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 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.

Fig. 1

Schematic representation of the CO₂ adduct in the Ni,Fe carbon monoxide dehydrogenase adapted from Dobbeck et al.,^, and diagram of the imidazolium-functionalized manganese bipyridine complex reported by Nippe and co-workers for CO₂-to-CO conversion in the presence of H₂O as the proton source.

Fig. 2

Schematic representation of the imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts that were investigated in this work. The lowest energy structure for ${\mathbf{3}}_{\mathbf{\text{C}}\mathbf{2}}^{+}$ features a hydrogen-bonding interaction between the methylene bridge and bromide.

Fig. 3

Computed free energy (kcal mol⁻¹) profile for yielding the metallocarboxylic acid species, ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}(\text{I}5)$. All free energies are calculated with respect to the separated reactants and the superscript corresponds to the spin multiplicity of a given species.

Fig. 4

Optimized geometries of the transition state 1_C2(TS1) for CO₂ addition (left) and computed metallocarboxylate intermediate 1_C2(I2) (right). Distances are given in angstroms, and bond angles are in degrees. Non-participating hydrogen atoms are omitted for clarity.

Fig. 5

Optimized geometries of 1_C2(I3) (left) and 1_C2(I4) (right). Distances are given in angstroms, and bond angles are in degrees. Non-participating hydrogen atoms are omitted for clarity.

Fig. 6

Comparison of the protonation-first pathway (left-side) and reduction-first pathway (right side) after the formation of the metallocarboxylic acid intermediate ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}(\mathbf{\text{I}}\mathbf{5})$.

Fig. 7

Computed free energy (kcal mol⁻¹) profile for the protonation-first and reduction-first pathway from the metallocarboxylic acid intermediate ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}(\mathbf{\text{I}}\mathbf{5})$ in the presence of water. All free energies are calculated with respect to the separated reactants and the superscript represents the spin multiplicity of a given species.

Fig. 8

Optimized geometries of the transition state 1_C2(TS3) (left) and neutral tetracarbonyl intermediate 1_C2(I7-R) (right). Distances are given in angstroms, and non-participating hydrogen atoms are omitted for clarity.

Fig. 9

(A) Computed free energy (kcal mol⁻¹) profile for forming the metal-hydride complex, ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}\left(\mathbf{\text{I}}\mathbf{11}\right)$. All free energies are calculated with respect to the separated reactants and the superscript corresponds to the spin multiplicity of a given species. (B) Optimized geometry of the transition state 1_C2(TS5), leading to the formation of the metal-hydride complex, ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}\left(\mathbf{\text{I}}\mathbf{11}\right)$. Distances are given in angstroms, and non-participating hydrogen atoms are omitted for clarity.

Fig. 10

(A) Computed free energy (kcal mol⁻¹) profile for H₂ formation from the metal-hydride species ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}(\mathbf{\text{I}}\mathbf{11})$ in the presence of water as the proton source. All free energies are calculated with respect to the separated reactants. The superscript corresponds to the spin multiplicity of a given species. (B) Optimized geometry of the transition state 1_C2(TS6) leading to the formation of the manganese dihydrogen complex, ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}(\mathbf{\text{I}}\mathbf{14})$. Distances are given in angstroms. Non-participating hydrogen atoms are omitted for clarity.

Fig. 11

Computed free energy (kcal mol⁻¹) profiles of one catalytic cycle for the CO₂ reduction reaction (CO₂RR in black) and the hydrogen evolution (HER in maroon) using ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}$ in the presence of H₂O as the proton source. All computed Gibbs free energies are reported with respect to the separated reactants at an applied potential of Φ = −1.82 V versus the Fc^0/+ couple.

Fig. 12

Computed free energy (kcal mol⁻¹) profiles of one catalytic cycle for CO₂-to-CO conversion using ${\mathbf{1}}_{\mathbf{\text{C}}\mathbf{2}}^{+}$, ${\mathbf{2}}_{\mathbf{\text{C}}\mathbf{5}}^{+}$, ${\mathbf{3}}_{\mathbf{\text{C}}\mathbf{2}}^{+}$ in the presence of H₂O as the proton source. All computed Gibbs free energies are reported with respect to the separated reactants at an applied potential of Φ = −1.82 V versus the Fc^0/+ couple.