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

Area of Science:

• Inorganic Chemistry
• Catalysis
• Computational Chemistry

Background:

• Ruthenium complexes with (^tBuCNC)Ru(bpy) ligands show high efficiency and selectivity for formic acid production via CO2 reduction.
• The precise molecular mechanisms and active species involved in this catalytic process remain incompletely understood.

Purpose of the Study:

• To investigate the reaction mechanism of CO2 photocatalytic reduction using L-Ru-CNC catalysts.
• To elucidate the origin of high formate selectivity in this ruthenium-catalyzed system.
• To identify the catalytically active species through computational analysis.

Main Methods:

• Employed Density Functional Theory (DFT) calculations to simulate and analyze the reaction pathway.
• Investigated the electronic structure and energy profiles of intermediates and transition states.
• Correlated computational findings with experimental observations.

Main Results:

• Identified a Ru0 intermediate formed via two single-electron reductions of the catalyst under photoirradiation.
• Detailed two competing pathways: one leading to formate (HCOO-) and another to carbon monoxide (CO).
• Established that the formate pathway exhibits significantly lower thermodynamic and kinetic barriers compared to the CO pathway, explaining the observed high formate selectivity.

Conclusions:

• The high selectivity for formate is attributed to the kinetically favored pathway involving a RuII-H hydride intermediate attacking CO2.
• An alternative formate pathway involving protonation of the bipyridine ligand and an orthogonal proton-electron transfer mechanism was also identified.
• The DFT calculations successfully reconcile experimental data and clarify the molecular basis for the high formate selectivity in this ruthenium catalytic system.