Computational study on the mechanism of hydroboration of CO2 catalysed by POCOP pincer nickel thiolate complexes: concerted catalysis and hydride transfer by a shuttle

Abstract

Hydroboration of carbon dioxide (CO₂) catalysed by bis(phosphinite) (POCOP) pincer nickel complexes is among the most efficient homogeneous processes for the reduction of CO₂ to the methanol level. Although both POCOP pincer nickel hydride and thiolate complexes are effective catalysts, the latter is far more effective under the same conditions. The mechanism for nickel hydride complexes catalysed reactions is well-established. However, that for nickel thiolate complex catalysed reactions remains elusive. In this work, the mechanism for the reduction of CO₂ catalysed by POCOP pincer nickel thiolate complexes was investigated using density functional theory. The calculated results indicated that the reaction occurs via a concerted catalytic process involving two active species and the hydride is transferred by a shuttle species. Specifically, the reaction proceeds through four cycles: formation of two active species (cycle I) followed by further reaction of these two species to form a hydride transfer shuttle which is responsible for hydride transfers CO₂→HCOOBcat (cycle II), HCOOBcat→CH₂O (cycle III) and CH₂O→catBOCH₃ (cycle IV). The calculated mechanism is in good agreement with the experimental observation that the reaction is exothermic with simultaneous HBcat degradation.