Highly Active Ruthenium CNC Pincer Photocatalysts for Visible-Light-Driven Carbon Dioxide Reduction

Five ruthenium catalysts described
herein facilitate self-sensitized carbon dioxide reduction to form
carbon monoxide with a ruthenium catalytic center. These catalysts
include four new and one previously reported CNC pincer complexes
featuring a pyridinol derived N-donor and N-heterocyclic carbene (NHC) C-donors derived from imidazole
or benzimidazole. The complexes have been characterized fully by spectroscopic
and analytic methods, including X-ray crystallography. Introduction
of a 2,2′-bipyridine (bipy) coligand and phenyl groups on the
NHC ligand was necessary for rapid catalysis. [(CNC)­Ru­(bipy)­(CH3CN)]­(OTf)2 is among the most active and durable
photocatalysts in the literature for CO2 reduction without
an external photosensitizer. The role of the structure of this complex
in catalysis is discussed, including the importance of the pincer’s
phenyl wingtips, the bipyridyl ligand, and a weakly coordinating monodentate
ligand.