Recent Advances in Polyoxometalate-Catalyzed Reactions

This review presents recent developments in some important industrial processes. For homolytic systems, the similarities and specificities of the transformation of cyclohexane into adipic acid (via the formation and isolation of the cyclohexanol–cyclohexanone mixture) and of the process for the preparation of terephthalic acid (by oxidation of p-xylene) are analysed from a prospective viewpoint. Innovations in heterolytic chemistry are also presented. For example, the epoxidation of propene currently performed by the Arco–Lyondell method, which uses a molybdenum catalyst and tert-butyl hydroperoxide, is in competition with other systems involving recycling of the co-product, or the use of hydrogen peroxide as oxygen donor. Access to diphenols, from phenol and H2O2, is becoming more selective. New oxoperoxo complexes of titanium, rhenium, tungsten and molybdenum are presented with their potential for catalytic oxidation. Achievements of our group in the development of catalysts for C–C bond cleavage by dioxygen, and for epoxidations by H2O2 are also given. Biomimetic oxidations and systems without metal catalysts feature among the innovations. These novel systems could compete with transition-metal-based chemistry for fine chemicals, but industry increasingly focuses research and development on fully inorganic systems, and there is little impact on the large scale of catalytic complexes with organic ligands.

Transition-metal complexes for liquid-phase catalytic oxidation: some aspects of industrial reactions and of emerging technologies

The use of polyoxometalates in protein crystallography - An attempt to widen a well-known bottleneck.

Synergistic directing roles of six lacunary fragments resulted in an unprecedented Zr24-cluster substituted poly(polyoxotungstate) Na10K22[Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4(GeW8O31)2]·85H2O (Na10K22·1·85H2O), which contains the largest [Zr24O22(OH)10(H2O)2] (Zr24) cluster in all the Zr-based poly(polyoxometalate)s to date. The most remarkable feature is that the centrosymmetric Zr24-cluster-based hexamer contains two symmetry-related [Zr12O11(OH)5(H2O)(W2O10H)(GeW9O34)2(GeW8O31)]16– trimers via six μ3-oxo bridges and was simultaneously trapped by three types of different segments of B-α-GeW9O34, B-α-GeW8O31, and W2O10. The other interesting characteristic is that there are two pairs of intriguing triangular atom alignments: one is composed of the Zr(2,4,6,8,11) and W21 atoms and the other contains the Ge(1–3), Zr(3,5,7,9,10,12) and W26 atoms, and the Zr5 atom is inside the triangle; a linking mode is unobserved. The oxygenation reactions of thioethers by H2O2 were evaluated when Na10K22·1·85H2O serve...

Synergistic Combination of Multi-ZrIV Cations and Lacunary Keggin Germanotungstates Leading to a Gigantic Zr24-Cluster-Substituted Polyoxometalate

The oxidation of alkenes by H2O2 catalyzed by Ti(IV)-containing polyoxometalates (POMs) as models of Ti single-site catalysts has been investigated at DFT level and has been compared with other early transition-metal-substituted polyoxometalates. We have studied in detail the reaction mechanism of the C2H4 epoxidation with H2O2 mediated by two different POMs, the Ti-monosubstituted Keggin-type POM [PTi(OH)W11O39]4− and the Ti-disubstituted sandwich-type POM [Ti2(OH)2As2W19O67(H2O)]8−. These species exhibit well-defined 6- and 5-coordinated titanium environments. For both species, the reaction proceeds through a two-step mechanism: (i) the Ti-OH groups activate H2O2 with a moderate energy barrier yielding either Ti-hydroperoxo (TiIV-OOH) or Ti-peroxo (TiIV-OO) intermediate, and (ii) the less stable but more reactive Ti-hydroperoxo species transfers oxygen to alkene to form the epoxide, this latter step being the rate-determining step. The higher activity of the sandwich anion was attributed to the absence ...

Mechanistic insights into alkene epoxidation with H2O2 by Ti- and other TM-containing polyoxometalates: role of the metal nature and coordination environment.

The previously unknown titanium(IV)-containing μ-hydroxo dimeric heteropolytungstate (Bu4N)7[(PTiW11O39)2OH] (TBA salt of H1) has been synthesized, starting from H5PTiW11O40, and characterized by elemental analysis, multinuclear (31P, 17O, 183W) NMR, IR, FAB-MS, cyclic voltammetry, and potentiometric titration. 31P NMR reveals that H1 (δ −12.76) readily forms in MeCN from the Keggin monomer (POM), PTiW11O405- (2, δ −13.34), upon the addition of 1.5 equiv of H+, via the protonated species, P(TiOH)W11O394- (H2, δ −13.44). The ratio of H1, 2, and H2, which are present in equilibrium in MeCN solution at 25 °C, depends on the concentration of both H+ and H2O. The Ti−O−Ti linkage readily reacts with nucleophilic reagents, such as H2O and ROH, to yield monomeric Keggin derivatives. μ-Hydroxo dimer H1 shows higher catalytic activity than 2 for thioether oxidation by hydrogen peroxide in acetonitrile. The reaction proceeds readily at room temperature and affords the corresponding sulfoxide and sulfone in ca. quant...

Martin A. Fedotov

Papers

A dimeric titanium-containing polyoxometalate. Synthesis, characterization, and catalysis of H2O2-based thioether oxidation.