Catalytic Hydride Transfer to CO2 Using Ru-NAD-Type Complexes under Electrochemical Conditions.

The catalytic performance of Ru-NAD-type complexes [Ru(tpy)(pbn)(CO)]2+ ([1]2+; tpy = 2,2';6',2″-terpyridine; pbn = 2-(pyridin-2-yl)benzo[b][1,5]naphthyridine) and the Ru-CO-bridged metallacycle [2]+ was investigated in the context of the electrochemical reduction of CO2 in H2O/CH3CN at room temperature. A controlled-potential electrolysis of [1]2+ and [2]+ afforded formate (HCOO-) as the main product, under concomitant formation of minor amounts of CO and H2. Metallacycle [2]+ showed a higher selectivity toward the formation of HCOO- than [1]2+ (HCOO-/CO for [1]2+, 2.7; HCOO-/CO for [2]+, 7). The generation of HCOO- via a catalytic hydride transfer from the NADH-type ligands of [1]2+ and [2]+ to CO2 was supported by the experimental results and a comparison with the reduction of CO2 catalyzed by [Ru(tpy)(bpy)(CO)]2+ under similar conditions. A mechanism for the catalytic reduction of CO2 by [1]2+ and [2]+ was proposed based on the experimental evidence. The thus-obtained results may help to expand the field of NADH-assisted reduction reactions.