Overall reaction mechanism of photocatalytic CO 2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)-Re(i) supramolecular photocatalyst.

Rhenium(i) complexes fa c-[Re I (diimine)(CO) 3 (L)] n + are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO 2 reduction. However, the selective reduction mechanism of CO 2 to CO is unclear, although numerous mechanistic studies have been reported. A Ru(ii)-Re(i) supramolecular photocatalyst with fac -[Re I (diimine)(CO) 3 {OC(O)OCH 2 CH 2 NR 2 }] (R = C 2 H 4 OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO 2 reduction reactions. In this work, the reaction mechanism of photocatalytic CO 2 reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e. , fac -[Re I (diimine)(CO) 3 (COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac -[Re I (diimine˙ - )(CO) 3 {OC(O)OCH 2 CH 2 NR 2 }] - unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1 H -benzo[ d ]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH - was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate-ester complex with a fac -[Re I (diimine)(CO) 3 (COOC 2 H 4 NR 2 )] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac -[Re I (diimine)(CO) 4 ] + unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e. , RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).