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

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 intermediate [I-1] lying 2.0 kcal/mol below the reagents and an associated free energy barrier of 16.0 kcal/mol. The overall reaction is thus found to proceed with rather small free energy barriers and with a sizable free energy gain, consistent with the low-temperature conditions employed experimentally. In the absence of Na+, the reaction proceeds with a concerted mechanism, with an energetic barrier 15.2 kcal/mol higher than the corresponding process with Na+, pointing at a very relevant role of the Na+ counterion in determining the reaction thermodynamics and preferred pathway. We also investigated the selective cleavage of the Nb-O bond and release of an acetate ligand occurring after reduction of the [Me-2] product by SmI2, finding this process to be energetically favored compared to the competitive Nb-N bond cleavage.