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Investigating new reactions for coordinated dinitrogen Park, Rosa 2008

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INVESTIGATING NEW REACTIONS FOR COORDINATED DINITRO GEN  by  ROSA PARK B. Sc. (Hons.), York University, 2005  A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (CHEMISTRY)  THE UNIVERSITY OF BRITISH COLUMBIA (VANCOUVER) April 2008 © Rosa Park, 2008  Abstract The chemistry of the tantalum dinitrogen complex ([NPN]fa)2(A-H)2(1A-1 1 :fi 2 N2 )Ta-[NPN], 1, (where [NPN] represents the acyclic tridentate ligand [(PhNSiMe2CH2) 2 PIN 2-), with primary alkenes, group IV and V metallocene complexes, and GaCp* is explored. The reaction of 1 with 1-pentene and 1-hexene occurs via olefin-insertion into the metal-hydride bond to give two new complexes, ([NPN]Ta(CH2)4CH3)2(1-11 1 :1 1 -N2), 10, and ([NPN]Ta(CH2)3CH3)2(1A-11 1 :11'-N2), 11, which were characterized using NMR spectroscopy. The solid-state structure of 11 was established and revealed that N2 has been transformed into a bridging end-on mode. In contrast to its G symmetry in solution, the solid-state structure of 11 is C, symmetric; a VT-NMR study was performed and showed that 11 exists as an equilibrium between two isomers in solution: a C, symmetric isomer 11A, which is the predominant isomer at room temperature, and a C, symmetric isomer 11B, which is the minor isomer. The reactivity of 1 with group IV and V metallocenes was investigated. The reaction of 1 with Cp2Hf(PMe 3 )(11 2 -Me 3 SiCCSiMe 3 ) produces [N(IA-P=N)N]Ta(vt-H)2(1A-N(HfCp2))Ta[NPN], 14, in which N2 is cleaved and new Hf-N and P=N bonds have formed. The reaction of 1 with group V metallocene hydrides Cp2MH 3 (M Nb, Ta) was also attempted however these complexes do not react with 1. The reaction of 1 with GaCp* produces a new complex, [NPN]Ta(vtN(GaCp*))Ta(=NPh)[NPIA,-N], 18, which was characterized using NMR spectroscopy. Complex 18 decomposes over several days in solution, and one product of decomposition  ii  that was isolated was [(PhNH)(NPvt-N)Tah, 19. The solid-state structure of 19 showed that the [NPN] ligand was cleaved at the N-Si bond, similar to that observed for the reaction of 1 with 9-BBN and HB(C6H 5 )2. A mechanism for the formation of complexes 18 and 19 is proposed.  iii  ^  Table of Contents Abstract ^ Table of Contents ^ List of Tables ^ List of Figures ^ Glossary of Terms ^ Acknowledgements ^ Dedication ^  ii  iv vi vii xiii xiv  Chapter 1: The Chemistry of Dinitrogen  ^1.1^Introduction ^  1  1.2^Nitrogen Fixation and the Haber-Bosch Process ^  5  1.3^The Chemistry of Side-on End-on Dinitrogen ^  7  1.4^Scope and Prospectus ^  14  Chapter 2: Olefin Insertion Reactions of the Side-on End-on Dinitrogen Complex  ^2.1^Introduction ^ 2.2^Olefin Insertion Reactions ^  15 17  2.2.1 Olefin Insertion Reactions of Bridging Hydride Complexes ^ 18 2.2.2 Hydrozirconation of Olefins ^  20  2.2.3 Olefin Insertion with 1 ^  22  2.3^Results and Discussion ^  23  2.3.1 Reaction of 1 with 1-pentene ^  23  2.3.2 Reaction of 1 with 1-hexene ^  26  2.4^Conclusions ^  33  2.5^Experimental ^  33  2.5.1 General Procedure ^  33  2.5.2 Materials and Reagents ^  34  2.5.3 Synthesis, Characterization, and Reactivity of Complexes ^ 35  iv  Chapter 3: The Reactivity of Group IV and V Metallocenes with Side-on End-on Coordinated N2 3.1^Introduction ^  39  3.2^The Chemistry of Cp2NbH 3 and Cp 2 TaH 3 ^  42  3.3^The Reaction of 1 with Cp2Zr(II) and Cp 2 Ti(II) ^ 44 3.4^Results and Discussion ^  47  3.4.1 Reaction of 1 with Cp2Hf(PMe3)(ti 2 -Me 3 SiCCSiMe 3 ) ^ 47 3.4.2 Attempted Reaction of 1 with Cp 2 NbH 3 and Cp 2 TaH 3 ^ 50 3.5^Conclusions ^  51  3.6^Experimental ^  52  2.6.1 General Procedure ^  52  3.6.2 Materials and Reagents ^  52  3.6.3 Synthesis, Characterization, and Reactivity of Complexes ^ 52  Chapter 4: The Reactivity of a Group 13 Complex with Side-on End-on Coordinated N2 4.1^Introduction ^  55  4.2^The Reaction of 1 with Lewis Acids ^  57  4.3^Results and Discussion ^  58  4.3.1 Reaction of 1 with GaCp* ^  58  4.4^Conclusions ^  65  4.5^Summary and Future Work ^  65  4.6^Experimental ^  67  4.6.1 General Procedure ^  67  4.6.2 Materials and Reagents ^  67  4.6.3 Synthesis, Characterization, and Reactivity of Complexes ^ 67 4.7^References ^  69  Appendix: X-ray Crystal Structure Data Al^Crystallographic Data ^  77  List of Tables Table 1.1^N-N bond lengths and stretching frequencies for some simple molecules and N2 complexes ^ .2  Table 1.2^The different bonding modes of N2 in mono- and bimetallic transition metal (M) complexes ^ 4  Table 1.3  ^  A comparison of selected bond lengths (A) for 1 and 2 ^ 10  Table 3.1^Group IV and V metallocene N2 complexes, their precursors and N-N bond lengths (A) ^ 42  Table A-1  ^  Table A-2  Crystallographic Data and Structure Refinement for ([NPN]Ta)2([1-H)4 (2) and UNPNrfa(CH2)5CH3)2(1-111:111N2) (11) ^ 77  ^  Crystallographic Data and Structure Refinement for [N(AP=N)N]Ta(i-H)2(wN(HfCp2))Ta[NPN] (14) and [(PhNH)(NPIA-N)Tab (19) ^ 78  vi  List of Figures  Figure 1.1^Some examples of activated early transition metal N2 complexes  and their N-N bond lengths ^  3  Figure 1.2^Structure of the FeMo cofactor extracted from the nitrogenase  enzyme in Azotobacter vinelandii ^  6  Figure 1.3^ORTEP depiction of the solid-state molecular structure of 2^  9  Figure 2.1^An illustration of the four-centered transition state formed in an  olefin-hydride insertion ^  15  Figure 2.2^Examples of hydride-olefin complexes and the activation energies  (kcal/mol) associated with the hydride-insertion into ethylene ^  16  Figure 2.3^Two complexes in which the hydride and olefin ligands are  17  cis^  Figure 2.4^Subsequent reactions of Cp 2 Zr(R)Cl ^ 21  vii  Figure 2.5^31P{' HI NMR spectrum of 10 in C 6 D 6 ^  24  Figure 2.6 300 MHz H NMR spectrum of 10 in C6D6 ^ ..24  Figure 2.7^13 P{' HI NMR spectrum of 10-  Figure 2.8  15 NPHI  15  N2 ^ 25  NMR spectrum of 10 in C6D6 ^ 26  Figure 2.9^15 N{'H} NMR spectrum of 11 in C6D6 ^ 27  Figure 2.10^ORTEP depiction of the solid state molecular structure 11 ^ 28  Figure 2.11 A stack plot of 31 P{'H} NMR spectra of 11 from 273 K to 203  K^  .30  Figure 2.12 Van't Hoff plot of 11 from 283 K to 203 K ^ 31  Figure 3.1^Examples of side-on and end-on N2 metallocene complexes ^ 40  Figure 3.2^A group V metallocene hydride ^  43  Figure 3.3^31P{'H} NMR spectrum of14 in C6D6 ^ 48  viii  Figure 3.4^An ORTEP depiction of the solid-state molecular structure of 14 ^  .49  Figure 4.1^31 P{'H} NMR spectrum of 1 and GaCp* in C 6 D 6 ^ 59  Figure 4.2^31 P{'H} NMR spectrum of 18 in C6D6 ^ 60  Figure 4.3 400 MHz 'H NMR spectrum of 18 in C6D6 ^ 61  Figure 4.4 °N{ 1 H} NMR resonances of 18 in C 6 D 6 ^  .62  Figure 4.5^An ORTEP depiction of the solid-state molecular structure of 19 ^  . .63  ix  Glossary of Terms A^Angstrom (10 10 m) Anal.^analysis atm^atmosphere 9-BBN^9-borabicyclononane (HB(C8H14))2 'Bu^isobutyl group (-CH2CH(CH3)2) "Bu^normal butyl group (-CH2CH2CH2CH3) tertiary butyl group (-C(CH 3 ) 3 ) °C^degrees Celsius 3  C^carbon -13  Calcd.^calculated -1 cm^reciprocal centimeters (wavenumbers) COSY^correlated spectroscopy Cp^cyclopentadienyl group ([C 5 H 5 ] -) Cp*^pentamethylcyclopentadienyl group ([C 5 Me 5 ] -) DIBAL^diisobutylaluminium hydride (HA1(CH2CH (CH3)2)2 d^doublet D^deuterium 2D^two-dimensional 8^delta heat dd^doublet of doublets deg (°)^degrees DFT^density functional theory dppe^bis(diphenylphosphino)ethane Ea^Activation Energy equiv^equivalent g^grams  HIPT^hexa-isopropylterphenyl substituent  HOMO^Highest Occupied Molecular Orbital HSQC^Heteronuclear Single Quantum Coherence Hz^Hertz nT  JAB^  coupling constant between nuceli A and B over n bonds  K^degrees Kelvin  kcal^kilocalories kJ^kilojoules L^ligand group  LUMO^Lowest Unoccupied Molecular Orbital M^metal  M-H^metal hydride Me^methyl substituent m^multiplet m^meta-position of aryl ring  mg^milligrams MgADP^adenosine diphosphate, magnesium salt MgATP^adenosine triphosphate, magnesium salt mol^mole mmol^millimoles MOs^molecular orbitals "N^nitrogen-15 nm^nanometer NMR^nuclear magnetic resonance [NPN]^[PhP (CH2S iMe 2 NP h)2] 11"^hapticity  of order n  o^ortho-position of aryl ring ORTEP^Oakridge Thermal Ellipsoid Plotting Program p^Para-position of aryl ring 31p^  phosphorous-31  Ph^phenyl group (-C6H5) 7C^  pi xi  ppm^parts per million py^pyridine group 'Pr^isopropyl group (-CH(CH3)2) R^alkyl or aryl group rt^room temperature s^singlet a^sigma t^triplet T^temperature THE^tetrahydrofuran TMS^tetramethylsilane ((CH 3 )4Si) VNN^  stretching freqency of N-N bond  X^halide group Z^number of formula units in the unit cell  Acknowledgements I would like to thank Professor Mike Fryzuk for giving me the opportunity to work in this group, and inspiring me with dinitrogen chemistry. His patience, encouragement and support were instrumental throughout my time at U.B.C. To my labmates, for all the fun beer drinking memories and insightful discussions, both chemistry related and otherwise; Fiona Hess, Bryan Shaw, Gabriel Menard, Kyle Parker, Nathan Halcovitch, Joerg Schachner, Owen Summerscales, and Yasuhiro Ohki. Special thanks are due to Howie Jong and M.J. Ferreira for solving the X-ray structures. Thank-you Dr. Erin MacLachalan for editing this entire thesis while being a full-time mommy - your comments and suggestions were invaluable in writing this thesis. Many thanks to the excellent staff at the Chemistry Department at U.B.C.: Ken Love and Milan for the countless hours in fixing and maintaining the old glove box - its old  but still going strong; Zorana Danilovic, Maria Ezhova, Nick Burlinson, Andrew Lewis (at S.F.U.), and Colin Zhang (at S.F.U.) and for all of their support with NMR spectroscopy; Brian Ditchburn for fixing my line and glassblowing all of the sealed NMR tubes and broken J-Young tubes; Noga Levit for being a great safety advisor; Brian Patrick for his expertise with the X-ray structures; Sophia Nussbaum for allowing me to TA the first year labs, and introducing me to Bucky; everyone at chem stores, mech shop and electronics shop - thank you. To my family: mom, dad, grandma, and grandpa, thank you for always encouraging me to do my best and having faith in everything I do; to my dear sisters, Domina and Agnes for being my cheerleaders - I love you guys! Thank you Sharon Lee, Haidy Abdel-Sayed and Michel Yun for being an incredible support group and being there for me whenever I was homesick.  Dedication  For my family - mom, dad, grandma, grandpa, Agnes and Domina - I am truly blessed to have such a loving family.  xiv  Chapter One The Chemistry of Dinitrogen. 1.1 Introduction. The earth's atmosphere is composed of about 78% N2 . 1 Reduced nitrogen is found in many biologically relevant compounds such as amines, nucleic acids and proteins, and nitrogen is an essential element for all life forms. 2 However, despite its abundance,  is  N2  extremely inert, and the challenge in trying to use transition-metal complexes to catalytically activate and functionalize  N2 still  remains.  Using N2 as a reagent industrially is appealing because it is abundant, accessible, and inexpensive; the development of a transition-metal catalyst that can convert  N2  into  nitrogen-containing compounds such as amines or N-heterocycles would be ideal. However, the intrinsic properties of N2 are a barrier to attaining such a cycle, so far. In contrast to isoelectronic carbon monoxide, which is a reactive molecule and an excellent ligand,  N2  is  incredibly stable and a poor ligand. 