Group 4 Half-Sandwich Tris(trimethylsilylmethyl) Complexes: Thermal Decomposition and Reactivity with...

The thermal decomposition of group 4 trimethylsilylmethyl derivatives [M(η⁵-C₅Me₅)(CH₂SiMe₃)₃] (M = Ti (1), Zr (2), Hf (3)) in solution and their reactivity with N,N-dimethylamine-borane were investigated. Heating of hydrocarbon solutions of compounds 2 and 3 at 130-200 °C results in the elimination of SiMe₄ and the clean formation of the singular alkylidene-alkylidyne zirconium and hafnium compounds [{M(η⁵-C₅Me₅)}₃{(μ-CH)₃SiMe}(μ₃-CSiMe₃)] (M = Zr (4), Hf (5)). The reaction of 2 and 3 with NHMe₂BH₃ (≥1 equiv) at room temperature affords the dialkyl(dimethylamidoborane) complexes [M(η⁵-C₅Me₅)(CH₂SiMe₃)₂(NMe₂BH₃)] (M = Zr (6), Hf (7)). Compounds 6 and 7 are unstable in solution and decompose with formation of the alkyl(dimethylamino)borane [B(CH₂SiMe₃)H(NMe₂)] (8), SiMe₄, and other minor byproducts, including the tetranuclear zirconium(III) octahydride complex [{Zr(η⁵-C₅Me₅)}₄(μ-H)₈] (9) in the decomposition of 6. Addition of NHMe₂BH₃ to the titanium tris(trimethylsilylmethyl) derivative 1 gives the trinuclear mixed valence Ti(II)/Ti(III) tetrahydride complex [{Ti(η⁵-C₅Me₅)(μ-H)}₃(μ₃-H)(μ₃-NMe₂BH₂)] (10) at 45-65 °C. While the complete conversion of 1 under argon atmosphere requires excess NHMe₂BH₃ (up to 15 equiv), complex 10 is readily prepared with 3 equiv of NHMe₂BH₃ under a hydrogen atmosphere indicating that the formation of 10 involves hydrogenolysis of 1 in the presence of (NMe₂BH₂)₂. In absence of amine-borane, the reaction of 1 with H₂ leads to the tetranuclear titanium(III) octahydride [{Ti(η⁵-C₅Me₅)}₄(μ-H)₈] (11), which upon addition of NHMe₂BH₃ and subsequent heating at 65 °C affords complex 10. The X-ray crystal structures of 2, 4, 5, 10, and 11 were determined.