Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands

Two series of ruthenium complexes with various polypyridyl ligands have been prepared. One series of complexes (5 examples) are featured with tetradentate polypyridyl ligands and two acetonitrile molecules at the axial positions of the coordination sphere; the other series (3 examples) include combinations of a tridentate polypyridyl ligand, one 2,2 '-bipyridine (bpy) or two picolines, and one acetonitrile ligand. All these complexes were fully characterized by their NMR spectra as well as X-ray single crystal structures. Their electronic absorption and redox data were measured and reported. Of the 8 complexes, three candidates effectively catalyze electrochemical CO2 reduction reaction (CO2RR) in wet acetonitrile medium, generating CO as the major product. All these three catalytically active complexes contain a 2,2 ':6 ',2 '':6 '',2 '''-quaterpyridine (qpy) ligand scaffold. A maximum turnover frequency (TOFmax) of>1000 s(-1) was achieved for the electrocatalytic CO2 reduction at a modest overpotential. On the basis of electrochemical and spectroelectrochemical evidences, the CO2 substrate was proposed to bind with the ruthenium center at the two-electron reduced state of the complex and then entered the catalytic cycle.