3 The reasons for its stability are: it is non polar, it has a high bond dissociation enthalpy (945 kJ moll, as well as a large HOMO-LUMO gap.' All of these properties make it difficult for N2 to bind to transition metals, and to be oxidized or reduced. When dinitrogen does bind to a metal complex it may become activated, and the degree of activation can be correlated from the N-N bond length (typically determined by Xray diffraction), or by the stretching frequency of N2 in the complex (determined by infrared or Raman spectroscopy). A comparison of N-N bond lengths of some organic compounds,  1  along with some examples of dinitrogen complexes is given in Table 1.1. Stretching frequencies determined by IR and Raman are also included, if data was available.  Table 1.1. N-N bond lengths and stretching frequencies (R = Raman, IR = Infrared) for some simple molecules, as well as for some dinitrogen metal complexes.  N2(0 5  N-N bond Length (A) 1.0975  N-N Stretching Frequency (cm') 2331 (R)  PhN=NPh 6  1.255  1442 (R)  H 2 N-NH 2 7  1.449  1111 (R)  CpFe(dippe) (N2)* 8  1.13(1)  2112 (IR)  {[Pe2PCH2SiMe2)2N]Zr(C1)}2(1-11 2 :1 2 -N2) 9  1 . 548 (7)  731 (R)  11^ -2, 1° [(11 5 -05H2-1,2,4-Me3)2Ti]^ 2 , 11 2- N 2 \r-2,-1  1.216(3)  {[(Me3S02N]2(THF)Y12(Kr1 2 :1 2 -N2)"  1.268(3)  {Cp*Ta[N(Tr)C(Me)N('Pr)112(vri'm'-N2)' 2  1.288(10)  Compound  (tBuNSiMe2N(CH2CH2FTr2)2Mn)2(p-ri^-N2)  3  1.208(6)  [Ir{C6H3-2,6-(CH2PBut2)2}12(11-11 '11 1 -N2) "  1.176(13)  [(11 5 C5Me4H)2Zr]2(p.-1 2 :r1 2 -N2) 1  1.377(3)  NRCH2)5C(cc-C4H2N)]4}Sm3Li2}(ptri 2 :i 2 N2)[Li(THF)2]-(THF)2 16 [(1) 5 C5Me4H)2Hf12(1A-11 2 :1 2 -N2) 17  1.502(5)  1685 (R)  1.423(11)  The degree of N2 activation in a complex depends on the identity of the metal, and its oxidation state, as well as on the nature of the ligands in the complex.' 8 Early transition metal N2 complexes typically display a greater degree of activation, while late transition metal N2 complexes usually contain weakly activated nitrogen.' Some examples  2  19-23  of  activated early transition metal  N2  complexes, along with their bond lengths (measured by  single crystal X-ray diffraction) are shown in Figure 1.1.  Mee  Si,^me2 iPr2  PMe3  (1  .298 A  PMe3 1.548(7) CHCMe3  CI,,  Ta— N—N=-Ta Me3CHC  1.379 (21)  Zr^P iPr2  1 PMe3 A Me 3 P  \ CI  A  SiMe3^SiMe3  NON \  ,0C1^Me3Si iPr2 P—Zrz.:C PiPr2 Me3Si (^ Me 2 Si—N  B  /^ „s` N^SiMe 3  •  SiMe3^c SiMe3  Me e  N=N R'  N  N  k. 1.0975 A Ph^Ph  H2N -4 NH2  R' s M-7eN.---m I  \-1.449  A•  R  A  SiMe2  Me 2 Si .  Ph  Me 2 Si  P\^  D M, X, R', R i. Hf, Me, Me, Et ii. Hf, Me, H, iPr iii.Hf, Me, Me, iPr iv. Hf, NMe 2 , H, iPr v. Hf, NMe 2 , Me, iPr vi. Zr, NMe 2 , Me, iPr  \ 2  Ph \ \/ N,,  Bond Length (A) 1.635(5) 1.630(4) 1.611(4) 1.600(6) 1.581(4) 1.518 (2)  0 Ph  1.503(3) A  E  Figure 1.1. Some examples of activated early transition metal  N2 complexes  and their N-N  bond lengths. For comparison, the bond lengths of dinitrogen and hydrazine are provided in the inset.  In Figure 1.1, N2 activation is evident from the increased length of the N-N bond, which either approaches or exceeds the N-N single bond in hydrazine. The various complexes from structure D show even more dramatic lengthening of the  N2 bond; 22  end-on  hafnium and zirconium complexes have N-N bond lengths that range from 1.518(2) to  3  1.635(5) A (structures Di - Dvi in Figure 1.1). Complex Di has an N2 bond length that is 1.635(5) A, and is the longest N-N bond measured to date for an  N2  complex. 22  The various binding modes of dinitrogen for both monometallic and bimetallic complexes, and the degree of  N2  activation are summarized in Table 1.2. The common  bonding mode for transition metal dinitrogen complexes is end-on in both bimetallic and monometallic  complexes. 24 It was not until 1988 when the first planar side-on bound  N2  complex (Cp*2Sm)  2 (11-41 2 -1 2 -N2)  N2  was communicated. 25 Since 1988, many other side-on  bound dinitrogen complexes have been reported, all of which have been structurally characterized as dinuclear metal systems, with the exception of one side-on monometallic  N2  Complex, [OS (NH3)5N2] [PFD] 2.26  Table 1.2. The different bonding modes of  N2  in mono- or bimetallic transition metal (M)  complexes. Only connectivity is indicated, with approximations in bond order and activation (N-N triple bond = weak activation, N-N single bond = strong activation). Weak Activation M  -  1\1  -  Strong Activation End-on Mononuclear  N  M  Side-on Mononuclear  III N  -..  IIIN m m •---..^..------  End-on Dinuclear  M=N—N=M  M—N=-N—M  .....-^N1^.....,  N ....I..,  M.-..., 1 1 ,-M N  M-..., 1 „-M N Ivi^M  N  4  \N1  ,,,\ N M"'N'Is'iV1  Side-on Dinuclear Side-on^Endon Dinuclear  N2  1.2 Nitrogen Fixation and the Haber-Bosch Process. In 1965, Allen and Senoff reported the first transition metal dinitrogen complex, [Ru(NH 3 ) 5 1\1 21 2 ., which resulted from the reaction of RuC1 3 with hydrazine hydrate, as shown in Equation 1.1. 27  _  NH 3`  N2H 4 .H 2 0  - 2-F  NH3  H 3 N—jd N___N^Cl2  RUC13.XH20  I  H 3 N^ NH3  Equation 1.1.  Since this discovery, the coordination chemistry of N2 has flourished. Some of this chemistry focused on modeling the active site in the nitrogenase enzyme, which converts N2 into ammonia, in a process known as nitrogen fixation (Equation 1.2). 4  N2( g ) + 8F1 + + 8e + 16MgATP 2NH 3 ( g ) + H2( g ) + 16MgADP + 16P; -  Equation 1.2  Nitrogen fixation is carried out by certain bacteria, including Azotobacter vinelandii and Clostridium pasteurianum, 28 and produces thousands of tons of ammonia every day  worldwide, (an estimated 170 x 10 6 tons/year) under ambient conditions (20°C, 1 atm). 29 The active site in nitrogenase, where N2 is reduced to NH 3 , is an iron-sulfur-molybdenum cofactor, or FeMoco. 4 A high-resolution crystallographic analysis of nitrogenase determined  5  at 1.16 A was performed in 2002, and revealed a light atom (X) in the centre of FeMoco (shown in Figure 1.2), that is compatible with being C, N, or  0.30  p  OMO  •Fe OS 00  •C  Figure 1.2. Structure of the FeMo cofactor extracted from the nitrogenase enzyme in Azotobacter vinelandii. 36  The identity of atom X in the centre of the cofactor remains elusive. Although theoretical investigations suggested that X is a nitrogen atom," 4 recent experimental evidence does not support these claims." Despite the fact that the structure of FeMoco has been scrutinized in great detail, the mechanism by which it transforms N, to NFli is still unclear. N, is converted into ammonia industrially via the Haber-Bosch process.' In this process, -N 2 reacts with three equivalents of H, to yield NH3 under high temperature (400550 °C) and pressure (270 atm) in the presence of an activated iron or ruthenium catalyst (Equation 1.3). 4 This process yields millions of tons of ammonia annually, and fertilizer that is made from this ammonia is responsible for sustaining approximately 40% of the world's  6  ^  population." Two Nobel Prizes were given for the discovery of the Haber-Bosch Process: the first was awarded to Fritz Haber in 1918, for discovering the nitrogen fixation reaction, and the second was awarded to Carl Bosch in 1931, for developing the high-pressure techniques that made this process industrially feasible.  Fe or Ru Catalyst N2( g ) + 3H2(g)^2NH3(g) 400-550 °C 270 atm  Equation 1.3  1.3 The Chemistry of Side-on End-on Dinitrogen. This thesis will focus on the chemistry of the side-on end-on  N2  complex  ([NPN]Ta)2(v-H)2(µ-11 1 :11 2 -N2) (where [NPN] = [(PhNSiMe2CH 2 ) 2 PIN 2-), which will be noted as 1 below." Complex 1 contains two tridentate [NPN] ligands that are each bound to two formally Ta(V) metal centers. Ph Ph Ph :^\P  \ —.  ---."  \,SiMe2 i \-.1',1 / Me S^„,.._ 7 SiMe2 ^ N T .^Ta ..,„,„ Me 2 Si^/ 1\ N /I ,,,,, NON^  .-p \^1^ Ph  ^Ph Ph  The two Ta(V) centers are also bound i1 1 -11 2 to the nitrogen molecule. The ligand synthesis is outlined in Scheme 1.1." The reaction of 2 equivalents of C1CH2SiMe 2 NHPh with 4  7  equivalents of n-BuLi and 1 equivalent of PhPH 2 produces Li z [NPN], which in turn reacts with Me 3 TaCl z to form [NPN]TaMe 3 via a metathesis reaction. Me 2 Si 2^CI  Me2 Si^Ph  Et 20 THF + 2LiHNPh  2  -78 ° C  CI  H  -2 LiCI^CI Et 2 0 THF -2 LiCI  PhPH 2 4 Bu n Li Ph  Me2 Si  Ph I I ., NI, Me 2 SiN^,Me Me 2 Si  NPh  TaMe 3 Cl 2 Et 2 0  ^"Me Me  Ph  -2 LiCI -2 THF  THF /  Nlph  Si Me 2  Ph  Scheme 1.1 Upon exposure of [NPN]TaMe 3 to 4 atm of H z , the purple, diamagnetic Ta(IV)Ta(IV) complex, ([NPN]Ta) 2 (A-H) 4 , 2, is produced (Equation 1.4). An ORTEP depiction of the solid-state molecular structure of this tetrahydride complex, along with selected bond lengths and bond angles is shown in Figure 1.3. Ph Ph^ P Ph  Ph I I,N  me2SI N^ 2  Me  Me 2 Si  -  Me  P—"\\  v SiMe 2  rj';  5 H2  Ph  /SiMe2 Me2Si^ ' "" ,1 " " *Ta—=Ta -6 CH ^ M e ^ ■INN ,/ ph H H^ Ph  Ph  Ph  Equation 1.4  8  2  Figure 1.3 ORTEP depiction of the solid state molecular structure of ([NPN]I a) 2 (1A-H)4, -  2 (ellipsoids at 50% probability). Hydrides were located on a difference map and refined  isotropically, and tantalum hydride bond lengths were determined by PARST. All hydrogen atoms and phenyl ring carbons other than ipso carbons have been omitted for clarity. Selected bond lengths (A) and bond angles (°): Tal-Tal' 2.5602(8), Tal-Hl 1.8471(290), Tal-H2 1.8542(258), Tal-Nl 2.093(2), Tal-N2 2.058(2), Tal-P1 2.5741(10), HI-TalH2 59.98(4), N1-Tal-H1 90.81(8), N1-Tal-H2 148.27(4), N2-Tal-H1 155.17(1), N2Tal-H2 96.03(7), P1-Tal-H1 86.65(7), P1-Tal-H2 83.45(8) N1-Tal-N2 110.69(10), N1-Tal-Tar 119.66(7), N2-Tal-Tal' 123.26(7), N1-Tal-P1 82.65(7), N2-Tal-P1 84.08(8), P1-Tal-Tal' 125.06(3).  The solid-state structure of 2 is C, symmetric, and the 'H and "P{' H} NMR spectra" indicate that 2 is also C, symmetric in solution; this is in contrast to that observed  9  for 1, which is Cs symmetric in solution. Selected bond lengths for 1 and 2 are shown in Table 1.3.  Table 1.3. A comparison of selected bond lengths (A) for 1 and 2. ([NPN]Ta(R-H))2({R-Til:12-N2), 1  ([NPN]Ta)2(R-H)4, 2 Atom  Atom  Distance (A)  Atom  Atom  Tal  Ta2  2.5602(8)  Tal  Ta2  2.8311(3)  Tal  N1  2.093(2)  Tal  N1  2.066(5)  Tal  N2  2.058(3)  Tal  N2  2.040(5)  Tal  P1  2.5741(10)  Tal  P1  2.625(2)  Distance (A)  The Tal-Tal' bond length in 2 is 2.5602(8) A, which is considerably shorter than the Tal-Ta2 separation for 1 (2.8311(3) A). A slightly smaller difference is observed with the Ta-N and Ta-P bonds. The Tal-N1 and Tal-N2 bond lengths for 2 (2.093(2) and 2.058(2) A, respectively) are slightly longer than that of 1 (Tal-N1 = 2.066(5), Tal-N2 = 2.040(5)), and the Tal-P1 for 2 (2.5741(10) A) is slightly shorter than that for 1 (Tal-P1 = 2.625(2) A). ([NPN]Ta) 2 (t-H) 4 reacts with N2 gas to form 1, (N-N bond: 1.319(4) A), as shown in Equation 1.5. The yield of the reaction is improved if a mixture of 90% gases are used, although the precise mechanism of this is unknown."  10  N2  and 10% H2  Ph  Ph  ^  Ph  Ph  P--SiMe2 90% N2 H 10% H2 /SiMe2 Me2Si'^/^  V:  Me2Si  N'Y^ '''Ph H H  -H2^  /  .1  Na■—P  Ph Ph  H ^/ SiMe2 • N Ta //a^ Me2Si^N^'Ph Mee Si  1.319(6) A Ph  Ph  2  1 Equation 1.5  The formation of 1 from 2 is interesting for many reasons. Firstly, the use of alkali metal reducing agents, (such as Na and KC 8 ), which are commonly used to form dinitrogen complexes, are avoided. Secondly, this is a rare example of forming an activated early transition metal dinitrogen complex by displacing H 2 , 41 ' 42 although this reaction is known for late transition metals."'" Complex 1 is also impressive in its wide scale of reactivity. In some cases, the sideon end-on N2 can act as a nucleophile (Equation 1.6); for instance, 1 reacts with benzyl bromide to give the N-benzyl derivative, I[NPN]Ta(Br)} 24t-H){[t-1 1 -i 2 -NN(CH2C 6 H 5 )}(Ta[NPN]), 3, (N-N bond: 1.353(4) A)."  