Ligand effects in rhodium complexes for chemical NADH regeneration

Various (pentamethylcyclopentadienyl)-rhodium(III) complexes comprising aromatic bidentate ligands [(Cp*)Rh(L)Cl]⁺ (Cp* = pentamethylcyclopentadienyl, L = 2,2′-bipyridine, 1,10-phenanthroline, and their derivatives) were prepared to compare their reactivities of chemical cofactor regeneration. When the catalytic NADH regeneration was performed with sodium formate⁻, the reaction rates were compared with six Rh(III) complexes. The kinetics of the reactions including kinetic isotope effects are also studied in the chemical NADH regeneration. The Rh(III) complexes react with formate efficiently to afford the intermediates [(Cp*)Rh(L)(H)]⁺ and CO₂. The electronic and steric effects of the ligands of the Rh complexes were observed on the reaction rates. The overall reaction rates of the NADH regeneration were also obtained using DCOO⁻ in H₂O. Such H/D exchange rates of [(Cp*)Rh(L)(H)]⁺ and the observation of a deuterium kinetic isotope provide valuable mechanistic insight into the catalytic NADH regeneration.