Oxo, imido, and borollide complexes of tantalum and zirconium

Citation

Quan, Roger Weihong (1994) Oxo, imido, and borollide complexes of tantalum and zirconium. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:07052012-161238837

Abstract

The preparation of reactive oxo, imido, and borollide complexes on tantalum and zirconium will be described. Metathesis of Cp*₂Ta(=O)Cl with AgPF₆ afforded [Cp*₂TaF₂HPF₆], which presumably formed from P-F activation of PF6^-. When AgS0₃CF₃ was used, [Cp*₂Ta(=O)][S0₃CF₃] was isolated; however, it was unreactive toward styrene, methane, or benzene. Exposure to water resulted in the 1,2-addition product, [Cp*₂Ta(OH)₂][S0₃CF₃]. Finally, treating Cp*₂Ta(=O)H with [Ph₃C][B(C₆F₅)₄] afforded [Cp*₂Ta(=O)][B(C₆F₅)₄]. Even with this non-coordinating counterion, the complex was found to be surprisingly unreactive. To prepare an imido complex, treating Cp*TaCl₄ with four equivalents of lithium anilide affords Cp*Ta(=NPh)(NHPh)₂. This complex reacts readily with substituted anilines to generate Cp*Ta(=NPh')(NHPh')₂ and with alcohols such as pinacol to afford Cp*Ta(OCMe₂CMe₂O)₂, products whereby both the imido and amido groups are exchanged. Exposing Cp*Ta(=NPh)(NHPh)₂ to C0₂ resulted in the isolation of Cp*Ta(OCONPh)(η⁴-OCONHPh)₂ with both the formal [2+2] addition of C0₂ across the imido group and insertion of C0₂ into the amido group. Finally, bis-imido complexes were prepared by treating Cp*Ta(=NPh)(NHPh)₂ to one equivalent of HCl to yield Cp*Ta(=NPh)₂ or by treating Cp*TaCl₄ with four equivalents of lithium 2,6-diisopropylanilide to yield Cp*Ta(=NPh")₂. The borollide ligand, (C₄H₄BNⁱPr₂)^2-, ligand is introduced by treating Cp*ZrCl₃ with one equivalent of Li₂(C₄H₄BNⁱPr₂)•THF to generate Cp*(C₄H₄BNⁱPr₂)ZrCl•LiCl. This complex contains a η^5- coordinated borollide ligand, as shown by an X-ray crystal structure analysis. Alkylation and arylation with trimethylsilylmethyllithium, benzylpotassium or phenyllithium yielded Cp*(C₄H₄BNⁱPr₂)ZrR complexes. These catalysts polymerizes ethylene, but only oligomerizes α-olefins. In addition, Cp*(C₄H₄BNⁱPr₂)ZrCl•LiCl has been found to exhibit the formal heterolytic cleavage of HCl and CH₃I, affording Cp*(C₄H₄BNHⁱPr₂)ZrCl₂ and Cp*(C₄H₄BNⁱPr₂)ZrCl(I), respectively. Finally, treatment of Cp*(C₄H₄BNⁱPr₂)ZrCl•LiCl with LiNH^tBu generated the zwitterionic complex, Cp*(C₄H₄BNHⁱPr₂)Zr=N^tBu.

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