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 ^•-)] ⁺ in aprotic and even protic media is slow compared to that of [Ru(1 ^•-)] ⁺. The net hydrogen-atom transfer between *[Ru(1)] ²⁺ and hydroquinone (H ₂Q) 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 ₂Q. Finally, the rate of the net hydride ion transfer from [Ru(1HH)] ²⁺ to [Ph ₃C] ⁺ is significantly slower than that of [Ru(2HH)] ²⁺ owing to steric congestion at the donor site.