Tuning the Photocatalytic CO2 Reduction through para-Substituents in Bipyridyl Rhenium Complexes.

Six fac-[Re-(NN)-(CO)3Cl] complexes, where NN is a 2,2'-bipyridine ligand with systematically varied electron-donating or -withdrawing groups at the 4,4' positions, were explored as photocatalysts for CO2 reduction to CO. The light absorption, redox potentials, and excited-state dynamics of the complexes, as well as the reactivity and stability of key catalysis intermediates, were correlated with the Hammett constants (σp) of the substituents, revealing their impact on the photocatalytic activity. Electron-donating substituents, such as -OCH3 and -CH3, resulted in slower excited-state quenching by sacrificial electron donors in comparison to the unsubstituted complex, but their corresponding one-electron-reduced species (OERS) reacted quickly with CO2. On the other hand, electron-withdrawing substituents, such as -Br, -COOH, and -CO2CH3, resulted in more favorable and faster reductive quenching, but at the cost of slower reactivity of the OERS. A comparison of the catalytic activity of the complexes employing two different sacrificial electron donors displayed how the interplay of these factors affects the CO2 reduction performance and how photocatalysis can be controlled by strategic manipulation of both the ligand environment and the reaction conditions.