Differences of pH-Dependent Mechanisms on Generation of Hydride Donors using Ru(II) Complexes Containing Geometric Isomers of NAD+ Model Ligands: NMR and Radiolysis Studies in Aqueous Solution

The pH-dependent mechanism of the reduction of the nicotinamide adenine dinucleotide (NADH) model complex [Ru(bpy)2(5)]2+ (5 = 3-(pyrid-2′-yl)-4-azaacridine) was compared to the mechanism of the previously studied geometric isomer [Ru(bpy)2(pbn)]2+ (pbn = 2-(pyrid-2′-yl)-1-azaacridine, previously referred to as 2-(pyrid-2′-yl)-benzo[b]-1,5-naphthyridine) in aqueous media. The exposure of [Ru(bpy)2(5)]2+ to CO2 •− leads to the formation of the one-electron reduced species (k = 4.4 × 109 M−1 s−1). At pH < 11.2, the one-electron reduced species can be protonated, k = 2.6 × 104 s−1 in D2O. Formation of a C−C bonded dimer is observed across the pH range of 5−13 (k = 4.5 × 108 M−1 s−1). At pH < 11, two protonated radical species react to form a stable C−C bonded dimer. At pH > 11, dimerization of two one-electron reduced species is followed by disproportionation to one equivalent starting complex [Ru(bpy)2(5)]2+ and one equivalent [Ru(bpy)2(5HH)]2+. The structural difference between [Ru(bpy)2(pbn)]2+ and [Ru(bpy)2(5)]2+ dictates the mechanism and product formation in aqueous medium. The exchange of the nitrogen and carbon atoms on the azaacridine ligands alters the accessibility of the dimerization reactive site, thereby changing the mechanism and the product formation for the reduction of the [Ru(bpy)2(5)]2+ compound.