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Polyoxotungstates [PW(4)O(24)](3-) (PW(4)) and [PW(12)O(40)](3-) (PW(12)) have been inserted into nanocages of the metal organic framework MIL-101. The hybrid materials PW(x)/MIL-101 (x = 4 or 12) containing 5-14 wt % of polyoxotungstate have been obtained and characterized by elemental analysis, N(2) adsorption, FT-IR, Raman, and (31)P NMR MAS spectroscopic techniques. Their catalytic performance was assessed in the selective oxidation of alkenes with aqueous hydrogen peroxide under mild reaction conditions ([H(2)O(2)] = 0.1-0.2 M, 50 degrees C, MeCN). PW(x)/MIL-101 enclosing 5 wt % of polyoxotungstate demonstrated fairly good catalytic activities in the epoxidation of various alkenes (3-carene, limonene, alpha-pinene, cyclohexene, cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters achieved with homogeneous PW(x). For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a higher level of olefin conversion was attained using PW(12)/MIL-101. Moreover, confinement of PW(12) within MIL-101 nanocages allowed us to reach higher epoxide selectivities at higher alkene conversions. The hybrid PW(x)/MIL-101 materials were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several times with the maintenance of the catalytic performance.

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The oxidation of alkenes by H(2)O(2) 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 C(2)H(4) epoxidation with H(2)O(2) mediated by two different POMs, the Ti-monosubstituted Keggin-type POM [PTi(OH)W(11)O(39)](4-) and the Ti-disubstituted sandwich-type POM [Ti(2)(OH)(2)As(2)W(19)O(67)(H(2)O)](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 H(2)O(2) with a moderate energy barrier yielding either Ti-hydroperoxo (Ti(IV)-OOH) or Ti-peroxo (Ti(IV)-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 of dimer formation, and its higher selectivity to the larger energy cost of homolytic O-O bond breaking in the hydroperoxo intermediate. We also propose several requisites to improve the efficiency of Ti-containing catalysts, including flexible and 5-fold (or lower) coordinated Ti environments, as well as reagent-accessible Ti sites. Calculations on other TM-containing Keggin-type POMs [PTM(OH)W(11)O(39)](4-) (TM = Zr(IV), V(V), Nb(V), Mo(VI), W(VI), and Re(VII)) showed that when we move from the left to the right in the periodic table the formation of the epoxide via peroxo intermediate becomes competitive because of the higher mixing between the orbitals of the TM and the O-O unit.

The previously unknown titanium(IV)-containing mu-hydroxo dimeric heteropolytungstate (Bu4N)7[(PTiW11O39)2-OH] (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 (delta -12.76) readily forms in MeCN from the Keggin monomer (POM), PTiW11O40(5-) (2, delta -13.34), upon the addition of 1.5 equiv of H+, via the protonated species, P(TiOH)W11O39(4-) (H2, delta -13.44). The ratio of H1, 2, and H2, which are present in equilibrium in MeCN solution at 25 degrees 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. mu-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. quantitative yield. The addition of H2O2 to H1 or H2 results in the formation of a peroxo complex, most likely the hydroperoxo complex P(TiOOH)W11O39(4-) (I), which has 31P NMR resonance at -12.43 ppm. The rate of the formation of I is higher from H2 than from H1. When H1 is used as a catalyst precursor, the rates of the thioether oxidation and peroxo complex formation increase with increasing H2O concentration, which favors the cleavage of H1 to H2. H2O2 in MeCN slowly converts 2 to another peroxotitanium complex, P(TiO2)W11O39(5-) (II), which has 31P NMR resonance at -12.98 ppm. Peroxo complexes I and II differ in their protonation state and interconvert fast on the 31P NMR time scale. Addition of 1 equiv of H+ completely converts II to I, while 1 equiv of OH- completely converts I to II. 31P NMR confirms that I is stable under turnover conditions (thioether, H2O2, MeCN). Contrary to two-phase systems such as dichloroethane/aqueous H2O2, no products resulting from the destruction of the Keggin POM were detected in MeCN in the presence of H2O2 (a 500-fold molar excess). The reactivity of I, generated in situ from II by adding 1 equiv of H+, toward organic sulfides under stoichiometric conditions was confirmed using both 31P NMR and UV-vis spectroscopy. This is a rare demonstration of the direct stoichiometric oxidation of an organic substrate by a titanium peroxo complex.

The protonated titanium peroxo complex [Bu(4)N](4)[HPTi(O(2))W(11)O(39)] (1) has been first prepared via interaction of the micro-oxo dimeric heteropolytungstate [Bu(4)N](8)[(PTiW(11)O(39))(2)O] (3) with an excess of 30% aqueous H(2)O(2) in MeCN. Peroxo complex 1 has been characterized by using elemental analysis, UV-vis, IR, resonance Raman (RR), (31)P and (183)W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The electronic and vibrational spectra of 1 are very similar to those of the well-known unprotonated titanium peroxo complex [Bu(4)N](5)[PTi(O(2))W(11)O(39)] (2), while (31)P and (183)W NMR spectra differ significantly. A compilation of the physicochemical techniques supports a monomeric Keggin type structure of 1 bearing one peroxo ligand attached to Ti(IV) in a eta(2)-coordination mode. The protonation of the titanium peroxo complex results in an increase of the redox potential of the peroxo group, E(1/2) = 1.25 and 0.88 V relative to Ag/AgCl reference electrode for 1 and 2, respectively. In contrast to 2, 1 readily reacts with 2,3,6-trimethylphenol (TMP) at 40 degrees C in MeCN to give 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (BP) and 2,3,5-trimethyl-p-benzoquinone (TMBQ). The proportion between BP and TMBQ in the reaction products depends on the TMP/1 ratio. When a 2-fold excess of TMP is used, the main reaction product is BP (90%), while using a 2-fold excess of 1 leads to TMBQ (95%). On the basis of the product study, a homolytic oxidation mechanism that implicates the formation of phenoxyl radicals is suggested. The RR deuterium labeling experiments show that the activating proton is most likely localized at a Ti-O-W bridging oxygen rather than at the peroxo group. Theoretical calculations carried out at the DFT level on the protonated and unprotonated titanium peroxo derivatives also propose that the most stable complex is formed preferentially after protonation of the Ti-O-W site; however, both Ti-OH-W and TiOO-H protonated anions could coexist in solution.

Mesoporous metal-organic frameworks Cr- and Fe-MIL-101 are highly efficient, true heterogeneous and recyclable catalysts for solvent-free selective oxidation of cyclohexane with molecular oxygen and/or tert-butyl hydroperoxide under mild conditions.