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Abstract:
Homogeneous CO2 reduction catalyzed by [NiI(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 η1-CO2 adduct is the precursor for CO evolution, whereas formate is obtained from an η1-OCO adduct. Our calculations show that generation of the η1-CO2 adduct is energetically favored by ∼14.0 kcal/mol relative to that of the η1-OCO complex, thus rationalizing the product selectivity observed experimentally. Binding of η1-CO2 to NiI 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.
