Showing papers on "Polyoxometalate published in 2005"

A 3D canted antiferromagnetic porous metal-organic framework with anatase topology through assembly of an analogue of polyoxometalate

Abstract: The unique porous metal−organic framework {KCo3(C6H4O7)(C6H5O7)(H2O)2·8H2O}8 (1), which exhibits an unprecedented infinite 3D (3,6)-connected decorated anatase net, has been obtained by hydrothermal reaction. Upon dehydration, the compound retains crystallinity and exhibits a type I N2 sorption isotherm, characteristic of a microporous solid with apparent Langmuir surface area 939 m2/g and pore volume 0.31 cm3/g. Magnetic measurements for both 1 and dehydrated 1 show the spin-canted antiferromagnetic state below 5 K and a magnetic hysteresis loop at 2 K. Thus, dehydrated 1 represents the first metal−organic framework for which microporosity and a spin-canted antiferromagnetic state coexist, which demonstrates that the self-assembly of organo-polymetal clusters and metal ions can provide a potential route to magnetic porous metal−organic frameworks.

Degradation of dye pollutants by immobilized polyoxometalate with H2O2 under visible-light irradiation.

Abstract: A Keggin polyoxometalate (POM, i.e., PW12O40(3-)) and its lacunary derivative are immobilized on an anionic exchange resin through electrostatic interaction at pH 4.6 in an aqueous dispersion. The resin-supported POM thus obtained catalyzes the efficient degradation of cationic dye pollutants in the presence of H2O2 under visible-light irradiation. To evaluate the photocatalytic system, degradation of a rhodamine B (RB) dye was investigated in detail using UV-visible spectroscopy, high performance liquid chromatography, and gas chromatography/mass spectrometry techniques to identify the intermediates and final products. Fluorescence lifetime measurements revealed the electron transfer from the visible-light-excited RB molecules to the POMs. Electron paramagnetic resonance measurements, investigation of the effects of *OH and *OOH scavengers on the photoreaction kinetics, and IR analysis indicated that de-ethylation of RB was due to *OOH radicals, but the decomposition of the conjugated xanthene structure was caused by the peroxo species formed by interaction of H2O2 with the lacunary POM loaded on the resin. A total organic carbon removal of ca. 22% was achieved, and the recycle experiment suggested excellent stability and reusability of the heterogeneous catalyst. On the basis of the experimental results, a photocatalytic mechanism is discussed.

Self-Assembly of a Series of Extended Architectures Based on Polyoxometalate Clusters and Silver Coordination Complexes

Abstract: Three unusual compounds based on polyoxometalate building blocks, [(H2O)5Na2(C6NO2H4)(C6NO2H5)3Ag2][Ag2IMo6O24(H2O)4]·6.25H2O (1), [(H2O)4Na2(C6NO2H5)6Ag3][IMo6O24]·6H2O (2), and (C6NO2H6)2[(C6NO2H5)2Ag][Cr(OH)6Mo6O18]·4H2O (3), have been synthesized and characterized by elemental analysis; IR, XPS, and ESR spectroscopy; TG analysis; and single-crystal X-ray diffraction. Compound 1 is constructed from the cationic two-dimensional (2D) coordination polymer sheets which are constituted of [(H2O)5Na2(C6NO2H4)(C6NO2H5)3Ag2]3+ and anionic [Ag2IMo6O24(H2O)4]3- chains as pillars, forming a three-dimensional (3D) supramolecular framework via weak Ag−O interactions. Compound 2 is composed of the well-defined [IMo6O24]5- building blocks, which are linked through trinuclear Ag-pyridine-3-carboxylic acid, [(C6NO2H5)6Ag3]3+, fragments into a one-dimensional (1D) hybrid chain; adjacent chains are further connected by sodium cations to yield a novel 2D network. Compound 3 has a 1D chainlike structure constructed from [C...

