Dicobalt-μ-oxo Polyoxometalate Compound, [(α₂‑P₂W₁₇O₆₁Co)₂O]^14–: A Potent Species for Water Oxidation, C–H Bond Activation, and Oxygen Transfer

High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbon–hydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt­(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt­(IV)-μ-oxo polyoxometalate compound [(α2-P2W17O61Co)2O]14–, [(POMCo)2O], has now been isolated and characterized by the oxidation of a monomeric [α2-P2W17O61CoII(H2O)]8–, [POMCoIIH2O], with ozone in water. The crystal structure shows a nearly linear Co–O–Co moiety with a Co–O bond length of ∼1.77 Å. In aqueous solution [(POMCo)2O] was identified by 31P NMR, Raman, and UV–vis spectroscopy. Reactivity studies showed that [(POMCo)2O]2O] is an active compound for the oxidation of H2O to O2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon–hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)2O]2O] is best defined as a compound having two cobalt­(III) atoms with two oxidized oxygen atoms.过渡金属的高价氧代化合物常被认定为通过碳氢键活化或氧转移完成烃类氧化，以及水氧化过程中的活性物种。近年来，已有多篇钴催化水氧化的相关研究报道，且钴(IV)物种被提议作为该过程的活性中间体。一种被正式归类为双核钴(IV)-μ-氧代多金属氧酸盐（polyoxometalate, POM）的活性化合物[(α2-P2W17O61Co)2O]¹⁴⁻（简记为[(POMCo)₂O]），现已通过在水溶液中用臭氧氧化单体型[α2-P2W17O61CoII(H2O)]⁸⁻（简记为[POMCoIIH2O]）得以分离并表征。晶体结构显示，该化合物存在近乎直线型的Co-O-Co结构单元，Co-O键长约为1.77埃。在水溶液中，[(POMCo)₂O]可通过31P核磁共振谱、拉曼光谱以及紫外-可见光谱进行鉴定。反应性研究表明，[(POMCo)₂O]是一种具有多重催化活性的化合物：可催化H₂O氧化为O₂，可直接将氧原子转移至水溶性亚砜与膦类化合物，可通过锰卟啉介导实现烯烃的间接环氧化，还可通过碳氢键活化实现醇类的选择性氧化。上述醇类选择性氧化过程似乎通过氢原子转移机制进行。密度泛函理论与完全活性空间自洽场（CASSCF）计算结果有力证实，[(POMCo)₂O]的电子结构最优描述为：包含两个钴(III)原子与两个氧化态氧原子的化合物。