Ligand-assisted Hydride Transfer: A Pivotal Step for CO2 Hydroboration Catalyzed by a Mononuclear Mn(I) PNP Complex.

A mononuclear Mn(I) pincer complex [Mn(Ph2 PCH2 SiMe2 )2 NH(CO)2 Br] was disclosed to catalyze the pinacolborane (HBpin)-based CO2 hydroboration reaction. Density functional calculations were conducted to reveal the reaction mechanism. The calculations showed that the reaction mechanism could be divided into four stages: (1) the addition of HBpin to the unsaturated catalyst C1; (2) the reduction of CO2 to HCOOBpin; (3) the reduction of HCOOBpin to HCHO; (4) the reduction of HCHO to CH3 OBpin. The activation of HBpin is the ligand-assisted addition of HBpin to the unsaturated Mn(I)-N complex C1 generated by the elimination of HBr from the Mn(I) pincer catalyst. The sequential substrate reductions share a common mechanism, and every hydroboration commences with the nucleophilic attack of the Mn(I)-H to the electron-deficient carbon centers. The hydride transfer from Mn(I) to HCOOBpin was found to be the rate-limiting step for the whole catalytic reaction, with a total barrier of 27.0 kcal/mol, which fits well with the experimental observations at 90 °C. The reactivity trend of CO2 , HCOOBpin, HCHO, and CH3 OBpin was analyzed through both thermodynamic and kinetic analysis, in the following order, namely HCHO>CO2 >HCOOBpin≫CH3 OBpin. Importantly, the very high barrier for the reduction of CH3 OBpin to form CH4 reconciles with the fact that methane was not observed in this catalytic reaction.