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

• Published in: Journal of the American Chemical Society 2013-02, Vol.135 (6), p.2080-2083
• Main Authors: Norris, Michael R, Concepcion, Javier J, Harrison, Daniel P, Binstead, Robert A, Ashford, Dennis L, Fang, Zhen, Templeton, Joseph L, Meyer, Thomas J
• Format: Article
• Language: English
• Subjects: Catalysis; Crystallography, X-Ray; Models, Molecular; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Oxidation-Reduction; Ruthenium - chemistry; Water - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja311645d

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, [Rua II-Rub II-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.