Electrochemical Carbon Dioxide Reduction Catalyzed by a Dinuclear Ruthenium Complex with a Flexible Bridging Ligand

A dinuclear Ru complex, [{Ru(tpy)Cl}2(µ-L)](PF6)2 (1), was synthesized by the reaction of the flexible bridging ligand L (= N,N′-bis(5-bipyridylhexyl)-cis-1,4-cyclohexanediamine) with [Ru(tpy)Cl3]. The complex contained two [Ru(tpy)(bpy)Cl]+ moieties linked by L, and the reaction of 1 with AgNO3 allowed for isolation of the ligand-exchanged product [{Ru(tpy)(Solv.)}2(µ-L)](PF6)4 (2). The behavior of 2 as a CO2 reduction electrocatalyst was investigated in acetonitrile. Under N2, two reduction peaks appeared at Ep,c = −1.08 (tpy/tpy•−) and −1.40 V (bpy/bpy•−) vs. NHE. In a saturated CO2 solution, a further reduction peak, at Ep,c = −1.60 V, showed significant current enhancement consistent with electrocatalytic CO2 reduction. The primary product of the CO2 reduction was CO, and the turnover frequency was calculated to be ca. 1.4 s−1.