Photoinduced four- and six-electron reduction of mononuclear ruthenium complexes having NAD+ analogous ligands

The ruthenium complexes [Ru(bpy)(pbn)2](PF6)2 ([2]2+; bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbn)3](PF6)2 ([3]2+) were synthesized. Photoirradiation (λ > 420 nm) of [2]2+ and [3]2+ in CH3CN/triethanolamine (TEOA) brought about proton coupled four- and six-electron reduction of the complexes to produce [Ru(bpy)(pbnH2)2](PF6)2 ([2·H4]2+; pbnH2 = 5,10-dihydro-2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbnH2)3](PF6)2 ([3·H6]2+), respectively. The photoexcited [RuIII(bpy)(pbn˙−)(pbnH2)]2+ intermediate is quenched by intermolecular electron transfer from TEOA to RuIII, while intramolecular transfer from pbnH2 to RuIII is negligible. As a result, novel photochemical four- and six-electron reduction of [2]2+ and [3]2+ is achieved through repetition of the two-electron reduction of the Ru-pbn group. The high efficiency photochemical two-, four- and six-electron reductions of [Ru(bpy)2(pbn)]2+ ([1]2+), [2]2+ and [3]2+, respectively, by taking advantage of proton coupled two electron reduction of NAD+ analogous type ligands such as pbn opens a general pathway for multi-electron reduction of metal complexes via illumination with visible light.