Mixed-Metal Hybrid Polyoxometalates with Amino Acid Ligands: Electronic Versatility and Solution Properties

Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal–oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal–oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled MoV centers in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal–oxo cores.

研究人员合成了8种带有氨基酸配体的新型混合金属（Mo、W）多金属氧酸盐（polyoxometalates, POMs）家族成员，并借助多种物理表征技术对其固态与溶液态的结构与性质展开了系统性研究。尽管此类POM的外围结构骨架保持恒定，不同的同系物在中心金属氧簇核的核数、整体氧化还原态、金属组成以及两性离子型α-氨基酸配体的种类上均存在差异。结构表征结果显示，存在Mo对W的位点选择性取代现象，且Mo表现出强烈的占据中心金属氧簇核的倾向。在蓝色还原态物种中，该核心结构单元为闭合四金属环结构；而在无色氧化态同系物中，则为开放三金属环结构。密度泛函理论（density functional theory）计算结果表明，该金属氧簇核是还原反应的优先位点，且相较于三金属核，四金属核对应的分子轨道能量显著更低。还原态物种均为反磁性，其四金属核中存在一对强反铁磁耦合的五价钼（Mo(V)）中心；而在氧化态配合物中，所有钼均为六价。溶液态小角X射线散射（small-angle X-ray scattering, SAXS）与圆二色光谱（circular dichroism, CD）研究表明，该杂化POM在特定浓度与pH范围内的水溶液中可稳定存在数天，且过量氨基酸的存在可提升其稳定性。圆二色光谱实验还发现，此类氨基酸配体可与其他α-氨基酸发生快速配体交换，且能够分离得到配体交换产物，这证实了POM骨架在交换过程中得以保持完整。还原态物种的循环伏安图（cyclic voltammograms）显示其在相对较低的电位下存在一个不可逆氧化过程，但氧化态同系物并未呈现出对应的还原过程。尽管氧化态与两电子还原态杂化POM整体结构相似，但由于二者中心金属氧簇核分别为开放环与闭合环结构，二者无法相互转化。