A Nonanuclear Copper(II) Polyoxometalate Assembled Around a μ-1,1,1,3,3,3-Azido Ligand and Its Parent Tetranuclear Complex

Abstract: Reaction of Cu I I , [γ-SiW 1 0 O 3 6 ] 8 - , and N 3 - affords three azido polyoxotungstate complexes. Two of them have been characterized by single-crystal X-ray diffraction. Complex KNaCs 1 0 [{γ-SiW 1 0 O 3 6 Cu 2 (H 2 O)-(N 3 ) 2 } 2 ].26H 2 O (1) is obtained as crystals in few hours after addition of CsCl. This linear tetranuclear Cu I I complex consists in two [γ-SiW 1 0 O 3 6 Cu 2 -(H 2 O)(N 3 ) 2 ] 6 - units connected through two W=O bridges. When the filtrate is left to stand for one night, a new complex is obtained. From both elemental analysis and IR spectroscopy, it has been postulated that this compound could be formulated K 1 . 5 Cs 5 . . 5 [SiW 1 0 O 3 7 -Cu 2 (H 2 O) 2 (N 3 )].14H 2 O (1a), showing the loss of one azido ligand per polyoxometalate unit. Finally, when no cesium salt is added to the reaction medium, the nonanuclear complex K 1 2 Na 7 [{SiW 8 O 3 1 Cu 3 (OH)(H 2 O) 2 (N 3 )} 3 -(N 3 )].24H 2 O (2) is obtained after three days. Compound 2 crystallizes in the R3c space group and consists in three {Cu 2 } units related by a C 3 axis passing through the exceptional μ-1,1,1,3,3,3-azido bridging ligand. Each trinuclear Cu" unit is embedded in the [γ-SiW 8 O 3 1 ] 1 0 - ligand, an unprecedented tetravacant polyoxometalate, showing that partial decomposition of the [γ-SiW 1 0 O 3 6 ] 8 - precursor occurs with time in such experimental conditions. Magnetically, complex 1 behaves as two isolated {Cu 2 (μ 1 , 1 -N 3 ) 2 } pairs in which the metal centers are strongly ferromagnetically coupled (J=+224 cm - 1 , g= 2.20), the coupling through the W=O bridges being negligible. The magnetic behavior of complex 2 has also been studied. Relatively weak ferromagnetic couplings (J 1 = +1.0 cm - 1 , J 2 = +20.0 cm - 1 , g=2.17) have been found inside the {Cu 3 } units, while the intertrimeric magnetic interactions occurring through the hexadentate azido ligand have been found to be antiferromagnetic (J 3 = -5.4 cm - 1 ) and ferromagnetic (J 4 =+1.3 cm - 1 ) with respect to the end-to-end and end-on azido-bridged Cu I I pairs, respectively.

Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups

Abstract: Five new materials built from polyoxotungstates and Cu(II) ions as linkers have been synthesized by hydrothermal reactions from a mixture of sodium tungstate, copper chloride and bipyridine. The value of the initial pH, the nature of the heteroelement (P or Si) and of the ligand (2,2′- and/or 4,4′-bipyridine) permit the control of the nature of the polyoxotungstate clusters and their connectivity via the copper ions, and hence the dimensionality of the framework. A single phase has been obtained with silicon as heteroelement at an initial pH of 5, namely the 2D material [SiW12O40][Cu(2,2′-bpy)2]2·10H2O (1) with saturated Keggin polyoxotungstates linked by {Cu(2,2′-bpy)2}2+ groups. With phosphorous as heteroelement and at the same initial pH, three different structures have been isolated according to the nature of the ligand. Indeed, the two 1D materials [{Cu5(2,2′-bpy)5(H2O)(HPO4)(PO4)}PW11CuO39]·6H2O (2) with 2,2′-bpy and [4,4′-Hbpy][{Cu2(2,2′-bpy)2(4,4′-bpy)2.5}PW11CuO39]·16H2O (3) with a mixture of 2,2′- and 4,4′-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na3[4,4′-Hbpy]{Cu4(4,4′-bpy)8(H2O)8}[PW11CuO39(H2O)][PW10Cu2O38(H2O)2]·38H2O (4) with 4,4′-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Na2[{Cu8(2,2′-bpy)8}(PW8O31)2]·15H2O (5) has been evidenced. Magnetic studies of compound 2 revealed that the predominant interactions involve only 4 paramagnetic centers, which are interacting within pairs, among the 6 Cu(II) centers. The χMT = f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H = −JS1×S2, with S1 = S2 = ½ and J = −105.4 cm−1, showing strong antiferromagnetic interactions within the two Cu(II) pairs.

