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

Cyclopentadienyl-free disilyl amide complexes [(Me2N)3M(SiButPh2)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(SiButPh2)2 (6), and (Me2N)3Ta(SiButPh2)[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 −SiButPh2- anions to give (Me2N)3Ta(SiButPh2)[Si(SiMe3)3] (7)
and (Me2N)3Ta(SiButPh2)2 (6). The silyl ligand in (Me2N)3Zr-Si(SiMe3)3 was found to undergo
a reversible exchange with SiButPh2-, probably through a disilyl intermediate, to reach the
following equilibrium:  (Me2N)3ZrSi(SiMe3)3 + SiButPh2- ⇌ (Me2N)3ZrSiButPh2 + Si(SiMe3)3-
with ΔH° = 4.6(0.5) kcal/mol and ΔS° = −7(2) eu. A similar exchange involving
[(Me2N)3M(SiButPh2)2]- (M = Zr, 1; Hf, 2) was observed:  (Me2N)3ZrSiButPh2 + SiButPh2-
⇌ 1 with the estimated free energy of activation ΔG⧧ = 14.1(0.5) kcal/mol. (Me2N)3Ta(SiButPh2)2 (6) and (Me2N)3Ta(SiButPh2)[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.