Ph Ph  Ph  Ph^P \P—SiMez  Me Si N^  Me 2 Si  Ph  Ph  f/e"^  ^` Ta N  \  SiMe2^PhCH2Br  i  "e2 Y‘j***''Ta ^...,N/ SiMe2  Me2Si t  Ph  --"*" \SIMe 2  ^1.\.N7  P^N \^I  Ph  Ph  Ph CH2P\  1.353(4) A  Ph  Equation 1.6  11  3  The reaction of 1 with simple hydride reagents E-H (E-H = 9-BBN (HBR 2 ), DIBAL (HAIR 2 ) and H 3 SiBun) results in the functionalization of N 2 . 4547 The first step is formation 4 via E-H addition across the Ta-N bond (Scheme 1.3). Complex 4 has been characterized in the solid-state for E = BR2 or SiH 2 Bun, and in solution by NMR spectroscopy for E = AlBu'2. 4 " 7 Second,  H2  elimination and cleavage of the N-N bond is common to these  processes to form intermediate 5, (Scheme 1.2). Ph  Ph  Ph  Ph 11%.H  Me  Ph  Ph ,SiMe 2  Me2Si^I  NO  Ph  SiMe2 Me 2 Si'^./A^„N SiMe2  E-H  .-^ N^A A/P—L\SiMe2  Ti  Me2Si^ —N —N^ N kw_ H  N  -  Ph ^  h  Ph E  4  Ph  Ph  ^Ph^N^  --"\s siimmee:  e2?' Me 2 Si I^di  NN^N/'/,ph ^NMI11 I^\ E  - H2  M  Ph  Ph  5 Scheme 1.2  Intermediate 5 will yield different products depending on the hydride source (Scheme 1.3). Hydroboration of 1 (where E = BC81-114) degrades the [NPN] ligand and eliminates H2 from the bridging hydrides to form to form [(PhNSiMe2CH2P(Ph)CH 2 SiMe2v-N)Ta(=NBC8F1 1 4)](1,t-N)(Ta[NPN]), 6, along with one equivalent of benzene from the N-Ph of [NPN]. In this process,  N2  is cleaved and functionalized. 12  45  ^  Hydroalumination of 1 (where E = Al 'B ) produces ([NPN]TaH)(i-H)(µ-1 1 :11 2. N2Al(p.-1-1)13u)(Ta[NPN]), 7." In this process, one equivalent of isobutene is eliminated, and N2 is cleaved and functionalized; however, although ligand degradation is not observed, one amide donor of [NPN] migrates from Ta to Al. Hydrosilylation of 1 produces ([NPN]Ta) 2 (t-NSiH 2 Bun) 2 , 8, a symmetrical disilylimide complex that has a cleaved and functionalized N 2 ; in this complex, [NPN] remains intact. 47 /in  SMe 2 Ph\. 7\ ,,,,,,,,, p^i  Me 2 SiPh  //SiMe2  \ N^  /sime2  1N / , "•Ta .N1 ,,,  Ph i  h  /^6^`h  BR2  E=B (-Ph H)  Ph  Ph  Ph^  /7\  Ph  Me2Si......1  Ph  i ^/P—"\sime2 ^I \J  E=A1  Me Sys^N SiMe2 Ta, / Ta•c"N/ Me2Si \ Ph p Fl^I  E  ( C4F 18) -  Ph  -  N  \P ,\ SiMe2  \ / .P---0.-"T\NzTa.SNNIf,,ph ^ phoo,—  ^/SiMe2  H H i^■ Me2Si, NA1^Ph NI' ' ■  Bui  Ph  Ph^7  5 E=Si  (-H2)  Ph Ph^SiRH2 N  . Wu". Ta 7 Me2Sy^z a  i / me 2 sV_ Ph  Scheme 1.3 13  ^SiRH 2  8  sime2  P—N iN /sime2 NPh  Ph  1.4 Scope and Prospectus. The central goal of this thesis is to explore new types of reactivity that lead to the cleavage and functionalization of N2 in ([NPN]Ta(1-H))2(1A-r01 2 -N2), 1. In Chapter Two, the reactivity of 1 with several alkene reagents is investigated. Migratory insertion with primary alkenes occurs to form an activated end-on N2 alkyl complex. The synthesis and characterization of these complexes is discussed. The focus of Chapter Three is the reactivity of 1 with group IV and V metallocenes. With group IV metallocenes, N2 is cleaved and functionalized. In contrast, group V metallocenes did not react with 1, and the reasons for this will be explained in this chapter. In Chapter Four, the reactivity of 1 with GaCp* forms a new complex, 18, that was characterized using NMR spectroscopy. Complex 18 decomposes into a number of different P-containing species over the course of several days, as evident in the  31  P{ 1 H} NMR  spectrum. One of these species, 19, was isolated and characterized using X-ray diffraction. The solid-state structure showed evidence of ligand rearrangement, a process that was previously observed with hydroboration reactions of 1. The mechanism and characterization of 18 will be discussed in this chapter, along with a general summary of this thesis, and some suggested future work.  14  Chapter Two Olefin Insertion Reactions of the Side-on End-on Dinitrogen Complex 2.1 Introduction. Olefin insertion of M-H bonds is an important elementary step in many catalytic reactions." In its simplest terms, an olefin-hydride insertion is the addition of a metalhydride bond across the  JI  bond of an unsaturated hydrocarbon." This addition is a  concerted process that occurs via the formation of a four-centered transition state (Figure 2.1). 5 °  R  M Figure 2.1. An illustration of the four-centered transition state formed in an olefin-hydride  insertion.  Molecular orbital studies have contributed to a better understanding of this process. 5 ' -54 These studies describe the key orbital interactions involved in olefin-hydride insertion; first, the alkene It orbital donates electrons to the LUMO antibonding G * MH orbital; second, back donation from the HOMO 0MH orbital to the olefin it orbital creates a cyclic transition state that activates the insertion of the olefin into the metal hydride, to generate an alkyl transition-metal complex.  15  For a concerted reaction, the activation energy is typically lower than the bond dissociation energies of the weakest bonds to be broken." Some examples of hydride complexes and the activation energies for insertion of ethylene are provided in Figure 2.2. 55" An explanation of the factors that affect the energy barrier for hydride-olefin insertion is beyond the scope of this chapter; however, the repulsion between non-bonding metal electrons and the electrons in the olefin, the type of ligands and metals that are used, and the electronic state of the olefin, have all shown to be factors in the activation barrier."'"'"'"  H  R 3 P/ , 'Rh R3P CO  C  A R = Me:  E a = 15.5  M =Ta: E a = 21.3  R = Ph:  E a = 11.7  M=Nb: E a = 17.3  R = H:  E a = 13.4  B E a = 19.2  Me 5  Me 3 P/, I +q  'Fu^H  I+  PMe 3  c=c  H^H C=C bond energy of ethene: 65.4 kcal/mol  L  E L=CI,q=O: E a =7.7  E a = 12.2  L = CO, q = 1: E a = 23.8  Figure 2.2. Examples of hydride-olefin complexes and the activation energies (kcal/mol)  associated with the hydride-insertion into ethylene. For comparison, the C=C bond energy for ethylene is shown in the inset.  16  The orientation of the olefin relative to the M-H bond is also important for the concerted reaction. The olefin must coordinate cis to the M-H bond in order to form a coplanar M-C-C-H fragment upon reaction (see Figure 2.3).  49  Although A and B in Figure  2.3 both have a cis arrangement of the alkene and the M-H bond, A undergoes insertion at least 40 times faster than B because the M-C-C-H moiety can be coplanar."  PPh3  1 „^  PPh 3  ,H  ,,,It'ss Ir^H I PPh 3  A^  B  Figure 2.3. Two complexes in which the hydride and olefin ligands are cis. Complex A undergoes insertion faster than B.  2.2 Olefin Insertion Reactions. Olefin-insertion is a crucial step in many reactions, and is commonly explored with terminal transition metal-hydrides. There are only a few reported cases of olefin insertion into bridging transition metal-hydrides, some of which will be discussed in section 2.2.1. Hydrozirconation also features an olefin-hydride insertion to generate new organic products; this process, along with the reaction of 1 with propene, will be described in sections 2.2.2, and 2.2.3, respectively.  17  2.2.1. Olefin Insertion Reactions of Bridging Hydride Complexes. The few reported cases of olefin-insertion reactions with bridging-hydride complexes have been limited to group 3 metal-hydrides. 61-" A kinetic study with (Cp* 2 YH) 2 revealed that this process can occur via an associative or dissociative mechanism (Scheme 2.1).  63  The  associative mechanism involves the attack of the olefin on the metal-hydride, to generate two intermediates: an olefin-coordinated intermediate that undergoes insertion, and a second mono-hydride intermediate, which binds to an olefin and undergoes insertion. In the dissociative mechanism, the bridged-hydride complex forms two mono-hydride intermediates, and then coordinates an olefin, which undergoes insertion. Both processes depend on several factors, including the rate of dissociation of the hydride-dimer, the steric environment of the olefin, and of the metal-alkyl complex. For instance, the reaction of (Cp* 2 YH) 2 with unhindered olefins, such as propene, butene and hexene, forms mono- or double- inserted products through an associative process. On the other hand, the reaction of (Cp* 2 YH) 2 with sterically hindered olefins, such as 3-methyl butene, 2-methyl propene, occurs through a dissociative process and only forms mono-inserted products.  18  Associative Process: Cp*,  / Y—H Cp* \  Cp*  Cp*  Cp^ oH H  Cp*  Cp*  -11■-  /) Y\ /Y \ CID* H^Cp*  \^H  Cp*  / Y \/ /"..  AR  Mono insertion product  R  R = Me R = Et R = Bu Cp / Double insertion product  Dissociative Process:  H CP2 * Y-o—H (  Cp*^[Cp*\  CP \  /Y \  H  ^2  Cp*^Cp*^Cp*  CP2*Y'c''' H^H  Scheme 2.1.  In some instances, olefin-insertion with group 3 bridged-hydride complexes can form bridging-alkyl complexes. For example, yttrium and scandium (Scheme 2.2) hydride dimers can react with primary olefins to form ri Land 11 2 - alkyl-bridged complexes.'''"  19  CMe3  THF 2 H 2 C=CHR  Me 2 Si^V, H  R = Et R = "Pr = "Bu  THE CMe3  s^ 2- iic^N^ sime2  H 2C= CH 2  Me2Si^“ PMe 3  PMe3 H ---  Me2Si—N Me3C  CMe Me 3  Sc CMe3 ^CH3  Me3P CMe 3  2 CH 2 =CHCH 3  H2C Me2Si^  C H2  CH2 H3C  CMe3  Scheme 2.2.  2.2.2. Hydrozirconation of Olefins. Hydrozirconation is the reaction of Cp2ZrHCI (Schwartz's reagent) and an unactivated* alkene to generate the alkylzirconium(IV) product, Cp 2 Zr(R)C1. 64 This process involves either the regiospecific addition of Zr-H to a terminal double bond, or Zr-H addition to an internal double bond, followed by rearrangement via Zr-H elimination and readdition to place the metal at the least hindered position of the alkyl chain. Subsequent reactions of Cp 2 Zr(R)Cl produce a wide range of organic compounds, depending on the reagent that is used. For example, electrophilic halogenation reagents, such as Br 2 , 1 2 , Nbromo- (NBS) or N-chloro-succinimide (NCS), to Cp2Zr(R)Cl affords organic halides (RX)  * The term 'unactivated' refers to olefins that are substituted with alkyl or proton substituents only. 20  and Cp2Zr(X)C1. 65,66 The reaction of Cp 2 Zr(R)C1 with oxidizing reagents such as basic hydrogen peroxide, tert-butyl hydroperoxide, and m-chloro-perbenzoic acid yields alcohols. 67 Cp2Zr(R)Cl undergoes an insertion reaction with carbon monoxide to yield an acylzirconium(IV) species that reacts subsequently with acids, Br 2 /CH 3 OH, hydrogen peroxide and NBS to generate aldehydes, esters, carboxylic acids, and acyl halides, respectively." These reactions are illustrated in Figure 2.4.  Halogenation  Oxidation  ROH Alcohols  RB  Br2  H 2 0 2 /H 2 0; NaOH  12^RI  iBuO0H  I  NBS^RBr  CI w COSH  NCS^RCI 'Cp2Zr(R)CI)  CO Insertion H30'  Cp2Zr\-- CI C—R ii 0  Br2  C H3O H H202  NBS  RCHO RCOOCH3 RCOOH RCOBr  Aldehydes, Esters, Carboxylic Acids, Acyl Halides  Figure 2.4. Subsequent reactions of Cp2Zr(R)Cl.  21  Alkyl Halides  2.2.3. Olefin Insertion with ([NPN]Ta(R-H) 2 (µ-1 1 :1 1 -N 2 ).  The formation of dark red aNPN1Ta(CH2)2CH3)2(1A-11 1 :1 1 -N2), 9, from 1 and propene over two weeks in toluene was described in 2001 (Equation 2.1). 69 In 9, N2 bridges the two tantalums end-on, and is moderately activated (N-N bond: 1.289(6) A). This was the first report of olefin insertion via hydride migration using 1.  