2]n (Cp' = Cp, Cp*; M = Mo, W; X = Cl, Br, I; n = 1,2) has recently been advanced.9b Electrochemical and ESR studies of these complexes have indicated that the reactions of these complexes with Grignard reagents proceed with electron transfer to form the radical anions, [Cp'Mo(NO)X2]". The studies showed that the stability of the radical anion increases in 42 the order [Cp'Mo(NO)I2] - < [Cp'Mo(NO)Br2]" < [Cp'Mo(NO)Cl2]- and suggest that the dichloro complexes, therefore, would be the reagents of choice for metathesis reactions with Grignard reagents. It seems reasonable to extend the conclusions of that study to reactions which involve metathesis with dialkylmagnesium reagents and the dihalonitrosyl precursors. Thus, it is probably a manifestation of the stability of the dichloro radical anion that the dichloro starting materials lead to better yields of the r/4-butadiene complexes. The analytical, mass spectral, infrared and 1 H and B C NMR data for the n4-trans-diene complexes (1-9) prepared as a part of this study are collected in Tables 2.1-2.6. These complexes are yellow, diamagnetic solids which are quite soluble in organic solvents, the Cp* analogues being noticeably more soluble than their Cp counterparts. When pure, they are thermally stable as solids but decompose slowly in solutions. These decompositions result in the precipitation of a brown solid which is only soluble in THF and other strongly solvating solvents12 and contains no nitrosyl ligands as evidenced by its IR spectrum as a Nujol mull. The Cp*Mo(NO)(r?4-rrans-diene) compounds are more thermally stable than their Cp analogues and decompose much more slowly in solutions. In solution 1-9 are air-sensitive, but as solids they may be handled in air for short periods of time with no deleterious effects. A single crystal X-ray crystallographic analysis of 3,251 CpMo(NO)(r/4-rran5-2,5-dimethyl-2,4-hexadiene) has shownit to be a monomer in the solid state and has confirmed that the diene ligand is attached to the metal center in a twisted, transoidal fashion. The IR spectra of 1-9 in CH 2 C1 2 solutions (Table 2.1) exhibit single, strong absorptions in the region 1565-1595 cm"1 attributable to linear, terminal nitrosyl ligands. The absorptions occur at progressively lower energies with increasing methyl substitution on the diene ligands. As well, the nitrosyl-stretching frequencies exhibited in the IR spectra of the Cp* analogues are 15-20 cm"1 lower in energy than those exhibited by the Cp Table 2.1. Elemental Analyses, Mass Spectral and Infrared Data for ther?4- Diene Complexes, 1 -10. Complex No. Yield (%) Analytical Data (%) C H found(calcd) found(calcd) N found (calcd) Low Resoln Mass Spectra ¥+,m/za " N O (CH 2 CI 2 ) (cm"1) (THF) CpMo(NO)(&a/w-2-methylbutadiene) 1 21 46.41(46.34) 5.24(5.02) 5.16(5.40) 261 1593 1610 CpMo(NO)(fr«ww-13-pentadiene) 2 21 4632(46.34) 532(5.02) 5.21(5.40) 261 1589 1605 CpMo(NO)(fra«j-24-dimcthyl-2,4-hexadiene) 3 60 51.95(51.83) 6.44(636) 4.66(4.65) 303 1584 1599 CpMo(NO)(&o/u-2,4-dimethyl- 1,3-pentadiene) 4 19 49.98(50.18) 5.85(5.92) 4.80(4.87) 289 1584 1589 CpMo(NO)(&«wiy-l,4-diphenyIbutadiene) 5 9 63.53(63.49) 4.82(4.79) 3.43(3.53) 399 1595 1612 Cp*Mo(NO)(fro/w-butadiene) 6 40 53.07(53.36) 6.57(6.66) 4.36(4.44) 317 1616* 1591 Cp*Mo(NO)(&B/Jj-E-13-pentadiene) 7A 31 54.68(54.74) 7.20(7.00) 4.29(4.26) 331 1568 1586 Cp*Mo(NO)(fra/ts-Z-1,3-pentadiene) 7B 11 54.68(5434) 7.23(7.00) 4.23(4.26) 331 1568 1586 Cp*Mo(NO)(fra«j-23-dimethyIbutadiene) 8 41 56.55(56.00) 7.63(7.28) 4.00(4.08) 345 1574 1593 Cp*Mo(NO)(fra«j-2^-dimethyI-2,4-hexadiene) 9 47 57.76(58.22) 7.70(7.87) 3.70(3.77) 373 1566 1585 Cp*Mo(NO)(cw-23-dimethyrbutadiene) 10 11 55.89(56.00) 7.11(7.28) 4.01(4.08) 289 1539 1548 _ no Assignments for Mo. Hexanes. Table 2.3 *H NMR Chemical Shifts for the Complexes, 1 - 1 0 . Complex Chemical Shifts (6 In ppm, CA), unless specified otherwise) Number Cp R l l R12 ^ 1 *32 R41 R42 1A° 5.49 (s) 3.54 (d) 2.76 (d) 1.25 (s) 3.17 (d,d) 2.73 (d,d) 3.74 (d,) 1B° 5.53 (s) 3.35 (d,d) 2.88 (d,d) 2.37 (d,d) 1.53 (s) 1.94 (d,d) 3.45 (d,d) 2A* 4.94 (s) 1.47 (s) 3.56 (d,q) 2.10 (d,d) 3.22(d,d,d) 3.37 (d,d) 2.71 (d,d) 2B* 4.95 (s) 2.84 (d,d) 2.71 (d,d) 1.99(d,d,d) 3.33 (d,d) 2.48 (d,q) 1.92 (s) 3 4.93 (s) 1.68 (s) 1.89 (s) 2.44 (d) 3.41 (d) 1.05 (s) 2.14 (s) 4AC 4.85 (s) 3.11(d) 2.64(d) 1.31 (s) 1.46 (s) 1.48 (s) 2.15 (s) 4BF 5.20 (s) 2.05 (s) 2.00 (s) 1.45 (s) 1.57 (s) 2.65 (d) 3.25 (d) 5 4.64 (s) 6.88-7.32 (m) 3.39 (d) 4.37 (d,d) 2.98 (d,d) 4.68 (d) 6.88-7.32 (m) 6 1.65 (s)d 2.45 (d,d) 3.08 (d,d,d) 1.57 (m) 3.57 (m) 1.21 (d,d,d) 3.47 (d,d,d) 7A 1.87 (s)d 2.43 (d,d) 2.85 (d,d) 1.51 (m) 3.44 (t) 2.06 (m) 1.90 (s) 7B 1.87 (s)d 2.07 (m) 1.89 (s) 3.44 (t) 1.52 (m) 2.85 (d,d) 2.44 (d,d) 8 1.69 (sf 3.27 (d) 1.76 (d) 1.00 (s) 1.74 (s) 2.66(d) 3.37 (d) 9 1.70 (s)d 1.68 (s) 1.84 (s) 2.34 (d) 3.59 (d) 1.13 (s) 2.03 (s) 10 1.68 (s)d -0.49 (d) 3.45 (d) 2.20 (s) 2.20 (s) -0.49 (d) 3.45 (d) a Isomers IA and IB exist in a 1:3 ratio. Recorded in CDCl-j. * Isomers 2A and 2B exist in a 1:2 ratio. c Isomers 4A and 4B exist in a 1:4 ratio. dThisisr/ 5-C 5Me 5 = Cp*. 4^ 46 Table 2.4. 1 H NMR Coupling Constants for the Diene Complexes, 1 -10. Complex Coupling Constants (in Hz, CfiD-- unless specified otherwise) Number Jll-12 J41-42 Jll-21 J32-42 J12-21 J32-41 J21-32 J21-41 lA f l 1.9 3.6 — 6.6 — 14.0 — 0.8 1B° 2.8 3.1 7.0 . . . 14.4 — — 0.8 2A — 2.7 — 6.2 12.8 13.8 10.0 — 2B 2.4 6.0 6.8 — 13.8 12.0 11.7 . . . 3 — . . . — — — — 11.8 . . . 4A 2.3 — . . . . . . . . . . . . . . . . . . 4B — 2.4 . . . — . . . . . . — . . . 5 . . . — — 12.0 . . . 13.5 10.5 . . . 6 2.3 2.5 6.5 6.0 14.0 12.5 10.5 — 7A 2.5 . . . 6.8 . . . 14.4 11.4 11.4 . . . 7B — 2.6 6.0 6.7 . . . 14.1 11.3 . . . 8 2.9 2.8 — . . . . . . . . . . . . . . . 9 — — — . . . . . . . . . 11.5 . . . 10 4.9 4.9 — — — — — — a Recorded in CDC13. Table 2.5. 1 3 C NMR Chemical Shifts for the Complexes, 1 -10. Complex Chemical Shift* (S In ppm, C^D^ nnless specified otherwise)0 Number Cp C l C 2 C 3 C 4 R l l R12 R21 R32 R41 R42 IA 94.88 (d) 53.95 (t) 105.22 (s) 92.81 (d) 54.15 (t) . . . . . . 21.28 (q) . . . . . . . . . IB 95.08 (d) 50.46 (t) 79.70 (d) 115.80 (s) 55.28 (t) — . . . . . . 18.10 (q) . . . — 2A 95.75 (d) 50.24 (t) 78.95 (d) 99.81 (d) 72.99 (d) 20.68 (q) . . . — . . . . . . . . . 2B 94.90 (d) 70.43 (d) 86.60 (d) 92.80 (d) 54.05 (d,d) — . . . . . . . . . . . . 20.23 (q) 3 97.43 (d) 86.18 (s)b 83.24 (d) 92.28 (d) 86.77 (s)b 31.01 (q) 23.32 (q) . . . — 23.44 (q) 31.04 (q) 4A 97.75 (d) 55.10 (t) 113.00 (s) 90.28 (d) 8X20 (s) — . . . 26.15 (q) . . . 32.40 (q) 33.15 (q) 4B 96.42 (d) 88.45 (s) 813 (d) 113.05 (s) 55.23 (t) 32.40 (q) 25.15 (q) . . . 18.15 (q) . . . . . . S 97.85 (d) 71.93 (d) 75.99 (d) 91.60 (d) 7557 (d) 124-131 (m) 142-144 (m) — . . . . . . . . . 124-131 (m) 142-144 (m) 6 10550 (d)c 58.92 (t) 91.28 (d) 96.71 (d) 61.43 (d,d) . . . . . . . . . . . . . . . . . . 7A 106.35 (s)c 58.50 (t) 88.42 (d) 99.05 (d) 79.51 (d) . . . . . . . . . . . . . . . 19.41 (q) 7B 10452 (s)c 79.45 (d) 99.45 (d) 88.50 (d) 58.51 (t) 19.41 (q) . . . . . . . . . . . . . . . 8 105.84 (s)c 60.57(t) 102.80 (s) 109.26 (d) 59.20 (t) . . . . . . 22.35 (q) 21.20 (q) . . . . . . 9 105.49 (s)c 83.44 (s)b 88.87 (d) 91.27 (d) 88.52 (s)* 28.29 (q) 24.03 (q) . . . . . . 20.48 (q) 29.58 (q) 10 104.79 (s)c 58.24 (d\d) 117.01 (s) 117.01 (s) 58.24 (d,d) . . . . . . 23.91 (q) 23.91 (q) . . . . . . a The letters in brackets denote V ^ - H "^'•plic' 1 ' 6 5-* Note: These Cj vs. C 4 or C2 vs. Cj are tentative. c This isfj 5 -C 5 Mc 5 , the methyl carbons produce quartet resonances at*: 6,10.51; 7A, 10.70; 7B, 10J9; 8,10.72; 9,10.08; 10,10.49 ppm. —1 Table 2.6. 1 3 C NMR ^Q-H Coupling Constants the Diene Complexes, 1 -10. Complex Number Cp Coupling Constants (in Hz, CgDg C l C 2 unless specified otherwise)0 c 3 c 4 R l l R12 "21 R32 R41 R42 l A b 174.7 150.1 . . . 148.0 150.9 . . . . . . 128.2 — — — l B b 175.7 157.5 154.9 . . . 154.2 . . . . . . . . . 128.0 . . . . . . 2A 176.0 157.2 156.0 161.4 147.0 127.3 . . . — - . . . . . . . . . 2B 178.0 155.0 143.0 162.1 162.0 . . . . . . . . . . . . . . . 123.8 3 174.6 . . . 153.1 1603 . . . 126.0 126.5 — . . . 1263 126.0 4A 175.0 153.0 . . . 149.0 . . . . . . . . . 1233 — 124.4 124.4 4B 175.0 . . . 149.9 . . . 154.0 124.4 127.3 . . . 127.0 . . . . . . 5 175.0 155.0 155.0 165.0 150.0 . . . . . . . . . . . . . . . . . . 6 c 156.0 152.0 164.0 150.0,160.0 . . . . . . . . . . . . . . . . . . 7A c 158.0 153.2 160.8 148.8 . . . . . . . . . . . . . . . 128.0 7B c 152.0 158.0 158.0 156.7 126.7 . . . . . . . . . . . . . . . 8 c 1523 . . . . . . 155.6 . . . . . . 127.5 1273 . . . . . . 9 c . . . 152.0 159.8 . . . 125.3 126.8 . . . . . . 126.1 125.5 10 c 161.3 . . . 161.3 . . . . . . 127.3 1273 . . . — » 1613 1613 Recorded using broad band gated decoupling. b Recorded in CDClj. c This isf?5-C5Me5, the methyl carbons have V & H : 6,126.0; 7A, 1273; 7B, 126.0; 8,125.0; 9,126.8; 10,126.7 Hz. 49 compounds, indicative ofthe greater electron density on the metal center in the Cp* compounds. Preparation of Cp*Mo(NO)(rj4-fra/ts-l,3-pentadiene). The preparation of complexes 7A and 7B is as outlined in the Experimental Section. It is deserving of special comment because two isomers of this complex may be isolated separately when the reduction of [Cp*Mo(NO)I2]2 is done in the presence of an isomeric mixture of E- and Z-1,3-pentadiene, i.e., [Cp*Mo(NO)I2]2 + 4 Na/Hg + 2 (1,3-pentadiene) -Cp * Mo(NO)(r/ 4-trans-E- 1,3-pentadiene) + Cp*Mo(NO)(r/4-/ra/w-Z-l,3-pentadiene) (5) + 4 Nal + Hg When prepared according to the method outlined for the preparation of the other n4-trans-dicne complexes and filtered through Florisil, 7A and 7B are not isolated independently. However, if a hexanes extract of the reaction mixture is eluted through a Florisil column with hexanes, the two complexes may be separated. The physical and analytical data for these complexes are included in Tables 2.1 - 2.6. As the data indicate, yellow 7A and red 7B are found to have very similar physical properties. A crystal of 7A suitable for an X-ray crystallographic analysis was obtained by filtration from a hexanes solution of 7A The X-ray data were collected and solved by Richard Jones and Fred Einstein at Simon Fraser University. The SNOOPI plot of the solid-state molecular structure is shown in Figure 2.1. Table 2.7 lists pertinent bond lengths and angles. Notable are the carbon-carbon bond lengths among the four coordinated carbons of the diene ligand where, as expected,73 the bond lengths are approximately equivalent within the estimated standard deviation limits. The delocalization of the JT-electrons of the diene have previously been observed in the structure of CpMo(NO)(f? 4 -C 1 7 C 1 6 C 1 8 C 1 5 C 2 . C 1 4 noi C 2 0 C 1 9 C I Figure 2.1. Solid-State Molecular Structure of 7A. 51 Table 2.7. Selected Bond Lengths () and Angles (deg) in the Molecular Structure of 7A. C1-C2 1.385(8) C2-C3 1.425(8) C3-C4 1.407(8) C4-C5 1.489(9) M o - N 1.767(4) N - O 1.213(5) Mo - N - O 170.7(4) M o - C l 2.333(6) M o - C 2 2.207(5) M o - C 3 2.238(5) Mo - C4 2.304(5) 52 fra/w-^S-dimethyl^^-hexadiene)23 and is consistent with the bonding rationale for all the Cp'Mo(NO)(rj4-rran5-diene) complexes isolated to date (vide infra). The most important feature of the molecular structure is the fact that the £-l,3-pentadiene ligand is coordinated to the molybdenum with the methyl pointing away from the cyclopentadienyl ring. Unfortunately, repeated attempts to obtain crystals of 7B suitable for structural analysis were unsuccessful. However, the NMR data for 7B are consistent with its formulation as CpMo(NO)(r)4-trans-Z- 1,3-pentadiene), i.e., The X H NMR spectra of 7A and 7B are shown in Figure 2.2 and illustrate the slight differences in the chemical shifts and coupling patterns. The chemical shifts of the protons of the diene ligands of 7A and 7B are very similar. Most notably, the chemical shifts of the resonances due to the methyl hydrogens of the diene ligands (7A, 1.90; 7B, 1.89 ppm) are very similar, indicating that the methyl group of each isomer is in a similar environment. Therefore, since the methyl group of the diene ligand of 7A is pointing away from the Cp ligand, it may be concluded that the same is true for 7B. Consistent with these formulations are the *H - *H coupling constants (Table 2.4). For 7A, it is expected that three trans *H - X H couplings (Jz2-2i> ^21-32' ^32-4l) a n £ * o n e c " *H - 1 H coupling ( / ^ i ) should be observed, while for 7B, it is expected that two trans * H - X H couplings (/2i-32» ^32-4i) a n ( * t w 0 c " ' X H couplings (Jn_2i> ^ 32-42) should be observed. Within the ranges of V J ^ J generally observed for these types of compounds (vide infra) C 3 / ^ . - 10 -15 Hz, *Jcis^ 5 - 8 Hz), 7 a this is indeed seen to be the case. N O 7A O 7B Figure 22. 300 MHz H NMR Spectrum of 7A (a) and 7B (b) in CDC13. _AAA i i n ~ p r 1 1 1 I 1 1 1 1 1 1 1 1 1 l 11 1 1 1 1 1 1 1 1 1 1 1 1 1 i 1 1 1 j 1 1 1 1 1 1 1 1 1 l 1 * 1 r p n T j T T T T ) 1 1 i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 M I ^  1 1 1 1 1 1 1 11 1^ I I I 1 I I M j ^  pp1^ — —> r\ *"i Q O R 2.4 2.2 C . U l .O * • 3.4 3.2 3.0 2.8 2.6 2.2 - i - n - r r r T T T T - i i i i i i i i i i i i | i i M i i i i i | n i i | i i i i | i i i i i i i i i | i i i i I T T T T T T T T T T T i ' M 1 1 1 1 | 1 1 1 1 I 1 1 1 ' ] 1 1 1 1 I 1 1 1 1 | 1 3.'4 3.2 3 0 2.8 2.6 2.,4 2.2 2.0 l . B 1.6 T 1| I I I I I I I I I I I I I M I I | I I I I I I I I I | iyx ' I . 1 . . ' -i n n u f J 1.4 1.2 PPM O J 54 It is interesting that for 7A and 7B no isomers are observed. Analogous to compounds 1,2, and 4, it may be expected that each 7A and 7B would exist in equilibrium in solution with their corresponding diastereomers, 7A* and 7B', i.e. in which the diene ligand has the opposite face bound to the metal (vide infra). These isomers are not observed by NMR spectroscopy of solutions of the two compounds. While it is possible that the other diastereomers are not formed, it is more likely that they are present in very small quantities and are, therefore, not observed. It is remarkable that 7A and 7B are such distinctly different complexes and, especially, that they may be separated on a Florisil column. This is in contrast to some other systems (e.g. 1,2, 4) in which diastereomers exist in the solutions but are not separable. A slight change in the electronic environment of the molybdenum might be the cause of the difference in color of the two isomers. It is somewhat notable, however, that these electronic differences are enough to affect the color of the complex but not significant enough to affect the frequency of the nitrosyl stretches in the IR spectra of 7A and 7B. It will be seen in Chapter 4 of this thesis that when the CpMo(NO) fragment coordinates to a ligand to allow the differentiation of the endo- and eto-allyl isomers of that ligand, the two isomers are separable on a Florisil column and the V^Q'S of the two isomers differ by almost 20 cm"1. Alternate Synthetic Approaches. Many attempts were made to prepare the ?74-diene complexes of Cp'Mo(NO) by other routes. Attempts to synthesize the tungsten analogues, as well as to extend the series to include cyclic and non-conjugated dienes were also made. N O 7A* O 7B' 55 Reduction of the dimers [ C p M o ^ O ) } ^ (X = Cl, Br) with sodium amalgam in the presence of acyclic, conjugated dienes led to the expected products in similar yields when X = Cl, but in very low yields when X = Br, eq 6. [CpMo(NO)X2]2 + 4 Na/Hg + 2 diene 2 CpMo(NO)(74-fro?w-diene) + 4 NaX + Hg (6) X = C1,* 20-40% X = Br,* 0-10% This is not easily rationalized, but presumably it reflects the dependence of the stability of reaction intermediates on the halide ligand. The diiodo nitrosyl complex is the simplest to prepare and thus is still the reagent of choice for these reductions. Attempts to effect the reductions 1 with reducing agents other than sodium amalgam led to other products. The reductions with Na(C gH 1 0 ) produced several products (by IR) which decomposed above "-50°C, i.e. -78°C [CpMo(NO)I2]2 + 4 Na(C gH 1 0 ) + diene • several products -50°C decom >osition (7) The reductions with zinc amalgam, a less powerful reducing agent, led to [CpMo(NO)I]2i3 as the major product (eq 8). [CpMo(NO)I2]2 + 4 Zn/Hg + 2 diene ^[CpMo(NO)I]2 + Znl + Hg (8) Many variations on the attempts listed above were used in efforts to prepare tungsten analogues of the molybdenum diene complexes; however, these met with no successes. It is surprising that they were not preparable, given the close relationship between the chemistry of compounds containing the CpMo(NO) and CpW(NO) 56 fragments.14 In particular, their phosphine and phosphite analogues, CpM(NO)L2 (M = Mo, W; L = phosphine or phosphite) are of comparable stability and ease of preparation.