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‐NM₂, 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‐NP₂, 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.