Quest for Active Species in Al/B-Catalyzed CO₂ Hydrosilylation

Abstract

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me2­CH2C6H4)­(C6H5)­Al­(μ-H)2B­(C6H5)2] (6) during carbon dioxide (CO2) hydrosilylation. Preliminary investigations into CO2 reduction using [(2-Me2­NCH2­C6­H4)­(H)­Al­(μ-H)]2 (1) and [Ph3C]­[B­(3,5-C6H3Cl2)4] (2) in the presence of Et3SiH and PhSiH3 resulted in CH2(OSiR3)2 and CH3OSiR3, which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B­(3,5-C6H3Cl2)3 (3), [(2-Me2­NCH2­C6H4)­(3,5-C6H3Cl2)­Al­(μ-H)2B­(3,5-C6H3Cl2)2] (4), and [(2-Me2­NCH2­C6H4)2­Al­(THF)]­[B­(3,5-C6H3Cl2)4] (5), were isolated. Among compounds 2–5, the highest catalytic conversion was achieved by 4. Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh3, respectively. 6 was found to be in equilibrium with 1 and BPh3, thus making the catalytic process of 6 more efficient than that of 4. Computational investigations inferred that CO2 reduction occurs across the Al–H bond, while Si–H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh3.