^ Since the CpW(NO)L2 complexes are stable complexes, the problem is believed to be in the stabilization of intermediates in the reaction. As yet, however, given the limited knowledge of the nature of the intermediates in the molybdenum reaction, it is difficult to speculate on the instability of any tungsten intermediates. Cp'Mo(NO)(r?4-cw-2,3-dimethyibutadiene) Complexes. When the diene employed in reactions 1 is 2,3-dimethylbutadiene, IR monitoring of the two conversions (Cp' = Cp or Cp*) indicates a band due to the formation of an intermediate nitrosyl-containing product in the IR spectra of each reaction in addition to the nitrosyl band due to the expected n4-trans-diene product. These other products (eq 9) [Cp'Mo(NO)I2]2 + 4 Na/Hg + 2 (2,3-dimethylbutadiene) • Cp'Mo(NC% 4-foz/iy-2,3-dimethylbutadiene) + Cp'Mo(NO)(r;4-cw-2,3-dimethylbutadiene) + 4 Nal + Hg (9) are the 774-c£s-2,3-dimethylbutadiene complexes, 10 (Cp' = Cp*) and 11 (Cp' = Cp), which may be separated from their n4-trans-diene isomers and isolated by chromatography and fractional crystallization. It should be noted that, despite extensive efforts, no spectroscopic evidence for the formation of any other r/4-as-diene complexes analogous to 10 and 11 has been observed during the syntheses of the other n4-trans-diene complexes isolated during our studies. The Cp'Mo(NO)(r/4-cy-diene) complexes, 10 and 11, are red, diamagnetic solids which have similar physical properties to their rj4-fra/u-diene isomers. As is seen for the j?4-fra/ty-diene isomers, the pentamethylcyclopentadienyl analogue is more soluble than the perhydrocyclopentadienyl species. The IR spectra of 10 and 11 exhibit f N O ' s in CH 2 C1 2 at 1539 and 1552 cm"1, respectively, energies which are 35-40 cm*1 lower than those of their 57 respective trans-diene isomers (1574 and 1590 cm*1, respectively). This indicates that the Mo d X - *NO JT * back-donation of electron density is significantly greater in the cw-diene complexes. This conclusion is in accord with the Fenske-Hall MO rationale of the bonding in these compounds.2b A single-crystal X-ray crystallographic analysis of 11,-* the Cp analogue, has confirmed its formulation as an 18-electron, monomeric species in which the 2,3-dimethylbutadiene ligand is attached to the metal center in a planar, cisoidal fashion. That 10 and 11 are the kinetic products of reactions 2 is established by the fact that they isomerize irreversibly to the trans-diene isomers in solutions (i.e., eq 10) Cp'Mo(NO)(r? 4-rra/w-2,3-dimethylbutadiene) < ^ • Cp'Mo(NO)(tj4-cis-2,3-dimethylbutadiene) (10) A study of this isomerization is discussed in Chapter 3 of this thesis. One final point concerning the reduction of [CpMo(NO)I2]2 by sodium amalgam in the presence of 2,3-dimethylbutadiene must be made. That point is that at least two equivalents of sodium per molybdenum must be used in order to obtain the desired r?4-diene complexes as the organometallic products. When a deficiency of sodium is used, the new complex, CpMo(NO)I(r?3-C6H11), 12, is obtained as an orange, microcrystalline solid in 24% isolated yield. [CpMo(NO)I2]2 + 2/3 (Na/Hg) + 2 (2,3-dimethylbutadiene) • CpMo(NO)I(^3-C6H1 1) (11) The spectroscopic properties of 12, discussed in the Experimental Section, are consistent with it having a monomeric structure similar to that established for the chiral CpW(NQ)X(r73-C3H5), (X = I,16a Cl 1 6 b ) . While the exact origin of 12 remains unknown, it is probably formed via hydrogen atom abstraction from the solvent by some radical species 58 present in the reaction mixture. It may give a clue as to the mechanism of formation of these diene complexes, or at least a hint as to the identity of an intermediate in the reaction. The most likely radical species which could abstract a hydrogen from the solvent to form 12 is "CpMo(NO)I(diene)", i.e., eq 12. CpMo(NO)I(diene)-+ H- ^ C p M o C N O ) ! ^ 3 - ^ ^ ) (12) Electrochemical studies915 of CpMo(NO)I2 have shown that upon reduction, a radical species is formed, i.e. +e~ CpMo(NO)I7 •CpMo(NO)I 2(THF)- (13) THF This radical anion could lose I" and coordinate a diene in an»?2-fashion, i.e., 1) -1" , THF CpMo(NO)I 2(THF)' • 2) + 2,3-dimethylbutadiene CpMo(NO)I(r?2-2,3-dimethylbutadiene)- (14) Ther?2-diene could then abstract H- from the solvent as in eq 12. We have observed spectroscopically (but not isolated) other Tj3-allyl complexes which invariably result when reactions 1 and 2 are effected with a dearth of sodium. Since the formation of these n3-allyl compounds is only observed when less than a stoichiometric amount of reducing agent is present, it is reasonable to suggest that the next step in the reaction to form the ij4-diene products in the presence of sufficient reducing agent may be a reduction followed by loss of I" and coordination of the other double bond of the diene, i.e. + e CpMo(NO)I(f?2-2,3-dimethylbutadiene)-CpMo(NO)I(rj 2-2,3-dimethylbutadiene)- (15) -r CpMo(NO)I(r/ 2-2,3-dimethylbutadiene)" • CpMo(NO)(r/4-2,3-dimethylbutadiene) (16) This is consistent with the fact that similar i / N O bands are seen for all the reductions of [CpMo(NO)I2]2 in the presence of acyclic, conjugated diene. The v N O bands of the final diene products vary slightly with the extent of substitution on the diene ligand. These observations would be expected if either the coordination of the diene takes place in the latter steps of the reaction or the intermediates of a particular reduction reaction are similar to those of another reduction reaction. Since the y N O bands assigned to intermediates vary slightly as the diene is varied, it is reasonable to suggest that the diene ligand, as the only variable, is involved early in the reaction. The formation of these r?3-allyl complexes is also consistent with the early involvement of the diene in the reaction. The *H and 1 3 C NMR Spectroscopic Properties ofthe r;4-Diene-containing Complexes, 1-11. We have carried out a detailed analysis of the NMR spectra of complexes 1 -11. To obtain the maximum amount of information from the spectra it was necessary to assign both the location on the diene ligand (i.e. C x vs C 4 , R n vs R^, see Table 2.2) and the orientation with respect to the cyclopentadienyl ring of each nucleus responsible for the observed * H and ^ C resonances. This task was accomplished with a series of 1- and 2D NMR experiments. The 1 H NMR spectra were used to determine the proton coupling networks (Tables 2.3 and 2.4). The orientations of the diene ligands with respect to the cyclopentadienyl ligand were obtained from NOE difference spectra. Carbon-13 (gated and broad-band decoupled) NMR experiments were used to obtain carbon-proton connectivities and coupling constants (Tables 2.5 and 2.6). Finally, 2-D HETCOR (2-Dimensional Heterocorrelation) experiments were utilized to produce X H and U C chemical shift correlation plots for each complex which permitted complete spectral 60 assignment. An illustrative example of the analysis of the spectra of CpMo(NO)(»? -trans-(E,Z)-2,4-hexadiene) has been presented as an example of the procedure.7 Particularly noteworthy aspects of this NMR study include the following: (1) The configurations of the diene ligands in these systems can be reliably established from the NMR spectra of the r?4-diene complexes. For example, complexes 10 and 11 contain a symmetrically substituted diene complexed to the Cp'Mo(NO) fragment in a cis fashion. Consequently, the *H and B C NMR spectra of these compounds reflect the fact that their molecular structures possess mirror symmetry. In contrast, complexes 1-9 (and other CpMo(NO)(r/4-rran5-diene) complexes prepared)7 contain dienes (both symmetrically and unsymmetrically substituted) coordinated to the metal center in a twisted, transoidal manner. Accordingly, the two ends of the diene ligand are inequivalent (see Table 2.2), this inequivalence being readily apparent in the NMR spectra. Examples of this are seen in Figures 2.3 and 2.4 which contain the 1 H NMR spectra of 8 and 10, respectively. (2) Each compound which contains an unsymmetrically substituted diene ligand (eg. 1,2, 4, 7) exists in solutions as a mixture of diastereomers.17 The NMR spectra of these complexes confirm that these diastereomers simply have opposite faces of the diene bound to the metal center. The ratio of the diastereomers in a particular solution does not change with time, but varies slightly from solution to solution. (3) Most importantly, the J H and ^ C NMR spectra of 1-10 and the other rj 4-diene complexes of Cp'Mo(NO) reported earlier7 provide valuable information about the nature of the molybdenum-diene bonding in these systems. In valence bond terms, the metal-diene bonding in the ds-diene complexes may be Figure 2.3. 300 MHz *H NMR Spectrum of 8 in C 6 D 6 . Figure 2.4. 300 MHz lH NMR Spectrum of 10 in C 6 D 6 . 63 represented by a resonance hybrid involving forms IA (the x, x form) and IB (the a2, x form)18 shown below. Mo Mo-V v IA IB Most cw-diene complexes of the later transition metals are best described as being classically x-bonded (i.e., mainly IA) while those of the early transition metals adopt principally the a2, x mode of interaction (i.e., IB).1 8 By analogy to these cw-diene complexes, a similar bonding description has been invoked for the r;4-/ra/w-diene compounds, i.e., IIA IIB IIA being the x, x form and IIB being the a 2 , x form. The differences between the A and B resonance forms should be readily detectable by NMR spectroscopy since the terminal carbons in the A forms are sp2 hybridized while they are sp3 hybridized in the B forms. The carbon-proton coupling constants evident in the D C NMR spectrum of 10 (VCH = 161.3 Hz) are similar in magnitude to what is expected for an sp2 hybridized carbon (VQJ* 140 -160 Hz) 1 9 and somewhat larger than that observed for Cp2ZT(n4-cis-C 4 H 6 ) , a a2, x system (i.e., lJCn = 144 Hz). 2 0 In addition, the protons of the terminal carbon of the diene ligand in 10 exhibit a geminal coupling constant of 4.9 Hz, a value intermediate to that observed for (OC)3Fe(r?4-cty-C4H6)21 (i.e./jjjj = 2.4 Hz) and Cp2Zr(T74-cw-C4H6) (i-c./jjjj = 10.0 Hz). It thus appears that the molybdenum-diene linkage in 10 (and also in 11) more closely resembles that extant in later transition-metal 64 diene complexes. Certainly, resonance form IA makes a larger contribution to the metal-diene bonding in the Cp'Mo(NO) systems than in the related Cp 2Zr systems. In contrast, the trans-dxent complexes 1-9 (and other C"pMo(NO)(rj4-frans-diene) complexes7) exhibit very small geminal coupling constants (i.e., Jn.^- J41-42 = 2-4 Hz) which approach those of free butadiene22 (i.e., 1.7 Hz) and are somewhat smaller than those exhibited by the Cp2Zr(r;4-fra/is-diene) complexes (4-6 Hz). In addition, the vicinal couplings displayed by the Cp'Mo(NO)(r74-fram-diene) complexes (/n_2i~ 3^2-41 = H -15 Hz) are smaller than those exhibited by the zirconocene fraro-diene complexes (i.e., 16 Hz) or free butadiene (i.e., 16.9 Hz). Finally, the coupling constants across the C2-C3 bond for the molybdenum r/4-fra/w-diene complexes C21-32 = 10-12 Hz) are similar in magnitude to that of free butadiene (/ = 10.3 Hz) but smaller than that measured for the zirconocene systems (i.e., 15 -16 Hz). These spectral properties indicate that the resonance form HB is a larger contributor to the molybdenum-diene bonding in the CpMo(NO)(r;4-rra/is-diene) compounds than it is in the analogous Cp2Zr(r?4-fra/u-diene) systems. To summarize the NMR studies then, the data suggests that the cis- and fra/ts-diene ligands are bonded to the Cp'Mo(NO) fragment in a similar manner, resonance hybrids A and B making approximately equal contributions to the metal-diene bonding in each case. This conclusion is supported by the fact that the diene carbon-carbon bond lengths in the solid state molecular structures of CpMo(NO)(r/4-fran5-2,5-dimethyl-2,4-hexadiene), CpMo(NO)(r?4-cis-2,3-dimethylbutadiene)2 and Cp*Mo(NO)(j?4-fro/ts-(£)- 1,3-pentadiene) are approximately equivalent (1.385 to 1.425 A) . This contrasts with the intramolecular dimensions of the related zirconocene compounds, as might be expected on the basis of the NMR properties. In the zirconium system, the cw-diene complexes exhibit short-long-short bond length alternation (i.e. principally IB) while the fra/ts-diene analogues display the short-long-short alternation (i.e. HA) similar to that of free butadiene. 65 Electrochemical Studies of CpMo(NO)fo4-diene) Complexes. In order to better understand the stability and reactivity of the n4<is- and r)4-trans-diene complexes, electrochemical studies were carried out on two representative complexes, CpMo(NO) (rj4-rro7ty-2,5-dimethyl-2,4-hexadiene), 3, and CpMo(NO)(r?4<£r-2,3-dimethylbutadiene), 11.' The cyclic voltammograms of these two complexes are shown in Figure 2.5. Qualitatively, they look quite similar. The cyclic voltammograms indicate that the diene complexes are reasonably stable with respect to reduction since no electroactive reduction behavior is observed to the limit of the solvent ("-2 V). This is consistent with the fact that each of these complexes has a large HOMO - LUMO energy gap, as shown by Fenske-Hall molecular-orbital calculations on the CpMo(NO) (butadiene) model systems.215 Since the compounds are formed under highly reducing conditions (i.e. Na/Hg), this is not a surprising result. It also may explain why these complexes are isolable as they are not readily reduced and therefore stable in the presence of sodium. The fact that the fra/w-diene complex reacts rather reluctantly with Lewis bases such as phosphines23 may also be partially explained on the basis of this result since it has been suggested that some such reactions may proceed via electron transfer rather than simple nucleophilic displacement.24 On the other hand, both 3 and 11 are quite readily oxidized and, in fact, each of them exhibits three irreversible oxidations before the solvent limit of " + 2V. In accord with these results are the Fenske-Hall molecular-orbital calculations which have shown that the second and third highest occupied molecular-orbitals are close in energy to the HOM0. 2 b Thus, it is not surprising that multiple oxidations are seen in the cyclic voltammograms of 3 and 11. In addition, the fact that the peaks are so close together is an indication that in their reactions with electrophiles, concomitant oxidations may occur. Indeed, preliminary studies to date of the rj4-/ra/ts-diene systems seem to confirm these predictions. 66 + 2 —r~ + 1 T -0 -2 Volts VsAg-Wire Figure 2.5. Cyclic Voltammograms of 3 (a) and 11 (b) in CH2C1. 67 REFERENCES Nurse, C. R. Ph. D. Thesis, University of British Columbia, 1983. (a) Hunter, A. D.; Legzdins, P.; Nurse, C. R.; Einstein, F. W. B.; Willis, A. C. /. Am. Chem. Soc. 1985,101,1791. (b) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. C ; Bursten, B. E.; Gatter, M. G. /. Am Chem Soc. 1986,708,3843. (a) Drezdzon, M. A.; Shriver, D. F. The Manipulations of Air-sensitive Compounds', 2nd ed.; John Wiley and Sons: New York, 1986. (b) "Experimental Organometallic Chemistry, a Practicum in Synthesis and Characterization"; Darensbourg, M. Y.; Wayda, A. L., editors; American Chemical Society : Washington, D. C , 1987. Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; 2nd ed.; Pergamon Press: Oxford, 1980. Shaka, A. J.; Keeler, J.; Freeman, R. /. Mag. Res. 1983,53, 313. Seddon, D.; Kita, W.G.; Bray, J.; McCleverty, J. A. Inorg. Synth. 1976,16, 24. (a) Christensen, N. J.; Hunter, A. D.; Legzdins, P. Organometallics 1989,8, 930. (b) Hunter, A. D. Ph. D. Thesis. University of British Columbia, 1985. Yasuda, H. ; Nakano, Y.; Natsukawa, K.; Tani, H. Macromolecules 1978,11, 586. (a) Legzdins, P.; Wassink, B. Organometallics 1984,3,1811. (b) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics 1989,8,1485. Nicholson, R. S. Anal Chem. 1966,38,1406. Holloway, J. D. L ; Geiger, W. E. /. Am Chem Soc. 1979,101,2038. Drago, R. S. Pure Appl Chem 1980,52,2261. James, T. A.; McCleverty, J. A. /. Chem Soc A. 1971,1068. For example: (a) Hoyano, J. K.; Legzdins, P.; Malito, J. T. Inorg. Synth. 1974,18, 126. (b) Legzdins, P.; Wassink, B. Organometallics 1988, 7,482, and references therein, (c) Legzdins, P.; Rettig, S.; Sanchez, L Organometallics 1988, 7,2394. 68 (15) Hunter, A. D.; Legzdins, P. Organometallics 1986,4,1001. (16) (a) Greenhough, T. J.; Legzdins, P.; Martin, D. T.; Trotter, J. Inorg. Chem. 1979, 11,3268. (b) Faller, J. W.; Shvo, Y. /. Am Chem Soc. 1980,102, 5396. (17) Each Cp'Mo(NO)(rj4-rram-diene) complex must exist as a mixture of enantiomers. (18) (a) Yasuda, H. ; Nakamura, A. Angew. Chem, Int. Ed. Engl 1987,26,123. (b) Erker, G.; Kruger, C. Adv. Organomet. Chem 1985,24,1. (19) (a) Levy, G. C.; Lichter, R. L.; Nelson, G. L. Carbon-13 Nuclear Magnetic Resonance Spectroscopy, 2nd ed.; John Wiley and Sons: New York, 1980. (b) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic Compounds; Academic Press: London, England, 1981. (c) Becker, E. B. High Resolution NMR: Theory and Chemical Applications, 2nd ed.; Academic Press: New York, 1980. (20) Erker, G.; Wicher, J.; Engel, K.; Kruger, C. Chem. Ber. 1982,115,3300. (21) Bachmann K.; von Philipsborn, W. Org. Magn. Reson. 1976,8, 648. (22) (a) Segre, A. L.; Zette, L.; DiCorato, A. /. Mol Spectrosc. 1969,32, 296. (b) Marais, D. J.; Sheppard, N.; Stoicheff, B. P. Tetrahedron 1962,17, 613. (23) Chapter 4 of this thesis. (24) Kochi, J. K. /. Organomet. Chem. 1986,300, 139, and references therein. 69 CHAPTER 3 A Preliminary Kinetic Study of the Isomerization of CpMo(NO)(n4-cw-2,3-dimethylbutadiene) to CpMo(NO) (r? 4-rra/w-2,3-dimethylbutadiene) 70 Introduction As outlined in Chapter 2, when the diene used in eq 1 is 2,3-dimethylbutadiene other products of reactions 1, namely the r;4-cty-2,3-dimethylbutadiene complexes, may be isolated in addition to the 7?4-/ranj-2,3-dimethylbutadiene compounds, i.e., [Cp'Mo(NO)I2]2 + 4 Na/Hg + 2 (2,3-dimethylbutadiene) Cp'Mo(NO)(r;4-cis-2,3-dimethylbutadiene) + Cp'Mo(NO)(r/4-fran5-2,3-dimethylbutadiene) (1) In these systems the r/4-cw-diene complexes are the kinetic products of the reactions 1 while the r?4-taz/w-diene complexes are the thermodynamic products, a fact which is established by the observation that the isolated r/4-cu-diene complexes isomerize to the 774-fra/u-diene complexes in solutions (eq 2). Once formed, ther?4-trans-2,3-dimethylbutadiene complexes do not convert to their cw-diene analogues. Cp'Mo(NO)(r/ 4-cw-2,3-dimethylbutadiene) « x * Cp'Mo(NO)(r> 4-tazra-2,3-dimethylbutadiene) (2) This behavior contrasts with that of the <"^Zr(»74-2,3-dimethylbutadiene) complexes for which the isomers having the »74-c£s-diene ligands are thermodynamically preferred, and the two isomers are in rapid equilibrium.1 A theoretical study using Fenske-Hall MO calculations comparing the model systems, CpMo(NO)(7j4-frans-butadiene) and CpMo(NO)(»74-c£s-butadiene)2 has shown that the principal factors responsible for the preferential trans attachment of butadienes 71 to the CpMo(NO) fragment are electronic in nature, the H O M O of the trans-diene complex being ca. 0.9 eV (86 kJ mol"1) lower in energy than that of the cw-diene compound. It is interesting that during the syntheses of all the Cp'Mo(NO)(^4-fra/is-diene) compounds evidence for cis-diene complexes is observed only for 2,3-dimethylbutadiene. There are at least two explanations which may be presented to account for this observation. Steric interaction between the substituents of the diene ligand and the Cp1 rings may play a very important part in determining the final mode of linkage of the diene ligand. As well, steric effects may prevent the formation of the intermediates necessary for the formation of the cis-diene complex. If this were the case, however, it might be expected that cis-diene complexes, as intermediates, would be observed for any of the reactions 1 where the diene is substituted at the second carbon (i.e., 2-methylbutadiene and 2,4-dimethyl-1,3-pentadiene metal complexes have been prepared).3 In these systems, the product ratios of the two isomers formed [i.e., with the substituent of C2 either up , toward the Cp* ring (i.e. A), or down, away from the Cp' ring (i.e. B)] 2-methylbutadiene 2,4-dimethyl-l,3-pentadiene A N O O B M o Mo O O 72 are A:B = 1:3 and 1:4 for the 2-methylbutadiene and 2,4-dimethyl-l,3-pentadiene complexes, respectively, indicating that perhaps steric factors are significant. However, no t-No bands assignable to cis-diene complexes are observed in the IR spectra obtained during the course of the reactions used to prepare these two diene complexes, even at low temperatures. It may be, however, that the V^Q bands due to the n4<is-diene