Early transition metal complexes supported by amidophosphine and amidocarbene ligands
Creators: name: Spencer, Liam Patrick
Publisher: name: University of British Columbia
Year: 2010

description: The reactivity of the tantalum dinitrogen complex ([NPN]Ta)₂(μ-H)₂(μ-η¹:η²-N₂) (where [NPN] = [(PhNSiMe₂CH₂)₂PPh]²⁻) with several zirconium hydride reagents is explored. The addition of [Cp₂Zr(Cl)H]x leads to the unanticipated reduction of the N-N bond without Zr-H addition. The coordinated N₂ ligand is cleaved to form a triply bridging nitride and a phosphinimide functional group that bridges between Ta and Zr centres. A series of experiments to determine the mechanism of this reaction reveals that a "Cp₂Zr" species promotes reduction of the N-N unit. This type of dinitrogen reduction is extended to include the insertion of a "Cp₂Ti" fragment into the N-N bond. The synthesis of early transition metal complexes employing a tridentate diamido N-heterocyclic carbene (NHC) ligand set (denoted [NCN]) is also investigated. Aminolysis reactions with diamino-NHC precursors and M(NMe₂)₄ (M = Ti, Zr, Hf) provide bis(amido)-NHC-metal complexes that can be further converted to chloro and alkyl derivatives. Alkyl elimination reactions with the diamino-NHC ligands and Zr(CH₂R)₄ (R=Ph, SiMe₃) yield dialkyl-NHC-zirconium complexes. The central position of the NHC donor in this tridentate architecture renders the carbene stable to dissociation from the metal centre in strongly coordinating solvents. The hafnium dialkyl complexes are thermally stable with the exception of the dialkyl complex, Mes[NCN]Hf(CH₂CH₃)₂, (where Mes[NCN] = (2,4,6-Me₃-C₆H₂NHCH₂CH₂)₂N₂C₃H₂) which undergoes β-hydrogen transfer and subsequent C-H bond activation with an ortho-methyl substituent on the mesityl group. Activation of Mes[NCN]M(CH₃)₂ (M = Zr, Hf) with [Ph₃C][B(C₆F₅)₄] yields {Mes[NCN]MCH₃} {B(C₆F₅)₄}, which is a moderately active ethylene polymerization catalyst. The hafnium dialkyl complexes also insert carbon monoxide, substituted isocyanides, and substituted cumulenes into a hafnium-sp³-carbon bond to yield expected insertion products. In some circumstances, further C-C bond coupling occurs to yield enediolate and eneamidolate metallacycles. Attempts to reduce Mes[NCN]ZrCl₂ in the presence of dinitrogen lead to mixtures of products. In one case, an ether cleavage product is isolated, which is a result of C-O bond activation of the solvent used in the reaction. Aminolysis and alkyl elimination reactions with the diamino-NHC ligand and tantalum(V) reagents provide complexes with an amide-amine donor configuration. Attempts to promote coordination of the remaining pendant amine donor have been unsuccessful. Metathesis reactions with the lithiated diamino-NHC ligand (Li₂Ar[NCN]) and Clx Ta(NR₂)₅-x derivatives provide a successful method to coordinate both amide donors, yielding the desired Ar[NCN]TaClx(NR₂)₃-x complexes. Attempts to prepare trialkyl tantalum complexes by this methodology resulted in the formation of a metallaaziridine derivative. DFT calculations on model complexes suggest the lowest energy pathway involves a tantalum alkylidene intermediate, which undergoes amido C-H bond activation to form the metallaaziridine moiety. This mechanism was confirmed by examining the distribution of deuterium atoms in an experiment between Mes[NCN]Li₂ and Cl₂Ta(CD₂Ph)₃. The preparation of chiral [NCN] group 4 complexes is achieved by aminolysis and alkyl elimination reactions with a chiral diamino-NHC ligand and suitable group 4 reagents. The titanium and zirconium derivatives are investigated in the asymmetric intramolecular hydroamination of an aminoalkene in an attempt to promote selectivity in the N-heterocycle synthesized. While the titanium [NCN] complex shows no activity, the zirconium [NCN] complex is an efficient catalyst for the intramolecular formation of a substituted pyrollidine. Examination of the steroselectivity in the N-heterocyclic product formed reveals very low enantioselective excess.