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Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

机译：探究铱催化氧演化过程中氧耦合途径的可行性

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A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine–pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.
机译：合成了一系列的Cp * Ir III 二聚体，以阐明自由基催化的氧耦合途径在铱催化的O2演化中的机理。已经研究了前体对纳米颗粒形成的氧化稳定性及其氧释放活性，并将其与合适的单体类似物进行了比较。我们发现带有单齿NHC配体的前体降解形成纳米颗粒（NPs），因此它们的O2释放速率不受二聚体前体中它们的核数或两种金属之间的距离的显着影响。双重螯合的双吡啶-吡唑化物配体提供了抗氧化的配体骨架，从而可以更有意义地比较二聚体及其相应单体的催化性能。使用高碘酸钠（NaIO4）作为氧化剂，二聚体每[Ir]的O2析出速率明显低于单体，表明在催化循环中相互作用为负而不是协同作用。对二聚体的电化学分析进一步证实了没有自由基可与氧偶联的途径。因此，我们得出结论，替代途径，即水对高价Ir-氧代物种的亲核攻击，可能是这些催化剂进行水氧化的首选机理，而双分子氧代耦合不是相关钌中的有效机理替代品。化学，至少在本系统中。

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