compounds are obscured by the i/^o bands of other intermediates and therefore are not observed. If it is the steric interaction which most directly determines the coordination mode of the diene ligand in any products or intermediates, it is clear that this effect is observed only when the diene used in reactions 1 is 2,3-disubstituted. This situation may be compared to the other systems which are known to coordinate dienes in an s-trans r?4-fashion, namely, Cp'2M(diene) (M = Zr, Hf) and Cp'M(diene)2 (M = Nb, Ta) where it has been shown that, to date, every complex containing a diene which is substituted at C2 exists mainly as the s-cis conformer due to steric conflicts between the C2 substituent and the Cp' ligands in the trans-dienc complexes.13 In fact, for the zirconocene and hafnocene systems, the solutions of the diene complexes contain only the cis-diene isomers at room temperature. Tatsumi and co-workers,4 in their molecular orbital analyses of Fe(CO)3(butadiene) and Cp2Zr(butadiene), have shown that the electronic properties of the metal fragments primarily determine the preference of the metal fragment for a 5-cis or s-trans diene ligand. Once the metal fragment has "decided" its electronic preference for dienes in the s-cis or s-trans conformation, then steric factors are influential in determining which conformer will be observed. Another explanation which may account for why the only »j4-cis-diene complexes obtained in the Cp'Mo(NO)systems are those with 2,3-dimethylbutadiene may be that uncomplexed 2,3-dimethylbutadiene is significantly different than the other 1,3-dienes studied. Tai and Allinger5 have calculated that the difference in energy (A E) 73 between s-mwis-2,3-dimefhylbutadiene and .s-ds-2,3-dimethylbutadiene, as free dienes, is 0.78 kcal mol"1 (33 kJ mol"1) and that the ds-diene conformer is not planar, but twisted by 43°. Furthermore, they determined that the twisted ds-diene conformer has a reduced C2 - C3 bond order and is lengthened by 0.015 while the CI - C2 and C3 - C4 bonds are correspondingly shortened by 0.005 . The A E values determined for some other dienes (i.e., butadiene, 2-methylbutadiene, and 1,3-pentadiene) are in the vicinity of 2.7 kcal mol"1 (11.3 kJ mol"1) in favor of the fra/zs-diene analogues. They state that if A E is 1.0 kcal mol"1 the s-cis/s-trans population ratio would be 0.2 at room temperature which implies that there is a significant population of s-ds-2,3-dimethylbutadiene present during the reductions 2. In contrast, a much smaller ds-diene population is available for the other acyclic, conjugated dienes studied. The energy difference between CpMo(NO)(»74-ds-2,3-butadiene) and CpMo(NO)(r74-fran5-2,3-butadiene), model ds- and fra/is-diene complexes, has been determined by Fenske - Hall molecular-orbital calculations to be 0.9 eV (86 kJ mol"1).2 This number is less than that determined for Fe(CO)3(r?4- s-ds-butadiene) and Fe(CO)3(»74-fra/ts-butadiene) (1.3 eV, 125 kJ mol"1) which exist only as the s-cis conformer and more than that calculated for Cp2Zr(r;4-cis-butadiene) and Cp2Zr(r?4-fra/ts-butadiene) (0.07 eV, 6.7 kJ mol"1, favoring the ds-diene complex) which exist as a mixture (55/45 cis/trans) at room temperature 4 While it is acknowledged that it is difficult to compare molecular orbital calculations done by different researchers, the numbers are consistent with experimental observations. In actuality, it is only possible to determine why the products are the isomers they are and why others are not observed only through a detailed analysis of the mechanism of formation of these Cp'Mo(NO)(r74-diene) compounds. This is presently impossible due to the fact that the method of formation of these species (eq 1,2) is very complex with several observable intermediates and a significant amount of 74 decomposition of organometallic compounds occurring in the reaction mixture. It is, however, possible to study the isomerizations 2 in order to better understand the relationship between Cp'Mo(NO)(f/4-ciy-2,3-dimethylbutadiene) and Cp'Mo(NO)(r?4-/ra/w-2,3-dimethylbutadiene). To this end, a preliminary kinetic study of the isomerization was carried out. The results of this work are presented in this chapter. 75 Experimental Section The isomerizations were monitored by 1 H NMR spectroscopy using a 25 s relaxation delay on a Bruker WP-80 MHz spectrometer at ambient temperatures. Sixteen FTD's were collected for each spectrum. The data collected were processed using the LOTUS 1-2-3 data processing program. The graphs, Fig. 3.1 - 3.8 were plotted with a Hewlett Packard Plotter, Model HP 7090A. Sample Preparation: In a glove box, a sample of a mixture of CpMo(NO)(r?4-rra/w-2,3-dimethylbutadiene) and CpMo(NO)(r?4-cis-2,3-dimethylbutadiene)6 was weighed accurately (0.0251 g, 0.0920 mmol) and dissolved in benzene-^ (2.0 mL). Two NMR tubes were loaded with this solution (0.046 M). In a similar fashion, two samples of concentration 0.031 M were prepared. The NMR tubes were flame-sealed under a vacuum, and a X H NMR spectrum of each one was obtained to measure the initial concentration of CpMo(NO)(rj4-cis-2,3-dimethylbutadiene). One sample at each concentration was maintained at 8°C (± 1°C) while the other was maintained at 20°C(± 1°C). Data Collection: Periodically, 1 H NMR spectra were collected for each of the samples. The concentration of the cis-diene complex was determined by calculation of a ratio of the integration values for the resonances of the hydrogens on the two different Cp rings [8: 5.13 (cis); 5.04 (trans) ppm] in the spectrum to determine the percentage of the total concentration which was the cis-diene complex. As an additional check, similar ratios of the integration of the signals due to the methyl hydrogen resonances [6: 2.22 (cis); 1.70, 0.90 (trans) ppm] of the two complexes were made, and the concentration of the cis-diene complex was determined. 76 (8) „ (Thousand*) TIME (h) 0 . 0 2 6 (b) B •xpfl (Thousands) TIME (h) ' r o Q r o s s i o n Figure 3.1. Plots of Concentration of CpMo(NO)(rj4-cis-23-dimethylbutadiene) (M) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 8°C. 77 5 (b) 0.04S 0.04 0.035 0.03 0.025 0.02 -0.015 -0.01 -'b coos -) 1 1 r 0.045 0.04 -0.035 0.03 -H 0.025 -0.02 0.015 -0.01 -0.005 • • ~l 1 1 1 1 1 1 1 1 — T 0.4 0.8 1.2 1.6 2 (ThouMnds) T H E (h) D wept* I • ntQ*~*m!on Figure 3.2. Plots of Concentration of CpMo(NO)(r?4<w-23-dimethylbutadiene) (M) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 8°C. 78 (a) ii (b> a ~i 1 r 0.4 0.6 fThouaonda) T 0.2 a «acp f l Figure 33. Plots of Concentration of C^Mo(NO)(r;4<is-2,3-dimethylbutadiene) (M) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 20°C. 79 (a) g 0.045 0.04 0.035 0.03 -0.025 0.02 -0.015 0.01 -0.005 -0.2 "i 1 r 0.4 0.6 (Thousands) TME (h) (b) m 5 0.6 (Thouaanda) Figure 3.4. Plots of Concentration of CpMo(NO)(r;4-cw-2,3-dimethylbutadiene) (M) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 20°C 80 (a) 8 (Thousand*) TIME (h) (b) = in 0 -| 1 • x p f i (Thousands) me (h) ilon Figure 3.5. Plots of the Natural Logarithm (ln) of the Concentration of CpMo(NO)(r/4-cts-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 8°C. 81 -3.1 » -4.6 H 1 1 1 1 1 1 1 1 1 1 1 1 1 r-0 0.4 0.8 1.2 1.6 2 2.4 2.8 TIME (h) T 1 1 1 1 1 1 1 1 1 1 1 1 1 r~ 0 0.4 0.8 142 1.6 2 2.4 2.8 (Thouaanaa) TIME (h) D wprl ragracslon Figure 3.6. Plots of the Natural Logarithm (In) of the Concentration of CpMo(NO)(r?4-c«-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 8°C. 82 (a) « o LJ >—' c 0.6 (Thousands) TIME (h) (b) in O Figure 3.7. Plots of the Natural Logarithm (ln) of the Concentration of CpMo(NO)(r?4-cw-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.031 M at 20°C. 83 -3 -5.6 H 1 1 1 1 1 1 1 1 1 r 0 0.2 0.4 0.6 0.8 1 (Thousand*) TIME (h) -3 -5.8 H 1 1 1 1 1 1 1 1 1 r 0 0.2 0.4 0.6 0.8 1 (Thousands) TIME (h) Figure 3.8. Plots of the Natural Logarithm (In) of the Concentration of CpMo(NO)(r?4<w-2,3-dimethylbutadiene) versus Time for the Isomerizations. (a) Data from Integration of Cp Proton Resonances, (b) Data from Integration of Methyl Proton Resonances for the Sample of Concentration 0.046 M at 20°C. Table 3.1. Kinetic Data for the Zero-Order Isomerizations from the Graphs in Figures 3.1 - 3.4. Solution Temperature Figure R2 Concentration °C Number 0.031 M 8 3.1a 0.9454 3.1b 0.9557 0.046 M 3.2a 0.8972 3.2b 0.9069 0.031 M 20 3.3a 0.9370 3.3b 0.9190 0.046 M 3.4a 0.8964 3.4b 0.9057 Table 32. Kinetic Data for the First-Order Isomerizations from the Graphs in Figures 3.5 - 3.8. Solution Temperature Figure k R2 Concentration °C Number (br1) 0.031 M 8 3.5a 3.12 xlO"4 0.9867 3.5b 3.07 xlO 4 0.9879 0.046 M 3.6a 4.45 xl ( H 0.9712 3.6b 4.55 x KT4 0.9792 0.031 M 20 3.7a 1.92 xl(r 3 0.9925 3.7b 1.88 x l(r 3 0.9931 0.046 M 3.8a 2.15 xl(r 3 0.9727 3.8b 2.27 x IO"3 0.9766 86 Kinetic Study Results The samples used to study the isomerizations (eq 2) were prepared as outlined in the Experimental Section. Figures 3.1 - 3.4 show the zero-order plots of concentration of CpMo(NO)(r74-5-cw-2,3-dimethylbutadiene) versus time for each ofthe reactions studied. These are presented to illustrate how badly the data fits a line when plotted as concentration versus time (zero-order). Table 3.1 contains the values for R 2 as determined by a least-squares linear regression of the data displayed in Figures 3.1 -3.4. It is clear from the Figures and the R 2 values (Table 3.1) that the reaction is not zero-order. The rate equation for the disappearance of the ds-diene complex in a first-order reaction is7 If the concentration of C is assumed to be [C]o at time zero (t = 0), equation 5 may be integrated between limits [C]Q to [C] and 0 to t to yield and thus a plot of ln [C] versus time should yield a line whose negative slope is equal to the rate constant, k. Figures 3.5 - 3.8 show the first-order plots of the natural logarithm of the concentration of CpMo(NO)(f?4-cw-2,3-dimethylbutadiene) versus time for each of the reactions. The values obtained from those graphs for k are shown in Table 3.2. If a first-order rate law is assumed (Figures 3.5 - 3.8, Table 3.2), fcave is found to be 1.05 x 10"7 s"1 at 8°C and 5.71 x 10"7 s"1 at 20°C. The dependence of the rate constants on the temperature may be represented by an empirical equation proposed by Arrhenius, - d[C]/dt = k[C] (3) ln[C] = -At + ln[C] 0 (4) 87 k = A e ^ / * 7 (5) where A is the pre-exponential factor, also called a frequency factor, and E a is the activation energy. Written in logarithmic form, eq 5 becomes In = In A - E a / R T (6) Differentiation of eq 6 with respect to temperature and integrating between limits, and T 2 , yields In (k2/k{) = [Ea/R][(T2 - TiVOIY^)] (7) Substitution of the first-order data into eqs 6 and 7 gives values of 96 kJ mol"1 (23 kcal mol"1) and 8.3 x 1010 s"1 for E a and A, respectively. An estimation of AG* may be made using the available data and eq 8.7 k = [kT/h ] exp(-A G* /RT) (8) Using this method, AG* for the first-order reaction is determined to be 110 kJ mol"1 (26 kcal mol"1). This number is similar in magnitude to those determined for the trans-to-cis isomerization in the zirconocene systems and consistent with the cis-to-trans isomerization in the molybdenum system not involving any bond breaking8 but rather, the expected slippage of the diene ligand to an ^ -coordination as proposed below. 88 Discussion The value of A G * 9 determined for the isomerization of CpMo(NO)(r/4-as-2,3-dimethylbutadiene) to CpMo(NO)(r;4-fram-23-dimethylbutadiene) is in the same range as those determined for the isomerizations of C^Zr^-fra/ts-diene) to Cp2Zr(»j4-ds-diene) [diene = butadiene (22.7 kcal mol"1), 2-methylbutadiene (19.6 kcal mol"1), 2,3-dimethylbutadiene (18.2 kcal mol"1), and 2,3-diphenylbutadiene (21.0 kcal mol"1)]. These AG* values are also in the same range as those determined for the metallacyclopentene ring inversion of some early transition-metal and actinide complexes,8 and significantly lower than the metal-carbon bond disruption enthalpies (£>M-C) determined experimentally for a series of thorium alkyl complexes.8 A plausible mechanism for the isomerization is outlined below. 1 2 3 4 Mo = CpMo(NO) Scheme 3.1. It is proposed that the diene ligand slips to an ^-coordination mode (2), twists to the thermodynamically preferred conformation5 (s-trans) (3), and then coordinates the double bond to attain an r/4-coordination of the diene ligand. Additional support for a mechanism such as this comes from the fact that CpMo(NO)(rj4-cw-2,3-dimethyibutadiene) reacts faster with phosphines than does its trans-diene analogue to form CpMo(NO)(r/2-2,3-89 dimethylbutadiene)(PMePh2).10 This provides some evidence for the equihbrium 1 *» 2 in Scheme 3.1, although it does not rule out slippage of the i75-C5H5 ring to rj3, a process for which there is some precedence.11 Neither of these proposed intermediates is observed by XH NMR spectroscopy. The above mechanism is similar to that proposed for the isomerization of Cp2Zr(r;4-trans-diene) to Cp2Zr(r;4-cw-diene)12 shown in Scheme 3.2. C p ' 2 M Cp' 2 M — Cp'2M<LV5 1 2 3 4 Scheme 3.2. In the zirconocene system, it was found that the overall isomerization 1 -» 4 follows a first-order rate law. The equilibrium 1» 2 is very facile while there is a substantial energy barrier between 2 and 3. This may be expected since the transformation of 2-» 3 involves the formation of two metal-carbon a-bonds. Erker and co-workers also presented evidence for x-allyl intermediates in the isomerizations, i.e.,13 CpsZr' These intermediates account for the isomerizations observed within the diene ligands, e.g., \ / -25 *C o, 90 Isomerizations of this type are not observed in the case of CpMo(NO)(r74-fnwty-diene) implying that no such radical isomerization is occurring in those systems.3 This study, which is admittedly very preliminary in nature, has nonetheless given us a starting point from which to base further investigations into this system. It is clear that a more comprehensive kinetic analysis is needed in order to produce more reliable data. Notwithstanding that, the present study merits some discussion. The isomerizations are very slow as evidenced by the fact that the first half of the cis-diene complex is not consumed until after approximately 1200 h (50 d) at 8°C and 700 h (30 d) at 20°C. A study of a reaction with a half-life of greater than -10 days using conventional techniques may not produce reliable data.7 In addition, the molybdenum diene complexes are known to be somewhat thermally sensitive.33 Although the samples looked to be clear solutions until the reactions were almost 75% complete, small amounts of decomposition products may have been present. Unfortunately, an internal standard was not used in these studies, and any decomposition compounds were not observed until they were present in quantities large enough to be visually observed, at which time data collection was discontinued. No soluble impurities were observed by 1 H NMR spectroscopy. It is noted that when the data is plotted as In [C] vs time, it fits a straight line much better than when it is plotted as [C] vs time. In fact, the values of R 2 , the coefficient of determination and a measure of how well the data fit a line, fall in the range of 0.896 -0.956 for the zero-order plots (Table 3.1) as compared to values of 0.971 - 0.997 for the first-order plots (Table 3.2). It is acknowledged that the range 0.971 - 0.997 is not a range which implies excellent data and that R 2 values of 0.99 or higher are more desirable. It is somewhat apparent from the graphs in Figures 3.5 - 3.8, however, that the initial data points are less scattered than the final data points, and perhaps a weighted regression analysis would yield lines of better fit. Because of significant experimental error in this study, however, a more complex treatment of the data was not performed. 91 It is rather difficult to estimate the error in this study, but it is important to attempt to do so. Several changes need to be incorporated in the study to yield more reliable data. The changes in the concentration of CpMo(NO)(f?4-cw-2,3-dimethylbutadiene) were measured using the integrals on the *H NMR spectra. The error in this measurement is estimated to be -1%. The use of an internal standard would help to determine if the total concentration of diene complexes ([cis] + [trans]) is indeed constant or if any of the starting or product complexes were lost due to decomposition during the experiment. It would be useful to monitor the isomerization over a larger range of concentrations to determine if the reactions then obey first-order kinetics at all concentrations. If reactions were monitored at higher temperatures, the isomerizations would not take so long to go to completion, and thus may yield more reliable data. To check consistency, more than one sample at each concentration and temperature should be monitored. It is expecially important to note that the temperature should be well controlled and a temperature range of ± 1°C (as was the case in this study) is not acceptable. It is important to document solvent effects, as well. It was initially suggested that the isomerizations are significantly faster in the coordinating solvent, THF, than they are in Q D 6 , 3 but this suggestion is now open to question. There are other spectroscopic techniques which may be more effective in monitoring the isomerization. Since the solution color changes from the red of the cis-diene complex to the yellow of the trans-diene compound during the isomerization, it may be possible to monitor the isomerization by UV-vis spectroscopy. The two complexes, CpMo(NO)(r>4-cis-2,3-dimethylbutadiene) and CpMo(NO)(r?4-rra/u-2,3-dimethylbutadiene) exhibit significantly different J^NO stretching bands in the IR spectra and it may be possible to use IR spectroscopy to monitor the isomerization. We are, at present, developing the methodology to monitor reactions by UV-vis and IR spectroscopy in our laboratories. It should be mentioned that the isomerizations proceed much more quickly in conventional lab glassware than in the NMR sample tubes. The reason for this is not clear, but it may be 92 speculated that the presence of adventitious air may affect the rate of isomerization. NMR sample tubes are flame-sealed under vacuum. Laboratory glassware is cleaned in a different fashion than the NMR sample tubes, and thus it would not be surprising if residues on the glass of either type of vessel helped or hindered the isomerization. However, since the isomerizations in the new NMR tubes give the longest reaction times, it is the method of choice for the kinetic study. The results which emanate from this analysis indicate that a thorough kinetic analysis will most likely yield some interesting and reliable data. The values obtained for E a and AG* are reasonable and in the range expected. It is clear, however, that future investigations of this system must incorporate careful precautions to prevent decomposition of the cis- and trans- diene complexes. The present study has given a base from which to build, and the suggested changes will no doubt allow for a successful study. 