Theoretical Study on Photocatalytic CO₂ Reduction to Formate by a Ruthenium CNC Pincer Complex

Despite the high efficiency and excellent formic acid selectivity of ruthenium complex [(^tBuCNC)Ru(bpy)] catalysts, the molecular details of their catalytically active species remain unclear. Density functional theory calculations, serving as a reliable supplement to experiments, were employed to investigate the reaction mechanism of CO₂ photocatalytic reduction by L-Ru-CNC and elucidate the origin of product selectivity. Under photoirradiation, the ruthenium catalyst undergoes two consecutive single-electron reductions to form a Ru⁰ intermediate. For the formate pathway, the Ru⁰ intermediate reacts with ETOA-H⁺ in solution to generate a Ru^II-H hydride. This species performs a nucleophilic attack on CO₂, forming a carboxylate intermediate, which is further reduced by the electron donor BI(OH)H to yield Ru^I-OCHO. Subsequent release of HCOO^- regenerates Ru^I for catalytic cycling. This pathway is thermodynamically favorable with low kinetic barriers. For the CO pathway, Ru⁰ mediates nucleophilic attack on CO₂ to form Ru^II-CO₂, which undergoes protonation by ETOA-H⁺ to generate Ru^II-COOH. Further protonation produces Ru^II-CO. The kinetic barrier for HCOO^- formation is significantly lower than that for CO release, rationalizing the high formate selectivity. Slow dissociation of CO enables an alternative formate pathway. Protonation of the bipyridine ligand's nitrogen forms an OC-Ru^II-NH intermediate, which binds CO₂ via an orthogonal proton-electron transfer mechanism (analogous to formate reductase), ultimately generating formate. These results reconcile experimental observations and clarify the high formate selectivity in this catalytic system.