Solid-State Crystal-to-Crystal Phase Transitions and Reversible Structure–Temperature Behavior of Phosphovanadomolybdic Acid, H₅PV₂Mo₁₀O₄₀

The crystal packing and secondary structure of H₅PV₂Mo₁₂O₄₀ was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H₅PV₂Mo₁₂O₄₀ becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H₅PV₂Mo₁₂O₄₀ to crystalline MoO₃ and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO₃. Importantly, H₅PV₂Mo₁₂O₄₀ can be easily recovered by dissolution in water at 80 °C

Solid-State Crystal-to-Crystal Phase Transitions and Reversible Structure–Temperature Behavior of Phosphovanadomolybdic Acid, H₅PV₂Mo₁₀O₄₀

The crystal packing and secondary structure of H₅PV₂Mo₁₂O₄₀ was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H₅PV₂Mo₁₂O₄₀ becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H₅PV₂Mo₁₂O₄₀ to crystalline MoO₃ and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO₃. Importantly, H₅PV₂Mo₁₂O₄₀ can be easily recovered by dissolution in water at 80 °C

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

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-P₂W₁₇O₆₁Co^II(H₂O)]^8–, [POMCo^IIH₂O], 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