93 REFERENCES (a) Yasuda, H. ; Nakamura, A. Angew. Chem. Int. Ed. Engl 1987,26,723. (b) Erker, G.; Kruger, G ; Muller, G. Adv. Organomet. Chem 1985,24,1. Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. C ; Bursten, B. E.; Gatter, M . G. / . Am. Chem. Soc. 1986,108,3843. (a) Christensen, N. J.; Hunter, A. D.; Legzdins, P. Organometallics 1989,8, 930. (b) Chapter 2 of this thesis. Tatsumi, K.; Yasuda, H. ; Nakamura, A. Isr. J. Chem. 1983,23,145. Tai, J. CV, Allinger, N. L. /. Am. Chem. Soc. 1976, 98, 7928. See Chapter 2, Experimental Section for the method of preparation of this mixture. Alberty, R. A. Physical Chemistry; John Wiley and Sons: Toronto, 1983; pp 602 - 619. Smith, G. M.; Suzuki, H. ; Sonnengerger, D. G ; Day, V. W.; Marks, T. J. Organometallics 1986,5, 549 and references therein. Unfortunately, due to the nature of the data, the errors in AG* are not known. Hunter, A. D.; Legzdins, P., unpublished observations. (a) Christensen, N. J.; Hunter, A. D.; Legzdins, P.; Sanchez, L Inorg. Chem. 1987,26, 3344, and references therein, (b) Chapter 5 of this thesis. Erker, G.; Wicher, J.; Engel, K.; Rosenfeldt, F.; Dietrich, W.; Kruger, C. /. Am. Chem Soc. 1980,102, 6344. Erker, G.; Engel, K.; Korek, U. ; Czisch, P.; Berke, H. ; Caubere, P.; Vanderesse, R. Organometallics 1985,4,1531. 94 Chapter 4 Reactivity Studies of the Cp'Mo(NO)(r/4-frarts-diene) Complexes 95 Introduction It is expected that coordination of a diene ligand to a metal center will change the electronic structure of the diene ligand and thus change its reactivity with electrophiles and nucleophiles, with respect to the reactivity of the free diene. It is generally found that the reactivity is varied, and, in several cases, the nature of the product is influenced significantly by the reaction conditions and the specific nature of the starting diene complex. It is therefore of interest to study the reactions ofthe Cp'Mo(NO)(Tj4-/rtms-diene) [Cp' = Cp ( r j 5 - C 5 H 5 ) or Cp* (r / 5 -C 5 Me 5 )] complexes with various electrophiles and nucleophiles. Electrochemical studies of a member of this class of diene complexes, namely CpMo(NO)(r/4-rrtmy-2,5-dimethyl-2,4-hexadiene),1 show that it is stable with respect to reduction to —2 V in CH 2C1 2 . This stability is not surprising as the Cp'Mo(NO)(n*-trans-diene) complexes are formed in the presence of a strong reducing agent (i.e. Na/Hg). It is expected that this property would be reflected in the reactions of these diene complexes with nucleophiles. On the other hand, the cyclic voltammogram of CpMo(NO)(j)4-trans-2,5-dimethyl-2,4-hexadiene) shows three close irreversible oxidation waves before the solvent limit of - + 2 V. This leads to the expectation that reactions of these molybdenum diene complexes with electrophiles may lead to oxidation of the metal complex. In this chapter, the results of some reactivity studies are summarized. Specifically, the reactions of some representative Cp'Mo(NO)(r/4-frarts-diene) complexes with phosphines, carbon monoxide, alkynes and ketones are described. In agreement with the electrochemical analysis, the products of these reactions are formed rather slowly in the reaction mixtures and are not the products of a reduction, but rather a coupling reaction. The results of a brief study of the reactions of two representative complexes with protonic acids are also described. 96 Experimental Section General experimental procedures are outlined in the Experimental Section of Chapter 2. Some additional NMR experiments used in these studies are described below. They were conducted with Varian's pulse sequences on a Varian XL300 spectrophotometer. Selective Insensitive Nuclei Enhancement through Polarization Transfer (SINEPT): These spectra were collected with a 90° U C pulse of 18 us. The low-power proton pulse from the decoupler was 46 us. The decoupler was on during acquisitions (0.4 s) and off between acquisitions (2 s). The number of incremental spectra was determined according to the concentration of the individual samples, and spectra with adequate signal-to-noise were obtainable in 4 -12 h. 2-Dimensional Correlation Spectroscopy (2D-COSY): These homonuclear correlation spectra were collected using a 90° 1 H pulse of 45 us. A delay period of 1 s was used between acquisitions (0.32 s). A 2-D Fourier transformation resulted in 2D-spectra with adequate signal-to-noise after 6 -12 h of data collection, depending on the concentration of the individual sample. Reaction of CpMo(NO)074-/ra/u-2,5-dimethyl-2,4-hexadiene) with CO. A yellow hexanes (50 mL) solution of CpMo(NO)(r;4-rran5-2,5-dimethyl-2,4-hexadiene) (0.10 g, 0.33 mmol) which exhibited a f N O at 1622 cm"1 in its IR spectrum was transferred by cannula into a Fisher-Porter vessel. The vessel was pressurized to 60 psig with CO, and the yellow solution was stirred at room temperature for 5 days during which time a small amount of a brown precipitate appeared. The final supernatant solution's IR spectrum displayed a v C ( at 1960 cm"1 and a « / N O at 1649 cm"1 after the pressure had been released. The final reaction mixture was filtered through a short (3x2 cm) column of Celite supported on a medium porosity frit, and the column was washed with hexanes. The filtrate and washings were combined and dried in vacuo to yield 0.09 g (81% yield) of CpMo(NO)(CO)(r?2-2,5-97 dimethyl-2,4-hexadiene) (1) as bright yellow crystals. Anal. Calcd. for C 1 4 H 1 9 N 0 2 M o : C, 51.07; H, 5.77; N, 4.26. Found: C, 51.30; H, 5.81; N, 4.40. IR (Nujol mull) * c o 1948 (s) cm'1; i / N O 1643 (s) cm"1. IR (Et^O) vCQ 1958 (s) cm"1; i / N O (s) 1640 cm"1; X H NMR (C 6D 6) 6 5.80 (d, 1H, = 10.5 Hz, C=CH uncoord.), 4.95 (s, 5 H, C^35), 3.81 (d, 1 H, = 10.5 Hz, C=CH coord.), 2.04 (s, 3 H, CH 3 ) , 1.79 (s, 3 H, CH 3 ) , 1.76 (s, 3 H, CH 3 ) , 1.35 (s, 3 H, CH 3 ) . D C NMR (C 6D 6) 5 151.2,127.4, 95.5,75.2,58.3,32.6,26.1,23.6,18.5; low-resolution mass spectrum (probe temperature 120°C) m/z 331 (P +). Reaction of CpMo(NO)(r?4-rram-2,5-dimethyI-2,4-hexadiene) with PMePh2 and PMe 3. Both of these conversions involved a 20 - 30% excess of the phosphine reagent, and both reactions proceeded similarly. Consequently, the one involving PMePh 2 is described as a representative example. A bright yellow solution of CpMo(NO)(r/4-rra«5-2,5-dimethyl-2,4-hexadiene) (0.10 g, 0.33 mmol) in hexanes (50 mL) was cooled to -78°C and was then treated with neat PMePh 2 (0.75 mL, 0.40 mmol). The reaction mixture was allowed to warm slowly to room temperature and was stirred at this temperature for 12 h whereupon its color darkened. IR monitoring of the progress of the reaction during this time revealed that the nitrosyl absorption characteristic of the initial diene complex (v N O 1622 cm"1) was gradually replaced by a lower energy absorbance at 1589 cm"1. Removal of the volatiles from the final reaction mixture under reduced pressure produced a brown residue. This residue was washed with hexanes (2 x 20 mL) and crystallized from Et^O to obtain 0.13 g (80%) of CpMo(NO)(r/ 2-2,5-dimethyl-2,4-hexadiene)(PMePh2), (2) as pale yellow feather-like crystals. Anal. Calcd. for C ^ H ^ P N O M o : C, 62.25; H, 6.38; N, 2.79. Found: C, 61.88; H, 6.18; N, 3.08. IR (Nujol mull) vNQ 1557 cm"1; IR (Etp) vNQ 1581 cm"1; X H NMR 6 6.9 - 7.0 (m, 10 H, Cgi5), 4.93 (s, 5 H, C ^ ) , 4.82 (d, 1 H, = 11.5 Hz, C=CH uncoord.), 3.19 (dd, 1 H, 3 / H H = 11.5 H z , / ™ = 9.4 Hz, C=CH coord.), 2.32 (s, 3 98 H, CH 3 ) , 1.98 (d, 3 H, ^ = 7.8 Hz, PCJJ3), 1.76 (s, 3 H, CH 3 ) , 1.53 (s, 3 H, CH 3), 1.29 (s, 3 H, CH 3 ) ; low-resolution mass spectrum (probe temperature 120°C), m/z 503 (P+). The reaction of CpMo(NO)(f?4-fran5-2,5-dimethyl-2,4-hexadiene) with PMe 3 was conducted in a similar manner, the total reaction time also being 12 h. The final r?2-diene complex, CpMo(NO)(rj2-2,5-dimethyl-2,4-hexadiene)(PMe3) (3) was isolated as a yellow, microcrystalline solid in 64% yield. Anal. Calcd. for C ^ H ^ P N O M o : C, 50.96; H, 7.44; N, 3.72. Found: C, 50.73; H, 7.27; N, 3.89. IR (Nujol mull) * N 0 1537 cm"1; IR (Et^O) * N O 1584 cm"1; 4 H NMR s 4.96 (s, 5 H, C ^ ) , 4.70 (d, 1 H, 3 / ^ = 11.5 Hz, C=CH uncoord.), 2.88 (dd, 1 H, 3 / H H = n.5 H z , / H P = 10.0 Hz, C=CH coord.), 2.21 (s, 3 H, CH 3 ) , 1.77 (s, 3 H, CH 3 ) , I. 72 (s, 3 H, CH 3 ) , 1.69 (s, 3 H, CJJ3) 1.06 (d, 9 H, 2 J m = 8.5 Hz, P(CJJ3)3). Reactions of CpMo(NO)(rj4-fra/w-2,5-dimethyl-2,4-hexadiene) with 1-Phenylpropyne and 1,7-Octadiyne. The treatment of CpMo(NO)(rj4-?ram-2,5-dimethyl-2,4-hexadiene) with the two alkynes was similar, and the reaction with 1-phenylpropyne is described below as a representative example. In a Schlenk tube, CpMo(NO)(r;4-frfln5-2,5-dimethyl-2,4-hexadiene) (0.3 g, 1.0 mmol) was dissolved in 1-phenylpropyne (2 mL) to give a brown solution whose IR spectrum exhibited a v N O at 1618 cm"1. The Schlenk tube was sealed and heated to 50°C for 12 h. Hexanes (40 mL) was added and the solution was filter-cannulated to the top of a Florisil column (3x3 cm) made up in hexanes. The excess of alkyne reagent was eluted with hexanes and discarded. A second yellow band was eluted with Et^O and collected. The solvent was removed under reduced pressure to yield analytically pure Cp(NO)Mo[r,3-C(Me)2CHCHC(Me)2C(Me)C(Ph)] (4) in 70% yield. A crystal suitable for X-ray crystallographic analysis was grown by cooling a concentrated solution of 4 at -20°C for 2 weeks. Anal. Calcd. for C ^ H ^ N O M o : C, 63.34; H, 6.47; N, 3.35. Found: C, 63.36; H, 6.38; N, 3.32. IR (hexanes) i / N O 1633 cm"1; IR (Nujol mull) z / N O 1628 cm"1. 99 (NMR assignments for both alkyne-inserted products are based upon the structure and labelling scheme below.) ^ ^ ^ = a * ^ » « ^ MeA X H NMR (C 6 D 6 ) 6 6.9 - 7.0 (m, 5 H, C^J 5), 5.43 (dd, 1 H , / ^ = 14.0 Hz, ~ C H A ~ ) , 4.78 (s, 5H, GjBs), 4.65 (dd, 1 H . / ^ = 14.0 Hz, «CH B - - ) , 1.68 (s, 3 H, (CJJ3)D), 1.52 (s, 3 H, (CH 3) E), 1.24 (s, 3 H, (CH 3)A), 1.22 (s, 3 H, (CH 3)B), 1.00 (s, 3 H, (CH 3)C). B C NMR (C 6 D 6)5 159.3 (s, = C-Me), 154.9 (s, =£-Ph), 121-131 (m, C 6 H 5 ) , 110.5 (d, V Q J = 147 Hz, ~ C H A - ) , 106.3 ((d, = 147 Hz, ~ C H B ~ ) , 102.2 (dp, V Q J = 176 Hz, 2 / C H = 7 Hz, C 5 H 5 ) , 69.5 (s, CMe 2), 47.5 (s, CMe 2), 29.3 (q, = 125.4 Hz, (CH 3)B), 29.2 (q, ^ = 125.0 Hz, (CH 3)D), 26.9 (q, V Q , = 126.1 Hz, (CH 3)C), 25.8 (q, V Q , = 122.0 Hz, (CH 3)A), 18.5 (q, I / Q J = 126.1 Hz, (CH 3) E); low-resolution mass spectrum (probe temperature 120°C) m/z 419 (P +). Treatment of CpMo(NO)(r;4-rra«s-2,5-dimethyl-2,4-hexadiene) (0.3 g, 1.0 mmol) with 1,7-octadiyne (0.5 g, excess) in hexanes (20 mL) followed by a similar work-up procedure led to the isolation of Cp(NO)N^o[r;3-C(Me)2CHCHC(Me)2CHC{(CH2)4CCH}] (5) in 60% yield. Anal. Calcd. for C ^ H ^ O M o : C, 61.94; H, 7.12; N, 3.44. Found: C, 61.73; H, 7.10; N, 3.48. IR (hexanes) vNO 1632 cm"1; IR (Nujol mull) i / N O 1606 cm"1. Two isomers arbitrarily designated as A and B exist in solution in a ratio of A:B = 7:1. (Labelling of H A and H B is as shown in above drawing.) lH NMR (C 6 D 6 ) Isomer A: S 6.32 (s, 1 H, =CH-), 5.35 (d, 1 H . / H H = 14.0 Hz, —CHA~), 4.99 (s, 5H, C ^ ) , 4.72 (d, 1 H . / ^ = 14.0 Hz, - C H B - ) , 2.1 - 2.4 (m, 5 H, -(-CJJ2-)- x 2,=CH), 1.84 (s, 3 H, CH 3 ) , 1.60-1.83 (m, 4 H, -(-CH2-)- x 2), 1.24 (s, 3 H, CH 3 ) , 1.21 (s, 3 H, CH 3 ) , 1.01 (s, 3 H, CH 3 ) . Ph. 100 Isomer B: 5 6.54 (s, 1 H, =CH-), 5.04, (d, 1 H . / ^ = 14.0 Hz, ~ C H A ~ ) , 4.61 (d, 1 U>JHH = 1 4 - ° m> " C -H B - - ) , 2.1 - 2.4 (m, 5 H, -(-CH2-)- x 2 ,HCH ) , 1.60 - 1.83 (m, 4 H, -(-CH2-)- x 2), 1.55 (s, 3 H, CH 3 ) , 1.50 (s, 3 H, CH 3 ) , 1.35 (s, 3 H, CH 3 ) , 0.85 (s, 3 H, CH 3 ) . B C NMR (C 6 D 6 ) Isomer A: 6 163.1 (s, = £ H ) , 158.0 (d, ^ = 144 Hz, =CH-Mo), 111.3 (d, = 156 Hz, - -CH A «) , 107.9 (d, V Q , = 148 Hz, —CHB—), 100.8 (dp, ^ C H = 1 7 6 Hz> 2 / C H ~ 3 / C H = 6 Hz .£5 H 5) - ( s » £ M e z ) » 7 7 ' 7 ( s » ^ M e 2 ) > 6 8 ' 6 (d> ^ C H = 2 4 4 H z » = ^ H ) . 66.9 (s ,=£-CH 2 ) , 46,4 (t, V Q J = 120 Hz, -CH 2-), 28.9 (t, ljCH = 1 3 2 ^ - £ % ) , 28.7 (t, lJCH = 136 Hz, -£H 2 - ) , 18.6 (t, V Q , = 128 H Z , -£H 2 - ) , 29.9 29.1,27.2,27.1 (overlapping multiplets, C H 3 x 4). Isomer B: (concentration too low for adequate signal-to-noise in the UC NMR spectrum) ^C^H] NMR (C 6D 6) 6 162.0,158.5,106.3,100.0, 97.2,46.7,43.7,31.7, 29.6, 29.4, 24.2, 23.9. Low-resolution mass spectrum (probe temperature 150°C) m/z 419 (P +). Reactions of CpMo(NO)(r?4-fran5-2,5-dimethyl-2,4-hexadiene) with Acetone. Method A: To a reaction flask containing 0.3 g CpMo(NO)(f74-rran5-2,5-dimethyl-2,4-hexadiene) (1.0 mmol) was added acetone (25 mL) by syringe. An IR spectrum of the yellow solution showed a band due to the nitrosyl ligand at 1591 cm"1. The reaction mixture was stirred for -1.5 h whereupon the color had changed to orange, and the IR spectrum indicated that the v N O had shifted to 1608 cm"1. The acetone was removed under reduced pressure to leave an oily brown residue. Extraction of this residue with hexanes (3 x 25 mL) produced a yellow solution ( f N O 1632,1616 cm"1) which was filter-cannulated to a new flask and concentrated in vacuo to -20 mL. This solution was then chromatographed through a Florisil column (3x3 cm) made up in hexanes using Et^O as eluant. The yellow band which developed was eluted, dried in vacuo, and recrystallized from hexanes to obtain 0.20 g (56%) of Cp(NO)Mo[r;3-C(Me)2CHCHC(Me)2C(Me)26], 8B, as a yellow, micTocrystalline solid. A crystal suitable for X-ray crystallographic analysis was obtained by maintaining a 101 concentrated hexanes solution of complex 8B at -20°C for one week. Physical and analytical data for this complex and the other analogues isolated during the study of the reactions of the trans-diene complexes with acetone are collected in Tables 4.2 and 4.5. After the first yellow band had been eluted from the column, T H F was used to develop a second yellow band which was collected separately and dried to obtain 0.10 g of a product (8A) which had the same elemental composition as the first product (8B) but quite different physical properties (see Tables 4.2 - 4.5). Attempts to obtain suitable crystals of 8A for X-ray analysis were thwarted by its isomerization to 8B in solutions. The hexanes-insoluble residue from the reaction mixture was dissolved in T H F to give a pale yellow solution with a f N 0 at 1593 cm"1, but it was produced in such small quantities ( <5%) that its identity was not determined. Method B: In a manner similar to that outlined in Method A, a solution of CpMo(NO)(rj4-frans-2,5-dimethyl-2,4-hexadiene) (0.3 g, 1.0 mmol) was stirred in acetone (25 mL), but the reaction mixture was stirred overnight. Similar work-up to that outlined in Method A allowed isolation of complex 8B in 2 0 % yield. Extraction of the hexanes-insoluble residue from the reaction with T H F (3 x 15 mL) yielded a yellow solution ( i / N O 1593 cm*1). Concentration of the THF solution at —20°C for several weeks produced yellow crystals of [CpMo(NO)]30*2: n2: rj 1 -C(Me 2 )-0 ) 3 THF, 9, in 60 % yield. A crystal suitable for X-ray crystallography was selected from this material. Anal. Calcd. for ( ^ H 4 1 N 3 0 7 M o 3 : C, 41.05; H, 5.00; N, 5.13. Found: C, 40.75; H, 4.89; N, 5.11. IR (Nujol mull) vNQ 1587,1576 cm*1. IR (THF) vNQ 1593 cm"1. J H NMR ( C D 3 N02) S 5.99 (s, 5 H, C ^ ) , 5.88 (s, 5 H, C^J 5), 5.63 (s, 5 H, C ^ ) , 3.65, (m, 8 H, -(-CH2-)- x 4), 2.04 (s, 3 H, C H 3 x 2), 1.95 (s, 3 H, CH 3 ) , 1.86 (s, 3 H, CJJ3), 1.80 (m, 8 H, -(-CH2-)- x 4), 1.59 (s, 3 H, CH 3 ) . B C NMR ( C D 3 N0 2 ) S 104.93 (dp, lJcu = 176 Hz, 2 / C H = 7 Hz, C 5 H 5 ) , 104.71 (dp, lJcu = 176 Hz, 2 / C H = 6 Hz, £ 5 H 5 ) , 104.16 (dp, V Q , = 176 Hz, 2 / C H = 7 Hz, C 5 H 5 ) , 96.70 (s, M e ^ - O ) , 94.04 (s, M e ^ - O ) , 92.27 (s, Me^-O) , 68.66 (t, V Q J = 140 Hz, CH 2 ) , 31.13,34.64,33.74,33.55,32.84 (overlapping multiplets due 102 to 6 x CH 3 ) , 26.59 (t, = 122 Hz, CH 2 ) ; low-resolution mass spectrum (probe temperature, 150°C) m/z 543 [P - CpMo(NO) 2 (Me 2CO)] +. Reaction of Cp'Mo(NO)(r/4-fraH5-butadiene) Complexes with Acetone. Treatment of Cp'Mo(NO)(r/4-fraAis-butadiene) (Cp1 = Cp or Cp*) with acetone followed by the work-up outlined in Method A above led to the isolation and characterization of the new complexes CpMo(NC% 3-e/zrfo-CH 2CHCHCH 2CMe 26) (6A), CpMo(NO)(r , 3 ^co-C H 2 C H C H C H 2 C M e 2 6) (6B), and Cp*Mo(NO)(f?3-e/uio-CH2CHCHCH2CMe26), (7). Physical and analytical data for complexes 6A, 6B, and 7 are included in Tables 4.2 - 4.5. In each case, there was no evidence for formation of any complexes analogous to the trimer 9 or any other nitrosyl-containing complexes. Reaction of Complex 8B with Acetone. A solution of complex 8B (0.15 g, 0.4 mmol) in acetone (25 mL) and THF (10 mL) was stirred at room temperature for 3 d. An IR spectrum of the final mixture showed that the i / N O of the starting complex at 1595 cm"1 was gone and one at 1587 cm"1 had developed. In addition, the color of the solution had changed from yellow to dark yellow. The acetone and THF were removed under reduced pressure, and the resulting brown residue was extracted with T H F (3 x 10 mL). The THF solution was filter-cannulated to another flask and dried to obtain 0.3 g of complex 9. Reaction of Complex 8B with lR3-Bis(diphenylphosphino)propane (dppp). A solution of 8B in T H F (0.15 g, 0.4 mmol, v N O 1593 cm*1) was treated with dppp (-1 g, excess), and the mixture was stirred for one week until the only J / n o evident in the IR spectrum of the solution was at 1561 cm*1. Solvent was removed in vacuo, and the resulting orange powder was washed with hexanes (1 x 15 mL) and then Et^O (2 x 10 mL). The orange precipitate was then dissolved in CH 2 C1 2 (20 mL) to yield an orange solution which was concentrated to -10 mL to induce precipitation and then put in the refrigerator (— 5°C) for several days. From this cooled mixture was isolated by filtration 0.20 g (0.3 mmol, 83 % yield) of CpMo(NO)(dppp) which was characterized by comparison of its physical properties with those of analogous complexes.2 103 Anal. Calcd for C3 2 H 3 1 NOP 2 Mo: C, 63.71; H, 5.14; N, 2.32. Found: C, 63.43; H, 5.25; N, 2.41. X H NMR (CDC13) S 5.10 (s, 5 H, C£l5), 6.9 - 7.8 (m, 20 H, [P(C^5)2]2), 2.3 - 2.8 (m, 6 H, -(CH2)-3). IR (CH2C12) « / N O 1549 cm"1. IR (Et^O) i ^ N O 1584 cm"1. Reaction of CpMo(NO)(r74-rran5-2,5-dimethyl-2,4-hexadiene) with HX, X = I, 0 2 C C F 3 , 0 3 S - p - C 6 H 4 C H 3 . The reactions of CpMo(NO)(r;4-/ran5-2,5-dimethyl-2,4-hexadiene) with the H X species are similar. The reaction with HI is presented as a representative example. To a reaction flask containing CpMo(NO)(r?4-fraAis-2,5-dimethyl-2,4-hexadiene) (0.30 g, 1.0 mmol) in CH 2 C1 2 (50 mL) (vNQ 1584 cm"1) was added HI (0.25 mL, -1 mmol) by syringe. The color of the solution immediately changed from yellow to red, and an IR spectrum of the solution showed a band due to uNQ at 1653 cm"1. The reaction mixture was stirred for 10 min, and then the solvent was removed under reduced pressure. The remaining orange residue was extracted with Et^O (3 x 50 mL), and the red extractions were filter-cannulated to a fresh flask where the solvent volume was reduced to approximately 70 mL in vacuo. This solution was slowly layered with hexanes and cooled to -0°C overnight to induce the crystallization of analytically pure, CpMo(NO)(»j3-CgH15)I (10) as an orange-red solid. Physical and analytical data for all the allyl complexes prepared by the reactions of CpMo(NO)(r;4-frfl^-2,5-dimethyl-2,4-hexadiene) with HI (10), trifluoroacetic acid (11), and /?ara-tolylsulfonic acid (12) are collected in Tables 4.9 - 4.10. Also contained in those tables are the data for a similar product complex prepared in a similar manner from the reaction of CpMo(NO)(r74-rra«5-butadiene) (13) with HI. 104 Results and Discussion Reactions of CpMo(NO)(r/4-rra/w-2,5-dimethyI-2,4-hexadiene) with Carbon Monoxide and Phosphines. Initial studies of the characteristic reactivities of some of the r/4-fra/z$-diene complexes with two electron ligands have afforded some interesting results. For instance, treatment of CpMo(NO)(r74-fra/u-2,5-dimethyl-2,4-hexadiene), a representative member of this class of compounds, with neutral Lewis bases, L, such as phosphines or carbon monoxide leads to novel r/2-diene complexes, i.e., CpMo(NO)(rj4-^an5-diene) + L •CpMo(NO)(L)(»? 