Disilyl complexes of zirconium, hafnium, and tantalum. Their synthesis, characterization, and exchanges with silyl anions

Article abstract of DOI:10.1021/om050302f

Cyclopentadienyl-free disilyl amide complexes [(Me₂N) ₃M(SiBu^tPh₂)₂]^- (M = Zr, 1; Hf, 2 as [Li(THF)₄]⁺ salts), K(18-crown-6) _3/2{(Me₂N)₃M[(Me₃Si) ₂Si-(CH₂)₂-Si(SiMe₃)₂]} (M = Zr, 3; Hf, 4), (Me₂N)₃Ta[Si(SiMe₃) ₃]₂ (5), (Me₂N)₃Ta(SiBu ^tPh₂)₂ (6), and (Me₂N) ₃Ta(SiBu^tPh₂)[Si(SiMe₃) ₃] (7) have been prepared. The structures of 1-4 have been determined by X-ray single-crystal diffraction. The two -Si(SiMe ₃)₃^- ligands in (Me₂N) ₃Ta[Si(SiMe₃)₃]₂ (5) were replaced sequentially by the -SiBu^tPh₂^- anions to give (Me₂N)₃Ta(SiBu^tPh₂)[Si(SiMe ₃)₃] (7) and (Me₂N)₃Ta(SiBu ^tPh₂)₂ (6). The silyl ligand in (Me ₂N)₃Zr-Si(SiMe₃)₃ was found to undergo a reversible exchange with SiBu^tPh₂^-, probably through a disilyl intermediate, to reach the following equilibrium: (Me₂N)₃ZrSi(SiMe₃)₃ + SiBu ^tPh₂^- (Me₂N)₃ZrSiBu ^tPh₂ + Si(SiMe₃)₃-with ΔH° = 4.6(0.5) kcal/mol and ΔS° = -7(2) eu. A similar exchange involving [(Me₂N)₃M(SiBu^tPh ₂)₂]^- (M = Zr, 1; Hf, 2) was observed: (Me ₂N)₃ZrSiBu^tPh₂ + SiBu ^tPh₂^- ? 1 with the estimated free energy of activation ΔG? = 14.1(0.5) kcal/mol. (Me ₂N)₃Ta(SiBu^tPh₂)₂ (6) and (Me₂N)₃Ta(SiBu^tPh₂)[Si(SiMe ₃)₃] (7) are thermally unstable. Kinetic studies give the activation parameters of the decomposition of 7: ΔH? = 22.8(1.3) kcal/mol and ΔS? = -3(5) eu.