Effects of isolobal heteroatoms in divanadium-substituted γ-Keggin-type polyoxometalates on (OV)2(µ-OH)2 diamond and (OV)2(µ-O) core structures and the transformation.

Effects of isolobal heteroatoms in divanadium-substituted γ-Keggin-type polyoxometalates, (TBA)(4)[γ-XV(2)W(10)O(38)(µ-OH)(2)] 1(X) and (TBA)(4)[γ-XV(2)W(10)O(38)(µ-O)] 2(X) (where X = Ge or Si), on (OV)(2)(µ-OH)(2) and (OV)(2)(µ-O) core structures and transformations from 2(X) to 1(X) have been investigated. X-ray crystallography of 1(X) and 2(X) reveals that larger Ge (covalent radius 1.22 Å; covalent radius of Si 1.11 Å) induces (a) expansion of (OV)(2)(µ-OH)(2) and (OV)(2)(µ-O) cores, (b) expansion of lacunary sites, and (c) deep location of divanadium cores inside their lacunary sites. Density functional theory (DFT) calculations for anionic moieties of 1(X) and 2(X) reveal that energy levels of the highest occupied molecular orbital (HOMO)-1 in 1(Ge) and HOMO in 2(Ge) are lower than those in 1(Si) and 2(Si), respectively, because of smaller contribution of p(z) orbitals of oxygen atoms in 1(Ge) and 2(Ge), which would result from shorter V···O(-Ge) distances. Compound 2(Ge) reacts with water vapor to form (TBA)(4)[γ-GeV(2)W(10)O(38)(µ-OH)(2)] 1'(Ge) via a crystal-to-crystal transformation, and the water dissociation proceeds heterolytically. DFT calculations reveal that the reaction proceeds through (1) coordination of water on a coordinatively unsaturated site of vanadium in the lowest unoccupied molecular orbital (LUMO), followed by (2) proton transfer to the bridging oxo moiety. The order is different from that in 2(Si), which would result from the lower energy level of HOMO of 2(Ge) (i.e., lower nucleophilicity toward a proton of water) than that of 2(Si).

Efficient epoxidation of electron-deficient alkenes with hydrogen peroxide catalyzed by [γ-PW10O38V2(µ-OH)2]3-.

A divanadium-substituted phosphotungstate, [γ-PW(10)O(38)V(2)(µ-OH)(2)](3-) (I), showed the highest catalytic activity for the H(2)O(2)-based epoxidation of allyl acetate among vanadium and tungsten complexes with a turnover number of 210. In the presence of I, various kinds of electron-deficient alkenes with acetate, ether, carbonyl, and chloro groups at the allylic positions could chemoselectively be oxidized to the corresponding epoxides in high yields with only an equimolar amount of H(2)O(2) with respect to the substrates. Even acrylonitrile and methacrylonitrile could be epoxidized without formation of the corresponding amides. In addition, I could rapidly (≤10 min) catalyze epoxidation of various kinds of terminal, internal, and cyclic alkenes with H(2)O(2) under the stoichiometric conditions. The mechanistic, spectroscopic, and kinetic studies showed that the I-catalyzed epoxidation consists of the following three steps: 1) The reaction of I with H(2)O(2) leads to reversible formation of a hydroperoxo species [γ-PW(10)O(38)V(2)(µ-OH)(µ-OOH)](3-) (II), 2) the successive dehydration of II forms an active oxygen species with a peroxo group [γ-PW(10)O(38)V(2)(µ-η(2):η(2)-O(2))](3-) (III), and 3) III reacts with alkene to form the corresponding epoxide. The kinetic studies showed that the present epoxidation proceeds via III. Catalytic activities of divanadium-substituted polyoxotungstates for epoxidation with H(2)O(2) were dependent on the different kinds of the heteroatoms (i.e., Si or P) in the catalyst and I was more active than [γ-SiW(10)O(38)V(2)(µ-OH)(2)](4-). On the basis of the kinetic, spectroscopic, and computational results, including those of [γ-SiW(10)O(38)V(2)(µ-OH)(2)](4-), the acidity of the hydroperoxo species in II would play an important role in the dehydration reactivity (i.e., k(3)). The largest k(3) value of I leads to a significant increase in the catalytic activity of I under the more concentrated conditions.

An efficient H2O2-based oxidative bromination of alkenes, alkynes, and aromatics by a divanadium-substituted phosphotungstate.

A divanadium-substituted phosphotungstate TBA(4)[γ-HPV(2)W(10)O(40)] (TBA = tetra-n-butylammonium) could act as an effective homogeneous catalyst for the H(2)O(2)-based oxidative bromination.

Efficient stereo- and regioselective hydroxylation of alkanes catalysed by a bulky polyoxometalate.

Direct functionalization of alkanes by oxidation of C-H bonds to form alcohols under mild conditions is a challenge for synthetic chemistry. Most alkanes contain a large number of C-H bonds that present difficulties for selectivity, and the oxidants employed often result in overoxidation. Here we describe a divanadium-substituted phosphotungstate that catalyses the stereo- and regioselective hydroxylation of alkanes with hydrogen peroxide as the sole oxidant. Both cyclic and acyclic alkanes were oxidized to form alcohols with greater than 96% selectivity. The bulky polyoxometalate framework of the catalyst results in an unusual selectivity that can lead to the oxidation of secondary rather than the weaker tertiary C-H bonds. The catalyst also avoids wasteful decomposition of the stoichiometric oxidant, which can result in the production of hydroxyl radicals and lead to non-selective oxidation and overoxidation of the desired products.