Sensitized and Self-Sensitized Photocatalytic Carbon Dioxide Reduction Under Visible Light with Ruthenium Catalysts Shows Enhancements with More Conjugated Pincer Ligands

A new method to synthesize complexes of the type [(CNC)Ru-II(NN)L](n+) has been introduced, where CNC is a tridentate pincer composed of two (benz)imidazole derived NHC rings and a pyridyl ring, NN is a bidentate aromatic diimine ligand, L=bromide or acetonitrile, and n=1 or 2. Following this new method a series of six new complexes has been synthesized and characterized by spectroscopic, analytic, crystallographic, and computational methods. Their electrochemical properties have been studied via cyclic voltammetry under both N-2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO was performed using these complexes both in the presence (sensitized) and absence (self-sensitized) of an external photosensitizer. This study evaluates the effect of different CNC, NN, and L ligands in sensitized and self-sensitized photocatalysis. Catalysts bearing the benzimidazole derived CNC pincer show much better activity for both sensitized and self-sensitized photocatalysis as compared to catalysts bearing the imidazole derived CNC pincer. Furthermore, self-sensitized photocatalysis requires a diimine ligand for CO2 reduction with catalyst 2(ACN) being the most active catalyst in this series with TON=85 and TOF=22 h(-1) with an electron donating 4,4'-dimethyl-2,2'-bipyridyl (dmb) ligand and a benzimidazole derived CNC pincer.

Automated summary

A new method for synthesizing complexes of the type [(CNC)Ru-II(NN)L](n+) has been introduced and a series of six new complexes have been characterized. The study shows that catalysts with benzimidazole derived CNC pincers exhibit better activity in photocatalysis, and a diimine ligand is required for self-sensitized photocatalysis.