Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)

We have recently reported a series of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts, [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2, and Me4), for CO2-to-CO conversion in the presence of H2O as the proton source [J. Am. Chem. Soc., 2019, 141, 6569]. These catalysts feature slightly acidic imidazolium moieties in the secondary coordination sphere and reduce CO2 at mild electrochemical potentials. Here, we employ density functional theory (DFT) calculations to understand the electronic structure and reactivity for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) using [Mn[bpyMe(ImMe)](CO)3Br]+ (1+). Our work indicates that, in the absence of water, the imidazolium ligand stabilizes the Mn-CO2 adduct through hydrogen bonding-like interactions, similar to the activated CO2 molecule in the C-cluster of the Ni,Fe-carbon monoxide dehydrogenase II, and assists the protonation steps during CO2RR and HER. More significantly, based on the energy span model, we demonstrate that the selectivity for CO2 fixation over proton reduction results from a higher activation energy for yielding the manganese dihydrogen intermediate before H2 release, which is the TOF determining transition state (TDTS) under an applied potential of Φ = -1.82 V versus Fc0/+. The calculated TOF also reflects the selectivity for CO2RR, which is four orders of magnitude larger than for HER, consistent with the CPE experiments that show no hydrogen was obtained. In the case of CO2 reduction, the TOF determining intermediate (TDI) corresponds to the doubly reduced active catalyst, 1C2(red2), which features a manganese(0) center that couples ferromagnetically with one unpaired electron in the π* orbital of bipyridine. On the other hand, for HER, the metal-hydride intermediate, 1C2(I11-R), is the TDI. Finally, second-order perturbation analyses imply that the strongest hydrogen bonding-like interaction at the C2 position in 1+ contributes to the higher catalytic activity with respect to [Mn[bpyMe(ImMe2)](CO)3Br]+ (2+) and [Mn[bpyMe(ImMe4)](CO)3Br]+ (3+) for CO2 fixation, consistent with the experimental data.