Steric effect for proton, hydrogen-atom, and hydride transfer reactions with geometric isomers of NADH-model ruthenium complexes

Two isomers, [Ru(1)](2+) (Ru = Ru(bpy)(2), bpy = 2,2'-bipyridine, 1 = 2-(pyrid-2'-yl)-1-azaacridine) and [Ru(2)](2+) (2 = 3-(pyrid-2'-yl)-4-azaacridine), are bio-inspired model compounds containing the nicotinamide functionality and can serve as precursors for the photogeneration of C-H hydrides for studying reactions pertinent to the photochemical reduction of metal-C-1 complexes and/or carbon dioxide. While it has been shown that the structural differences between the azaacridine ligands of [Ru(1)](2+) and [Ru(2)](2+) have a significant effect on the mechanism of formation of the hydride donors, [Ru(1HH)](2+) and [Ru(2HH)](2+), in aqueous solution, we describe the steric implications for proton, net-hydrogen-atom and net-hydride transfer reactions in this work. Protonation of [Ru(2(center dot-))](+) in aprotic and even protic media is slow compared to that of [Ru(1(center dot))](+). The net hydrogen-atom transfer between *[Ru(1)](2+) and hydroquinone (H(2)Q) proceeds by one-step EPT, rather than stepwise electron-proton transfer. Such a reaction was not observed for *[Ru(2)](2+) because the non-coordinated N atom is not easily available for an interaction with H(2)Q. Finally, the rate of the net hydride ion transfer from [Ru(1HH)](2+) to [Ph3C](+) is significantly slower than that of [Ru (2HH)](2+) owing to steric congestion at the donor site.