2-diene) (1) where diene = 2,5-dimethyl-2,4-hexadiene. The various »?2-diene product complexes (i.e., L = CO (1), PMePh2 (2), or PMe 3 (3)) are isolable in good yields as yellow, crystalline solids which are only sparingly soluble in hexanes and Et^O, and more soluble in toluene, CH 2 C1 2 and THF. The NMR spectral data of these compounds are presented in the Experimental Section and are indicative of their possessing "piano-stool" molecular structures, i.e. L-*' 1 ""'NO W Me Mr>< Thus, when L = PMePh 2 or PMe3, the proton on the coordinated double bond of the diene ligand, (i.e., H A ) exhibits substantial coupling to the phosphorus (/H_P = 9.4 Hz). Molecular structures similar to that depicted above have been established for related CpMo(NO)(CO)(>? 2-olefin) complexes.3 105 For L = PMePh2, conversion 1 requires 12 h at 20°C to go to completion. It is thus somewhat more sluggish than previously reported displacements of r;4-dienes from metals' coordination spheres by stronger Lewis bases.4 Interestingly, further reaction between the r?2-diene product complex and another equivalent of phosphine to form the known2 CpMo(NO)(PMePh2)2 compound is even slower, requiring an additional 8 days. This reluctance of the r/2-diene complex to liberate the diene is probably a manifestation of the increased Mo-» rj2-diene back-bonding caused by the presence of the phosphine ligand. As expected, the electron density at the metal center in the r/2-diene complex [ i / N O (CHjCy 1553 cm"1] is intermediate between that of the rj4-fraro-diene complex [ i / N O (CH2C\^) 1584 cm"1] and the bis(phosphine)complex [vN O (CHjClj) 1539 cm"1].2 Finally, it should be noted that the rj2-diene carbonyl complex, 1, requires somewhat more forcing conditions for its preparation than do the related phosphine compounds, 2 and 3. Thus, when L = CO in reaction 1, a hexanes solution of the organometallic reactant must be subjected to 60 psig of carbon monoxide at room temperature for 5 days in order to effect complete conversion. This observation again gives a clear indication of the relatively great inertness of the Mo-r/4-fra/ts-diene linkage in CpMo(NO)(r;4-rran5-2,5-dimethyl-2,4-hexadiene) to undergo substitution reactions. Even more surprising is the fact that no CpMo(NO)(CO)2, the disubstituted product, is formed during this transformation. The product of the reaction between the related zirconocene diene complexes and carbon monoxide is a compound in which the diene ligand and the CO have coupled5 (eq 2). 106 The difference in reactivity between the zirconocene diene complexes and the molybdenum diene compounds discussed here is presumably a reflection of different types of metal-diene interactions as well as different properties exhibited by these complexes in solution. Reaction of CpMo(NO)(r/4-rranj-2,5-dimethyl-2,4-hexadiene) with Alkynes. Treatment of CpMo(NO)(r?4-fra«5-2,5-dimethyl-2,4-hexadiene) with alkynes proceeds quite cleanly to afford interesting products which are the products of a coupling reaction between the diene ligand and the alkyne (eq 3). The reactions are carried out in the presence of an excess of the alkyne according to the procedures outlined in the Experimental Section. The alkyne-coupled products are yellow, diamagnetic solids which are soluble in common organic solvents and may be recrystallized conveniently from hexanes or Et^O. As solids, they are moderately air-stable and may be handled in air for short periods of time with no detectable decomposition. In solution, they are somewhat more sensitive to air and decompose to produce insoluble brown precipitates and pale brown solutions. The *H and NMR spectra of the product of the reaction between CpMo(NO)(r/4-fra/is-2,5-dimethyl-2,4-hexadiene) and 1-phenylpropyne, i.e. complex 4, are shown in Figures 4.1 and 4.2, respectively. These spectra are diagnostic in that they illustrate that the product has incorporated a molecule of the alkyne. However, they do not unambiguously indicate whether the diene and the alkyne have coupled. The products which might be expected to be formed in this type of reaction are shown below. (3) Figure 4.1. 300 MHz *H NMR Spectrum of 4 in C 6 D 6 . U Figure 42. 75 MHz " C NMR Spectrum of 4 in C 6 D 6 . A B C Mo = CpMo(NO) Analogous to the products of reactions 1, it might be expected that the products of reactions 3 are the CpMo(NO)(rj2-2,5Klimethyl-2,4-hexadiene)(T;2-alkvne) complexes (i.e. A). If the diene and the alkyne have coupled, the products to be expected are the 18-electron <7,r;3-allyl complex (i.e. .6) or the 16-electron a, a complex (i.e. C), both of which have precedent.6 The most diagnostic piece of information which may be obtained is from the B C NMR spectrum in which two carbon resonances are seen in the olefinic carbon range at 159.3 and 155.4 ppm. The chemical shifts of these resonances are indicative of a change in the chemical environment of the alkyne carbons which would be expected to resonate at a different field upon coordination to the molybdenum. This, however, does not differentiate between an »72-acetylene and a a-bound vinyl ligand, i.e. In order to determine the carbon-carbon connectivity extant in these complexes, SINEPT (Selective Insensitive Nuclei Enhancement through Polarization Transfer) NMR experiments were conducted. These experiments are quite useful if the carbon-proton connectivities are known. In a SINEPT experiment, proton resonances are irradiated and the carbon resonances are observed. The polarization transfer through bonds may be Mo Ph Me Mo = CpMofNO) M c / \ 110 detected two, three, four and (sometimes) five carbons away, and in this fashion the carbon connectivities may be determined. As an example, the analysis of the spectra obtained for the 1-phenylpropyne coupled complex, 4, are detailed here. The 2D-HETCOR (2-Pimensional Heterocprrelation) NMR spectrum of 4 is presented in Figure 4.3. Using the 1-dimensional *H and B C and the 2D-HETCOR NMR spectra, the proton resonances may be assigned to their respective V carbon resonances. The SINEPT NMR spectrum obtained when the AB-type proton resonance at 5.41 ppm is irradiated is shown in Figure 4.4. From this spectrum a few items may be noted. First, the delay periods used in the pulse sequence of the experiment were not adjusted properly, and this accounts for the signal seen for the carbon only one bond from the irradiated proton. Normally, adjustment of the delay periods causes the signal due to the one-bond carbon to be absent from the spectrum. It is seen in the spectrum, however, that there is some signal enhancement of the carbon resonance at 106 ppm. Usually, it was quite difficult to adjust the pulse delays so as to obtain no signal for the carbon only one bond away from the irradiated protons, and the SINEPT experiments conducted were quite "hit-and- miss" as far as that parameter was concerned. Second, the signals of the carbons in the spectrum are pointed up and down. It is possible to adjust the pulse delays in order to cause the signals due to carbons bearing one or three protons to point down and the signals due to carbons attached to zero or two protons to point up. This is very useful in the analysis of more complex spectra, but it was not done in these experiments. It is evident that irradiation of the resonance at 5.41 ppm causes a polarization transfer to be observed for several carbons. Signal enhancement is observed for the carbon due to the other CH-fragment, the cyclopentadienyl ring carbons, each of the end carbons of what was once the diene ligand (CMe^ and two methyl carbons. Figure 4.5 shows that irradiation of the AB proton which resonates at 4.62 ppm enhances the signals due to the Cp-ring carbons, the CMe 2 carbons, and two methyl carbons in addition to the carbon of the other C H fragment. While these two F1 (PPM) F 2 ( P P M ) Figured . 2D-HETCOR NMR Spectrum of 4. Figure 4.4. 300 MHz SINEPT NMR Spectrum of 4: Irradiation at 5.41 ppm. Figure 4.5. 300 MHz SINEPT NMR Spectrum of 4: Irradiation at 4.62 ppm. 114 with the alkyne, there is an interesting observation to be made at this point. It is notable that the polarization transfer is seen for the cyclopentadienyl ring carbons. This means that the polarization transfer has occurred through molybdenum-carbon bonds. This is a phenomenon which has not been previously documented in the literature and we were surprised to see it. The experiment which establishes the coupling reaction is that in which the resonance due to the methyl protons which resonate at 1.87 ppm in the proton NMR spectrum is irradiated. This spectrum is shown in Figure 4.6. Enhancement of the signals at 159.3 and 155.4 ppm is seen. In addition, enhancement of the signals due to both of the C M e 2 carbons is observed. This is likely the most important effect seen in this experiment and it indicates that coupling between the alkyne and the diene ligand has indeed taken place. In other words, the methyl proton resonance which was irradiated is that due to the methyl group of the alkyne fragment. Irradiation of this signal is expected to enhance the signals due to the olefinic carbons. If the structure of the product was enhancement of perhaps one of the CMe 2 signals may be expected since it is known that the polarization transfer may go through the molybdenum. That effect would be one which was passed through four bonds. In order to see the enhancement of the signal due to the other C M e 2 carbon, the transfer would need to occur through six bonds which is beyond the sensitivity of the experiment. In agreement with the conclusion of a coupling reaction is the observed enhancement of only two methyl carbon signals (in addition to a signal for the i — i — p p — i — i — i — i — i — i — i — i — r 1«;0 1 4 0 1—I—rj—(—|—i—i i i | i i i— i — j — i — i— r — i — ( i—i—i—i—|—l—r—i—I—J— i—I—i—I—|—I—i—i—r~j—M I I I—|—I I I I TP i f l i | r~i I r 120 100 SO 6 0 40 2 0 P P M 116 carbon to which the irradiated protons are bonded). A polarization transfer through seven bonds would be required if the product was the 16-electron a,a bound coupled-ligand product (i.e. C). The net conclusion is that one of the following structures is exhibited by this complex, the carbons which exhibited the enhanced signals being noted with black dots. The SINEPT experiments imply that B is more likely than A since this decreases the average number of bonds between the irradiated protons and the enhanced carbons. In this fashion, it is possible to determine the structure of compounds of this type using NMR spectroscopy alone. To confirm the conclusions drawn from the NMR analyses, an X-ray crystallographic analysis of this complex (4) was undertaken. The X-ray diffraction data were collected and solved by Richard Jones and Fred Einstein at Simon Fraser University. The SNOOPI plot of the molecular structure is shown in Figure 4.7. Selected bond lengths and angles are listed in Table 4.1. The SNOOPI plot shown in Figure 4.7 confirms the coupling reaction of the diene and the alkyne to form a a, r/3-allyl ligand in which the allyl portion of the ligand is oriented endo with respect to the cyclopentadienyl ring. Additional interesting structural features include the similar carbon-carbon bond lengths exhibited by the allyl carbons (C16 - CIS, C15 - C14) which are intermediate between that expected for a carbon-carbon single bond (e.g. C14 - C13, C13 - C12) and a carbon-carbon double bond (e.g. C12 - Cll ) . This may be compared to the allyl ligand in CpW(NO)X(r>3-C3H5) (X = I,615 CI60) in which the allyl linkage is asymmetrically (a,*) bound to the metal, i.e. A B 117 C3 C4 C131 Figure 4.7. Solid-State Molecular Structure of 4. 118 Table 4.1. Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 4. Mo- C16 2.222(2) C16 -C15 1.412(3) C15 -C14 1.382(3) C14 -C13 1.514(3) C13 -C12 1.517(3) C12 - C l l 1.337(3) Mo- C l l 2.365(2) Mo- C12 2.313(2) Mo- C13 2.357(2) Mo-•N 1.769(2) N - O 1.200(2) Mo- N - O 174.0(2) 119 An intriguing feature is the fact that the alkyne has inserted into the molybdenum-diene-carbon bond so as to end up with the bulky phenyl group of the alkyne on the carbon nearer to the molybdenum and therefore the cyclopentadienyl ring (i.e. structure B as determined by the SINEPT experiments). This is undoubtedly due to steric interaction between the phenyl group and the methyl groups on the end of the diene ligand which has been coupled to the alkyne. Molecular models of this complex illustrate that while the phenyl group may orient itself so as to minimize conflict with the Cp ligand when it is attached to the carbon nearest to the molybdenum, this process is somewhat limited when the phenyl group is nearer to the two methyl groups, and avoidance of contact with the methyl groups usually results in contact with the cyclopentadienyl ligand. The analysis of the NMR spectra obtained for 5, produced in the reaction between CpMo(NO)(r?4-fra/is-2,5-dimethyl-2,4-hexadiene) and 1,7-octadiyne, is analogous but made difficult by the presence of a second isomer in solution. The probable structures of the two isomers are . / Mo = CpMo(NO) in which the alkyne-coupled product has the dangling alkyl either on the carbon bonded to the diene fragment or on the carbon bonded to the molybdenum. By analogy to the product 120 from the coupling with 1-phenylpropyne (4) it is expected that the major isomer is the one in which the dangling alkyl group is attached to the carbon bonded to the molybdenum. The isomers do not interconvert in solution, but maintain the ratio in which they are formed. That ratio is always in favor of the same isomer but varies from reaction to reaction. This complex is interesting and leaves one to wonder if it will react with a molecule of CpMo(NO)(r;4-/rcwts-2,5-dimethyl-2,4-hexadiene) to couple again, i.e. Mo = CpMo(NO) Studies are in progress to determine the feasibility of this reaction. Reactions ofthe Cp'Mo(NO)(r?4-trans-diene) Complexes with Acetone. A. Reaction of CpMo(NO)(n4-fmns-butadiene) with Acetone. When CpMo(NO)(r74-fra/u-butadiene) is treated with acetone according to Method A in the Experimental Section, a ligand coupling reaction occurs (eq 4). 6A: endo 6B: exo 121 Two products result from conversion 4, one in which the allyl portion of the coupled ligand is endo (6A) and one in which the allyl is exo (6B) with respect to the Cp ligand, i.e. 6A: endo 6B: exo These two complexes, 6A and 6B, are initially identifiable by their different v N O 's (1632,1616 cm"1) in an Et^O extraction of the reaction mixture once the acetone has been removed under reduced pressure. Then, the two complexes may be separated by elution through a Florisil column. If Et^O is used as eluant, the exo-allyl complex (6B) may be isolated; if Et^O/THF (1:1) is employed, the endo-a\\y\ complex, 6A may be isolated. The physical and analytical data for these complexes, and the other monomeric complexes characterized in these studies, are contained in Tables 4.2 to 4.5. The two complexes, 6A and 6B, are yellow, microcrystalline solids which dissolve in a host of organic solvents to give bright yellow solutions. They are moderately air-stable as solids and may be handled in air for short periods of time with no noticeable decomposition. In solutions, however, they are very much more air-sensitive and decompose to give colorless solutions with sludgy, brown precipitates. The IR spectra of 6A and 6B in Et^O exhibit I^Q'S at 1634 and 1616 cm"1, respectively, indicative of terminal linear nitrosyl ligands. The fact that the J / n o of 6A is higher than that of 6B indicates that there is more electron density available at the metal center for back-bonding to the NO ligand in 6B than in 6A This presumably reflects the difference in bonding of the allyl moiety of the coupled ligand to the metal center. 122 Table 42. Elemental Analyses, Mass Spectral and Infrared Data for the Acetone Coupled Products, 6-8. Complex Number C found(calcd) Analytical Data (%) H N found(calcd) found(calcd) Infrared Data J/NO (cm-1) hexanes Nujol mull Mass Spectral Data* P+(T,°C)b 6A 47.61(4735) 5.73(5.61)' 4.62(4.62) 1623 1587 305(180) 6B 47.48(47.55) 536(5.61) 4.67(4.62) 1603 1574 305(180) 7 54.62(54.72) 7.33(7.24) 3.80(3.75) 1616 1588 375(170) 8A 53.70(53.49) 7.00(6.96) 3.77(3.90) 1632 1630,1603 303(100)c 8B 53.23(53.49) 6.88(6.96) 3.85(3.90) 1616 1614,1578 303(100)c flAssignments for ^ M o . ^Probe temperature. cHighest mass peak observed, (P + - Me2CO). Table 4.3. 