Partnering a Three‐Coordinate Gallium Cation with a Hydroborate Counter‐Ion for the Catalytic Hydrosilylation of CO 2

A novel β‐diketiminate stabilized gallium hydride, ( Dipp L)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 κ 1 ‐formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C 6 F 5 ) 3 , ( Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO 2 . Under stoichiometric conditions, the addition of one equivalent of B(C 6 F 5 ) 3 to ( Dipp L)Ga(Ad)H leads to the formation of a 3‐coordinate cationic gallane complex, partnered with a hydroborate anion, [( Dipp L)Ga(Ad)][HB(C 6 F 5 ) 3 ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO 2 to the formaldehyde oxidation level at 60 °C in the presence of Et 3 SiH (yielding H 2 C(OSiEt 3 ) 2 ). When catalysis is undertaken in the presence of excess B(C 6 F 5 ) 3 , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H 2 C(OSiEt 3 ) 2 , CH 4 and O(SiEt 3 ) 2 . While this system represents proof‐of‐concept in CO 2 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 ( Dipp L)Ga(Ad){OC(H)OB(C 6 F 5 ) 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 [( Dipp L)Ga(Ad)] + cation (AN=69.8).