Disilyl Complexes of Zirconium, Hafnium, and Tantalum. Their Synthesis, Characterization, and Exchanges with Silyl Anions

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(SiMe₃)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(SiMe₃)₃] (7) and (Me₂N)₃Ta(SiBu^tPh₂)₂ (6). The silyl ligand in (Me₂N)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^tPh₂)₂]^- (M = Zr, 1; Hf, 2) was observed:  (Me₂N)₃ZrSiBu^tPh₂ + SiBu^tPh₂^- ⇌ 1 with the estimated free energy of activation ΔG^⧧ = 14.1(0.5) kcal/mol. (Me₂N)₃Ta(SiBu^tPh₂)₂ (6) and (Me₂N)₃Ta(SiBu^tPh₂)[Si(SiMe₃)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.