Ph Ph Ph^\P \ \ ^ SiMe2 N tt, H^---" Me2,6■, -- . :VA I SiMe2 Nia \N „ ph  i  7\  Mee Me2Ph „s Ph^Ph Si....,„, -S i ,^ t ,^ - Ph 1 N\ N  <  2 ,N  Ph  Ph  ■N p ---11.- Ta-----N—N--/ 1.289(6)  A  SiMe2 siMe2 ,P■,41  /  Ph  Ph  9  1 Equation 2.1  It is likely that the bridging hydrides in 1 force N2 into the side-on end-on configuration. Thus, the hydride migration reaction of 1 with olefins is likely to provide products in which the bonding mode of  N2 has  been transformed, and the extension of this  reaction to other alkenes is of interest. The reaction of 1 with 1-pentene and 1-hexene was investigated to see if it would affect the activation of coordinated dinitrogen in 1.  22  2.3 Results and Discussion. 2.3.1. Reaction of 1 with 1-pentene. Given that hydride migration was observed with propene for the formation of 9, a similar reaction was attempted with 1-pentene. Stirring a brown mixture of 1-pentene and 1 in benzene overnight at 65°C generated a yellow-red solution, from which red ([NPN]Ta (CH2)4CH 3 )2(1A-1 1 :i1 1 -N2), 10, was isolated in 82% yield (Equation 2.2).  Ph  Ph Ph^--.  Smie2SMie.2,Ph.sPh  V  I  SiMe2  Me2?<;:. '" / SiMee T^ a^a N Me2Si NON bNA ^P Ph Ph  2 /.\/\  Ph Ph/  1^  10  Equation 2.2  The 31 1){' HI NMR spectrum of 10 reveals two singlets at 8 4.69 and 20.0 (Figure 2.5), which indicate two different  31  P environments. The 1 H NMR spectrum of 10 is C,  symmetric, with inequivalent pentyl and silyl methyl resonances (Figure 2.6). The presence of two different ligand and alkyl environments was determined using H-COSY and 'H, ' 3 CHSQC spectroscopy. The absence of a peak attributable to a bridging or terminal hydride and the presence of alkyl ligand resonances in the 'H NMR spectrum, indicate that olefin insertion into the metal hydride has occurred in the formation of 10.  23  vv,wpftwArviv  i VAleivkAmOtiveviti'uv,^604#*04,A.P.44  20^ l'5^ 10  ^  5^ 0  (PPrn)  Figure 2.5.  me Me2 , 2  s'  31  P{ 1 1 1} NMR spectrum of 10 in C6D6. -  Phi^  —SIGH  Ph ^aMe 2 - siNie2  4 PnPh  <\--.^tsts,,il ^,  -' Ph ,N.,  t Ph  Loss of Hydride Peak  —SiCH 2 and Alkyl H —  10^8^ 6  (P Pm)  Figure 2.6. 300 MHz 1 H NMR spectrum of 10 in C6D6.  24  .2  3.".—  The ' 5 N 2 labeled species ([NPN]ra(CH2)4CH3)2(A-11 1 :TI'-' 5 N2), 10- 15 N2, was prepared from the reaction of 1- 15 N 2 and 1-pentene. The "'Pt' HI NMR spectrum of 1015  N2 shows  two resonances, a singlet at 8 20.0, and a resonance at 8 4.69 coupled to two  different 15 N2 environments, with coupling constants of 30.5 and 6.5 Hz (Figure 2.7).  20  25  15  10  5  0  (ppm)  Figure 2.7  13  P{' H} NMR spectrum of 10- 15 N 2 in C6D6.  The "NV HI NMR spectrum of 10- 15 N 2 has two doublets of doublets at 8 14.1 and 25.9 (Figure 2.8). The resonance at 8 14.1 is coupled to one Hz, and also to one ''P with  2 JPN  15  N with a U NN value of 11.5  30.5 Hz. The second resonance at 8 25.9 exhibits the  same one-bond coupling to 15 N where I = 11.5 Hz, and a three-bond coupling to 3 'P with a coupling constant of 6.5 Hz. The coupling constants are similar to those observed in the ' 5 1\11 1 H1 NMR spectrum of 9: UNN = 11.3 Hz, and a two- and three-bond N-P coupling at 30.5 and 6.6 Hz respectively. X-ray quality crystals could not be obtained for 10, but the  25  NMR data suggests that in solution, 10 is C, symmetric and has two chemically different tantalum centers, like 9.  26^ 24^22^20  ^ ^ 10  15^ 14  (PM)  Figure 2.8.  15  N{i1-1} NMR spectrum of 10 in C6D 6  .  2.3.2. Reaction of 1 with 1 hexene. -  Complex aNPNYFa(CH2)5CH3)2(i-1 1 :11 1 -N2), 11, was prepared in the same manner as 10, except 1-hexene was used instead of 1-pentene. Upon heating the mixture, the solution changed from brown to dark red, and a red precipitate ([NPN]Ta(CH2)5-  CH3)2(/ - * :i 1 - N2), 11, was collected in 86% yield (Equation 2.3).  Ph  KA Me Ph ime2 S—ie.,_i 2- SP \, ,hs• Ph 21\../Ph ime2 t ___ __ \i'.^ I \ 'N ►<( ,SiMe2 f\1^F--1•', 1-1 2 /\/\/^ Me^ 2>e A., '^SiMe2 ^ p --•,-Ta=N—N=-Td :"" N "• — 1- 6^Ta. IN ^ Me Si /NON ^ \1 \ ' '''p h Ph 2 \ ^/NI^ P Ph^  Ph .  Ph^  A..1"-, i/ -  I^  Ph^  P hi  11  ^1^ Equation 2.3  26  'H NMR spectroscopy, in conjunction with 'H-COSY and 'H, ' 3 C-HSQC spectroscopy, showed the absence of upfield resonances attributable to hydride ligands, and the presence of two chemically inequivalent ligand and alkyl environments. The  31  P{'H}  NMR spectrum of 11 in C6D6 shows two singlets 8 6.46 and 21.75. The 'H NMR spectra of 11 shows four inequivalent silyl methyl resonances, and two chemically different ligand and alkyl environments. ([NPN]Ta(CH2)5CH3)  2 (11-11 1 M I -1 5 N2) 3  11-15N2 was also  prepared, and the 31 P{ 1 HI NMR spectrum of 11- 15 N 2 shows two resonances - a singlet at 8 22.0, and a doublet of doublets 8 6.93 with 2JrN = 29.5 Hz, and 3JrN = 6.5 Hz. ''N NMR spectroscopy for 11 showed two doublets of doublets at 8 14.15 and 25.94 (Figure 2.9). The resonance at 8 14.15 has a one-bond ''N coupling of  I  ji\rtv =  11.3 Hz, and a two-bond  coupling to 3 'P at 2JPN = 29.5 Hz. The peak at 8 25.94 showed the identical one-bond coupling to ''N of 'J NN = 11.3 Hz, and a three-bond coupling to  31  P of 6.5 Hz. X-ray quality  crystals were formed from benzene by slow evaporation, and the solid state molecular structure of 11 was determined by X-ray diffraction. An ORTEP depiction of 11 with selected bond lengths and bond angles is shown in Figure 2.10.  26^24^22 20^18^16^14 (ppm)  Figure 2.9. "NPHI NMR spectrum of 11 in C6D6.  27  1 I C1.00  Ta1'  N3^  ;..  N2  C1' N3 '^Tat  P1 N1  Figure 2.10. ORTEP depiction of the solid state molecular structure 11, (ellipsoids at  50% probability). All hydrogen atoms and phenyl ring carbons other than ipso carbons have been omitted for clarity. Selected bond lengths (A) and bond angles (°): Tal-N1 2.043(3), Tal-N2 2.042(3), Tal-N3 1.820(3), Tal-Cl 2.200(4), Tal-P1 2.6851(10), N3-N3' 1.272(6), N1-Tal-N2 115.42(13), N1-Tal-N3 100.06(14), N2-Tal-N3 104.37(13), NITA-Pl. 75.50(9), N1-Tal-C1 121.68(17), C1-Tal-N2 113.94(17), Cl-Tal-N3 95.95(16), P1-Tal-N3 175.20(10), P1-Tal-N2 79.41(9), Cl-Tal -P1 85.05(12).  The solid-state structure of 11 has an N-N bond distance of 1.272(6)  A, which is  similar to the N-N bond length found in 9 (N-N = 1.289(6) A). The stereochemistry about the tantalum metal is distorted triganol bipyramidal, where atoms P1 and N3 occupy the  28  axial positions, and atoms C1, N1 and N2 occupy the equatorial. In contrast to its C, symmetry in solution (C6D6, rt), the solid-state structure of 11 is C, symmetric. A possible explanation for this anomaly is that 11 exists as an equilibrium (see Equation 2.4) between the predominant cis-trans isomer (11A) at room temperature, and the minor trans-trans isomer (11B).  Me2^ 2Phph Si sr\rthF, h •^ P  \ ..*  2  PhPh N'\1  Ph  <7 p^Ta=N — N^p  f jiMe2  Ph^Ph`'‘N'N-"Si" % Si 2 Ph Mee  Ph  11A cis-trans phosphines C s symmetric  11B trans-trans phosphines C 1 symmetric Equation 2.4  The possibility that the equilibrium in Equation 2.4 could occur suggested that a 31  P {'H} VT-NMR study would be informative for this process. Figure 2.11 shows a low-  temperature stack-plot of 31 P{'H} spectra of 11 in toluene acquired every 30 minutes with 10 K degree intervals, from 273 K (bottom) to 203 K (top). Integration of the peaks in Figure 2.11 with respect to temperature (K) shows an inverse relationship between 11A (8 6.46 and 8 21.75) and 11B (8 4.96); at lower temperatures, the resonance for 11B (8 4.96, purple) begins to emerge, and the resonances at 8 21.75 (green) and 8 6.46 (pink) assigned to the cis- and trans- phosphorus atoms of 11A respectively, begin to decrease. The new resonance at 8 4.96 is a singlet (consistent with a symmetric phosphine complex) and close  29  in range to the trans-phosphorus resonance of 11A at 6 6.46, supporting the formation of 11B. 7 Me2 Mee^Ph Mee i - SI /ss P h  Ph 711^ Me2SI"...^Ta–=N—N=1-a..--p Me 2 Si– N ""7 . isr.\Js‘^I\I NI ...._ 5> Ph -^ -.7.■ • ,,,_,•,/.,,.^Si Pl'i^rriph^ol^ me _  " ^: i  Me e  V‘ hN p  Ph  I --...Ta=N —N —Tal--p  ^ 4^ I Ph  2  Ph`‘'N''Si:  /Si^  Ph^Me2  2  11A  24^23  Figure 2.11.  31  P{IFI} spectra of 11 in toluene from 273 K (bottom) to 203 K (top) in -10  K increments.  A Van't Hoff graph of In(K,,,) vs 1/T (K ') shows the correlation of K g as a function of temperature (Figure 2.12). K e ,, was taken at each temperature as the integral of 11B divided by the average integral of 11A, according to Equation 2.5 (below). The graph shows a positive linear correlation in the formation of 11 B with decreasing temperature.  Keg = Integration of 11B (T)/ Average Integration of 11A (T) Equation 2.5  30  Ln( Keg) vs 1/T 0.00 -0.10  -  -  -0.20 -0.30 -0.40 Ln(K0-0.50 -0.60 -0.70  -  -0.80  -  -0.90  -  -1.00 ^ ^ ^ 0.0035 0.0040^0.0045 0.0050 1/T (K ) -,  Figure 2.12 A Van't Hoff plot, ln(IC,) vs 1/T from 283 K to 203 K.  From the Van't Hoff plot of ln(IC,) vs 1/T (Figure 2.12), the enthalpy (AH°) and entropy (AS°) were determined using the slope of the line and the y-intercept (Equation 2.6 and Equation 2.7). The thermodynamic parameters for the equilibrium are AH° = -1.4 kJ/mol and AS° = -9.8 J/K, which are relatively small and consistent with an equilibrium between two very similar species. Although the data supports the existence of an equilibrium, the reason as to why 11 B is preferred over 11A in the solid-state is unknown, and it is presumed that packing effects of the alkyl groups in the crystal favour the C, symmetric conformer. y = 172.8x — 1.2 (Equation 2.6) ln(K eq) = AH°/R(1/T) + AS°/R (Equation 2.7) -  AH°: 172.8 = -AH°/R; therefore, AH° = -1.4 kJ/mol  AS°: -1.2 = AS°/R; therefore, AS° = -9.8 J/ mol•K  31  A proposed mechanism for the olefin-insertion of 1 is shown in Scheme 2.3. The initial step is the attack of the olefin on the metal-hydride, followed by hydride-migration (i). In step two, the remaining bridging hydride is transformed into a terminal hydride, and the nitrogen moiety is rearranged from side-on end-on, to end-on (ii). In the third step, a concerted reaction occurs at the terminal hydride: a second alkene donates electrons from its Tu orbital into the G* TaH orbital; back donation from the 0 Ta - H orbital into the a* orbital of the alkene creates a four- centered four-electron transition state (iii). The final step is the insertion of the hydride into the beta position of the alkene, to form an alkyl-transition metal complex (v).  Ph  Ph  Ph  Ph  Ph^zs  Ph  V  Me SI -^  Me2 Si^  \ it M e^N • ••. CI - I--Ta^Ta. Me2Si^// I N.. .-11... / I N -N N  I)  Ti^ SiMe2 %,"›..SiMe2  N Ph  P^1 IPh Ph^ ii)  Ph  Ph Ph  R^  Ph^'s  SiMe2 sime2  R  Ph  Ph Ph, Slez sMie2  Ph  N Me 2y  Me, ^• .‹" Ta=N—Nr=Ta-4— P MezSi H^Ph  83/4*-..:•.r a  Ph  Ph  M,,, I^Sie2 ..e2 N •^IL-Si  N— N=Ta  Me2S1  ()  Ph  Ph iv)  Ph  Ph Ph Me M e Ph^ -,^A\ --;,/Si Si N 'N V ----7 ..' 1 \I .._ •-^ .. Me2y --•••<• Ta=-N —N =-Ta^P, `. Me2Si _......„... /^ Ph  VN,  V  \.---"P \  Ph  Scheme 2.3  32  P\  Ph  2.4 Conclusions. The olefin-insertion of 1 with the primary alkenes pentene and hexene produces the pentyl and hexyl complexes 10 and 11, respectively. The insertion of pentene and hexene with 1 occurs almost immediately and is completed overnight; this is evident from the change in colour of the solution, which turns from dark brown to red. The NMR spectra of 10 and 11 suggest that they are structurally analogous in solution, with C, symmetry. The  31  P{ 1 H} NMR spectra of the ' 5 N2 labeled species shows  two distinct phosphorus resonances, where only one resonance is coupled to the end-on bound N2 moiety. The solid-state structure of 11 was analyzed using X-ray diffraction and revealed an end-on bound N2 unit with an N-N bond length of 1.272(6) A. In contrast to its symmetry in solution, the solid-state structure of 11 is C, symmetric. A VT NMR study in conjunction with a Van't Hoff plot revealed that 11 forms an equilibrium between isomers 11A and 11 B in solution, and that the solid-state structure of 11 was isomer 11B; however, it is still undetermined as to why this particular isomer is favoured in the solidstate.  