Electronic effects on the mechanism of the NAD+ coenzyme reduction catalysed by a non-organometallic ruthenium(ii) polypyridyl amine complex in the presence of formate

Abstract

In the present study, electronic effects on the mechanism of the NAD⁺ coenzyme reduction in the presence of formate, catalysed by a non-organometallic ruthenium(II) polypyridyl amine complex, were investigated. The [Ru^II(terpy)(ampy)Cl]Cl (terpy = 2,2′:6′,2′′-terpyridine, ampy = 2-(aminomethyl)pyridine) complex was employed as the catalyst. The reactions were studied in a water/ethanol mixture as a function of formate, catalyst, and NAD⁺ concentrations at 37 °C. The overall process was found to be 11 to 18 times slower than for the corresponding ethylenediamine (en) complex as the result of π-back bonding effects of the ampy ligand. The mechanistic studies revealed a complete set of reactions that accounted for the overall catalytic cycle based on a formate-induced hydride transfer reaction to form the reduced coenzyme, NADH. The geometries of the ruthenium(II)-ampy complexes involved in the catalytic cycle and free energy changes for the main steps were predicted by DFT calculations. Similar calculations were also performed for the analogues ruthenium(II)-en and ruthenium(II)-bipy complexes (bipy = 2,2′-bipyridine). The DFT calculated energies show that both the solvent-formato exchange and the formato-hydrido conversion reactions have negative (favourable) energies to proceed spontaneously. The reactions involving the en complex have the more negative (favourable) reaction energies, followed by the ampy complex, in agreement with faster reactions for en complexes and slower reactions for bipy complexes than for ampy complexes.