Microscopic insight into the electroreduction of carbon dioxide by N- or C-substituted Ni(cyclam) complexes: the issues of CO2 activation and CO bond cleavage.

The nickel cyclam complex (cyclam = 1,4,8,11-tetraazacyclotetradecane) remains one of the most selective and efficient catalysts for the electrochemically driven conversion of CO2 into CO in aqueous media. Although the mechanisms associated with CO2 electroreduction by this complex have been well investigated in homogeneous and heterogeneous cases, few computational studies so far have compared the effect of N- or C-substitution on the catalytic properties. Moreover, the effect of explicit water or counter-anion molecules has very rarely been considered in theoretical studies with this family of complexes. In that context, we have performed computational density functional theory investigations on the electrocatalytic reduction of CO2 by considering three different Ni complexes bearing cyclam-based ligands, which have been previously investigated. In-depth analysis for each step of the catalytic cycle has allowed us to draw general structure-reactivity relationships for both trans-I and trans-III isomers of each calculated structure.