2.5 Experimental. 2.5.1. General Procedure.  Unless otherwise stated, all manipulations were performed under an atmosphere of dry, oxygen-free argon or nitrogen by standard Schlenk or glovebox techniques (Vacuum Atmospheres HE-553-2 glovebox equipped with a MO-40-2H purification system and a -35 °C freezer). Ar and N2 were dried through passage of a column with activated molecular  33  sieves and MnO. Hexanes, toluene, tetrahydrofuran, pentane, benzene and diethyl ether were purchased anhydrous from Aldrich, sparged with N2 and passed through columns containing activated alumina and Ridox catalyst. THE-d 8 , C6D6, and toluene-d 8 were dried over Na/K alloy under partial pressure, trap-to-trap distilled, and freeze-pump-thaw degassed three times. 'H,  1 {'H}, ' 3 C, 'H/l 3 C-HSQC and 'H/'H-COSY spectra were  31 3  performed on a Bruker AV-300, Bruker AV-400 or Bruker AV-600 spectrometer, operating at 300.1, 400.0 and 600 MHz respectively. The ' 5 1\1{'H} spectra were performed on a Bruker 600 MHz spectrometer by Dr. Andrew Lewis and Colin Yilin Zhang at the Department of Chemistry at Simon Fraser University. Unless otherwise noted, all spectra were recorded at room temperature. The 'H, ' 3 C NMR spectra were referenced with C 6 D 6 at 8 7.16 and 128.0, respectively. The  1 {'H} NMR spectra were referenced to external  31 3  P(OMe) 3 (8 141.0 with respect to 85% H 3 PO 4 at 8 0.0), and the ' 5 N{ 1 1-1} spectra was referenced to an external solution of 100mM NH4C1 in 90:10 H20:D20 at 8 -352.9. Chemical shifts (8) listed are in ppm, and absolute values of the coupling constants are in Hz. Elemental Analyses were performed by David Wong at the Department of Chemistry at the University of British Columbia.  2.5.2. Materials and Reagents. Reagents 1-pentene and 1-hexene were purchased from Aldrich and distilled before use. Complexes 1 and ' 5 1\124 were prepared using literature methods."  34  2.5.3. Synthesis, Characterization, and Reactivity of Complexes. Synthesis of UNPN]Ta(CH2)4CH3)2(R-1':i'-N2), 10. A solution of 1 (0.80 g, 0.634 mmol) and 1-pentene (2 mL, 18.28 mmol) in 50 mL benzene was sealed in a glass vessel equipped with a Teflon valve and stirred overnight at 65°C, at which time the colour of the solution changed from dark brown to yellow-red. The vessel was brought into a glovebox where the solvent was removed by vacuum to leave a red residue. The residue was triturated three times with hexanes and decanted. The red solid was collected on a fit 82% yield (0.73 g). NMR (C6D6, 400 MHz): 8 0.08, 0.31, 0.42, and -0.21 (s, 24H total, SiCH3 ), 0.0 (br m, 2H, TaBCH2CH2CH2CH 2 CH 3 ), 0.92 (t, 3H, TaBCH2CH2CH2CH2CH3 ), 1.08 (t, 3H, TaACH2CH2CH2CH2CH3 ), 1.16 and 1.24 (m, 4H, SiCH2 P), 1.24 and 1.36 (m, 4H, SiCH2 P), 1.42 (m, 2H, Ta B CH 2 CH 2 CH2 CH 2 CH 3 ), 1.45 (m, 2H, TaBCH2CH2CH 2 CH2 3  ), 1.51 (m, 2H, TaACH2CH2CH2CH2CH 3 ), 1.58 (m, 2H, TaACH2CH2CH2CH2CH -CH 3 ),  1.75 (m, 2H, TaBCH2CH2CH2CH2CH 3 ), 1.83 (m, 2H, TaACH2CH2CH2CH2CH 3 ), 2.38 (m, 2H, TaACH2CH2CH2CH2CH 3 ), 6.67, 6.98 (t, 4H, p-NC6H5), 6.74, 7.32 (m, 8H, mNC6H5), 6.91, 7.08 (d, 8H, o-NC6H5), 7.13, 7.23 (m, 2H, p-PC6145), 7.31, 7.48 (m, 4H, m-PC6H5), 7.74, 7.91 (dd, 4H, o-PC 6 H5 ). 31  P{ 1 1-1} NMR (C6D 6 ): 8 4.69 (s), 20.0 (s).  "CPHI NMR (C6D6): 8 0.54, 2.16, 4.33, and 6.13 (SiCH3), 14.96, and 17.67 (SiCH2P), 15.06 (TaBCH2CH2CH2CH2CH 3 ), 15.13 (TaACH2CH2CH2CH2CH3), 23.33 (TaBCH2CH2CH2CH2CH 3 ), 23.69 (TaACH2CH2CH2CH2CH 3 ), 27.87 (TaBCH2CH 2 CH2CH2CH 3 ), 32.22 (TaACH2CH2CH2CH2CH 3 ), 39.95 (TaBCH2CH2CH2CH2CH 3 ), 40.91 (TaACH2CH2CH2CH2CH 3 ), 56.20 (TaACH2CH2CH2CH2CH 3 ), 73.50 (TaBCH235  CH2CH2CH2CH 3 ), 122.15, 122.17 (p-NC6H5), 128.23, 128.39 (m-NC6H5), 126.63, 127.77 (o-NC6H 5 ), 131.01, 130.48 (p-PC6H5), 129.40, 129.30 (m-PC6H5), 132.10, 134.53 (o-P C6F15). HSQC NMR (C 6 D 6 ): 8 ('H; ' 3 C) (-0.21; 0.54), (0.0; 73.5), (0.8; 2.16), (0.31; 4.33), (0.42; 6.13), (0.92; 15.06), (1.08; 15.13), (1.16; 17.67), (1.24; 17.67), (1.24; 14.96), (1.36; 14.96), (1.42; 27.87), (1.45; 23.33), (1.51; 23.69), (1.58; 32.22), (1.75; 39.95), (1.83; 56.20), (2.38; 40.91), (6.67 ; 122.15), (6.74; 128.23), (6.91; 126.63), (6.98; 122.17), (7.08; 127.77), (7.13; 131.01), (7.23; 130.48), (7.31; 129.40), (7.32; 128.39), (7.48; 129.30), (7.74; 132.10), (7.91; 134.53). Analytical Calc'd for C58F1 84N6Si4Ta2: C 49.20; N 6.00; H 6.04; Found: *C 45.46; C 6.13; H 6.10. *All carbon analysis were found to be low, possibly due to the formation of tantalum carbide (TaC).  Synthesis of ([NPN]Ta(CH2)4CH 3 )2(R 1 1 :1 1 " 5 N2), 10 -  -  -  15  N2.  By the method outlined above, 1-' 5 N 2 (0.83 g, 0.658 mmol) and 1-pentene (2 mL, 18.28 mmol) were reacted to obtain 0.70 g of the red product (76% yield). 31  pl 1 HI NMR (C 6 D 6 ): 8 6.81 (dd, 2JPN= 30.4 Hz, 3 JPN = 6.5 Hz), 22.1 (s).  ' 5 N NMR (C6D6): 8 14.13 (dd,IJNN= 11.5 Hz, 3 JpN. 6.5 Hz), 25.92 (dd, 2 JPN= 30.4 Hz, I NN = 11.5 Hz). Synthesis of ( [NPN]Ta(CH2) 5 CH 3 )2(R 1 1 :11 1 N2), 1 1 . -  -  A mixture of 11 (0.9 g, 0.714 mmol) and 1-hexene (1.5 mL, 12.14 mmol ) was stirred in 45 mL of benzene. The dark brown solution was stirred overnight at 65°C and 36  turned red. The vessel was brought into the glovebox, where the solvent was removed via vacuum and left a red residue that was triturated three times with hexanes, filtered and collected on a glass fit in 86% yield (0.88 g). 1  11 NMR (C6D6, 400 MHz): 8 -0.11, 0.10, 0.26, and 0.39 (s, 24H total, SiCH3), 0.97  and 1.12 (m, 4H, SiCH2 P), 1.12 and 1.37 (m, 4H, SiCH2P), -0.02 (br m, 2H, TaBCH2CH2CH2CH2CH2CH3), 0.81 (m, 2H, TaACH2CH2CH2CH 2 CH 2 CH 3 ), 0.87 (t, 3H, TaACH2CH2CH2CH2CH2CH3 ), 1.14 (m, 2H, TaBCH2CH2CH2CH 2 CH2 CH 3 ), 1.28 (t, 3H, TaBCH2CH2CH2CH 2 CH 2 CH3 ), 1.35 (m, 2H, TaACH2CH2CH2CH2CH2 CH 3 ), 1.61 (m, 2H, TaBCH2CH2CH2CH2CH 2 CH 3 ), 1.75 (m, 2H, TaACH2CH2CH 2 CH 2 CH 2 3  ), 1.86 (m, 2H, TaBCH2CH2CH2CH2CH2CH 3 ), 1.93 (m, 2H, TaBCH2CH2CH2CH-CH 2  2  CH 3 ), 1.96 (m, 2H, TaACH2CH2CH2CH 2 CH 2 CH 3 ), 2.39 (m, 2H, TaACH 2 CH2 CH 2 -CH  2  CH 2 CH 3 ), 6.68, 6.96 (t, 4H, p-NC 6 H5 ), 6.75, 7.34 (m, 8H, m-NC6 H5 ), 6.93, 7.09 -CH  (d, 8H, o-NC6 115 ), 7.12, 7.28 (m, 2H, p-PC6H5), 7.36, 7.51 (m, 4H, m-PC6H5), 7.78, 7.93 (dd, 4H, o-PC 6 H5 ). 31  P{'H} NMR (C6D6): 8 6.46, 21.75  13  c1 1 1-11 NMR (C6D6): 8 0.46, 1.73, 4.34, 6.17 (SiCH 3 ), 14.31, 17.75 (SiCH2P), 14.68  (TaBCH2CH2CH2CH2CH2CH 3 ), 15.06 (TaACH2CH2CH2CH2CH26H 3 ), 15.93 (TaBCH2CH2CH2CH2CH2CH 3 ), 24.00 (TaACH2CH2CH2CH2CH2-CH 3 ), 28.21 (TaBCH2CH2CH2CH2CH 2 CH 3 ), 30.55 (TaACH2CH2CH2CH2CH2CH 3 ), 32.07 (TaBCH2CH2CH 2 CH 2 CH 2 CH 3 ), 32.30 (TaACH2CH2CH2CH2CH2CH 3 ), 38.47 (TaBCH2CH2CH2CH2CH2CH 3 ), 32.63 (TaACH2CH2CH2CH2CH2CH 3 ), 63.16 (TaACH2CH2CH2CH 2 CH 2 CH 3 ), 74.31 (TaBCH2CH2CH2CH2CH2CH 3 ), 117.12, 118.56  37  (p-NC6H 5 ), 122.19, 123.01 (m-NC6H5), 126.67, 127.02 (o-NC6H5), 129.94, 130.76 (pPC6H5), 129.23, 129.37 (m-PC6H5), 133.01, 134.23 (o-PC6H5). HSQC NMR (C6D6): 8 ('H; ' 3 C) (-0.11; 0.46), (1.73; 0.10), (4.34; 0.26), (6.17; 0.39), (0.97; 14.31), (1.12; 14.31), (1.12; 17.75), (1.37; 17.75), (1.28; 14.68), (0.87; 15.06), (1.14; 15.93), (1.35; 24.00), (1.86; 28.21), (0.81; 30.55), (1.93; 32.07), (1.96; 32.30), (1.61; 38.47), (2.39; 32.63), (1.75; 63.16), (-0.02; 74.31), (6.68; 117.12), (6.96; 118.56), (6.75; 122.19), (7.34; 123.01), (6.93; 126.67), (7.09; 127.02), (7.12; 129.94), (7.28; 130.76), (7.36; 129.23), (7.51; 129.37), (7.78; 133.01), (7.93; 134.23).  Synthesis of UNPN]Ta(CH2) 5 CH 3 )2(1 ii i :TV -  By the method outlined above, 1  -  15  -  15  N2), 11  -  15  N2.  N2 (0.77 g, 0.610 mmol) and 1-hexene (1.5 mL,  12.14 mmol) were reacted to give 0.63 g (72% yield) of 11-  15  N2.  31  P{ 1 1 1} NMR (C6D6): 8 6.93 ppm.(dd, 2JPN = 6.5 Hz, 3JpN = 29.5Hz), 22.0 ppm.  15  N NMR (C6D6): 8 14.15 (dd, 'JNN= 11.3 Hz, 3 JpN.. 6.5 Hz), 25.94 (dd, 2JPN= 30.4 Hz,  -  '^ J =  11.3 Hz).  38  Chapter Three The Reactivity of Group IV and V Metallocenes with Side-on End-on Coordinated N2. 3.1 Introduction. Metallocene dinitrogen complexes, and their reactivity patterns with other reagents have contributed to the current knowledge of  N2 activation  discoveries involved the protonation of early metallocene  and fimctionalization. 7 ° Seminal N2  complexes to form ammonia  and hydrazine. 71 ' 72 For instance, in 1972, Shilov et al. reported the formation of ammonia from protonation of (Cp2T02(L2-i ti N2) with HC1. 7 ' Similarly, Bercaw et al. discovered the formation of hydrazine from the reaction of [(i 5 -05Me5)2ZrN2h(p.2-11 1 :11 1 -N2) with excess HC1. 3 Inspired by these discoveries, investigations for using metallocene complexes to transform N2 into useful, organonitrogen products has emerged. The extent of activation and the bonding mode of  N2  to group IV metallocenes is  dictated by the nature of the substituents on the cyclopentadienyl (Cp) ligands (see Figure 3.1). 7 ° For instance, the reduction of (lf-0 5 Me 5 )2ZrC1 2 with sodium amalgam (Na/Hg) in the presence of N2 forms the end-on bridged dinitrogen complex with two terminal end-on bound dinitrogen units, Rif-05Me5)2ZrN212(1l 2 -il:TC-N2), A; 73 in contrast, when the Cp ring has only four methyl substituents, as in (i 5 -05 Me4H)2ZrC12, and is reduced under similar conditions, the side-on  N2  bridged complex, Rif C5Me4H)2Zrh(p,2-1) 2 :i 2 -N2), B,  forms. 74 The trend that is apparent in this example is that fewer methyl groups attached to the cyclopentadienyl ring seem to encourage an i 2 :11 2 -N2 hapticity. This trend is also  39  observed with the titanocene derivative.  N2  is bound in the end-on bridging mode for both  the penta- and tetramethylated Cp complexes [(rr-05Me5)2Ti]2(11 2 -1 1 1-1 1 -N2) (C) and [(r1 5 1  -N2), (D); 75 ' 76 the side-on bridging  N2  is observed for the-C5Me4H)2Ti](1Arl  trimethylated Cp complex, Ri 5 -05H2-1,2,4-Me3)2Ti12(12-11 2 :11 2 -N2) (E) . 77  End-on bound dinitrogen  Side - on bound dinitrogen  Ti-N=N Ti  1.160(14) A  D  C  1.170(4)A  Figure 3.1. Side-on and end-on N2 zirconocene and titanocene complexes.  Although there are several group IV metallocene dinitrogen complexes, to date, only one example of a group V metallocene-dinitrogen complex is known; the synthesis of complex F is shown in Equation 3.1. 78 The reaction of Cp* 2 TaC1 2 in THE with sodium amalgam under 1 atm moderately activated  N2  N2  forms [(1 5 -05 Me 5 )2TaCl] 2 (t2 -11 1 :11 1 - N2) , F, which has a  unit (N=N = 1.235(13) A).78  40  2^T  „CI  S 1 , `CI  ---  Na/Hg THE  . ....  Equation 3.1  Harsh alkali metal reducing agents, such as Na/Hg amalgam or KC8 under an  N2  atmosphere, are typically employed to generate dinitrogen complexes from a metallocene halide, Cp 2 MX2 (X = Cl or I). 79 In a few cases, dinitrogen complexes have been synthesized without the use of strong reductants via reductive elimination of  H2  from a dihydride  complex; 75 ' 77 '” for example, [(/ 5 -05 Me4 H) 2Ti] 2 (1,12 - 11 1 11 1- N2) is formed from (Ti 5 C 5 Me 4 H) 2 TiH by elimination of  H2  under an N2 atmosphere. 75 A selection of group IV and  V metallocene N2 complexes, their precursors, and N-N bond lengths (A) are listed in Table 3.1.  