DFT Investigation of Ligand-Based Reduction of CO2 to CO on an Anionic Niobium Nitride Complex: Reaction Mechanism and Role of the Na+ Counterion

Abstract

We have investigated the energetics and reaction mechanism of the recently reported ligand-based reduction of CO2 to CO in the coordination sphere of the [Na][NNb(N[tBu]Ar)3] (Ar = 3,5-C6H3Me2) Nb nitride complex, [Na][1], by means of density functional theory calculations. We investigated in detail the four steps of the catalytic cycle proposed by Cummins and co-workers (Silva, J. S.; Cummins, C. C.J. Am. Chem. Soc. 2010, 132, 2171). We pointed our attention to the second reaction step, involving the reaction between [Na][O2C-1] and Ac2O to lead to the [Me-2] product, highlighting the role of the Na+ counterion in the reaction profile. We find coordination of CO2 to [Na][1] to be highly exothermic, without any energy barrier, suggesting that this process is highly favored and possibly controlled by an entropic barrier. Calculation of the free energy profile for the rate-determining second reaction step has shown a stepwise pathway to be favored over a concerted one by 10.0 kcal/mol, with the reaction int...