New Ir Bis-Carbonyl Precursor for Water Oxidation Catalysis

This paper introduces IrI(CO)2(pyalc) (pyalc = (2-pyridyl)-2-propanoate) as an atom-efficient precursor for Ir-based homogeneous oxidation catalysis. This compound was chosen to simplify analysis of the water oxidation catalyst species formed by the previously reported Cp*IrIII(pyalc)­OH water oxidation precatalyst. Here, we present a comparative study on the chemical and catalytic properties of these two precursors. Previous studies show that oxidative activation of Cp*Ir-based precursors with NaIO4 results in formation of a blue IrIV species. This activation is concomitant with the loss of the placeholder Cp* ligand which oxidatively degrades to form acetic acid, iodate, and other obligatory byproducts. The activation process requires substantial amounts of primary oxidant, and the degradation products complicate analysis of the resulting IrIV species. The species formed from oxidation of the Ir­(CO)2(pyalc) precursor, on the other hand, lacks these degradation products (the CO ligands are easily lost upon oxidation) which allows for more detailed examination of the resulting Ir­(pyalc) active species both catalytically and spectroscopically, although complete structural analysis is still elusive. Once Ir­(CO)2(pyalc) is activated, the system requires acetic acid or acetate to prevent the formation of nanoparticles. Investigation of the activated bis-carbonyl complex also suggests several Ir­(pyalc) isomers may exist in solution. By 1H NMR, activated Ir­(CO)2(pyalc) has fewer isomers than activated Cp*Ir complexes, allowing for advanced characterization. Future research in this direction is expected to contribute to a better structural understanding of the active species. A diol crystallization agent was needed for the structure determination of 3.本研究介绍了IrI(CO)2(pyalc)（其中pyalc代表(2-吡啶基)-2-丙酸酯，英文全称为(2-pyridyl)-2-propanoate）作为铱基均相氧化催化的原子经济性前驱体。选择该化合物旨在简化对此前报道的Cp*（五甲基环戊二烯基）IrIII(pyalc)OH水氧化预催化剂所形成的水氧化催化剂物种的分析工作。本文对这两种前驱体的化学与催化性能展开了对比研究。 既往研究表明，采用高碘酸钠（NaIO4）对基于Cp*Ir的前驱体进行氧化活化时，会生成蓝色的四价铱（IrIV）物种。该活化过程伴随占位配体Cp*的脱除，Cp*经氧化降解生成乙酸、碘酸盐及其他必然副产物。此活化过程需消耗大量一级氧化剂，且降解产物会干扰对所生成IrIV物种的分析工作。 相较之下，由Ir(CO)2(pyalc)前驱体氧化所生成的物种则无此类降解产物（CO配体在氧化过程中易于脱除），这使得我们能够从催化与光谱学角度对所得Ir(pyalc)活性物种开展更细致的研究，尽管目前仍难以完成完整的结构解析。 Ir(CO)2(pyalc)经活化后，体系需添加乙酸或乙酸盐以防止纳米颗粒形成。对活化后的二羰基配合物的研究还表明，溶液中可能存在多种Ir(pyalc)异构体。通过氢核磁共振波谱（1H NMR）分析可知，活化后的Ir(CO)2(pyalc)所存在的异构体数量少于活化后的Cp*Ir配合物，从而支持了更深入的表征工作。 该方向的后续研究有望助力我们更清晰地理解活性物种的结构。本次研究中，为解析化合物3的结构，需使用二醇类结晶剂。