41  Table 3.1. Group IV and V metallocene dinitrogen complexes, their precursors and N-N bond lengths (A) (measured via X-ray diffraction).  Precursors [(11 5 - 05H2 - 1,2,4 Me3)2TiElb, or [( 11 5C 5 H2-1,2,4 - Me3)2TiI] [(1 5 -05H2-1,2-Me2-4'Pr) 2 TiI] rac-Me 2 Si(11 5 -05 H 2 -2SiMe 3 -4-13u) 2 TiC1 2 Me2S01 5 -05Me4)(1 5 -  C5H3-3-13u)TiC12 (11 5 -05Me4H)2TiH (11 5 -05Me5)2TiC12 (11 5 -05 Me 5 )2ZrC12 Me2Si(11 5 -05Me4)(1 5 C5H3-3-1311)ZrC12 (11 5 -05Me4H)2ZrC12  Group IV and V metallocene N2 complexes  N-N bond length (ii)  R115 - 05H2 - 1,2,4 - Me3)2Tili (k -11 2 :11 2 - N2)  1.216(3)77  [(15-05H2-1,2-Me2-4-1Pr)2Ti] 2 (A241 1 : 11 1- N2)  1.168(3)77  1.174(3)77  rac-[Me2Si-T1 5 -0 5 H2-2-SiMe 3 -4-PU)2Ti] 2(112 - 1i ' :1 'N2) [Me2Si(i5-05Me4)(115-05H3-3-`Bu)Ti] 2 (A2-11 1 :11-N2)  1.165(3)77  [(115-05HMe4)2Ti]2(1A2-11'1V-N2) R115-05Mes)2Ti12(122-1 ' :fi LN2) [(115-05Me5)2ZrN21 2 t2 41 1 11 1 -N2) [Me2S015-05Me4)(115-05H3-3-tBu)Zr]2([4,2-12:112-N2)  1.170(4)75 1.160(14)76 1.182(5)73 1.406(4) 8 '  [(T15-05Me4H)2Zr(11--N2)] 2 ([12 -11 2 : 11 2- N2)  1.377(3)74  (  Me2Si(11 5 -05H3 - 2SiMe3-4-CMe3)2ZrH2 (1 5 -0 5 Me4H)2Hf1 2  [Me2Si(115-05H3-2-S1Me3-4-CMe3)2Zr] 2 (1- 1,2 -11 2 : 11 2- N2)  1.2450(38) 80  [(115-05Me4H)2FIfl2(12-112:12-N2)  1.423(11)82  (11 5 -05Me5)2TaCl2  Ri5-05Me5)2TaCfl2(112-111:111-N2)  1.235(13)78  3.2 The Chemistry of Cp2NbH 3 and Cp2TaH 3 . In this chapter, the reactivity of 1 with group V metallocene hydrides, Cp 2 TaH 3 and Cp 2 NbH 3 is explored. Cp2NbH 3 and Cp2TaH 3 have similar chemical properties, but differ in their reactivity; the former is far more reactive than the latter." For example, in their reactions with HSiMe2Ph, Cp2TaH 3 requires a higher temperature and longer reaction time compared to Cp2NbH 3 ." Structurally, these complexes exhibit a distorted trigonal bipyramidal structure; the two Cp rings are bent back, causing a mixing of the d, s, and p orbitals to form three hybrid orbitals in the equatorial plane (Figure 3.2). 85  42  H  M = Nb, Ta  Figure 3.2. An illustration of a group V metallocene hydride.  Heating Cp 2 MH 3 (M = Nb or Ta) in the presence of a ligand (L) is a common route to forming Cp 2 MHL complexes;"'" it is believed that this process occurs via H2 elimination from Cp2MH 3 to generate a reactive intermediate, [Cp2MH], although this intermediate has never been isolated or detected (Equation 3.2).  [Cp2MH]^H2  Cp2MH3  Equation 3.2  Some reactions of Cp2MH 3 are outlined in Scheme 3.1. Heating Cp 2 NbH 3 or Cp 2 TaH 3 in the presence of triethyl phosphine, carbon monoxide, alkenes and alkynes (L) gives Cp2MHL." The reaction of HSiMe 2 Ph with niobocene and tantalocene hydrides oxidatively adds HSiMe 2 Ph to afford the silyl derivatives." When Cp 2 NbH 3 or Cp 2 TaH 3 are heated in absence of a ligand and eliminate a second equivalent of H2, a cyclometallated dime r fo rms . 86  43  MHCp  M = Nb, Ta  Scheme 3.1  3.3. The Reaction of 1 with Cp2Zr(II) and Cp2Ti(II). As mentioned in Chapter 1, the reaction of 1 with main group hydride reagents (EH) has previously been studied in the Fryzuk group.  89-92  The extension of this type of E-H  addition for metallocene hydrides was of particular interest. The zirconocene hydride reagent, [Cp 2 ZrH 2 ] 2 , reacts with 1 to give [N(1.1,-P=N)N]Ta(i-H)2(µ-N(ZrCp 2 ))Ta[NPN], 12, along with the evolution of H2 (' I 1 NMR (C6D6) = 8 4.47) (Equation 3.3)." Complex -  12 was characterized using X-ray analysis.  Ph Ph Ph^ P [Cp2Z 01212 - H2  P  \P  SiMe2  „;•••^Ta  p2 -Ta • •  Me Si^ \NN/^„  I P\^  h  Ph  Ph  1  12 Equation 3.3  44  /SiMe2  N  \ Ph Ph  To determine the origin of liberated Hz, deuterium labeled 1, ([NPN]fa)2(t-D)2(An 1 :11 2 -N2 ) was reacted with [Cp2ZrH2]2; the 'H NMR spectrum revealed free  H2  (no HD),  and the formation of 12 with loss of the signal at 8 11.4 due to the two bridging tantalum hydrides. This suggested that there was no HD exchange, and that the source of hydrogen was from [Cp2ZrHz] 2 to provide a Cp2Zr(II) species that was responsible for cleaving the NN bond in 1. To test this idea, 1 was reacted with a Cp 2 Zr(II) precursor, Cp2Zr(py)Me 3 SiCCSiMe 3 ). In this reaction, the formation of 12 was nearly quantitative (90% yield), and strongly suggested that the hydrides of [Cp2ZrHz]z reductively eliminate H2  to form Cp2Zr(II) before forming 12. The reaction of 1 with a Cp z Ti(II) precursor,  Cp2Ti(Me 3 SiCCSiMe 3 ), was also explored, and produced the analogous complex, 13, which was characterized by NMR spectroscopy. A proposed mechanism for the formation of 12 and 13 is shown in Scheme 3.2; this mechanism initially involves the formation of an adduct (B), followed by an inner-sphere electron transfer to cleave the N-N bond to form a bridging nitride species (C), one of which is transformed into a phosphinimide via nucleophilic attack from the phosphine donor in [NPN] (D).  45  ^  P  Ph Ph Ph :^..-----\  Ph Ph Ph  V  ..... . ^pl...,H / ----n"\ SiMe2 CP2M(II) Me2Si N....1. V.1^/SiMe2 ^ Y^Ta ,Ta ;N / Me2Si INN / \ //^ M = Ti, Zr ^Ni" NON 'Ph  \A P _^, ,',. I-1 Me27 Si N,....I.,T n_r i a/ Me Si/ I / \ N^  \Ph  \.--p  Ph  A  ^  ,SiMe2 N/ /SiMe 2 .,,,, N\ Ph  \....—P  Ph  I CI : —^Ph^  —  B N-N bond cleavage  Ph Ph \ ;^ Ph "^  Ph Ph^ \ Ph 7 ^H 7 ------—n\  PR3 attack at nitride  ^\,r;, ^ Me2Si INN.^IN / \,„„p ly, I^N^N ".• \...._ ::,..‹..N.,.... I Ph \^ P^\ MCP2 Ph Ph  D  \\  11 N\:,Ph NNI \.—P ....-1 MC ^ P2^Ph /^  ^  M Zr, M = Ti,  12 13  ^  S i Me2  Me2Si N...:::: .V1^/ SiMe2 40?^Ta._ A ja ,;.„. ::. ,. ,,., N./ Me2Si -.  Ph  C  Scheme 3.2  In this chapter, the reaction of 1 with a Cp 2 Hf(II) precursor, Cp2Hf(PMe3)( 11 2Me 3 SiCCSiMe 3 ), was explored. In contrast to zirconium and titanium, hafnium has been infrequently used in  N2  activation because it is the most difficult group IV metal to reduce."  Only recently has it shown the potential to form highly activated  N2  complexes; for example,  extreme lengthening of the N-N bond is observed in hafnium guanidinate and amidinate complexes;" also, hafnium on  N2  N2  complexes have the tendancy to form the more reactive side-  complex as opposed to end-on." The reaction of 1 with Cp 2 Hf(PMe 3 )(11 2 -  Me 3 SiCCSiMe 3 ) will be described in section 3.4.1. In addition, the observation that Cp 2 ZrH 2 is involved in the cleavage and functionalization of  N2  (described above), led us to  investigate the reaction of group V metallocene hydrides, in particular Cp 2 TaH 3 and Cp2NbH 3 . Would these metal hydrides undergo the same reductive elimination of 46  H2 to  generate a Cp2MH(III) species that would be capable of cleaving and functionalizing N2 in 1? These results will be described in section 3.4.2.  3.4. Results and Discussion. 3.4.1. Reaction of 1 with Cp2Hf(PMe3)(1 2 -Me3SiCCSiMe 3 )• A benzene solution of 1 and Cp2Hf(PMe 3 )(11 2 -Me 3 SiCCSiMe 3 ) was heated for three days at 75°C. This generated a dark purple mixture, from which a purple solid, [N(p.P=N)N]Ta(t-H)2(µ-N(HfCp 2 ))Ta[NPN], 14, was isolated in 73% yield (Equation 3.4).  Ph  Ph Ph  Ph^  Ph  Ph .171,,,^sime2  Me2Si N^ r^/SiMe2 16.'"'T^Ta ,,,, N'i Me 2 Si NON\  Cp2Hf(II)  ,SiMe2  Me2Si N .„„L".  ,, 1 (, hS M e 2 fci ap • ,,,^ -1 H 7 1 I T^. Me 2 Si^NN/ N NN  Ph Ph  Ph  Ph 1  14  Equation 3.4  The 31 P 1 H} NMR (C6 D6 ) spectrum of 14 (Figure 3.3) has two singlet resonances at {  8 15.64 and 54.89, indicating that there are two different phosphorus environments. These resonances are similar to those observed for 12 (8 7.3, 46.7) and 13 (8 8.6, 46.4).  47  T11111111111111 ^ 56^54^52^SO^42^46 44 42^40^3i^36^34^32^30 22^26 24 22^20^1 18 16^14^12^10^I^6  Figure 3.3.  31  PP HI NMR spectrum of14 in C6D6.  The 'H NMR spectrum of 14 in C6 D 6 shows four inequivalent silyl methyl resonances, a single Cp resonance at 8 5.55, and a doublet for the bridging hydrides at 8 11.48. The ' 5 1\1 2 labeled derivative, [N(µ-P="N)N] Ta(lx-H) 2 (ift-' 5 N(HfCp 2 ))Ta [NPN] , 14 "N 2 , was also prepared. The  15  NPHI NMR spectrum reveals a doublet at 8 -194.3  31.6 Hz), and a doublet at 204.25  (JPN =  ('JPN =  5.6 Hz); these two resonances are not mutually  coupled, and implies that N-N cleavage has occurred. The 15  -  1  31 3 { 1  H} NMR spectrum of 14  -  N2 also shows two resonances: a singlet at 8 11.15, and a doublet at 8 50.43, with a one-  bond P-N coupling constant of 31.6 Hz. Crystals of 14 were grown from an evaporated benzene solution, and analyzed using X-ray diffraction. The solid-state molecular structure of 14, along with selected bond lengths and bond angles, is shown in Figure 3.4.  48  Figure 3.4. An ORTEP depiction of the solid-state molecular structure of 14 (ellipsoids at 50% probability). Hydrides were located on a difference map and refined isotropically. All hydrogen atoms and phenyl ring carbons other than ipso carbons have been omitted for clarity. Selected bond lengths (A) and bond angles (°): Tal-Ta2 2.6819(4), Tal-Nl 2.094(4), Ta I-N2 2.133(4), Ta2-N2 1.941(4), N1-Hfl 2.110(4), N2-Hfl 2.020(4), N1-P1 1.594(4), Tal-N3 2.094(4), Tal-N4 2.057(4), Ta2-N5 2.122(4), Ta2-N6 2.114(4), Ta2P2 2.6115(14), N1-Tal-N2 85.95(15), Tal-N2-Ta2 82.18(14), N1-Hfl-N2 88.47(16), P1-N1-Tal 121.1(2), P I-NI-Hfl 146.8(3), N3-Tal-N4 106.96(17), N5-Ta2-N6 105.10(17), Tal-Ta2-N2 52.00(11).  The solid-state structure of 14 shows a cleaved N2 unit with a bridging hafnocene fragment between two nitrogen atoms (N1 and N2). Complex 14 is structurally analogous  49  to 12; both complexes have a cleaved and functionalized N2 unit, and a phosphinimide bond (P1=N1). This can suggest that 14 was formed via the same mechanism that was proposed for 12 (Scheme 3.5). The bond lengths of 14 are nearly identical to those of 12. The Ta 1Ta2 distance is 2.6819(4)  A, similar to other Ta(IV)-Ta(IV) bond lengths that have been  reported.% The phosphinimide bond length (P1-N1) is 1.594(4)  A, and is within a range  that is commonly found for groups 4 and 5 phosphinimide complexes. 97 '" The Hf-N bond lengths (Hfl-N1 = 2.110(4), Hfl-N2 = 2.020(4) A) are slightly shorter than the Zr-N bond lengths in 12, (Zrl-N1 = 2.146(9), Zrl-N2 = 2.040(9) A), as expected."  3.4.2. Attempted Reaction of 1 with Cp 2 TaH 3 and Cp2NbH 3 . A 1:1 mixture of 1 and Cp2TaH 3 was stirred in d8 -toluene at room temperature for 2 weeks. The reaction was monitored by 3 '13 {' HI and 'H NMR spectroscopy, and no new species were observed by periodically monitoring the reaction over 2 weeks. To promote the loss of H2 to generate a [Cp2TaH] species,'" the same mixture was heated at 100°C for 2 weeks, but produced no visible change. The mixture was also heated under a flow of N2 for 6 hours, but again there was no change. High-temperature 'H NMR spectroscopy was also used to monitor the progress of the reaction at 100°C, however, no new products were observed using this technique. The more reactive Cp2NbH 3 83 was also attempted. A 1:1 mixture of 1 and Cp 2 NbH 3 was stirred in benzene at room temperature for 1 week. The progress of the reaction was monitored by  31 PP  HI and 'H NMR spectroscopy in C6D6, but  like the Cp 2 TaH 3 reaction above, did not show the formation of any new products. The same mixture was heated and stirred at 90°C for 5 hours under a flow of N2 to promote the loss of H2 from Cp2NbH 3 and form [Cp2NbH]; 1 ° 1 however, this did not produce any visible  50  change. High-temperature 'H NMR spectroscopy at 90°C was also used to observe the progress of the reaction at this temperature, but did not show the formation of any new product. As mentioned earlier in this chapter, Cp 2 MHL complexes are formed by heating Cp 2 MH 3 in the presence of a ligand, (L). It is believed that heating Cp 2 MH 3 eliminates H2 to form an equilibrium with a reactive intermediate, [Cp2MH]; however this intermediate has never been isolated or detected. Therefore, a possible explanation for the lack of reactivity between 1 and Cp 2 MH 3 might be because the reverse reaction of  H2  with  [Cp2MH] occurs quicker than the reaction between Cp 2 MH 3 with 1 (Equation 3.5).  - H2  M—H + H2  M = Nb, Ta  Equation 3.5  3.5 Conclusions. The observation that Cp 2 Zr(II) reacts with 1 to form 12 compelled us to investigate the reaction of 1 with other group IV and group V metallocene complexes. The reaction of 1 and Cp2Hf(PMe 3 )(11 2 -Me 3 SiCCSiMe 3 ) forms 14, which contains a cleaved and functionalized N2 unit, and the formation of a new phosphinimide bond. Complexes 12 and 14 are structurally analogous, and it is strongly suggested that both complexes are formed  via the same proposed mechansim for 12 (Scheme 3.5).  51  Unfortunately, the group V metallocene hydrides Cp 2 TaH 3 and Cp2NbH 3 do not react with 1. Cp2MH 3 (M = Ta, Nb) was heated to provide a reactive intermediate [Cp2MH], but no reaction was observed. Since the [Cp 2 MH] intermediate cannot be isolated or detected, a possible explanation for lack of reactivity between 1 and Cp2MH3, might be because the reverse reaction of  H2  and [Cp2MH] occurs quicker than the forward  reaction of [Cp2MH] and 1.  3.6 Experimental. 3.6.1. General Procedure. Unless otherwise stated, general procedures were performed as described in Section 2.4.1. 3.6.2. Materials and Reagents. Cp2HOPMe 3 )(11 2 -Me 3 SiC2SiMe 3 ),' 02  1 103  and  15N 2 1 103  were prepared using  literature methods.  3.6.3. Synthesis, Characterization, and Reactivity of Complexes. Synthesis of [N(µ-P=N)N]Ta(R-H) 2 (µ-N(HfCp 2 ))Ta[NPN], 12. ([NPN]Ta(l.t-H)2(11-11 1 : riLN2) (0.65g, 0.515 mmol) and Cp2Hf(i 2 -Me 3 SiCCSiMe 3 ) (0.29g, 0.515 mmol) was dissolved in 50 mL benzene at room temperature, and sealed in a glass vessel equipped with a Teflon valve. The brown mixture was stirred for three days at 75°C, and the colour of the solution changed from brown to purple. The vessel was brought into a glovebox and the solvent was removed under vacuum, leaving a purple residue  52  that was triturated with hexanes and filtered to collect a purple powder, 14 (Yield = 0.59 g, 73%). 1  1-1 NMR (C 6 D 6 , 25°C, 400 MHz): -0.13, 0.12, 0.19, 0.28 (s, 6H each, SiCH3 ), 1.32  (dd, 2 J HH = 10.9 Hz, 2JPH = 22.5 Hz, 4H, SiCH2P), 1.94 (dd, 2JHH = 14.0 Hz, 2JPH = 39.2 Hz, 4H, SiCH2P), 5.54 (s, 10H, ri 5 -05 H 5 ), 6.52, 6.76, 6.99, 7.08, 7.29 (d, t, 20H total, NC6 H6 ), 7.13, 7.21, 7.26, 7.38 (d, t, 6H total, PC6H6), 7.54, 7.88 (dd, 4H, o-PC 6 H6 ) 11.48 (d, 2H, 2J-ram = 16.6 Hz) 1  31 3 { 1 13  H} NMR (C6D6): 8 15.64 (s) 54.89 (s).  C{ 1 11} NMR (C6D6): 0.33, 3.75, 4.44, 5.18 (SiCH3), 14.6, 20.33 (SiCH2P), 108.63,  (i 5 -05 H 5 ), 117.10, 118.53, 120.56, 122.86, 126.67, 129.51 (NC6H6), 129.02, 129.27, 129.95, 131.30 (PC6H6), 131.83, 133.42 (o-PC6H6). Analytical Calc'd for C 58 H 7 4N6P 2 Si4Ta 2 Hf: C 44.37; H 4.75; N 5.35; Found: C 44.67; H 5.11; *N 4.81. *Nitrogen analysis were found to be low, possibly due to the formation of HfN or TaN.  Synthesis of [N(R-P= 15 N)N]Ta(R-H)2(R- 15 N(HfCp2))Ta[NPN], 12- 15 N2. By the same method outlined above, 1- 15 N2 (0.88 g, 0.698 mmol) and Cp 2 Hf(i 2 Me 3 SiC2 SiMe 3 ) ( 0.393 g, 0.698 mmol) were reacted to give (0.84 g, 87% yield) of 1215  N2.  1  31 ) { 1  H} NMR (C6D6): 8 11.15 (s), 50.43 (d,'JpN = 31.6 Hz).  "N NMR (C6D6): 8 -194.3 (d, 1 JpN = 31.6 Hz), 204.25 (d, JPN = 5.6 Hz).  53  3.6.4. Attempted Reactions of 1 and Cp 2 MH 3 . Complex 1 (102 mg, 0.08 mmol) and Cp 2 MH 3 (1 equivalent) was dissolved in 1.5 mt in toluene and sealed in a J-Young tube and heated at 100°C (for M = Ta) and 90°C (for M = Nb). Complex 1 (1 g, 0.8 mmol) and Cp l MH 3 (1 equivalent) was dissolved in toluene and heated at 100°C for 6 hours (M = Ta) and 90°C for 5 hours (M = Nb) under a flow of N2.  54  Chapter Four The Reactivity of a Group 13 Complex with Side-on End-on coordinated N2. 4.1 Introduction. When nitrogen is activated in a transition-metal complex, the N2 unit may be susceptible to electrophilic attack. The most common electrophilic addition reaction with coordinated dinitrogen is protonation. 104 Protonation of transition-metal N2 complexes is of interest because it is invoked in biological nitrogen fixation.'  05  '" 6 One example of a  homogeneous catalyst that is capable reducing and protonating dinitrogen to ammonia is the mononuclear molybdenum complex, [HIPTN 3 N]Mo(N 2 ), ([HIPTN3 N] 3- = [{3,5-(2,4,6-iPr3-C6H2)2C6H3NCH2CH2}3NP - ) shown below."' This complex contains a tetradentate triamidoamine ligand, and can reduce N2 to NH 3 under ambient conditions (at room temperature and 1 atmospheric pressure).  i-Pr  i-Pr i-Pr  N III  HIPT^N  Jo N  i-Pr  —  N  55  i-Pr  The catalytic production of organonitrogen compounds from molecular nitrogen has become a "holy grail" in dinitrogen research; 108 thus, in addition to protonation reactions, the reactions of coordinated N2 with other electrophilic reagents have also been explored. Some reactions of N2 complexes with group 13 electrophiles, (E), afford new N-E bonds. For example, cis-[W(N2) 2 (L) 3 L 1 l (A) reacts with GaCI 3 109 and AlC1 3 "° to form trans,trans[{WC1(PMe2Ph)4([13-N2)12(GaC12)2i(N-N = 1 .3 2 ( 2 ) (B) and [WC1(Py)(PMe2-Ph)3(13-  N2)]2(A1C12)2 (N-N = 1.46(4) and 1.25(3) A) (C), respectively; the reaction of trans[NBud[W (NCS)(N2)(dppe)2] (D) with H2BCMe2CHMe 2 "' forms the boryldiazenido  complex, trans-[W(NCS)(N=NBHCMe2CHMe 2 )(dppe) 2 ] (N-N = 1.262(7) A) (E). These reactions are illustrated in Scheme 4.1. Cl Cl L^  MCI3 M=GaorAl  L ?M \ CI—W^N=N= yv —CI L  ^M  12  CI CI  M = Go: L = L1 = PMe 2 Ph M = Al: L = PMe 2 Ph, L 1 = Py  M = Ga: B M=AI:C  A  BHCMe2CHMe2 I III^ [^ r—P,„ ,,.. 11 .00 ,P-1 [NBu 4  II N  Me2CHCMe2BH2  ]  L P ."'  ^P^ NCS^  D^  P  ^P NCS  E  Scheme 4.1  Electrophilic group 14 complexes can also react with coordinated dinitrogen (see Scheme 4.2). For instance, the reaction of trans-[W(N 2 ) 2 (dppe) 2 ] (A) with alkyl bromides  56  and acyl chlorides forms new N-C bond complexes (B and C).112,113 Silicon electrophiles can react with coordinated  N2  to form N-Si bonds; the reaction of cis-[W(N2)2(dppe) 2 ] (D) with  Me 3 SiI affords a silyldiazenido complex, trans-[WI(NNSiMe 3 )(PMe 2 Ph) 4 ] (E); 1 ' similarly, the reduction of [N 3 N]MoCI ([N3N = N(CH2CH2NSiMe3)3) (F) with magnesium generates {[N 3 N]Mo-N=N} 2 Mg(THF) 2 , which reacts with Me 3 SiC1 to form [N3 N]MoN=NSiMe 3 (G). 1 "  ,SiMe3 INI I  III N  C  COR P Ii^ P ^  CICOR  N2  -  Me3SiI •.^.•  L•l. I  ^iiN I---. we  A  N  L^L  L = PMe 2 Ph D  ''4rp.-J  cl  Mg(THF)2  SiMe 3 12  R Mg  N R^II N R Ni„,. I  N2  N  t  F Scheme 4.2  4.2. Reaction of 1 with Lewis Acids. Lewis acids XR 3 , (XR3 = AIMe 3 (15), GaMe 3 (16), and B(C6 F5 ) 3 (17) react with 1 to form the adducts, [(NPN)Ta]2(t-H) 2 (µ.-11 1 :11 2 -NNXR3 ) (see Equation 4.0.' 16 The solidstate structures of 15, 16, and 17 have N-N bond lengths longer than that of 1, at  57  1.363(7), 1.356(18), and 1.393(7) A, respectively. In this chapter, the reactivity of 1 with GaCp* is explored.  Ph  Ph Ph Ph^-3, SiMe2 N I:I', H .-6 -Me29/ 44‹. : A^/ SiMe 2 Me2SiTd \ Ta. IN/  P; , SiMe2 Me2y-Tr I Me2Si ^V^SiMe2 ‘, Ta .. rxPh „Té\^ /^  X R3  I^// IV\ Ph ...._ / N'N^  N  P^N  P^Ph  \p  h  Ph XR 3  Ph  XR 3 = AIMe 3 (15), GaMe 3 (16), B(C 6 F 5 ) 3 (17)  1  Equation 4.1.  4.3. Results and Discussion. 4.3.1. Reaction of 1 with GaCp*. The initial reaction of 1 and GaCp* (1:1 equivalent) at room temperature is immediate but incomplete, as indicated by 3 '13 { 1 H} NMR spectroscopy (Figure 4.1); in addition to the resonances of 1 (8 11.7 and 13.8), the doublets at 8 14.3 and 22.3  Opp =  31  P{'H} NMR spectrum shows two  14.67 Hz), that are reminiscent of the  31  spectrum of the adduct complex [(NPN)Ta] 2(t-H)2(wri af-NNGaMe3), 16. '  58  PPHI NMR  ^  25  Figure 4.1.  31  ^ ^ ^ ^ 20 15 5 10 (PPm)  P{'H} NMR spectrum of 1 and GaCp* in C6 D 6 . The two new doublets are  represented by (*).  Since the reaction is not complete after stirring for several days at room temperature, the same mixture was heated overnight at 65°C. This resulted in the conversion of 1 into a new complex, [NPN]Ta(p.-N(GaCp*))Ta(=NPh)[NPI.1,--N], 18. Attempts to isolate 18 were unsuccessful; however, the complex was characterized in situ using 'H/ 13 C-HSQC, 13 C{ 1 1-1}, 31  1TH! and ' 5 NPHI NMR spectroscopy, and based on the data provided by these spectra,  a proposed structure for 18 is shown in Equation 4.2.  Ph Ph^ PhN,k H, H^  me2  /SiMe2 ONNN<  Me N  Ph  Ph^Ph Me 2^Ph Me2Si— –Si ` GaCp* H2  C4  ^N I  /NN  -a NNz  SiMe2  Ph7 N / Si Me2 Ph ' "^GaCp*  Ph Ph  1  18 Equation 4.2.  59  The 31 13 { 1 H} NMR spectrum of 18 no longer shows the two mutually coupled doublets that were observed at the onset of the reaction (i.e. Figure 4.1), but reveal two new singlets at 6 -2.65 and 20.38, indicative of two different 31 P environments (Figure 4.2). The 'H NMR spectrum of 18 shows no peaks in the region where the resonances for the bridging hydrides typically occur (at -6 11), but show eight singlets assigned to silyl methyl protons, and a singlet at 8 1.89 assigned to GaCp* protons (Figure 4.3); these findings suggest that 18 is C1 symmetric in solution. Since 1 does not decompose or eliminate  H2  even in refluxing in toluene, the loss of the bridging hydrides in the formation of 18 appears to require heating 1 in the presence of GaCp*.  •".0  13 (rpm)  Figure 4.2. 3 ' P{l H} NMR spectrum of18 in C6D6.  60  GaCp*  (PPrn) Figure 4.3. 400 MHz 'H NMR (C6D6) spectrum of 18. The trace amount of Et 2 0 is an  impurity in the deuterated solvent.  The 15 N2 labeled derivative, [NPN]Ta(p,- 15 N(GaCp*))Ta(=NPh){NP12-' 5 N], 18 15  -  N 2 , was also prepared and characterized by NMR spectroscopy, and supports the structure  proposed for 18 in Equation 4.3. The 31 1){'H} NMR spectrum of 18 doublets at 8 -0.97 ( 2JPN = 19.27 Hz) and 22.08  ( 2 JPN =  -  15 N 2  shows two  11.06 Hz); these resonances are  different from those reported for the metallocene complexes 12, 13, and 14 in Chapter 3, which show only one ' 1 P resonance coupled to an 15 N nucleus and indicates that 18 is structurally different from these complexes. The ' 5 N{' HI NMR spectrum (Figure 4.4) shows two doublet of doublets at 8 -23.2 ( 2JPN = 19.27 Hz, 1 JNsj = 3.01Hz) and at 8 291.2 ( 2JPN = 11.06 Hz, I JNsi = 2.01 Hz); these values are in contrast to those reported for 1 20.4 (JNp = 6.6, 24.6 Hz;  JNN =  21.6 Hz) and 163.6  61  (JNp =  3.5, 21.2 Hz;  -  15 N 2 ,  at 8 -  JNN = 2 1 .6)),  indicating that the side-on end-on N2 in 1 has changed in 18. Furthermore, the splitting in the ' 5 1\1{ 1 E1} NMR resonances of 18 reveals two different nitrogen atoms that are not mutually coupled, but coupled to different phosphorus and silicon environments (the  2 jpN  and 'J Ns , values found herein are within the range of other [NPN] complexes that have a similar structure to 18 and exhibit a two-bond N-P and one-bond N-Si coupling)' 6 implying that N-N bond cleavage has occurred, and that each ''N atom has a unique chemical environment, thereby supporting the structure proposed for 18 in Equation 4.3.  291.5^291.0^ ^ (ppm)  -23.0 (ppm)  ^  -23.5  Figure 4.4 ' 5 N{' HI NMR resonances of 18 in C6 D6 .  