1 H N M R Chemical Shifts for the Complexes, 6 - 8. Complex Chemical Shifts (S in ppm) Number Cp M e A H A H B R A R B 6A 5.22 (s) 1.60 (s) 1.44 (s) 5.62 (m) 4.75 (m) 2.08 (m) 1.20 (m) 1.88 - 2.20 (m) 6B 5.22 (s) 1.43 (s) 0.99 (s) 5.72 (m) 4.81 (m) 2.02 (m) 1.50 (m) 1.55 -1.75 (m) 7 1.59 (s) 1.44 (s) 1.41 (s) 5.95 (m) 4.13 (m) 1.98 (m) 0.96 (m) 1.92 (m) 1.69 (m) 8A 5.22 (s) 1.58 (s) 1.43 (s) 5.85 (df 4.80 (df 1.53 (s)c 1.28 (s) 1.04 (s)c 0.78 (s) 8B 5.24 (s) 1.65 (s) 0.96 (s) 5.84 (df 5.16 (d)e 1.26 (s)c 0.86 (s) 1.07 (s)c 1.04 (s) °JHH = 13.7 Hz. b / u u = 13.7 Hz. c Assignments of R A vs Rg are tentative. DJHH = 7 0 HZ-E J M I = 7.0 Hz. Table 4.4. NMR Chemical Shifts for the Complexes, 6 - 8. Complex Chemical Shifts (6 in ppm) Number Cp Cj C 2 C3 C 4 C / C M e ( a ) b RA/RB 6A 102.6 (d\p) 41.4 (t) 131.1(d) 104.7(d) 48.9 (t) 107.8 (s) 34.1 (q) 29.7 (q) 6B 101.8 (d,p) 42.2 (t) 117.0(d) 113.0(d) 46.9 (t) 105.0 (s) 33.8 (q) 29.4 (q) 7* 103.1 (s) 47.7(d) 118.8(d) 117.0(d) 44.1 (t) 111.5 (s) 35.6 (q) 33.7 (q) 8A 104.6 (d,p) 71.8 (s) 129.3(d) 103.7(d) 46.2 (s) 106.2 (s) 31.0 (q) 30.4 (q) 24.6 (q) 24.4 (q) 20.8 (q) fl Quaternary carbon of acetone fragment. * Methyl carbons of acetone fragment. c 5 This is Cp* = r? -C^Me .^ The methyl carbons resonate at 10.1 ppm. Table 4.5 1 3 C NMR Coupling Constants for the Complexes, 6 - 8. Complex Coupling Constants (in Hz) Number Cp C j C 2 C 3 C 4 C M e ( a ) R A /R B 6A 177.0° 155.2 156.0 156.0 129.0 124.0 125.5 6B 177.0° 153.7 162.0 150.0 127.5 122.5 124.5 • -7 b 150.5 160.0 150.0 122.5 124.2 125.0 -8A 175.6° 144.0 153.0 125.0 124.0 124.7 128.0 123.3 7 ^ ~ / C H observed for the Cp complexes are as follows: 6A, 7.5; 6B, 6.0; 8A, 6.7 Hz. This is Cp* = r? 5-C sMe < 5 . The methyl carbons exhibit V™ = 126.7 Hz. 126 NMR Spectral Studies of 6A and 6B. The complexes 6A and 6B exhibit comparable spectral properties. The proton NMR spectra of the two compounds are shown in Figure 4.8. The spectra differ slightly in the chemical shifts of the resonances and the values. Due to the complex coupling networks extant in these systems, for most of the multiplets may not be measured directly from the spectra. In fact, some of the multiplets look much less complex than they are. This fact is illustrated by 1H{1H} NMR experiments. The proton multiplets exhibited by the endo-allyl complex, 6A, were selectively irradiated with the low-power homonuclear decoupler. It was hoped that this experiment would allow measurement of the coupling constants from the spectra. The spectra obtained from the six irradiations are shown in Figure 4.9. Comparison of these spectra with the undecoupled spectrum in Figure 4.8a illustrates the experiment. Thus, as shown in Figure 4.9a, when the multiplet at 5.72 ppm is irradiated, the apparent triplet of doublets of doublets at 4.81 ppm becomes a broad doublet of doublets while the apparent doublet of doublets at 2.02 ppm becomes a broad singlet, and the doublet of multiplets at 1.50 ppm becomes a simple-looking multiplet. Unfortunately, because of this, it is not possible to measure the from the spectra in this experiment, but selective irradiation of each of the multiplets due to each of the protons of the diene fragment allows assignment of the coupling network, (i.e., which protons are coupled with one another). This task is much more easily accomplished with a 2D-COSY (2-Dimensional .Correlation Spectroscopy) NMR experiment, however, and the 2D-COSY NMR spectrum of 6A is shown in Figure 4.10. This spectrum allows immediate assignment of the coupling networks and illustrates the complex nature of the coupling exhibited by this compound. In a similar fashion, the multiplet resonances in the 1 H NMR spectrum of 6B were selectively irradiated. The multiplets are very complex and the only information gained from the experiment is coupling networks. Again, this is more easily done with a 2D-COSY A •r1 Ji Figure 4.9 (a - f). lH{lH} NMR Spectra of 6A. J U L JL. JL "I i | i i i i | i i i i — | — i — i — i — i — | — i — i — i — i — | — i — i — i — i — j — i — i — T — i — I — i — i — r 5.5 5 .0 4.5 4.0 3 .5 3 . 0 2 . 5 AJ I I UL — | — i — i — i — r — | — i — i — i — i — | 2 . 0 1.5 PPM 1.0 Figure 4.9 (continued). ^H^H} NMR Spectra of 6A. A J L J A IX-~ i — i — i — i — I — i — i — i — i — I — i — r ~ i — i — i — | — i — i — i — r — | — i — l — i — i — | — i — i — i — r 5.5 5.0 4.5 4.'0 3.5 3.'0 -i—I—i—i—i—i—]—i—i—i—T—1 i r 2.5 2.0 1.5 PPM 1-0 £ Fl (PPM) JLJUL F2 (PPM) Figure 4.10. 300 MHz 2D-Cosy Spectrum of 6A. 131 NMR experiment, and the spectrum obtained from that experiment is shown in Figure 4.11. The assignments of the *H NMR spectra are listed in Table 4.3. The NMR spectra of 6A and 6B are shown in Figures 4.12 and 4.13. Qualitatively, again, they look very similar. Assignment of the resonances to the carbons of 6A and 6B are contained in Table 4.4. The most remarkable feature of these spectra is the ^ C H measured from the gated decoupled NMR spectra (Fig. 4.12b and 4.13b). Specifically, the triplets in the spectra, due to C x and C 4 (Table 4.5), indicate that one of the carbons is sp2-hybridized (V^ : 6A; 42.2 ppm, 153.7 Hz. 6B; 41.4 ppm, 155.2 Hz) while the other is sp3-hybridized (-/QJ: 6A; 46.9 ppm, 127.5 Hz, 6B; 48.9 ppm, 129.0 Hz.). This is the first indication that the ligands have coupled. If the products of the reaction were complexes in which the two ligands had not coupled, all the carbons of the diene ligand would still be sp2-hybridized as they are in the starting diene complex.7 The remainder of the B C NMR spectra are as expected, each exhibiting two quartets assignable to the methyl carbons of the acetone fragment, a doublet of pentets due to the cyclopentadienyl carbons, two doublets assignable to carbons 2 and 3 of the diene fragment, and a singlet due to the O-bound carbon of the acetone fragment. The chemical shifts of the resonances of the two isomers are different, indicative of slight differences in the chemical environments of the carbons. Most notable is the chemical shift of carbon 2 of the coupled ligand. In 6A this carbon resonates at 117.0 ppm while in 6B the resonance is at 131.1 ppm. Carbon 2 in 6A, the ero-allyl complex, is nearer to the cyclopentadienyl ligand than carbon 2 in the endo-allyl compound. This may be the cause of the different environment exhibited by these two carbons. Another likely cause for the difference between the chemical shifts of those two resonances is a difference in the bonding in which those carbons are involved. If the two allyl moieties are asymmetrically bonded to the metal, it is likely that the asymmetry is different in 6A than it is in 6B. F l (PPM) F2 (PPM) Figure 4.11. 300 MHz 2D-C0SY Spectrum of 6B. Figure 4.13. 75 MHz U C {*H} (a) and Gated Decoupled , 3 C (b) NMR Spectrum of 6B. 135 Final assignment of the H and NMR spectra of these two compounds is made possible by a 2D-HETCOR NMR experiment. In this fashion, the proton resonances are cross assigned to the carbon resonances. The partial 2D-HETCOR NMR spectrum of 6A is shown in Figure 4.14, the spectrum only encompassing -0.6 - 2.2 ppm in the proton range and -24 - 50 ppm in the carbon range. From these experiments it is clear that the complete structural assignment of these complexes is possible using NMR spectroscopy alone. B. Reaction of Cp*Mo(NO)(r?4-fm/ty-butadiene^ with Acetone. Treatment of Cp*Mo(NO)(r/4-/ra/u-butadiene) with acetone proceeds in a manner similar to that for the Cp analogue. However, the reaction takes longer (12 h) to go to completion. A similar work-up results in the isolation of a single product, namely Cp*Mo(NO)(a,rj3-CH2--CH~CH-CH2-CMe2-6), 7, in which the allyl is coordinated in an e/nio-fashion to the molybdenum. This complex may be characterized in the same manner as that described above for the Cp-analogue and the characterization data are included in Tables 4.2 - 4.5. Crystals of compound 7 were obtained from a concentrated hexanes solution of the complex, and one of the crystals was used for an X-ray crystallographic analysis. The X-ray data were collected and solved by Vivien Yee and James Trotter of U.B.C. The ORTEP plot obtained is shown in Figure 4.15 and selected bond lengths and angles are listed in Table 4.6. Notable structural features are those which describe the allyl portion of the coupled ligand. First, the allyl is noted to be oriented endo with respect to the cyclopentadienyl ligand. The three carbons of the allyl fragment (C3, C4, C5 in Figure 4.15) are somewhat asymmetrically bonded to the molybdenum center. This is evident as the C3 - C4 bond (1.340 (12) A), in the range of a carbon-carbon double bond, is slightly shorter than the C4 - C5 bond (1.401 (12) A) which is intermediate between a carbon-carbon single bond and a carbon-carbon double bond. Figure 4.14. 2D-HETCOR Spectrum of 6A. Figure 4.15. Solid-State Molecular Structure of 7. 138 Table 4.6. Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 7. Mo -02 2.032(4) 02 •Cl 1.415(7) C l - C2 1.549(11) C2- C3 1.480(12) C3- C4 1.340(12) C4- C5 1.401(12) Mo -C5 2.233(8) Mo -C4 2.316(6) Mo -C3 2.417(6) Mo -N 1.768(5) N-< 01 1.201(6) Mo - N - O l 167.1(13) 139 Asymmetrical bonding of a r-allyl ligand to the CpM(NO) fragment has been noted in other systems.615'0 As anticipated, the C - C bond of the allyl fragment in 7 which has more double bond character (i.e., C3 - C4) is situated trans to the NO group, a good r -acceptor ligand. The molybdenum - allyl-carbon distances are also illustrative of the asymmetrical bonding of the allyl fragment of the coupled ligand in 7. Thus, the Mo - C5 bond length is within the range expected for a Mo - C single bond8 and, indeed, very close to the length of the Mo - C a-bond seen for the Mo - CI bond in 4 (Table 4.1). The Mo - C4 and Mo - C3 bonds are significantly longer than the Mo - C5 bond, and this is consistent with asymmetrical bonding proposed for the allyl portion of the ligand, i.e. Mo = CpMo This is in contrast to the symmetrical allyl bonding seen in the molecular structure of 4, where the Mo - allyl-carbon bond lengths (Table 4.1) are greater than 2.3 A and the molybdenum - allyl-carbon and the carbon-carbon bond distances extant in the allyl fragment are not indicative of any asymmetry in the binding of the allyl moiety to the molybdenum. That the Mo - allyl linkage is tending toward asymmetry is evident in the ^ C NMR spectrum of 7 where it is seen that the chemical shift of C5 (C t in Table 4.4) is 47.7 ppm which is in the chemical shift range usually associated with -sp3-hybridized carbon atoms in transition-metal alkyl complexes while C4 (Cj in Table 4.4) resonates at 118.8 ppm, in the region expected for -sp2-hybridized carbons bound to transition metals.9 The X / C H (150.5 Hz) associated with C5, however, indicates some sp2-character in solution which implies that there is probably some molecular fluxionality in solution. 140 Other structural features to note are the linearity of the Mo - NO linkage (167.1 (3) deg) and the Mo - N (1.768 (5) A) and N - O (1.201 (6) A) distances which are in the same range as those seen for compound 4 and are indicative of a normal Mo - NO linkage. It is of some interest that the Mo - NO linkages are so similar since the molecular structures are quite different. In 4 the ir -allyl fragment is bonded to the Mo center in a symmetrical fashion. In 7 the slight asymmetry of the r-allyl fragment and the presence of a permethylated cyclopentadienyl ligand should, in principle, cause more electron density to be transferred to the NO ligand. While this is reflected in the y N O 's of these two complexes in their IR spectra (4,1634 cm"1; 7,1616 cm"1; hexanes) it is not apparent in their solid-state molecular structures. C. Reaction of CpMo(NO)(r?4-fran5-2.5-dimethyl-2.4-hexadiene) with Acetone. Method A: The reaction between CpMo(NO)(r/4-fra/w-2,5-dimethyl-2,4-hexadiene) and acetone is fairly quick, being complete in 1 - 3 h. The products of the reaction are the endo-a\\yl (8A) and eco-allyl (8B) complexes, i.e., which are initially present in varying ratios ( A B * 3:1 - 4:1) depending on the reaction time. The complexes exhibit different t-NO's in IR spectra of their solutions, (8A, 1634 cm"1; 8B, 1616 cm"1 in hexanes). Interestingly, in this system, the endo-isomer isomerizes quickly ( < 3 h) to the eco-allyl isomer at room temperature. The X H NMR spectra of 8A O 8A: endo 8B: exo Figure 4.16. 300 MHz *H NMR Spectra of 8A (a) and 8B (b) in C 6 D 6 . (a) j — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — \ — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i i — i i i i • i i i p - 1 > • r 6.0 • 5.5 5.0 4.5 4.0 3.5 3.0 2.5 ; 2.0 1.5 1.0 PPM (b) UL | — i — i — i — i — | — i — i — i — i — | — i — i — r 6.0 5.5 5.0 — | - — r " l — i — r — r — i — i — i — i — | — l — I — i — I — | — I — i — I — I — | — I — i I • | i I ' ' | r 4.5 4.0 3.5 3.0 2.5 2.0 1.5 ~ i — i — | — i — r 1.0 P P M 142 and 8B are shown in Figure 4.16. Notable are the resonances assignable to 8B in the spectrum of 8A even though the spectrum was obtained immediately after preparation of the NMR sample from pure 8A. The rapid isomerization of the endo-allyl complex to the exo-isomer is presumably due to the strong steric interactions between the methyl groups of the diene fragment and the cyclopentadienyl ligand. The source of these interactions is evident in molecular models of 8A and 8B which illustrate the relief of steric strain which occurs upon isomerization to the ero-allyl isomer. This explanation may also account for the fact that this isomerization is not observed in the system where the diene ligand is not substituted (i.e., 6A and 6B). The NMR spectra of 8B are shown in Figure 4.17. It is not possible to obtain an acceptable NMR spectrum of 8A due to the isomerization discussed above. Notable is the high chemical shift of the resonance due to carbon 2 (Table 4.4) at 129.3 ppm, indicative of the presence of an eco-allyl ligand. Substitution of four methyl groups on the diene ligand (as compared to 6A and 6B, above) precludes complete assignment of each of the signals of the spectra to individual carbon atoms in the structure. For example, Cj and C 4 of the coupled ligand exhibit singlets in the gated decoupled NMR spectrum (Fig. 4.17b) and are assignable to the signals at 46.2 and 71.8 ppm. The hybridization of those carbons is not determinable from the NMR signals. Thus, the physical and analytical data do not show conclusively that the diene and the acetone ligands have coupled. An X-ray crystallographic analysis of a crystal of 8B was undertaken on a crystal grown from a concentrated hexanes solution of the complex. The X-ray data were collected and solved by Vivien Yee and James Trotter of U.B.C. The ORTEP plot of the molecular structure obtained from this analysis is shown in Figure 4.18, and selected bond lengths and angles are listed in Table 4.7. As illustrated by the ORTEP plot of the molecular structure, the two ligands have coupled to form a cr,r/3-exo-allyl complex. The structural features pertaining to the allyl fragment indicate that the allyl moiety may be Figure 4.17. 75 MHz " c ^ H } (a) and Gated Decoupled 1 3 C (b) NMR Spectra of 8B. Figure 4.18. Solid-State Molecular Structure of 8B. 145 Table 4.7. Selected Bond Lengths (A) and Angles (deg) from the Molecular Structure of 8B. Mo -02 2.105(11) 02 •CI 1.42(2) C l - C2 1.64(2) C2- C3 1.47(2) C3- C4 1.34(2) C4- C5 1.40(2) Mo -C5 2.395(18) Mo -C4 2.296(17) Mo -C3 2.448(16) Mo - N 1.74(2) N - i 01 1.23(2) Mo - N - O l 169.2(14) 146 tending toward asymmetry as the C4 - C3 bond (1.43 (2) A) is somewhat shorter than the C4 - C5 bond (1.40 (2)A). As in compound 7, the shorter bond (C3 - C4) is trans to the nitrosyl ligand, as expected. In solution, however, the chemical shift of C5, which would be -sp3-hybridized in an asymmetric allyl ligand, resonates at 71.8 ppm (Cj in Table 4.4), indicating some sp2-character at that carbon in solution. The molybdenum - allyl-carbon bond distances are not indicative of any asymmetry in the molybdenum - allyl bonding and are more consistent with the results obtained from the B C NMR experiments. Thus, in 8B the bonding of the allyl fragment to the molybdenum is not asymmetric. This is in contrast to that which observed for 7 (vide supra). The molybdenum - nitrosyl linkage is almost linear (169.2 (14) deg) and the Mo - N (1.74 (2) A) and N - O (1.23 (2) A) bond distances are indicative of a normal nitrosyl ligand. The molybdenum-oxygen bond distance of 2.015 (11) A is similar to that seen in the molecular structure of 7 and is in the range expected for a Mo - O a-bond.10 The 02 -C l bond distances in both 7 and 8B (1.415 (7) and 1.42 (2) A , respectively) are in the range expected for a C - O single bond.8 The reactivity of these molybdenum diene complexes may be compared to that of the Cp2Zr(r/4-fra/w-diene) compounds which has been extensively studied. In these systems, experimental conditions may be controlled such that the reacting species (i.e. n4-cw-diene or n4-trans-(liene) may be known. The zirconocene complex, Cp2Zr(r}4-fra/u-isoprene), reacts with 2-butyne to form the a, rj3-syn-allyl complex according to eq 5.11 CpjZr Cp2Zr, (5) 147 The alkyne always couples with the diene at the carbon furthest from the methyl substituent of the diene ligand. This product is very similar to those produced in alkyne reactions with CpMo(NO)(r;4-fra«j-2,5-dimethyl-2,4-hexadiene). In the zirconocene systems, if the alkyne is substituted with a bulky group (e.g., phenyl), the product of the reaction is one in which two alkynes are coupled and the diene ligand is released,12 i.e. Cp2Zr (diene) These types of products are not observed in the molybdenum systems, the structurally characterized product being one in which 1-phenylpropyne is coupled to the diene ligand (i.e. 4). In its reactions with ketones, Cp2Zr(r/4-fra/ts-isoprene) yields different products depending on the reaction conditions.13 These insertion products vary in stereochemistry and regiochemistry, i.e. R n p Thus, the thermal reaction of Cp2Zr(rj4-/ra/u-isoprene) yields both the Z- and E- isomers with preferential insertion occurring at the sterically most crowded position while the photochemical reaction effects insertion at the sterically least crowded position.14 These reactions are believed to proceed via an rj2-diene zirconium complex which coordinates a ketone ligand and couples the ligands, i.e. products Zr = Cp 2Zr R 2CO = benzophenone, acetone, cyclodecanone, acetophenone, pinacolone, isobutyraldehyde That 16-electron complexes are isolated as the final products of these reactions is remarkable. This is interesting because the products of other ligand coupling reactions between zirconocene diene complexes and unsaturated organic molecules (alkynes, isocyanates, heterocumulenes, ketenes) are the 18-electron a,»j3-allyl compounds, e.g. for CO> A possible explanation for this phenomenon may be that C5 in the ketone-inserted product is sp3-hybridized while in the other insertion products it is sp2-hybridized, a factor which may