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Oxo, imido, and borollide complexes of tantalum and zirconium

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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 η5- 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 LiNHtBu generated the zwitterionic complex, Cp*(C4H4BNHiPr2)Zr=NtBu.

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