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–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 H2Q proceeds by one-step electron-proton transfer (EPT), rather than sequential EPT. Such a reaction was not observed for *[Ru(2)]2+ because the non-coordinated N atom is not readily 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.
Last Modified: September 13, 2012