Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2 and Me4) for CO2-to-CO conversion over H2 formation

Abstract

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 Fc^0/+. 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, 1_C2(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, 1_C2(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.