The Mechanism of Homogeneous CO₂ Reduction by Ni(cyclam): Product Selectivity, Concerted Proton–Electron Transfer and C–O Bond Cleavage

Homogeneous CO2 reduction catalyzed by [Ni(I)(cyclam)](+) (cyclam = 1,4,8,11-tetraazacyclotetradecane) exhibits high efficiency and selectivity yielding CO only at a relatively low overpotential. In this work, a density functional theory study of the reaction mechanism is presented. Earlier experiments have revealed that the same reaction occurring on mercury surfaces generates a mixture of CO and formate. According to the proposed mechanism, an η(1)-CO2 adduct is the precursor for CO evolution, whereas formate is obtained from an η(1)-OCO adduct. Our calculations show that generation of the η(1)-CO2 adduct is energetically favored by ∼14.0 kcal/mol relative to that of the η(1)-OCO complex, thus rationalizing the product selectivity observed experimentally. Binding of η(1)-CO2 to Ni(I) only leads to partial electron transfer from the metal center to CO2. Hence, further CO2 functionalization likely proceeds via an outer-sphere electron-transfer mechanism, for which concerted proton coupled electron transfer (PCET) is calculated to be the most feasible route. Final C-O bond cleavage involves rather low barriers in the presence of H3O(+) and H2CO3 and is therefore essentially concerted with the preceding PCET. As a result, the entire reaction mechanism can be described as concerted proton-electron transfer and C-O bond cleavage. On the basis of the theoretical results, the limitations of the catalytic activity of Ni(cyclam) are discussed, which sheds light on future design of more efficient catalysts.