An Analytical Study of the Redox Behavior of 1,10-Phenanthroline-5,6-dione, its Transition-Metal Complexes, and its N-Monomethylated Derivative with regard to their Efficiency as Mediators of NAD(P)+ Regeneration

The synthesis as well as the chemical and electrochemical properties of homoleptic and heteroleptic (trispyridyl-methylamine as coligand) transition-metal complexes (Ru and Co) of 1,10-phenanthroline-5,6-dione (PD) and of its N-monomethylated derivative (PDMe+) are described. In particular, their ability to abstract hydride ions was studied. Electrochemical investigations with cyclic voltammetry, rotating disk electrode experiments, and spectroelectrochemical methods at different pH values gave an insight into the complex electrochemistry of the compounds described, which is strongly influenced by a hydration pre-equilibrium. The electrochemically active quinone form of the transition-metal complexes can be reduced to the hydroquinone state in acidic solution and to their transition-metal-stabilized semi-quinone states for neutral and basic solutions, whereas PDMe+ is reduced to the hydroquinone state in both acidic and neutral solutions. The compounds can also be reduced chemically, and are efficient catalysts for the indirect oxidation of the enzymatic cofactor NAD(P)H. For the indirect aerobic NAD(P)H oxidation, up to 900 turnovers per hour can be observed, an achievement yet to be reached by other catalyst systems.