Structure Function Relationships in Ruthenium Carbon Dioxide Reduction Catalysts with CNC Pincers Containing Donor Groups

Abstract

Seven ruthenium catalysts with the general formula [(CNC)Ru(CH3CN)2Cl]OTf have been used to understand structure function relationships in the sensitized photocatalytic CO2 reduction reaction. Herein, CNC is a pincer ligand containing imidazole-based N-heterocyclic carbenes (NHCs) attached to a central pyridyl ring with R groups at the 3- or 4-position. Two new complexes (R = 3-OMe, 4-NPh2) have been fully characterized by analytical and spectroscopic methods and single-crystal X-ray diffraction. Furthermore, three previously synthesized complexes (R = 4-Me, 4-NMe2, and 4-OH) are used for photocatalysis for the first time. Methoxy and methyl donor groups in the 4-position lead to durable catalysts (TON up to 1000) with high TOFs (up to 174 h–1) for CO production. In contrast, R = 4-NPh2, 3-OMe, and 4-H substituents lead to modest TOF values (up to 32 h–1), yet these systems are still robust and achieve up to 565 TON. In contrast, low activity is observed with 4-NMe2 and 4-OH substituents, which resemble sacrificial donors and may be leading to side reactions. Thus, remote substituents were found to greatly influence catalyst activity and longevity.