Efficient Self-Sensitized Photochemical CO2 Reduction Using [Re(bpy2+)(CO)3(I)]2+ and [Re(bpy2+)(CO)3(CH3CN)]3+ Photocatalysts with Pendent Ammonium Cations

Rhenium(I) tricarbonyl complexes fac-[Re(bpy)(CO)3(L)]n+ are the classic examples of self-sensitized photocatalysts capable of the dual roles of light absorption and catalysis. In this work, a series of dicationic halido or tricationic solvento complexes fac-[Re(bpy2+)(CO)3X]n+ (PF6)n (where X = Cl– or I– (n = 2), or CH3CN (n = 3) and bpy2+ is bipyridine modified by two −CH2–(NMe3)+ tetra-alkylammonium cations) have been investigated as self-sensitized and sensitized CO2 reduction photocatalysts. Four structural isomers differing in the cation position have been tested in N,N′-dimethylacetamide solvent (DMA) using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole (BIH) as the electron donor, and the position of the cationic pendants has a significant impact on the catalyst turnover number and quantum efficiency (ϕ). Up to 455 self-sensitized turnovers of CO and a high photon efficiency (ϕCO) of 22% have been achieved. Time-resolved infrared spectroscopy and theoretical calculations were used to characterize the catalytic cycle including the ligand exchange between one-electron reduced (OER) halido and solvento species as well as the binding of CO2 to the putative two-electron reduced (TER) species. The CO2-reactive TER catalyst was formed by disproportionation or intramolecular electron transfer between two forms of the OER catalyst as indicated by the formation of the fully oxidized catalyst concurrent with CO2 binding. When [Ru(bpy)3]2+ was used as a sensitizer, catalyst durability improved, and the selectivity toward formate increased as high as 3.3:1 over CO (total TON = 1370) due to acidification of the reaction, which promotes formation of the hydride intermediate, as BIH was consumed and deprotonated.