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.