Redox Mediator Effect on Water Oxidation in a Ruthenium-Based Chromophore–Catalyst Assembly

The synthesis, characterization, and redox properties
are described
for a new ruthenium-based chromophore–catalyst assembly, [(bpy)2Ru­(4-Mebpy-4′-bimpy)­Ru­(tpy)­(OH2)]4+ (1, [RuaII-RubII-OH2]4+; bpy = 2,2′-bipyridine;
4-Mebpy-4′-bimpy = 4-(methylbipyridin-4′-yl)-N-benzimid-N′-pyridine; tpy = 2,2′:6′,2″-terpyridine),
as its chloride salt. The assembly incorporates both a visible light
absorber and a catalyst for water oxidation. With added ceric ammonium
nitrate (CeIV, or CAN), both 1 and 2, [Ru­(tpy)­(Mebim-py)­(OH2)]2+ (Mebim-py = 2-pyridyl-N-methylbenzimidazole), catalyze water oxidation. Time-dependent
UV/vis spectral monitoring following addition of 30 equiv of CeIV reveals that the rate of CeIV consumption is
first order both in CeIV and in an oxidized form of the
assembly. The rate-limiting step appears to arise from slow oxidation
of this intermediate followed by rapid release of O2. This
is similar to isolated catalyst 2, with redox potentials
comparable to the [-Rub-OH2]2+ site
in 1, but 1 is more reactive than 2 by a factor of 8 due to a redox mediator effect.