Electrochemical CO2 Reduction Using a New Ru-NAD Type Bis Carbonyl Complex

Abstract

A new ruthenium(II) NAD (NAD=Nicotinamide Adenine Dinucleotide) type bis-carbonyl complex, [Ru(bpy)(pbn)(CO)₂]²⁺ [pbn = 2-(pyridin-2-yl)benzo[b]-1,5-naphthyridine; bpy = 2,2′-bipyridine] [1]²⁺, was successfully synthesized and fully characterized by single-crystal X-ray structural analysis, ESI-MS, IR and NMR spectroscopy. Complex [1]²⁺ together with [Ru(bpy)(pbn)(CO)(COOH)]⁺ and/or [Ru(bpy)(pbn)(CO)(CO₂)]⁰ complexes exist as equilibrium mixtures in aqueous solutions as evident from spectroscopic study. Chemical reduction of [1]²⁺ resulted the formation corresponding NADH form i. e., [Ru(bpy)(pbnHH)(CO)₂]²⁺ (pbnHH = 2-(pyridin-2-yl)-5,10-dihydrobenzo[b][1,5]naphthyridine) [1.HH]²⁺ as a two-electron-reduced species. The electrochemical behavior of complex [1]²⁺ in the presence of acid was investigated based on cyclic voltammetry analysis. A control potential electrolysis of [1]²⁺ afforded formate (HCOO⁻) as the major product with a lesser amount of CO and H₂, whereas that of [Ru(bpy)₂(CO)₂]²⁺ complex produced CO as the main product with a lesser amount of HCOO⁻ and H₂. The experimental results suggest that the selectivity of HCOO⁻ over CO should be due to catalytic hydride transfer from the NADH-type ligands of [1]²⁺ to CO₂.