The Course of (R₂R‘SiO)₃TaCl₂ (R = ^tBu, R‘ = H, Me, Ph, ^tBu (silox); R = ⁱPr, R‘ = ^tBu, ⁱPr) Reduction Is Dependent on Siloxide Size

Various sized siloxides (Cy3SiO > tBu3SiO > tBu2PhSiO > tBu2MeSiO ∼ iPr2tBuSiO > iPr3SiO > tBu2HSiO) were used to make (R2R‘SiO)3TaCl2 (R = tBu, R‘ = H (1-H), Me (1-Me), Ph (1-Ph), tBu (1); R = iPr, R‘ = tBu (1-iPr2); R = R‘ = iPr (1-iPr3); R = R‘ = cHex (Cy)). Product analyses of sodium amalgam reductions of several dichlorides suggest that [(R2R‘SiO)3Ta]2(μ-Cl)2 may be a common intermediate. When the siloxide is large (1-tBu), formation of the Ta(III) species (tBu3SiO)3Ta (6) occurs via disproportionation. When the siloxide is small, the Ta(IV) intermediate is stable (e.g., [(iPr3SiO)3Ta]2(μ-Cl)2 (2)), and when intermediate sized siloxides are used, solvent bond activation via unstable Ta(III) tris-siloxides is proposed to occur. Under hydrogen, reductions of 1-Me and 1-Ph provide Ta(IV) and Ta(V) hydrides [(tBu2MeSiO)3Ta]2(μ-H)2 (4-Me) and (tBu2PhSiO)3TaH2 (7-Ph), respectively.