CO2 photoreduction with rhenium based porphyrin sensitized homogeneous two-component systems

Rhenium diimine tricarbonyl complexes bearing bromide, acetonitrile, 3–picoline or triethylphosphite have been synthesized and employed for selective photocatalytic CO2 conversion into carbon monoxide. To avoid employing high–energy radiation, multicomponent visible light sensitized systems have been developed combining zinc(II) meso‒tetrasubstituted porphyrins with the rhenium catalysts. Electron transfer from the sensitizer to the metal complexes was suspected to be a major controlling element and chemical variability was exploited by inclusion of electron donating groups on the sensitizer, employing porphyrins with phenyl, tolyl, tert‒butyl or methoxyphenyl groups and electron withdrawing substituents on the metal complexes, using 2,2’‒bipyridine and 4,4’‒bis(methoxycarbonyl)‒2,2’‒bipyridine (BMCbpy), spanning a wide range of electron transfer driving force values aiming to favor this step. This project was focused on the study of the performance of the different combinations and the understanding of the underlying chemical and thermodynamic factors which control it.
Mixtures with BMCbpy derivatives were found to be essentially non–active, whereas bipyridine complexes with picoline or acetonitrile formed the most active combinations with all porphyrins. These multicomponent systems have proved to be as efficient as similar supramolecular dyads, even better in some cases. We have gained an insight into their catalytic behavior, showing that the chemical nature of the catalyst and not the sensitizer or the electron transfer driving force controls the life span and the catalytic ability of the two component mixtures. Mechanistic information has also been obtained, indicating that chlorin, a reduced species of the parent porphyrin, needs to be present in order to achieve efficient catalysis. Porphyrin decomposition and chlorin formation seems to be strongly correlated with the structure of the catalyst employed. In addition, porphyrin decomposition only occurs when chlorin is present and its kinetics depends on the photoinduced electron transfer driving force.