Reaction mechanism of the selective reduction of CO₂ to CO by a tetraaza [Co^IIN₄H]²⁺ complex in the presence of protons

Abstract

The tetraaza [CoIIN4H]2+ complex (1) is remarkable for its ability to selectively reduce CO2 to CO with 45% Faradaic efficiency and a CO to H2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO2 reduction. The selectivity for CO2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO2, and bicarbonate. After reduction of the catalyst by two electrons, formation of [CoIN4H]+-CO2- is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [CoIN4H]+-CO2H, followed by reaction with CO2 to form [CoIIN4H]2+-CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO2 as a Lewis acid to assist in C-O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1. Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO2.

Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal–Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways

“…The comparison with experimental entropies (obtained via Henry's Law) indicate differences of up to 9 kcal/mol for the free energy of CO 2 in solution. 95,96 This implies that calculated CO 2 binding free energies are too endergonic.…”

Solvation Entropy Made Simple

“…However, the use of gas phase entropies to model processes that change molecularity (e.g., adsorption and binding) in solution often exaggerates entropy changes during the reaction, ∆S reac . [1][2][3][4][5][6][7][8] In condensed media, the translational and rotational motions of a molecule are hindered, reducing the entropy as compared to the gas phase. Rearrangement of the solvent to form a cavity for the solute further lowers the entropy of the system.…”

“…Furthermore, in actual applications, implicit solvation models often do not improve the too-high binding free energies caused by the use of gas phase entropies. [1][2][3][4][5][6][7][8][9] Molecular dynamics and alchemical free energy methods can in principle determine free energies in solution without relying on gas phase formulas. [23][24][25] Nonetheless, such calculations require significant additional work from the user and are computationally demanding, which makes them unsuitable for routine or high-throughput calculations.…”

Übergangsmetallkomplexe als Katalysatoren für die elektrische Umwandlung von CO2 – eine metallorganische Perspektive

“…[236] comp. [237] IL, IR DFT [235][236][237][238] [242] ET M post. [242] n.v. [120] 72 [120] 20 [231] ET M exp.…”