Hydrogenative Carbon Dioxide Reduction Catalyzed by Mononuclear Ruthenium Polypyridyl Complexes: Discerning between Electronic and Steric Effects

The preparation and isolation of a family of Ru-Cl complexes containing the deprotonated anionic tridentate meridional ligand (1Z,3Z)-N-1,N-3-di(pyridin-2-yl)isoindoline-1,3-diimine (Hbid) and 1,3-di(2-pyridyl)benzene) (Hdpb), namely, [Ru(bid)(acac)Cl], 1d, [Ru(bid)(6,6'-Me-2-bpy)Cl], 1e, trans-[Ru(bid)(py)(2)Cl], 2, [Ru(dpb)(bpy)Cl], 3a, and [Ru(dpb)(4,4'-(COOEt)(2)-bpy)Cl], 3b, are reported. All these complexes have been thoroughly characterized in solution by NMR spectroscopy and for 1d and 1e by single-crystal X-ray diffraction analysis. Furthermore, the redox properties of all complexes have been investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The capacity of the various complexes to catalyze hydrogenative CO2 reduction was also investigated. Compound le is the best catalyst, achieving initial turnover frequencies above 1000 h(-1). Kinetic analysis identifies a relationship between Ru(III/II) couple redox potentials and initial turnover frequencies. Finally, DFT calculations further characterize the catalytic cycle of these complexes and rationalize electronic and steric effects deriving from the auxiliary ligands.