Synthesis and Characterization of a RuPt-Based PhotoHydrogen-Evolving Molecular Device Tethered to a Single Viologen Acceptor

A dinuclear (RuPtII)-Pt-II complex tethered to a methyl viologen (MV2+) unit {RuPt-MV2+; [Ru-II(bpy)2{phen-bpy(MV2+)}Pt(II)Cl2](4+), bpy = 2,2'-bipyridine, phen = 5-amino-1,10-phenanthroline} has been synthesized to investigate the effect of tethering a single pendant MV2+ acceptor on its photohydrogen-evolving (PHE) activity. Upon photoirradiation of RuPt-MV2+ in the presence of EDTA as a sacrificial electron donor, the one-electron-reduced species of RuPt-MV2+ {i.e., [RuII(bpy) 2{phen-bpy(MV+.)} PtIICl2](3+)} is generated as a major photoproduct by intramolecular electron transfer (IET) from the (MLCT)-M-3 excited state of the [RuII(bpy) 2(phen)](2+) unit to the MV2+ unit, followed by subsequent reduction of the [Ru-II(bpy) 2(phen)](3+) unit by EDTA. Although RuPt-MV2+ shows PHE activity in the presence of EDTA, the estimated turnover number (0.4) is smaller than that for the parent RuIIPtII system, which has no MV2+ tether. The relatively small driving force (0.08 eV) for the IET from the (MLCT)-M-3 excited state of the [Ru-II(bpy)(2)(phen)](2+) unit to the (PtCl2)-Cl-II(bpy) unit, which leads to the H-2 formation, has been judged to be the major cause of the exceptionally low PHE activity observed for the present system.