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

Abstract

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(II)H2O], 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 (31)P 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.

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

“…The Co IV oxidation state of [(TAML)Co IV (O)Sc] 2+ was confirmed by EPR and XAS measurements. It should be noted that the CoO distance of 1.67 Å for [(TAML)Co IV (O)Sc] 2+ determined by EXAFS is significantly shorter than the CoO distances of 1.79 Å and 1.85 Å determined for [(α‐P 2 W 17 O 61 Co IV ) 2 O] 14− by X‐ray diffraction72 and [(TMG 3 tren)Co IV (O)(Sc)(OTf) 3 ]] 2+ (TMG 3 tren=tris[2‐( N ‐tetramethylguanidyl)ethyl]amine) by EXAFS 73. The latter species was later suggested to contain a Co III (OH) unit rather than a Co IV (O) unit,74 and further investigation is needed to clarify the nature of the CoO(H) moiety.…”

Reactivity and O2 Formation by Mn(IV)- and Mn(V)-Hydroxo Species Stabilized within a Polyfluoroxometalate Framework

“…In this area a Mn(V)-oxo species was identified as an effective oxygen donor in an organic solvent, 11 and a dimeric Co(III)-oxyl species was efficient for O 2 formation and C–H bond activation in water. 12 Transition metal substituted polyfluoroxometalates of the quasi Wells–Dawson structure, Figure 1, are a subclass of the analogous polyoxometalate compounds, where an electron withdrawing but π -donating fluorine atom is an axial ligand to the substituted transition metal and trans to the purported reaction site. These polyfluoroxometalates have been only sparsely studied, for example, as epoxidation catalysts with H 2 O 2 (TM = Ni(II)) 13 and as catalysts for aerobic hydroxylation and oxidative dehydrogenation of alkylated arenes (TM = V(V)).…”

Reversible Temperature Dependent Dimerization of Transition Metal Substituted Quasi Wells–Dawson Polyfluoroxometalates

“…In comparison, α 1 ‐[P 2 M(H 2 O)W 17 O 61 ] q– polyoxometalates of the Wells–Dawson structure are easily converted into the thermodynamically more stable α 2 isomer as evidenced also by the apparent absence of crystal structures of α 1 ‐[P 2 M(H 2 O)W 17 O 61 ] q– . The monomeric α 2 metal substituted Wells–Dawson have been often characterized, but dimers with a single M–µO–M bridge are rare and different than the two M–µO–W bridges formed in these PFOM compounds . Reversible dimerization has not been reported.…”

“…Thus, dimerization via formation of oxo‐bridges between addenda atoms such as Nb V has been described, although this appears not to be so viable for the more common Mo VI and W VI addenda atoms . Formation of dimers via formation of bridges between substituting transition metals such as Fe III and Mn II is also known, as is the insertion of a lower valent transition metal between terminal addenda‐oxo moieties . To the best of our knowledge no equilibria between monomeric and dimeric species have been documented as a function of temperature.…”