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

A novel beta-diketiminate stabilized gallium hydride, (L-Dipp)Ga(Ad)H (where (L-Dipp)={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 kappa(1)-formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C6F5)(3), (L-Dipp)Ga(Ad)H catalyses the reductive hydrosilylation of CO2. Under stoichiometric conditions, the addition of one equivalent of B(C6F5)(3) to (L-Dipp)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(L-Dipp)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 degrees 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 (L-Dipp)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 [(L-Dipp)Ga(Ad)](+) cation (AN=69.8).