Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2

A novel β-diketiminate stabilized gallium hydride, (^DippL)Ga(Ad)H (where ( ^Dipp L)={HC(MeCDippN)₂}, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga?H bond under mild conditions. In this case, treatment of the resulting κ¹-formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆F₅)₃, (^DippL)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂. Under stoichiometric conditions, the addition of one equivalent of B(C₆F₅)₃ to (^DippL)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, (^DippL)Ga(Ad)]HB(C₆F₅)₃]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃SiH (yielding H₂C(OSiEt₃)₂). When catalysis is undertaken in the presence of excess B(C₆F₅)₃, appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂C(OSiEt₃)₂, CH₄ and O(SiEt₃)₂. While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^?1). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^DippL)Ga(Ad){OC(H)OB(C₆F₅)₃}, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the (^DippL)Ga(Ad)]⁺ cation (AN=69.8).