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

The ruthenium complexes [Ru(bpy)(pbn)(2)](PF(6))(2) ([2](2+); bpy = 2,2'-bipyridine, pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbn)(3)](PF(6))(2) ([3](2+)) were synthesized. Photoirradiation (λ > 420 nm) of [2](2+) and [3](2+) in CH(3)CN/triethanolamine (TEOA) brought about proton coupled four- and six-electron reduction of the complexes to produce [Ru(bpy)(pbnH(2))(2)](PF(6))(2) ([2·H(4)](2+); pbnH(2) = 5,10-dihydro-2-(2-pyridyl)benzo[b]-1,5-naphthyridine) and [Ru(pbnH(2))(3)](PF(6))(2) ([3·H(6)](2+)), respectively. The photoexcited [Ru(III)(bpy)(pbn˙(-))(pbnH(2))](2+) intermediate is quenched by intermolecular electron transfer from TEOA to Ru(III), while intramolecular transfer from pbnH(2) to Ru(III) 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.