Visible-Light-Driven Photocatalytic CO2 Reduction by Re(I) Photocatalysts with N-Heterocyclic Substituents

The properties and reactivity of metal complex photocatalysts for solar-to-fuel conversion can be conveniently controlled by the molecular design of their ligands. In this work, rhenium(I) tricarbonyl polypyridyl compounds, fac- [Re(NN)(CO)3Cl], where NN is a 1,10-phenanthroline ligand having pyrrole (Re-pyr), indole (Re-ind), or carbazole (Re-cbz) groups attached to its 4 and 7 positions, were investigated as photocatalysts for reducing CO2 to CO. These complexes can harvest light in the visible region and displayed an exceptionally high photocatalytic performance, with an up to 10-fold increase in the TONCO values compared to the unsubstituted Re-phen parental complex. Re-pyr was the best performing photocatalyst in this series, with a TONCO of 125 after 24 h and phi CO = 0.22. The TONCO of Re-pyr, Re-ind, and Re-cbz surpassed even the most efficient Lehn-type photocatalysts reported to date to promote the photoreduction of CO2 using triethanolamine as the sacrificial electron donor, without a second molecule working as a light absorber. The excellent photocatalytic performance of this series was intimately related to the presence of N-heterocyclic substituents, which enhanced the visible-light absorption properties of the compounds, favored the kinetics and thermodynamics of each individual step of the photocatalysis, inhibited deactivation of the photocatalysts by dimerization, stabilized reduced intermediates, and unlocked an alternative CO2 photoreduction pathway that proceeds predominantly through a two-electron reduced species [Re(NN)(CO)3]-.

Automated summary

The properties and reactivity of metal complex photocatalysts for solar-to-fuel conversion can be controlled by the molecular design of their ligands. In this study, rhenium(I) tricarbonyl polypyridyl compounds with different N-heterocyclic ligands were investigated as photocatalysts for reducing CO2 to CO. These complexes showed exceptional photocatalytic performance, with significantly increased TONCO values compared to the unsubstituted parental complex. The presence of N-heterocyclic substituents enhanced the visible-light absorption properties, favored the kinetics and thermodynamics of the photocatalysis, inhibited deactivation of the photocatalysts, and unlocked an alternative CO2 photoreduction pathway.