Copper-Mediated Reduction of CO₂ with pinB-SiMe₂Ph via CO₂ Insertion into a Copper–Silicon Bond

Reaction of [(IPr)Cu-OtBu] (1) with pinB-SiMe2Ph (2) leads to the Cu-silyl complex [(IPr)Cu-SiMe2Ph] (3). Insertion of CO2 into the Cu–Si bond of 3 is followed by transformation of the resulting silanecarboxy complex [(IPr)Cu-O2CSiMe2Ph] (4) to the silanolate complex [(IPr)Cu-OSiMe2Ph] (5) via extrusion of CO. As 5 reacts readily with 2 to regenerate 3, a catalytic CO2 reduction to CO is feasible. The individual steps were studied by in situ 13C NMR spectroscopy of a series of stoichiometric reactions. Complexes 3, 4, and 5 were isolated and fully characterized, including single-crystal X-ray diffraction studies. Interestingly, the catalytic reduction of CO2 using silylborane 2 as a stoichiometric reducing agent leads not only to CO and pinB-O-SiMe2Ph but also to PhMe2Si-CO2-SiMe2Ph as an additional reduction product.