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

Abstract

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–C₁ 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 (H₂Q) 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 H₂Q. 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.