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.

本研究报道了一种新型钌基发色团-催化剂组装体（ruthenium-based chromophore–catalyst assembly）的合成、表征及氧化还原性质，该组装体以氯化物盐形式存在，化学式为[(bpy)₂Ru(4-Mebpy-4′-bimpy)Ru(tpy)(OH₂)]⁴⁺（化合物1，[RuaII-RubII-OH2]⁴⁺；其中bpy为2,2′-联吡啶（2,2′-bipyridine），4-Mebpy-4′-bimpy为4-(甲基联吡啶-4′-基)-N-苯并咪唑-N′-吡啶，tpy为2,2′:6′,2″-三联吡啶（2,2′:6′,2″-terpyridine））。该组装体同时整合了可见光吸收单元与水氧化催化位点。在添加硝酸铈铵（ceric ammonium nitrate，Ce⁴⁺，简称CAN）的体系中，化合物1与化合物2（[Ru(tpy)(Mebim-py)(OH₂)]²⁺，其中Mebim-py为2-吡啶基-N-甲基苯并咪唑（2-pyridyl-N-methylbenzimidazole））均可催化水氧化反应。向体系中加入30倍当量的CAN后，通过时间分辨紫外-可见（UV/vis）光谱监测结果显示，Ce⁴⁺的消耗速率对Ce⁴⁺与组装体的氧化态形式均呈一级动力学。该过程的限速步骤推测为该氧化中间体的慢速氧化过程，随后伴随氧气的快速释放。这一反应机制与单独分离得到的催化剂2相似，二者的氧化还原电位与化合物1中的[-Rub-OH2]²⁺位点相当，但得益于氧化还原介导效应（redox mediator effect），化合物1的催化活性为催化剂2的8倍。