affect the flexibility of the metallacycle. The steric requirements of the sp3-hybridized carbon may limit the ability of the x-orbitals of the ligand to interact with the metal 149 orbitals (in the plane bisecting the two Cp rings)15 of x -symmetry. On the other hand, the steric restrictions of the sp2-hybridized carbon may hold the rest of the ligand in such a way that the JT-orbitals may interact with those of the metal center. Close inspection of the molecular structures of representative examples of three classes of these compounds (ketone, heterocumulene and alkyne inserted complexes)13 shows that while the allyl fragments of the heterocumulene and alkyne inserted products lie in the plane bisecting the two cyclopentadienyl ligands, the carbon-carbon double bond of the ketone inserted products is out of that plane. This explanation appears to fail when applied to the ethylene-inserted product which has been structurally characterized as a a, rj3-allyl complex.16 However, in solution, this complex interconverts with the a, a complex, i.e. Cp2Zr I I Cp2Zr That the 16-electron ketone inserted complex is stable enough to be isolated may be due to the fact that extra electron density is available to the zirconium from the oxygen atom. In fact, the Zr-O bond length (1.946(4)A, Ph 2CO; 1.964A, ' p r 2 C O ) B in the ketone-inserted products is significantly shorter than the Zr-O bond length (2.144A)13 in the carbon dioxide-inserted product, perhaps indicating a Zr-O bond order of greater than one in the ketone-inserted products. The products of the reactions of CpMo(NO)(r/4-frans-diene) are the a, Tj3-allyl complexes. The frontier orbitals of the CpMo(NO) fragment are believed to be pointing away from the cyclopentadienyl ring down the "legs of the piano stool" opposite the nitrosyl ligand,17 and in each case of ligand coupling, the position of the allyl fragment is consistent with this. The position of the frontier orbitals on the two fragments, Cp 2Zr and CpMo(NO), appears to determine the nature of coordination of the coupled ligand . This effect may be significant in affecting the subsequent chemistry of these complexes and the 150 stereochemistry of any organic products obtained when the ligand is removed from the metal center. These studies of the coupling of organic molecules in the presence of the Cp'Mo(NO) fragment have thus far been fruitful. Preliminary studies indicate that these diene complexes react very quickly with acetonitrile to form at least two products (by IR spectroscopy). Future studies will explore the coupling of butadienes with other unsaturated organic molecules. Studies are currently in progress to remove the coupled ligands from the molybdenum and to determine the nature of the organic products of these reactions. Method B. If the reaction of CpMo(NO)(r74-/ran5-2,5-dimethyl-2,4-hexadiene) with acetone is allowed to continue for extended periods of time (>5 h), the emfo-allyl isomer complex 8A is isolated in significantly reduced yields. Indeed, if the reaction is left for 15 h, then there is no 8A observed in the final reaction mixture. The only product which may be isolated from the hexanes extraction of the reaction mixture is 8B. There is also a significant amount of a hexanes-insoluble product produced. This brown precipitate dissolves in THF to give a pale yellow solution whose IR spectrum exhibits a single nitrosyl band at 1593 cm"1. Concentration of this T H F solution followed by cooling to —20°C overnight allows isolation of [CpMo(NO)]3(/i2: v2' v 1-OCMe2) 3,9, as a T H F solvate. The elemental analysis data for this trimer (Experimental Section) is consistent with this formulation although the parent ion is not observed in the mass spectrum. Rather, there is a peak assignable to the trimer minus [CpMo(NO)2(Me2CO)]. The trimer, 9, is isolated as a bright yellow powder from THF. It is insoluble in hexanes and E ^ O , very soluble in CH 2C1 2 , and less so in THF. It is not air-sensitive as a solid, being unchanged (as monitored by 1 H NMR spectroscopy) after exposure to air for 72 h. A crystal of 9 suitable for X-ray structural analysis was obtained from a T H F solution of 9. The X-ray data were collected and solved by Vivien Yee and James Trotter of U.B.C. 151 The ORTEP plot of the molecular structure which was obtained is shown in Figure 4.19, and selected bond lengths and angles are contained in Table 4.8. The ORTEP plot shows that the structure of 9 consists of three 3-legged piano stool structures connected by Mo-acetone-Mo linkages, i.e. The nitrosyl ligands are all approximately linear (Mo-N-O (ave) = 171.5°). The most notable structural features are those exhibited by the u2:v2'-v 1-acetone ligands. The angles are approximately 112° (ave), consistent with a tetrahedral sp-hybridized carbon. The Mo - C - O angle, however, is about 67° (ave), indicating significant distortion of the hybridization of that carbon from the expected sp3-hybridization and from the sp2-hybridization of that carbon in free acetone. The angle about the oxygen (Mo-O-C = 74°, ave) is also indicative of distortion as it deviates from the expected 120°. The bond lengths within the Mo -r?2-acetone linkage are also noteworthy. The carbon-oxygen length of 1.40A (ave) is close to the range expected for a carbon-oxygen single bond and is not lengthened as it is in strained-ring epoxides.8 It is also similar to that seen in the molecular structures of 7 (Table 4.6) and 8B (Table 4.7). It is longer than those exhibited by (PPh3)4Mo(r?2-PhCHO) (1 .33A) 1 8 or Cp 2Mo(CH 20) (1.36A)19 which have partial carbon-oxygen double bond character. The Mo - C bond distance of 2.17A is shorter than those exhibited by the »?2-ketone ligands in the Mo(r;2-PhCHO) (2.26A)18 and Mo(r/2-H2CO) (2.25A)19 complexes. These numbers are significantly smaller than that Figure 4.19. Solid-State Molecular Structure of 9. 153 Table 4.8. Selected Bond Lengths (A) and Angles (deg) in the Molecular Structure of 9. M o l - 04 2.076(6) Mo2 - 05 2.133(6) Mo3-06 2.040(6) M o l - CI 2.16(1) Mo2 - C4 2.17(1) Mo3-C7 2.188(9) M o l - 0 6 2.088(6) Mo2 - 04 2.121(6) Mo3 - 05 2.130(6) 0 4 - C 1 1.40(1) 05 -C4 1.42(1) 0 6 - C 7 1.39(1) M o l - N l 1.759(8) Mo2 - N2 1.752(9) Mo3 - N3 1.752(8) N l - O l 1.23(1) N2-02 1.22(1) N3-03 1.21(1) M o l - N l - O l 173.2(8) Mo2 - N2 - 02 169.3(9) Mo3 - N3 - 03 172.0(8) CI - M o l - 04 38.5(3) C4 - Mo2 - 05 38.4(3) C7 - Mo3 - 06 38.2(3) M o l -CI - 04 67.4(4) Mo2 - C4 - 05 69.4(5) Mo3 - C7 - 06 65.2(4) CI - 04 - M o l 74.1(5) C4 - 05 - Mo2 72.5(5) C7 - 06 - Mo3 76.7(4) C2 - CI - C3 111.0(8) C5 - C4 - C6 113(1) C 8 - C 7 - C 9 114.3(9) M o l - 04 - Mo2 131.8(3) Mo2 - 05 - Mo3 131.8(3) Mo3 - 06 - M o l 149.0(3) 154 seen in the molecular structure of 4, the alkyne-coupled product, but still within the range of a molybdenum - carbon a-bond. The Mo - O bond distance (a) of 2.08A (ave) is close to those of the other molybdenum »?2-ketone complexes, and indeed, the 0-» Mo distance (b) is in that same range. The reaction to form the trimeric complex (eq 7) is most remarkable. The fact that [CpMo(NO)]30i2:f?2:r;1-O-CMe2)3 ^ a trimer of molybdenum bridged by r; 2-acetone ligands is formed in the presence of excess acetone instead of a monomer with two acetone ligands bonded in an r? ^ fashion, i.e., Mo JO Y Q N O Me 2 Me 2 is most surprising given the ease with which other Lewis bases form analogous complexes. It can only be surmised that due to the strength of the Mo - O single bonds, the trimer (however it is formed) is a thermodynamic sink which, once formed, does not readily break up. The trimeric complex may be broken, however, with 1,3-bis(diphenylphosphino)propane (dppp) to form the dppp complex, analogues of which have been prepared previously.2 i.e. [CpMo(NO)]30* 2: n 2: n ^ O - C M e ^ j + dppp — • CpMo(NO)dppp + ? (8) The mode of formation of [CpMo(NO)]3(/J2: n2: r ^ - O - C M e ^ is not known. An interesting observation may be made, however. The endo-allyl complex, CpMo(NO)(a,r/3-CMe2--CH--CH-CMe2-CMe=CPh) (4), formed from the reaction of 155 (l^Mo(NO)(rj4-frans-2,5-dimethyl-2,4-hexadiene) and 1-phenylpropyne (vide supra) does not react with acetone, even after being stirred in acetone for more than two weeks. Thus, at present it is believed that the trimeric complex is most likely formed from reaction of the endo-ally\ complex with acetone. So far, no trimeric complex has been characterized in the butadiene systems (6 or 7). For this reason, it is believed that the steric repulsions extant in the tetra-methyl substituted diene system contribute to the thermodynamic driving force of this reaction and cause the ligand to somehow dissociate from the metal center. We have not as yet been able to ascertain the nature of the organic by-products of the reaction. We are presently designing the methodology for isolation and identification of these species. It will be especially interesting to see if the organic by-products of the reaction have incorporated more than one molecule of acetone. A Preliminary Investigation into the Reactivity of some CpMo(NO)(r? 4-fra/ty-diene) Complexes with Electrophiles: Reactions of CpMo(NO)(r?4-fra/ts-diene) with Acids, HX. The products of the reactions between CpMo(NO)(r;4-frans-diene) and H X (diene = 2,5-dimethyl-2,4-hexadiene, H X = HI, H 0 3 S C 6 H 4 C H 3 , H 0 2 C C F 3 : diene = butadiene, H X = HI) are the r?3-allyl complexes, CpMo(NO)(r;3-aUyl)(X), i.e. CpMo(NO)(^ L ) + HX - C p M o t N O X T ^ ^ J X (°) C p M o ( N O ) ( ^ ) + HX C p M o ( N O ) ( ^ ) X <10> Reactions 9 and 10 are nearly instantaneous with mixing and proceed cleanly in dichloromethane. The analytical data for the allyl complexes, 10 -13, are collected in Tables 4.9 and 4.10. Compounds 10 -13 are quite soluble in CH 2C1 2 , Et^O, and THF and not very soluble in hexanes, and thus they were recrystallized from E^O/hexanes. They Table 4.9. Elemental Analyses, Mass Spectral and Infrared Data for ther;3- Allyl Complexes, 10 -13. Complex No. Yield <%) Analytical Data (%) C H found(calcd) found(calcd) N found(calcd) Low Resoln Mass Spectra P^m/z 0 "NO (CH2C12) (cm1) (E^O) CpMo(NO)I(r/3-C8H15) 10 95 36.54(36.41) 4.40(4.66) 3.07(3.27) 431 1653 1667 CpMo(NO)0-O3SC6H4CH3)(r/ ^ CgH^) 11 77 51.02(50.75) 6.00(5.70) 2.69(2.96) 365 1657 1665 CpMo(NO)(02CCF3)(r; ^ CgH^) 12 61 43.37(43.40) 4.82(4.82) 3.27(3.37) 303 1665 1661 CpMoCNO)!^3-^!^) 13 98 28.89(28.98) 3.23(3.22) 3.68(3.76) 375 1652 1665 — g o Assignments for Mo. on OS Table 4.10. 1 H NMR Chemical Shifts for the Allyl Complexes, 10 -13. ISOMER A ISOMER B Complex Isomer Chemical Shifts Multiplicity Integration / Assignment Chemical Shifts Multiplicity Integration / Assignment Number Ratio (5 in ppm)" # H Hz ( £ i n p p m ) a # H Hz 10 1:0 11 13:1 12 6:1 13d 4:1 5.05 s 5 — 4.12 d 1 103 2.97 d,d 1 10.5,105 230 m 1 — 221 s 6 — 1.09 d 3 6.5 0.99 d 3 65 5.29 s 5 4.94 d 1 10.5 2.32 m 1 135 m 1 — 1.71 s 3 — 1.58 s 3 — 0.95 d 3 6.5 0.60 d 3 6.5 5.15 s 5 4.91 d 1 105 3.76 d,d 1 9.8,10.5 236 m 1 — 1.45 s 3 — 1.10 s 3 — 1.05 d 3 63 0.75 d 3 63 5.88 s 5 5.04 m 1 — 4.95 m 1 — 3.31 d 1 3.0,6.5 2.47 d 3 6.3 235 m 1 — Cp UA c M e . Me,, Cp H A c M e . A A M e B Cp H A « C M e , M e , M e , M e n Cp Uc H A Me 5.26 s 5 — 4.32 d 1 10.5 3.07 d,d 1 10.0,10.0 135 m 1 1.63 s 3 — 1.09 s 3 0.91 d 3 6.2 0.67 d 3 6.5 5.05 s 5 433 d 1 13.0 3.25 d,d 1 9.0,9.0 2.36 m 1 — 1.64 s 3 — 1.59 s 3 — 0.95 d 3 6.4 0.64 d 3 63 5.78 s 5 . . . . 4.62 m 2 — 3.45 d,d 1 2.5, 9.0 2.61 m 1 — 2.55 d 3 5.5 Cp "I Me. A M e , M e p Me, B Cp k " c A A M e B M e B Cp H A ' H C " D H B Me a C f i D 6 , unless otherwise specified. CDCI3 158 are red-orange microcrystalline solids which may be handled in air for short periods of time with no deleterious effects. The analytical and physical data for 10 -13 are consistent with their possessing 18-electron three-legged piano-stool structures analogous to the 17 3-allyl complexes prepared by Legzdins6b and Faller.60 The IR spectra of 10 -13 show strong nitrosyl absorptions in the region expected for these types of complexes (i.e. 1650 - 1670 cm"1). These nitrosyl-stretching frequencies are 60 - 70 cm"1 higher in energy than those of their respective diene starting materials. This indicates that the metal center in these species is less electron rich than that in the n4-diene complexes. The *H NMR spectra of 10 -13 are very complex, as expected6b'c (Table 4.9). Figure 4.20 shows the labelling scheme for the allyl ligand in complexes 10 -12 and 13. The spectra of 11 -13 show evidence for at least two isomers in solution. There is a possibility of four isomers in solution, the endo and exo isomers with the isopropyl fragment of the allyl ligand on the same side as the X ligand or on the same side as the nitrosyl ligand, i.e., for 10-12. Faller6 0 has shown that in some of these systems, the allyl complexes are fluxional in solution and interconvert when heated. Assignment of the 41 NMR spectra of 10 -13 was Figure 420. Labelling Scheme for the Allyl Complexes, 10 -13, Showing the Orientation of the Allyl Ligands With Respect to the Cyclopentadienyl Ligand of the CpMo(NO)X Fragment at the Top of the Page. 160 therefore limited to the designation of a resonance to a proton on the carbon chain with no attempt being made to assign the isomers. These rj3-allyl products are the expected products from the reaction of a diene complex and an electrophile, E X (E = electrophile; X = coordinating anion). The products of organometallic diene complexes [LnM(r/4-diene)] and EX, where X is a non-coordinating anion are expected to be the t;3-allyl cation complexes, i.e. LnM(r;4-diene) + E X ^ [LnM(r,3-allyl)E] + X" (11) In fact, Wink 2 0 has shown that in other systems this is a useful method to direct regio- and stereospecific electrophilic attack on a diene ligand. Our attempts to isolate the products of the reaction between the molybdenum trans-diene complexes and H B F 4 have been very frustrating. There appears to be more than one product of the reaction, as evidenced by a very broad (-100 cm"1) v^Q band in the infrared spectrum of the reaction mixture and several resonances in the Cp region of the proton NMR spectrum of the reaction residue. In addition, attempts to separate and purify the mixture lead to decomposition of all nitrosyl-containing species. An interesting addendum to these observations is that whatever the species are in the reaction mixture, they are stable with respect to attack by H" (e.g. NaBH 4 and Red-Al). However, while we have met with some initial failures while investigating this line of reactivity, it still appears that more study of this area is needed in order to define the limits of the reactions of the trans-diene complexes with electrophiles. 161 REFERENCES (1) Chapter 2 of this thesis. (2) Hunter, A. D.: Legzdins, P. Organometallics 1986,4,1001. (3) Faller, J. W.; Chao, K. H. ; Murray, H . H. Organometallics 1984,3,1231. (4) Johnson, B. F. G. ; Lewis, J.; Twigg, M. V. /. Chem Soc., Dalton Trans. 1974,2546, and references therein. (5) Nakamura, A.; Yasuda, H. ; Tatsumi, K.; Noda, I.; Mashima, K.; Akita, M.; Nagasuna, K. in Organometallic Compounds: Synthesis, Structure and Theory; Shapiro, B. L., ed.; Texas A & M University Press: College Station, Texas, 1983; pp 29 - 45, and references therein. (6) (a) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394. (b) Greenhough, T. J.; Legzdins, P.; Martin, D. T.; Trotter, J. Inorg. Chem. 1979,11, 3268. (c) Faller, J. W.; Shvo, Y. /. Am. Chem Soc. 1980,102,5396. (7) Christensen, N. J.; Hunter, A. D.; Legzdins, P. Organometallics 1989, 9, 930. (8) CRC Handbook of Chemistry and Physics; 61st ed.; R. C. Weast, ed. M. J. Astle, asst.ed.; CRC press: Boca Raton, Florida, 1980; p F-219. (9) Faller, J. W.; Chen, C. C ; Mattina, M. J.; Jakubowski, A. /. Organomet. Chem. 1973, 52, 361. (10) Jacobson, S. E.; Tang, R.; Mares, R. Inorg. Chem 1978,17,3055. (11) (a) Yasuda, H. ; Kajihara, K.; Nagasuna, K.; Mashima, K.; Nakamura, A. Chem. Lett. 1981, 719. (b) Erker, G. ; Engel, K.; Dorf, U. ; Atwood, J.; Hunter, W. E. Angew. Chem. Int. Ed. Engl. 1982, 21, 914. (12) (a) Kai, Y.; Kanehisa, N.; Miki, K.; Kasai, N.; Mashima, K.; Nagasuna, K.; Yasuda, H. ; Nakamura, A. Chem Lett. 1982,1979. (b) Skibbe, V.; Erker, G. /. Organomet. Chem. 1983,241,15. (13) Yasuda, H. ; Nakamura, A. Angew. Chem. Int. Ed. Engl 1987,26,123, and references therein. 162 (14) (a) Kai, Y.; Kanehisa, N.; Miki, K.; Kasai, N.; Akita, M. ; Yasuda, H. ; Nakamura, A. Bull Chem. Soc. Jpn. 1983,56, 3735. (b) Erker, G.; Dorf, U . Angew. Chem. Int. Ed. Engl 1983,22, 111. (15) Lauher, J. W.; Hoffmann, R. /. Am Chem. Soc. 1976,98,1729. (16) Erker, G.; Engel, JC; Dorf, U. ; Atwood, J.; Hunter, W. E. Angew. Chem. Int. Ed. Engl 1982,27, 914. (17) Hunter, A. D.; Legzdins, P.; Einstein, F. W. B.; Willis, A. G ; Bursten, B. E.; Gatter, M. G. /. Am. Chem. Soc. 1986,108,3843. (18) Clark, G. R.; Headfold, C. E. L.; Marsden, K.; Roper, W. R. /. Organomet. Chem. 1982,231,335. (19) Gambarotta, S.; Floriani, C ; Chiesi-Willa, A,; Gaustini, C. /. Am. Chem Soc. 1985, 101, 2985. (20) Wink, D. J.; Wang, N. -F.; Springer, J. P. Organometallics 1989,8, 259. 163 Chapter 5 Reduction of [ ( T ? 5 - C 5 H 5 ) M ( N O ) I 2] 2 Dimers by PMe 3: Synthesis and Properties ofthe Novel Complexes M ( N O ) ( P M e 3 ) 4 I , ( M = Mo, W) 164 Introduction Trimethylphosphine (PMe3) often exhibits distinctive behavior during its reactions with organo transition-metal complexes because of its relatively small cone angle and strong Lewis basicity.1 Particularly interesting in this regard are the reactions of PMe 3 with cyclopentadienyl-containing compounds. Previous work by Casey and co-workers2 has established the occurrence of some or all of the sequential substitution reactions summarized in eq 1 for a variety of such compounds, e.g., + PMe 3 . +PMe, (r ? : >-C 5H 5)M - (r?J-C5H5)M(PMe3) -PMe 3 -PMe 3 + PMe. (r ? 1-C 5H 5)M(PMe 3) 2 - » [M(PMe 3 ) 3 ] + C 5 H 5 - (1) -PMe 3 M = Re(CO)3, Re(CO)(NO)Me, Re(PMe3)(NO)(Me), Mo(CO)2(NO), W(CO)2(NO), etc. In 1987, we reported3 the first examples of another mode of reactivity involving these reactants, namely displacement of the cyclopentadienyl ligand with concomitant reduction of the organometallic reagent, i.e., ( r ; 5 -C 5 H 5 )M + + 3 PMe 3 • M(PMe 3) 2 + [(C5H5)PMe3] + (2) The specific examples of reactions 2 that we have observed involve the compounds having M = Mo(NO)(PMe3)2I or W(NO)(PMe3)2I. These compounds, in turn, result from the treatment of the complexes [(r;5-C5H5)M(NO)I2]2 (M = Mo or W) with trimethylphosphine. In this chapter the chemistry of these compounds will be described. The chemistry of the tungsten complexes, most of which was done by Luis Sanchez of our group, is included for completeness of discussion. 