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

• Published in: Chemistry : a European journal Vol. 27; no. 6; pp. 2138 - 2148
• Main Authors: Caise, Alexa, Hicks, Jamie, Ángeles Fuentes, M., Goicoechea, Jose M., Aldridge, Simon
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 26.01.2021
• Subjects: Acidity; Anions; beta-diketiminate; Binding; Carbon dioxide; Catalysis; Cations; Chemistry; Dipping; Gallium; hydride; Hydrides; Hydrogen bonds; Hydrosilylation; Oxidation; Reduction; Regeneration; Selectivity
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202004408

A novel β‐diketiminate stabilized gallium hydride, (DippL)Ga(Ad)H (where (DippL)={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). Selective: Addition of one equivalent of B(C6F5)3 to (DippNacnac)Ga(Ad)H leads to the formation of a 3‐coordinate cationic gallane, partnered with a hydroborate anion, [(DippNacnac)Ga(Ad)][HB(C6F5)3], which catalyses the selective reduction of CO2 to the formaldehyde oxidation level, yielding H2C(OSiEt3)2.