CO2 reduction by a Mn electrocatalyst in the presence of a Lewis acid: a DFT study on the reaction mechanism

The addition of a Lewis acid (Mg2+) has been shown to improve the efficiency of CO2 reduction by homogeneous electrocatalysts. Recently, a CO2 reduction protocol involving a Mn electrocatalyst with a bulky bipyridine ligand [Mn(mesbpy)(CO)(3) MeCN](mesbpy = 6,6 '-dimesityl-2,2 '-bipyridine) in the presence of Mg(OTf)(2) was reported (Sampson et al., J. Am. Chem. Soc., 2016, 138, 1386-1393). However, a detailed mechanistic understanding of this reaction is lacking. Here we present the details of the reaction mechanism based on thermodynamic and kinetic data derived from density functional theory (DFT) calculations. The DFT calculations demonstrate that the primary role of Mg(OTf)(2) is to stabilize a two-electron reduced Mn intermediate through Lewis pair binding. Furthermore, Mg(OTf)(2) makes the reaction thermodynamically and kinetically feasible. In our presented mechanism, two molecules of CO2 and Mg(OTf)(2) contribute to the C-O bond cleavage reaction. The demonstrated roles of Mg(OTf)(2) in this catalytic process are important for the design of novel multimetallic catalysts for CO2 conversion under milder reaction conditions.