Bulky Ligand-Induced Hindrance in Photocatalytic CO2 Reduction over Various Tris(bipyridine)cobalt(II) Chloride Complexes

Photocatalytic CO₂ conversion is one of the ideal approaches to address both topics of solar energy shortage and carbon neutrality. Cobalt(II) centers coordinated with bipyridines have been designed and evaluated as catalysts for CO₂ conversion under light irradiation. Herein, we report a series of pyridine-based cobalt complexes with alkyl substituents as molecular photocatalysts, aiming to elucidate the effects of alkyl type and substitution position on catalytic performance through spectroscopic and electrochemical measurements. The substitution of the hydrogen at 4,4'-positions on the bipyridine ring with a methyl group, a tert-butyl group, and a nonyl group led to a decrease in the conversion rate of CO₂ by 13.2%, 29.6%, and 98%, respectively. The methyl substituents at the 5, 5'-positions of the bipyridine ring resulted in a 71.1% decrease in the CO₂ conversion rate. The usage of either 6, 6'-Me₂-2,2'-bipy, 2,4-bipy, or 3,3'-bipy resulted in no detectable activity for CO₂ conversion in the current system. Both photo- and electrochemical analyses have been employed to reveal the relationship between changing ligands and photocatalytic performance on the molecular scale. These results demonstrate that bulky ligands significantly hinder CO₂ reduction by cobalt complexes due to steric interference with coordination and active-site accessibility. This study demonstrates that the substituent effect of ligands on photocatalytic reactions for CO₂ conversion provides valuable insight into a deeper understanding of molecular catalysis.