Complex 18 is unstable in solution, and can be observed by  31  P{ 1 H} NMR  spectroscopy to convert into a number of other P-containing species over several days. One of the products of this reaction was determined to be [(PhN)(NPII-N)Tah, 19. The solidstate molecular structure of 19 was established by X-ray crystallography (Figure 4.5), and can support the structure of 18 proposed in Equation 4.3.  62  Figure 4.5. An ORTEP depiction of the solid-state molecular structure of 19 (ellipsoids at 50% probability). All silyl methyl groups, hydrogen atoms and phenyl ring carbons other than ipso carbons have been omitted for clarity. Selected bond lengths (A) and bond angles (°): Tal-Tal' 3.0878(5), Tal-N3 2.0264(15), Tal-N1 1.8006(16), Tal-N2 2.0686(16), Tal-P1 2.6172(6), N3-Si2 1.1776(16), N3-Tal-N3' 80.82(6), N3-Tal-N2 103.48(6), N1Tal-N3 109.99(7), N3-Tal-P1 146.19(15), N1-Tal-P1 101.35(5), N1-Tal-N2 108.97(7), Si2-N3-Tal 129.06(9), Tal-N3-Tal' 99.18(6). The geometry about the tantalum center is best described as distorted square pyramidal, with the axial position occupied by N1, and the base vertices occupied by P 1, N2, N3 and N3'. Decomposition of the [NPN] ligand at the N-Si bond is evident from the  63  structure. A square M 2 N2 unit (M = Ta) is also apparent in the solid-state, and is a shape that has previously been observed with other group V  N2  complexes.' "• 18  The solid-state structure of 19 has a center of inversion, and it is apparent that there is no GaCp* in the complex. It is possible that GaCp* is responsible for the different symmetry observed between complexes 18 and 19, and that complex 18 is merely a GaCp* adduct of 19. The solid-state structure of 19 shows a cleaved N-N bond where the bridging N atoms are each bound to an Si atom; the "NI' HI NMR spectrum of 18 shows two distinct ' 5 N environments that each display J N_s, coupling, and support the formation of a NSi bond, like 19. The 'H NMR spectrum of 18 also shows loss of the bridging hydride resonance at 8 11.4, and is in agreement with the solid-state structure of 19, which shows no bridging hydride atoms. Therefore, the bonding information provided by the solid-state structure of 19 further supports the structure of 18 proposed in Equation 4.3. Decomposition of the [NPN] ligand at the N-Si bond has previously been observed with hydroboration reactions of 1 (Equation 4.3) with HB(C6H11)2 (20) and 9-BBN (21)." 9 The mechanism for this process has been investigated by MacKay et al.;" 9 the reaction of deuterium labeled 1, ([NPN]Ta)2(IA-D)2(µril m 2 -N2), with 9-BBN and HB(C6H6) revealed that N-N bond cleavage in 1 was correlated to H2 elimination from the bridging hydrides; "N NMR studies revealed that the bridging nitrogen atoms in 20 and 21 originated from the side-on end-on  N2  in 1; 29 Si NMR studies showed that the Si atom from  [NPN] migrates to the bridging N atom.  64  P me2^Ph Ph Ph^ i Ph Me2Si— Si`^r\; i ...,^P--"k \,N,, ^% H .\/^\ (^-:^N \.SiNle2^ HBL2^ Me 2>N ■,:td./.1-ar ,N/iSiMe2 ^ Me2Si^ L / \ N \',,,,,,, ^"‘ Ph^ Ph7P1-42/TXNLV2Ta' ‘)-1'';:—SiMiMe2e2 \---F) N / Ph Ph Ph Ph^  Ph  1^  20: L = C6H11 21: L = C 8 1-1 14  Equation 4.3  4.4. Conclusions. The reaction of 1 with GaCp* results in a cleaved and functionalized  N2  complex,  [NPN]Ta(ILL-N(GaCp*))Ta(=NPh)[NP!,t-N], 18. Although 18 could not be isolated, a possible solution structure was assigned by NMR spectroscopy and isotopic labeling. Complex 18 converts to 19, [(PhN)(NP1.1-N)Tah, spontaneously in solution over several days. Both 18 and 19 have undergone ligand decomposition, most likely because of the reactivity of the N-Si bond in [NPN]; Si is bound to the bridging nitride, and a phenylamide group is bound to Ta. A similar decomposition of [NPN] has been observed upon hydroboration of 1, and it is presumed that the formation of complex 18 and 19 occurs through a similar mechanism.  4.5 Summary and Future Work. This thesis investigates the reactivity of the tantalum dinitrogen complex, ([NPN]Ta)2(A-H)2(11-rl :Y1 2 -N2), 1, with primary alkenes, group IV and V metallocenes, and with GaCp*. In Chapter 2, the reaction of 1 with the 1-pentene and 1-hexene produced  65  ([NPN]Ta(CH2)4CH3)2([1-1'11 1 -N2), 10, and aNPN]Ta(CH2)5CH3)2(t T 1 ni N2), 11, -  -  each with two new Ta-C bonds, and a change to the bonding mode of N2 in 1. This reaction likely occurs through the olefin-insertion of the primary alkene into the Ta-H bond. Complexes 10 and 11 were characterized by NMR spectroscopy, and the solid-state structure of 11 revealed that the  N2  unit is in the bridging end-on mode, with an N-N bond  length of 1.272(6) A. The symmetry of 11 in solution is different from its symmetry in the solid-state. Complex 11 forms two isomers in solution; 11A which is the C, isomer and the predominant isomer in solution at room temperature, and 11B, which is the minor C, isomer. Therefore, the solid-state structure represents isomer 11 B, although it is unclear as to why this particular isomer is favoured in the solid-state. In Chapter 3 the reactivity of 1 with group IV and V metallocenes was investigated. The reaction of 1 with Cp2Hf(PMe 3 )(n 2 -Me 3 SiCCSiMe 3 ) produces [N(µ-P=N)N]Ta(p,H)2(µ-N(HfCp2))Ta[NPN], 14, in which  N2  is cleaved and new Hf-N and P=N bonds have  formed. The cleavage of N2 in 1 may be accomplished by the donation of two electrons from a Cp2Hf(II) species. In addition, the reaction of 1 with group V metallocene hydrides Cp2MH 3 (M = Nb, Ta) was attempted, however, no reaction was observed. It is believed that the 16 electron intermediate, [Cp 2 MH(III)] reacts quicker with  H2  than with 1, and  does not produce any useful result. In Chapter Four, the reaction of 1 with GaCp* to produce [NPN]Ta(A,N(GaCp*))Ta(=NPh)[NPIA-N], 18, was reported. Complex 18 decomposes over several days, and one product of decomposition that was isolated was [(PhN)(NP[I-N)Tab, 19. The solid-state structure of 19 showed that the [NPN] ligand was cleaved at the N-Si bond, similar to that observed for the reaction of 1 with 9-BBN and HB(C6H 5 )2.  66  This thesis has discussed the reaction of 1 with some early transition metal complexes in Chapter 4. In addition to many other reactions of 1 with early transition-metal complexes that remain to be explored, the reaction of 1 with certain late-transition metal complexes should be attempted. For example, Pd2(dba) 3 or (COD)2Ni may form adduct complexes or cleave and fiinctionalize N2 in 1. The reaction of 1 with other organic reagents such as cyclic alkenes and allenes and is another area that can be explored, and may possibly result in the formation of a carbon-nitrogen bond.  4.6. Experimental. 4.6.1. General Procedure. Unless otherwise stated, general procedures were performed as described in Section 2.4.1. 4.6.2. Materials and Reagents. GaCp*, 120 1 121 and 15 N2-1 121 were prepared using literature methods.  4.6.3. Synthesis, Characterization, and Reactivity of Complexes. Synthesis of [NPN]Ta(p,-N(GaCp*))Ta(=NPh)[NPp.-N], 18. GaCp* (16.5 mg, 0.081 mmol) was added dropwise to a solution of 1 (102 mg, 0.081 mmol) in 1.5 mL of C6 D 6 at room temperature. The mixture was sealed in a J-Young tube and heated overnight at 65°C. NMR (C6D6, 400 MHz): -0.78, -0.34, -0.19. -0.16, 0.08, 0.27, 0.39, 0.51 (s, 3H each, SiCH3), 0.78, 0.88, 1.05, 1.26 (m, 2H each, SiCH2P), 1.98 (s, 15H, GaC10H13), 6.81,  67  6.84, 6.87, 6.92, 6.94, 6.96, 7.01, 7.05, 7.08, 7.23, 7.30, 7.38, 7.42 (d, t, 20H, NC6H5 ), 7.53, 7.66, 8.15, 8.38 (m, dd, 10H, PC6H5 ). 31  P{'H} NMR (C6D6, 161 MHz): 6 2.65 (s) 20.38 (s). -  "CPHI NMR (C6D6, 101 MHz): 0.05, 1.29, 2.11, 2.46, 3.12, 3.27, 5.28, 6.57  (SiCH 3 ), 10.3 (GaC5(C5Hi5)), 113.4 (GaC5(C5H15)), 13.6, 15.8, 20.4, 21.8 (SiCH2P), 119.6, 121.8, 121.3, 121.9, 122.1, 123.5, 124.1, 125.7, 125.9, 129.1, 129.4, 129.8, 131.3 (phenyl ring carbons), 131.8, 133.7, 134.3 and 135.2 (PC6H5). *Note some proton and carbon resonances were eclipsed by solvent. 41/ 13 C HSQC NMR (C6D 6 ): 6 ('H; ' 3 C) (-0.78; 2.11), (-0.34; 1.29), (-0.16; 2.46), (0.08; 0.05), (0.27; 3.12), (0.39; 5.38), (0.51; 6.57); (0.78; 13.6), (0.33; 20.4), (1.05; 21.8), (1.26; 15.8); (1.98; 10.3); (6.81; 121.8), (6.84; 122.1), (6.87; 119.6), (6.92; 123.5), (6.94; 119.6), (6.96; 124.1), (7.01; 121.9), (7.05; 125.9), (7.08; 129.1), (7.23; 129.4), (7.30; 131.3), (7.38; 129.8), (7.53; 131.8), (7.66; 133.7), (8.15; 134.3), (8.38; 135.2). Synthesis of [NPN]Ta(1.4.  -  15  N(GaCp*))Ta(=NPh)[N11  By the method outlined above, 1  -  15  -  15  N], 18 '5N 2 . -  N2 (0.93 g, 0.737 mmol) and GaCp* (0.151 g,  0.737 mmol) were reacted in 1.5 mL C6D6. 1  31 3 { 1  H} NMR (C6D6, 161 MHz): a -0.97 (d, 2JPN = 19.27 Hz) and 22.08 (d, 2JPN =  11.06 Hz). "N NMR (C6D6, 60 MHz): b -23.2 ( 2JPN = 19.27 Hz, 1 JNs, = 3.01 Hz) and at 6 291.2  ( 2 J PN = 11.06 Hz, 1 JNs, = 2.01 Hz).  68  4.7 References. ' Jolly, W. L., Inorganic Chemistry of Nitrogen, W. A. Benjamin Inc, New York, New York, 1964, p. 4. 2  Burgess, B. K.; Lowe, D.J. Chem. Rev. 1996, 96, 2983.  3  MacKay, B. A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 385. Fryzuk, M. D.; Shaver, M. P. Adv. Synth. Catal. 2003, 345, 1061. Stoicheff, B. P. Can. J. Phys. 1954, 32, 630. Bouwstra, J. A.; Schouten, A.; Kroon, J. Acta Cryst. C. 1983, 39, 1121.  6  Collin, R. L.; Libscomb, W. N. Acta Cryst. 1951, 4, 10. de la Jara Real, A.; Tenorio, M. J.; Puerta, M. C.; Valerga, P. 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([NPN]Ta)2(t-H)4 (2) Empirical formula Formula weight Colour, habit Crystal size, mm Crystal system Space group  a,A b,A c, A  a, deg [3, deg y, deg Pcalc, gicrn 3 F(000) radiation 20 ,-„,„, deg total no. of reflections no. of unique reflections merge  no. with I n6(I) no. of parameters R (F 2 , all data) R„, (F 2 , all data) R (F, I>no(I)) R,„, (F, I>na(0) gof residual dens, e/ A3  ^  ([NPI\T]Ta(CH2)5CF13)2(xii:*-N2) (11)  ^ C481466N4P2Si4Ta2 ^ C60H88N6P2Si4Ta2 1429.56 1235.25 ^ red, prism purple, tablet ^ 0.10 x 0.22 x 0.31 0.22 x 0.13 x 0.03 ^ triclinic triclinic ^ P-1 P-1 11.658(4) 11.140(5) 11.918(5) 13.744(5) 12.911(7) 18.708(5) 105.49(3) 86.215(5) 100.10(2) 87.146(5) 90.47(2) 66.143(5) 2 2 1.397 1.570 722 1228 Mo Mo 54.21 56.02 30810 51577 8132 12539  0.0376 8132 334 0.0285 0.0912 0.0322 0.0936 1.125 3.132, -0.865  0.0333 12539 557 0.0223 0.0474 0.0364 0.0510 0.979 1.076, -1.012  R1 (F 2 , I>2a(I)) = EllF01-1Fcli/EFol; IL, (all data) = (2W(IF.2 1 - IF,2 1) 2 /XWIF.2 1 2 ) "2  77  Table A-2. Crystallographic Data and Structure Refinement for [N([1-P=N)N]Ta(v,H)2(x-N(HfCp2))Ta[NPN] (14) and [(PhNH)(N4t-N)Tah (19). [N(µ-P=N)1\1]Ta(vt-F1)2(1-t- ^i(PhNH)(NP p,-N)Tal 2 N(HfCp2))Ta[NPN] (14)^(19) Empirical formula Formula weight Colour, habit Crystal size, mm Crystal system Space group  a,A b,A c, A cc, deg  p, deg  y, deg 3  Pcalc, g/cm F(000) radiation 20., deg total no. of reflections no. of unique reflections R.„ge no. with I n6(I) no. of parameters R (F2 , all data) R,, (F 2 , all data) R (F, I>no(I)) RW (F, I>no(I)) gof residual dens, e/k  N6P 2 Si4Ta 2^C43H62N6P2Si4Ta2 ^ 1259.24 1726.14 ^ purple, needle yellow, chip ^ 0.45 x 0.50 x 0.26 0.24 x 0.56 x 0.10 triclinic^triclinic ^ P1 P-1 13.4808(14) 9.2542(15) 11.3559(18) 14.8372(13) 13.367(2) 17.7810(19) 81.516(4) 70.398(8) 78.401(7) 87.329(5) 81.041(7) 82.437(4) 1 2 1.645 1.620 624 1704 Mo Mo 55.8 50.3 22423 14440 5927 12837 0.0846 0.0262 5927 12837 284 782 0.0333 0.0145 0.0792 0.0339 0.0162 0.0501 0.0345 0.0860 1.075 1.047 1.628, -0.915 0.422, -0.378 C70 H86Hfl  R 1 (F 2 , I>2a(I)) =^IlF01-1Fc11//Fol; IL (all data) = (Ew(IF. 2 1-1F c2 1) 2 /Ew1F02 1 2 ) 112  78  

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