Mechanism of Si–H Bond Activation for Lewis Acid PBP-Ni-Catalyzed Hydrosilylation of CO₂: The Role of the Linear S_N2 Type Cooperation

Abstract

Lewis acid transition-metal-complex-catalyzed hydrosilylation of CO2 has attracted vast interests. Yet, the Si–H bond activation mechanism is still ambiguous and needs further elucidation. Herein, DFT studies were performed to study the reaction mechanism of CO2 hydrosilylation catalyzed by the PBP–Ni–OCHO·B(C6F5)3. Different from previously proposed B–Ni-bond-assisted nonlinear Si–H cleavage and boron-promoted Si–H bond activation, two other patterns of linear SN2 type Si–H bond activation were proposed: the B–Ni-bond-assisted Si–H bond cleavage and the Ni-promoted Si–H bond cleavage. Our results demonstrate the important role of the linear SN2 type cooperation in Si–H bond activation. As for the Si–H cleavage assisted by the B–Ni bond, the linear SN2 type proposed in this work is more favored than the nonlinear SN2 type. Specifically, the Ni-center-promoted linear SN2 type Si–H cleavage is the most plausible mechanism for Si–H bond activation because of its linear geometry, small deformation energy, and avoidance of B–O bond dissociation in the transition state. With regard to the whole hydrosilylation reaction, four steps are involved: initial Si–H activation, hydride transfer, second Si–H activation, and reduction of CO2. The C═O reductions of both hydride transfer and CO2 are preferred to be promoted by the B–H via the cooperation of the PBP–Ni···HB(C6F5)3 Lewis pairs. The initial Si–H activation with the free-energy barrier of ∼28 kcal/mol is the rate-determining step in the whole reaction. This research highlights the important role of the linear SN2 type cooperation in Si–H bond activation by Lewis acid transition-metal systems, which should provide important guidance to mechanistic understanding and catalyst design in the future.