Evidence for Oxidative Decay of a Ru-Bound Ligand during Catalyzed Water Oxidation

In the evaluation of systems designed for catalytic water oxidation, ceric ammonium nitrate (CAN) is often used as a sacrificial electron acceptor. One of the sources of failure for such systems is oxidative decay of the catalyst in the presence of the strong oxidant CAN (Eox = +1.71 V). Little progress has been made in understanding the circumstances behind this decay. In this study we show that a 2-(2′-hydroxphenyl) derivative (LH) of 1,10-phenanthroline (phen) in the complex [Ru­(L)­(tpy)]+ (tpy = 2,2′;6′,2″-terpyridine) can be oxidized by CAN to a 2-carboxy-phen while still bound to the metal. This complex is, in fact, a very active water oxidation catalyst. The incorporation of a methyl substituent on the phenol ring of LH slows down the oxidative decay and consequently slows down the catalytic oxidation. An analogous system based on bpy (2,2′-bipyridine) instead of phen shows much lower activity under the same conditions. Water molecule association to the Ru center of [Ru­(L)­(tpy)]+ and carboxylate donor dissociation were proposed to occur at the trivalent state. The resulting [RuIII–OH2] was further oxidized to [RuIVO] via a PCET process.在催化水氧化体系的评估研究中，硝酸铈铵（ceric ammonium nitrate, CAN）常被用作牺牲性电子受体。此类体系的失效诱因之一，便是在强氧化剂CAN（氧化电位Eox = +1.71 V）存在的条件下，催化剂发生氧化降解；目前学界对该降解过程的具体成因仍缺乏深入认知，相关研究进展甚微。本研究证实，配合物[Ru(L)(tpy)]+（其中tpy为2,2′;6′,2″-三联吡啶，2,2′;6′,2″-terpyridine）中1,10-邻菲啰啉（1,10-phenanthroline, phen）的2-(2'-羟基苯基)衍生物（LH），可在仍与金属中心配位的状态下被CAN氧化为2-羧基邻菲啰啉，而该配合物本身便是一种活性极高的水氧化催化剂。若在LH的酚环上引入甲基取代基，可延缓其氧化降解进程，进而降低催化氧化反应的速率；以2,2'-联吡啶（2,2′-bipyridine, bpy）替代邻菲啰啉构建的类似体系，在相同实验条件下的催化活性要低得多。研究提出，[Ru(L)(tpy)]+的Ru中心与水分子配位、以及羧酸根配体解离的过程，均发生于三价钌氧化态阶段；所得中间体[RuIII–OH2]可通过质子耦合电子转移（Proton-Coupled Electron Transfer, PCET）过程进一步被氧化为[RuIVO]。