Pentamethylcyclopentadienyl Aminoborole Complexes of Hafnium

The tricyclophosphate ligand, (P₃O₉)³⁻, was investigated as an oxidation resistant ligand for ruthenium chemistry. The silver salt of tricyclophosphate, Ag₃(P₃O₉).H₂O, was found to be a useful synthon in the preparation of organic soluble tricyclophosphate salts. The complex [(P₃O₉)Ru(C₆H₆)]⁻ was prepared and characterized. In water, it was found that the tricyclophosphate ligand dissociated from the [(C₆H₆)Ru]²⁺ dication. Cyclic voltammatry in acetonitrile shows only a small irreversible oxidation at 0.95 V (Vs ferrocene). The rhenium carbonyl complex [(P₃O₉)Re(C0)₃]²⁻ was also investigated as a precursor to high valent tricyclophosphate rhenium complexes. Based on these results as well as related work by Klemperer and coworkers, it is concluded that the tricyclophosphate ligand is an exceptionally hard one that binds only weakly to transition metals. The preparations of chloro and allyl derivatives of pentamethylcyclopentadienyl aminoborole hafnium complexes are described. The chloro derivative, Cp*[C₄H₄BN(CHMe₂)₂]HfCl•LiCl, is prepared by treatment of Cp*HfCl₃ with Li₂(THF){C₄H₄BN(CHMe₂)₂}. The structures of the chloro derivatives Cp*[C₄H₄BN(CHMe₂)₂]HfCl•LiCl(Et₂O)₂ and {Cp*[C₄H₄BN(CHMe₂)₂]HfCl-LiCl}₂ were determined by single crystal X-ray analysis. Treatment of Cp*[C₄H₄BN(CHMe₂)₂]HfCl•LiCl with allyl magnesium bromide yields Cp*[C₄H₄BN(CHMe₂)₂]Hf(η³-C₃H₅), whose structure was determined by X-ray analysis. The allyl species was active for the polymerization of ethylene, but not for the polymerization of α-olefins. Addition of ligands to the allyl derivative results in the formation of Cp*[C₄H₄BN(CHMe₂)₂]Hf(C₃H₅)(L) (L = PMe₃, pyridine, CO). The structure of Cp*[C₄H₄BN(CHMe₂)₂]Hf(η³-C₃H₅)(CO) was determined. Treatment of Cp*[C₄H₄BN(CHMe₂)₂]Hf(η³-C₃H₅)(CO) with PMe₃ results in the formation of the dieneolate complex Cp*[C₄H₄BN(CHMe₂)₂]Hf(OCHCHCHCH₂) (PMe₃). The electronic spectra of several aminoborole complexes were investigated and the low energy transitions assigned as borole-metal LMCT transitions. The amphoteric complex Cp*{η⁵-C₄H₄BN(CHMe₂)₂}HfCl•LiCl heterolytically cleaves H-X bonds to form Cp*{η⁵-C₄H₄BNH(CHMe₂)₂}HfCl(X) (X = Cl, CCR). Cp*{η⁵-C₄H₄BNH(CHMe₂)₂}Hf(CCTMS)₂ is prepared from Cp*{η⁵-C₄H₄BN(CHMe₂)₂}Hf(η³-C₃H₅) and two equivalents of (trimethylsilyl)acetylene. Methyl iodide reacts with Cp{η⁵-C₄H₄BN(CHMe₂)₂}HfCl•LiCl to form Cp*{η⁵-C₄H₃MeBNH(CHMe₂)₂}HfClI. Control experiments using deuterium labelled substrates show heterolysis occurs with no incorporation of deuterium into the 2,5 positions of the borole heterocycle. The X-ray structure determinations of Cp*{η⁵-C₄H₄BNH(CHMe₂)₂}HfCl₂, Cp*{η⁵-C₄H₄BNH(CHMe₂)₂}HfCl(CCTMS), and Cp*{η⁵-C₄H₃MeBN(CHMe₂)₂}HfClI are reported.