165 Experimental Section All reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions in a well-ventilated fume hood. General procedures are described in Chapter 2. The iodo nitrosyl reagents, [(r?5-C5H5)Mo(NO)I2]24, and [(r/5-C 5H 5)W(NO)I 2] 2 5 , and PMe 3 6 were prepared by published procedures. Synthesis of (rj5-C5H5)M(NO)I2(PMe3) (M = Mo or W). To a rapidly stirred, purple suspension of [(r/5-C5H5)Mo(NO)I2]2 (2.00 g, 2.25 mmol) in CH 2 C1 2 (100 mL) (yNO 16 7 7 cm"1) at room temperature was added PMe 3 (0.45 mL, 4.5 mmol) dropwise. The purple solid was gradually consumed over 30 min, and the final reaction mixture consisted of a clear, red solution whose IR spectrum exhibited a i / N O a t 1665 cm"1. This red solution was passed through a Florisil (60-100 mesh) column (2x3 cm) supported oh a medium-porosity frit. The Florisil was washed with CH 2 C1 2 (2 x 15 mL) and the washings were combined with the filtrate. Dropwise addition of hexanes (20 mL) to the combined filtrates over 1 h induced the precipitation of a red crystalline solid. This precipitate was collected by filtration, washed with hexanes (3 x 25 mL), and dried in vacuo (5 x 10"3mm) for 3 h at room temperature to obtain 1.50 g (65% yield) of (r/5-C5H5)Mo(NO)I2(PMe3) as an analytically pure, brick red powder. The tungsten analogue was obtained similarly as a red powder in 82% yield. The analytical, mass spectral, IR, and 1 H NMR data for these and other new complexes synthesized during this work are collected in Tables 5.1 and 5.2. Synthesis of [(»?5-C5H5)M(NO)I(PMe3)2]I (M = Mo or W). To a red solution of (r>5-C5H5)Mo(NO)I2(PMe3) (1.00 g, 1.92 mmol) in THF (90 mL) (vNQ 1643 cm"1) was added PMe 3 (0.20 mL, 2.0 mmol), and the mixture was stirred at room temperature for 2 h. The red solution gradually became pale orange, and a bright orange solid precipitated. The final solution exhibited no absorptions attributable to nitrosyl-containing products in its IR spectrum. The precipitate was collected by filtration, 166 washed with hexanes (3 x 10 mL), and dried under a vacuum to yield 1.00 g (87%) of [(f?5-C5H5)Mo(NO)I(PMe3)2]I as a bright yellow powder. The tungsten congener was obtained analogously as a yellow powder in 75% yield based on (r/5-C5H5)W(NO)I2(PMe3). Synthesis of MCNOXPMe^I (M = Mo or W). To a stirred yellow slush of [(775-C5H5)Mo(NO)I(PMe3)2]I (1.70 g, 3.00 mmol) in C H 3 C N (80 mL) at -453C was added PMe 3 (0.90 mL, 9.0 mmol). The mixture became orange immediately. The cooling bath was removed, and a yellow microcrystalline solid began to precipitate after "2 min. The reaction mixture was allowed to warm to room temperature, was stirred for 3 h, and was then taken to dryness in vacuo. The resulting solid was transferred to a Soxhlet extractor and was extracted with E ^ O (400 mL) for 4 days to obtain a yellow crystalline solid suspended in a yellow solution in the receiving flask. Removal of solvent from the extracts under reduced pressure afforded 1.44 g (86% yield) of Mo(NO)(PMe3)4I as an analytically pure, yellow crystalline solid [^Pf/H} NMR ( C 6 D 5 C D 3 , (PC) 6 -10.7]. The complex, Mo(NO)(PMe3)4I, could also be synthesized directly by treating [(r; 5-C 5H 5)Mo(NO)I 2] 2 with 10 equiv PMe 3 in C H 3 C N at -453C. Workup of the final reaction mixture in the manner described in the preceding paragraph produced the desired product in 72% yield. The analogous tungsten complex, W(NO)(PMe3)4I, was synthesized directly from [(r?5-C5H5)W(NO)I2]2 in a similar manner by adding the PMe 3 to an acetonitrile solution of the diiodide dimer at -45^ and then stirring the reaction mixture at room temperature for 5 days. The usual workup afforded the product in 50% yield. The Soxhlet apparatus from the original synthesis of Mo(NO)(PMe3)4I (vide supra) was transferred to a second receiving flask charged with CH 2 C1 2 (350 mL), and extraction was continued for a further 18 h. This operation produced a white powder suspended in a pale brown solution in the receiving flask. The white solid was collected by filtration on a medium-porosity frit, washed with CH 2 C1 2 (2 x 20 mL), and 167 recrystallized from EtOH (200mL) to obtain 0.23 g (29% yield) of [(C5H5)PMe3]I as a white, hygroscopic, crystalline solid. Anal. Calcd for C g H 1 4 PI: C, 35.84: H, 5.23; I, 47.37. Found: C, 36.21: H, 5.39; 1,47.00. DCIMS (700°C): m/z 141 ( C g H 1 4 P + ) [(M + 1)+ peak not observed]. Synthesis of (r/5-C5H5)M(NO)(PMe3)2 (M = Mo orW). These new complexes were synthesized from the [(r?5-C5H5)M(NO)I2]2 dimer in the manner previously described for the preparation of their PMePh2 analogues7. Both complexes, (r/5-C 5H 5)Mo(NO)(PMe 3) 2 and (r?5-C5H5)W(NO)(PMe3)2, were isolated as orange, crystalline, solids in 88% and 80% yield, respectively. Their spectroscopic and analytical data are included in Tables 5.1 and 5.2 for comparison with the other PMe 3-containing compounds prepared. 168 Table 5.1. Analytical, IR and Mass Spectral Data for the Phosphine Complexes Low-resolution Analytical Data, % IR Data Mass Spectral v N n (cm"1) Data3 C H N ComDlex m/zb foundfcalcd) foundfcalcd) found (calcd) ( C H ^ C U (r> 5-C5H5)Mo(NO)I2(PMe3) 447° 18.78(18.45) 2.85(2.69) 2.50(2.69) 1665 (r?5-C5H5)W(NO)I2(PMe3) 533° 15.56(15.71) 2.31(2.29) 2.19(2.29) 1638 [(r/ 5-C5H5)Mo(NO)I(PMe3)2]I 22.37(22.11) 3.77(3.85) 2.30(2.35) 1668 [(r,5-C5H5)W(NO)I(PMe3)2]I 19.51(19.22) 3.52(3.34) 2.04(2.03) 1645 Mo(NO)(PMe3)4I 559 26.14(25.87) 6.48(6.51) 2.75(2.51) 1539 W(NO)(PMe3)4I 643 22.34(22.33) 5.54(5.58) 2.27(2.17) 1522 (r) 5-C5H5)Mo(NO)(PMe3)2 345 38.37(38.49) 6.61(6.70) 4.11(4.08) 1523 (r/5-C5H5)W(NO)(PMe3)2 431 30.40(30.62) 5.30(5.33) 3.25(3.24) 1510 aProbe temperatures 100-150°C. Assignments involve the most abundant naturally occurring isotopes in each species (e.g. ^Mo, 184W). Attributable to [M - PMe^+ 169 Table 5.2. *H NMR Data for the Phosphine Complexes. Chemical Shift, 8 (ppm) Complex PMe 3 (77 5-C5H5)Mo(NO)l2(PMe3) isomer A 5.90 (d, 7 H p = 2.8 Hz) 1.84(d, 2/H.p = 10.5 Hz) a isomer B 5.92 (d, 3 / H p = 2.8 Hz) 1.88(d, 2/H.p = 10.5 Hz) a (r/ 5-C 5H 5)W(NO)I 2(PM e 3) 5 .96(d, 3 / H p = 2.6 Hz) 1.91(d, 2/H.p = 10.6 Hz) a [(r? 5-C 5H 5)Mo(NO)I(PM e 3) 2]I 6.09 (d, 3 / H . p = 2.09 Hz) 1.97 (m, W* = 10.8 Hz) c [(r,5-C5H5)W(NO)I(PMe3)2]I 6.91 (t, 3/H.p = 1.9 Hz) 2.07 (m, N*3 = 10.3 Hz) c Mo(NO)(PMe3)4I 1.45 (m)d W(NO)(PMe3)4I 1.52 (br s)e (r; 5-C 5H 5)Mo(NO)(PM e 3) 2 4.88 (t, 3 / H . p = 0.92 Hz) 1.16 (m, TV* = 7.44 Hz) e (rj 5-C 5H 5)W(NO)(PM e 3) 2 5.02(t,3/H.p = 1.5 Hz) 1.61 (m, /V* = 7.5 Hz)d 8 . c C D 3 N 0 2 . fl(CD3)2CO *C6D6-170 Results and Discussion In the absence of an added reducing agent such as sodium amalgam, PMe 3 reacts with the [(r/ 5-C 5H 5)M(NO)I 2] 2 (M = Mo or W) dimers in a sequential manner. The first two steps of this sequence involve initial cleavage of the dimers, i.e., [(r; 5 -C 5H 5)M(NO)I 2] 2 + 2 PMe 3 • 2 (r, 5-C 5H 5)M(NO)I 2(PMe 3) (3) followed by displacement of one of the iodo ligands from the inner coordination sphere of the metal, i.e., (i- 5 -C 5 H 5 )M(NO)I 2 (PMe 3 ) + PMe 3 5-C 5H 5)M(NO)I(PMe 3) 2]I (4) The optimum conditions for effecting conversions 3 and 4 independently are presented in the Experimental Section, and the indicated products of both transformations are isolable in high yields as analytically pure solids. The brick red, neutral mono(phosphine) and the yellow, cationic bis(phosphine) complexes exhibit spectroscopic properties similar to those described previously for their PMePh2 analogues.7,9 These properties (Tables 1 and 2) are consistent with their possessing four legged piano-stool molecular structures. Interestingly, (r/5-C 5H5)Mo(NO)I2(PMe3) exists in CD 2 C1 2 as a mixture of isomers (arbitrarily designated as A and B) in a ratio of A:B = 2:3 at ambient temperature. Presumably, one isomer has mutually cis iodo ligands, whereas in the other isomer these ligands are trans to one another. The congeneric tungsten complex, on the other hand, exists in CD 2 C1 2 exclusively as one isomer. Unfortunately, the 3 J H . P values exhibited by the signals due to the cyclopentadienyl protons in all three cases are approximately the same (Tables 1 and 2), thereby precluding definite structural assignments to the individual isomers.7 On 171 the other hand, the J H and 3 1 P NMR data for both [(r?5-C5H5)M(NO)I(PMe3)2]I (M = Mo and W) salts indicate that the cations exist in solutions as the single isomers having mutually trans PMe 3 ligands, i.e., The final step in the sequential reactions of PMe 3 with the [(?j -C 5H 5)M(NO)I 2] 2 dimers10 involves the displacement ofthe cyclopentadienyl rings from the [(r?5-C 5H 5)M(NO)I(PMe 3) 2] + cations formed via conversions 4, i.e., [(r;5-C5H5)M(NO)I(PMe3)2]I + 3 PMe 3 —^M(NO)(PMe3)4I + [(C5H5)PMe3]I (5) These transformations are particular examples of the generalized reaction represented by eq 2, the formal oxidation states of the transition metals decreasing by 2 units in going from reactant to product. Reactions 5 are most cleanly effected with CH 3 CN as solvent, the conversion having M = Mo proceeding more rapidly to completion (3 h) than that having M = W (5 days). The new M(NO)(PMe3)4I product complexes are isolable as yellow, diamagnetic solids in excellent yields. These solids are very air- and moisture-sensitive and decompose at room temperature in chlorinated organic solvents, particularly rapidly in CHC13 but more slowly in CH 2C1 2. The very low nitrosyl stretching frequencies evident in the IR spectra of these compounds (i.e. v N O ( C r ^ C y of 1539 and 1522 cm*1 for the molybdenum- and tungsten-containing species, respectively) indicate the presence of very electron-rich metal centers. Indeed, these frequencies are only slightly higher in energy than those exhibited by the related (n5-C 5H 5)Mo(NO)(PMe 3) 2 compounds (vide infra). + M The X H NMR spectra of the M(NO)(PMe3)4I complexes (formal AA'A"A , "X Q X' 9 X" 9 X'" 9 spin systems) are quite simple in appearance . Furthermore, the 3 1P{1H} NMR spectrum of Mo(NO)(PMe3)4I in C 6 D 5 C D 3 consists of a sharp singlet from + 25°C to -75°C. Taken together, these spectroscopic properties are in accord with the tetrakis(phosphine) complexes possessing octahedral molecular structures with mutually trans iodo and nitrosyl ligands, i.e., Analogous molecular structures have been previously proposed for the related compounds Mo(NO)(dppe)2Cln, Mo(NO)(PPh 2H) 4Cl n , and M(NO)(CO) 4 X 1 2 (M = Mo, W; X = Cl, Br, I). The fates of the cyclopentadienyl ligands displaced during reactions 5 were established by isolation and characterization of the byproduct [(C5H5)PMe3]I. Similar salts have been previously prepared.13 The nucleophilic displacement reactions14 summarized by eq 5 proceed through short-lived orange intermediates, which we have not as yet been able to characterize spectroscopically. However, by analogy to the substitution process outlined in eq 1, we believe that the first step in reactions 5 is probably the formation of anr; 1 -C 5 H 5 complex, the orange intermediate, i.e., 0 N [(r;5-C5H5)M(NO)I(PMe3)2]I + 2 PMe 3  [(r,1-C5H5)M(NO)I(PMe3)4]I (6) The seven-coordinate cationic complex thus formed could then undergo nucleophilic 173 attack by PMe 3 at the n -cyclopentadienyl ring to afford the final products, i.e., [(r71-C5H5)M(NO)I(PMe3)4]I + PMe 3 • M(NO)(PMe3)4I + [(C5H5)PMe3]I (7) It is interesting to compare reactions 7, in which the C 5 H 5 group is formally displaced as a cation, with the transformation involving a valence isoelectronic rhenium reactant, i.e,.2 0?1-C5H5)Re(NO)(PMe3)3Me + PMe 3 • [Re(NO)(PMe 3) 4Me] +C 5H 5" (8) in which the C 5 H 5 ligand is liberated from the coordination sphere of the metal as an anion. It thus appears that for these systems, the mode of reactivity of PMe 3 with the cyclopentadienyl-containing reactant is governed by the requirement that the final tetrakis(phosphine) product complexes satisfy the familiar 18-valence-electron rule. From a practical point of view, the most convenient method for the synthesis of the M(NO)(PMe3)4I complexes is the direct treatment of the dimers with 10 equiv of PMe 3, i.e., [(r;5-C5H5)M(NO)I2]2 + 10 PMe 3 • 2 M(NO)(PMe3)4I + 2 [(C5H5)PMe3]I (9) Obviously, reactions 9 are simply the sums of the conversions shown in eq 3-5, and they do afford the desired M(NO)(PMe3)4I products in good yields. In closing, one final point concerning the reactions of the [(r?5-C5H5)M(NO)I2]2 dimers with PMe 3 must be made. In the presence of an added reducing agent such as sodium amalgam, the two reactants do not produce the M(NO)(PMe3)4I complexes 174 considered above. Instead, the reaction that occurs under these experimental conditions is as shown in eq 10, i.e., [(r;5-C5H5)M(NO)I2]2 + 4 Na/Hg + 4 PMe 3 «-2 (r,5-C5H5)M(NO)(PMe 3)2 + 4 Nal + Hg (10) the final (r/5-C5H5)M(NO)(PMe3)2 complexes being isolable in excellent yields as orange, crystalline solids. Conversions 10 are particular examples of a general mode of reactivity that we have described in some detail previously.7 The new (n5-C 5H 5)M(NO)(PMe 3) 2 are formally related to the M(NO)(PMe3)4I complexes and, like them, are very air- and moisture-sensitive and decompose in chlorinated organic solvents. The spectroscopic properties of these compounds (Tables 5.1 and 5.2) are consistent with their possessing the familiar three-legged piano stool molecular structures.7 Of particular interest are the very low nitrosyl-stretching frequencies evident in their IR spectra [e.g. f N O ( C H 2 C l 2 ) 1523 and 1510 cm"1 for M = Mo and W, respectively], a feature that again emphasizes the electron richness of the group 6 metal centers in these complexes. 175 References 1. Tolman, C. A. Chem. Rev. 1977, 77,313. 2. O'Connor, J. M ; Casey, C. P. Chem. Rev. 1987,87,307 and references cited therein. 3. Christensen, N. J.; Hunter, A. D.; Legzdins, P.; Sanchez, L. Inorg. Chem. 1987, 26, 3344. 4. James, T. A,; McCleverty, J. A. /. Chem Soc. A 1971,1068. 5. Legzdins, P.; Martin, D. T.; Nurse, C. R. Inorg. Chem. 1980,19,1560. 6. Wolfsberger, W.; Schmidbaur,H. Synth. React. Inorg. Met.-Org. Chem. 1974,4, 149. 7. Hunter, A. D.; Legzdins, P. Organometallics 1986,5,1001. 8. Becker, E. B. High Resolution NMR: Theory and Chemical Applications, 2nd ed.; Academic Press: New York, 1980; pp 167 -171. 9. Hunter, A. D.; Legzdins, P.; Martin, J. T.; Sanchez, L. Organomet. Synth. 1986, 3, 58. 10. Phosphines that are less basic then PMe3 only undergo reactions analogous to conversions 3 and 4 when treated with the [(r/5-C5H5)M(NO)I2]2 dimers. 11. King, F.; Leigh, G.J. /. Chem. Soc, Dalton Trans. 1977,429. 12. (a) Legzdins, P.; Malito, J. T. Inorg. Chem 1975,14,1875. (b) King, R. B.; Saran, M. S.; Anand, S. P. Inorg. Chem 1974,13,3038. (c) Barraclough, C. G.; Bowden, J. A.; Colton, R.; Commons, C. J. Aust. J. Chem. 1973,26,241. 13. (a) Jutzi,P.; Kuhn, M. /. Organomet. Chem. 1979,177,349. (b) Jutzi, P.; Saleske, H. ; Nadler, D.J. Organomet. Chem. 1976,118, CS. 14. Related displacements of a cyclopentadienyl ligand by other nucleophiles have been reported; for representative examples, see: (a) Mawby, R. J.; White, C. /. Chem. Soc, Chem. Commun. 1968, 312. (b) White, C ; Mawby, R. J. Inorg. 176 Chim. Acta, 1970,4,261. (c) Kita, W. G. ; McCleverty, J. A.; Patel, B.; Williams, J.J. Organomet. Chem. 1974, 74, C9. (d) McCleverty, J. A.; Williams, J. / . Transition Met. Chem. (Weinheim, Ger.) 1976,1,288. (e) Aviles, T.; Royo, P. /. Organomet. Chem. 1981,221,333. (f) Slocum, D. W.; Engelmann, T. R.; Fellows, R. L.; Moronski, M.; Duraj, S. /. Organomet. Chem. 1984,260, C21 and references cited therein. 177 EPILOGUE This work has demonstrated firstly the preparation and characterization of a series of molybdenum diene complexes, Cp'Mo(NO)(rj4-diene). In every case except one, the diene is coordinated to the metal in the novel r\4-trans fashion, the exception being the case where the diene is 2,3-dimethylbutadiene, and it is coordinated to the molybdenum in the n4-cis fashion. The cis-diene complexes isomerize irreversibly to their fra/is-diene analogues in solutions, and a preliminary kinetic study of the isomerization in the Cp system has shown that the isomerization is either zero- or first-order, a factor which is difficult to ascertain due to the fact that the isomerization is very slow in an NMR tube, allowing other factors to become significant. A more thorough study taking into account the changes suggested in Chapter 3 may give a definitive result. Electrochemical analyses of CpMo(NO)(r/4-mvis-2,5-dimethyl-2,4-hexadiene) and CpMo(NO)(r?4-cis-2,3-dimethylbutadiene) has shown that they are difficult to reduce and undergo sequential oxidations. These properties are illustrated by the reactivity of these complexes with nucleophiles and electrophiles. In their reactions with unsaturated organic molecules, the r/4-fra/is-diene complexes produce products which have the diene coupled with the unsaturated molecule as new carbon - carbon bonds are formed. The structural analyses of these complexes indicate that the new coupled ligand is coordinated to the molybdenum in a a, r/3-allyl fashion in which the jj3-allyl moiety is bound to the Mo center in an endo- or exo- orientation (with respect to the cyclopentadienyl ligand), a factor which appears to be controlled by steric interactions within the molecule. Further studies will attempt to determine if the mode of linkage of that allyl fragment to the metal affects the stereochemistry of the product molecules upon their removal from the metal center. 178 And finally, in Chapter 5, it was shown that PMe 3 is a sufficiently potent reducing agent to reduce the [CpM(NO)I2]2 (M = Mo, W) dimers to M(NO)(PMe3)4I, [C5H5PMe3]I being the by-product and the reaction being shown to go through sequential intermediates, two of which have been isolated and characterized. Interestingly enough, reduction of the diiodo precursor complexes with sodium in the presence of PMe 3 does not lead to the tetrakis(phosphine) complexes, but rather it leads to the CpM(NO)(L)2 complexes, analogues of the diene complexes. In closing, interesting facts to note are: (1) Although much effort has been put into the attempted syntheses of the tungsten analogues of the molybdenum (»j4-diene) complexes, none have been observed thus far. This is despite the fact that the CpW(NO)(L)2 complexes (L = phosphine or phosphite) are known compounds. (2) The molybdenum (r74-diene) complexes may be useful reagents in organic synthesis as they readily form new carbon-carbon bonds with unsaturated organic molecules. This utility may be limited somewhat by the thermal sensitivity of the diene complexes. Further studies will determine the scope of the ligand coupling reactions and their applications to organic synthesis. 

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