Covalent Immobilization of (2,2′-Bipyridyl) (Pentamethylcyclopentadienyl)-Rhodium Complex on a Porous Carbon Electrode for Efficient Electrocatalytic NADH Regeneration

Covalent bonding of (2,2'-bipyridyl) (pentamethylcyclopentadienyl)-rhodium complex at the surface of a carbon-based porous electrode was achieved by combining diazonium electrografting, Huisgen cycloaddition, and metal complexation. The immobilized catalyst was applied to electrochemical regeneration of the reduced form of nicotinamide adenine dinucleotide (NADH). The different steps of surface functionalization were characterized by X-ray photoelectron spectroscopy and electrochemistry. The Faradaic efficiency of NADH regeneration was 87%. The chemical bonding provided good stability in solution under convection over 14 days, which is much better than the simple adsorption of the Rh complex on the electrode surface. Finally, the system was tested in the presence of NAD-dependent dehydrogenases that were immobilized in a sol gel film on the top of the functionalized porous carbon electrode. A total turnover of 3790 and turnover frequency of 164 h(-1) was observed. Two enzymatic reactions were considered: D-fructose reduction to D-sorbitol catalyzed by D-sorbitol dehydrogenase and hydroxyacetone reduction to 1,2-propanediol with galactitol dehydrogenase. The electrocatalytic regeneration of 1 mM NADH by the immobilized rhodium species was kept after 90 h electrolysis in the conditions of electroenzymatic synthesis.