Sinha Woormileela, Mizrahi Amir, Mahammed Atif, Tumanskii Boris, Gross Zeev
Schulich Faculty of Chemistry, Technion-Israel Institute of Technology , Haifa 32000, Israel.
Chemistry Department, Nuclear Research Centre Negev , Beer-Sheva 84190, Israel.
Inorg Chem. 2018 Jan 2;57(1):478-485. doi: 10.1021/acs.inorgchem.7b02696. Epub 2017 Dec 19.
A detailed investigation of the cobalt corrole Co(tpfc) as molecular catalyst for electrochemical water oxidation uncovered many important mechanism-of-action details that are crucial for the design of optimally performing systems. This includes the identification of the redox states that do and do not participate in catalysis and very significant axial ligand effects on the activity of the doubly oxidized complex. Specifics deduced for the electrocatalysis under homogeneous conditions include the following: the one-electron oxidation of the cobalt(III) corrole is completely unaffected by reaction conditions; catalysis coincides with the second oxidation event; two catalytic waves develop in the presence of OH, and the one at lower overpotential is dominant under more basic conditions. Comparative spectroelectrochemical measurements performed for Co(tpfc) and Al(tpfc), the analogous corrole chelated by the nonredox active aluminum, revealed that the second oxidation process of Co(tpfc) is much more significantly metal-centered than the first one. EPR studies revealed that shift from fully corrole-centered to partially metal-centered in the singly oxidized complex [Co(tpfc)] is achievable with fluoride as axial ligand. The analogous experiment, but with hydroxide instead of fluoride, could not be performed because of a surprising phenomenon: formation of a cobalt-superoxide complex that is actually relevant to oxygen reduction rather than to water oxidation. Nevertheless, fluoride and hydroxide induce very similar effects in terms of the appearance of two catalytic waves, lowering of onset potentials, and increasing the catalytic activity. The main conclusions from the accumulated data are that the apparent pH effect is actually due to hydroxide binding to the cobalt center and that π-donating axial ligands play pivotal and beneficial roles regarding the main factors that are important for facilitating the oxidation of water.
对钴卟啉Co(tpfc)作为电化学水氧化分子催化剂的详细研究揭示了许多重要的作用机制细节，这些细节对于设计性能最优的系统至关重要。这包括确定参与和不参与催化的氧化还原状态，以及双氧化配合物活性上非常显著的轴向配体效应。在均相条件下电催化得出的具体情况如下：钴(III)卟啉的单电子氧化完全不受反应条件影响；催化作用与第二次氧化事件同时发生；在OH存在下会出现两个催化波，在碱性更强的条件下，过电位较低的那个催化波占主导。对Co(tpfc)和Al(tpfc)（由非氧化还原活性铝螯合的类似卟啉）进行的比较光谱电化学测量表明，Co(tpfc)的第二次氧化过程比第一次氧化过程更以金属为中心。电子顺磁共振研究表明，以氟化物作为轴向配体时，单氧化配合物[Co(tpfc)]中可实现从完全以卟啉为中心向部分以金属为中心的转变。由于一个令人惊讶的现象，即形成了实际上与氧还原而非水氧化相关的钴超氧化物配合物，所以无法进行用氢氧化物代替氟化物的类似实验。然而，就出现两个催化波、降低起始电位以及提高催化活性而言，氟化物和氢氧化物诱导的效果非常相似。积累的数据得出的主要结论是，明显的pH效应实际上是由于氢氧化物与钴中心结合，并且π供体轴向配体在促进水氧化的重要主要因素方面发挥着关键且有益的作用。