Synthesis of main-chain polyoxometalate-containing hybrid polymers and their applications in photovoltaic cells

Abstract: Hexamolybdate clusters have been embedded through covalent bonds into the main chain of poly(phenylene acetylene)s. These hybrid polymers were synthesized by palladium-catalyzed coupling reactions of a diiodo functionalized cluster with a diethynylbenzene derivative or a diethynyl functionalized cluster with a diiodobenzene derivative. These polymers are soluble in organic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and free-standing films can be spin-coated or cast from solutions. While hybrid monomer 2a exhibits a sharp melting transition at 246 °C, polymers 5a and 5b show glass transitions at 125 and 102 °C, respectively. Cyclovoltammetry studies of the hybrid polymers revealed a reversible reduction wave at 1.19 V versus Ag/Ag+, comparable to those of bifunctionalized imido derivatives of hexamolybdates. These polymers show intense absorption in the visible range but with little fluorescence emissions, indicating efficient fluorescence quenching of the embedded polyoxom...

Polyoxometalate-supported Y- and YbIII-hydroxo/oxo clusters from carbonate-assisted hydrolysis.

Abstract: Carbonate-assisted hydrolysis of Y or Yb I I I ions in the presence of the trivacant Wells-Dawson polyoxo-anion, α-[P 2 W 1 5 O 5 6 ] 1 2 - , produced two polyoxometalate-supported Y- or Yb I I I -hydroxo/oxo clusters, which have been characterized by single-crystal X-ray structure determination. The structure of the Y complex consists of a distorted Y 4 (OH) 4 cubane cluster encapsulated by two lacunary α-[P 2 W 1 5 O 5 6 ] 1 2 - units, while the Yb cluster features a hexametallic core centered around a μ 6 -oxo atom with each Yb I I I 3 triangular face capped by an oxo or a hydroxo group. Magnetization measurements of the ytterbium(III) derivative suggested that intermolecular dipolar exchange is present at low temperatures (below 15 K). Despite its absence in the structures themselves, control experiments show that carbonate not only functions in the hydrolysis, it also influences the structure of the complexes by complexation to yttrium and the f-block elements.

Neutral and Cationic VIV/VV Mixed-Valence Alkoxo-polyoxovanadium Clusters [V6O7(OR)12]n+ (R = −CH3, −C2H5): Structural, Cyclovoltammetric and IR-Spectroscopic Investigations on Mixed Valency in a Hexanuclear Core

Abstract: The alkoxo-polyoxovanadium clusters [V6O7(OR)12] (R = −CH3, −C2H5) are fully alkylated polyoxometalate derivatives comprising a hexavanadate core with the vanadium ions organized in an octahedral fashion, a classic isopolyoxometalate structure (Lindqvist) which as an entity is not known for vanadium. The clusters are highly redox-active compounds, displaying a large number of thermodynamically stable redox isomers of which the chemical syntheses and structural characterization of the neutral and cationic VIV/VV mixed-valence species [VIV(4-n)VV(2+n)O7(OR)12]n+[SbCl6]n (R = −CH3, n = 0, 1; R = −C2H5, n = 0, 1, 2) are presented here. Neutral and positively charged clusters remain exceptional in the field of polyoxometalate chemistry. Results obtained from cyclic voltammetry, infrared spectroscopy, and from valence sum calculations conducted on X-ray structural data classify these clusters as class II mixed-valence compounds. Their highly symmetrical molecular structures make them particularly interesting as...

Strategies for oxidation catalyzed by polyoxometalates at the interface of homogeneous and heterogeneous catalysis

Abstract: A basic premise behind the use of polyoxometalates in oxidation chemistry is the fact that polyoxometalates are oxidatively stable. This, a priori, leads to the conclusion that for practical purposes polyoxometalates would have distinct advantages over widely investigated organometallic compounds that are vulnerable to decomposition due to oxidation of the ligand bound to the metal center. Since polyoxometalate synthesis is normally carried out in water by mixing the stoichiometrically required amounts of monomeric metal salts and adjusting the pH to a specific acidic value many structure types are accessible by variation of the reaction stoichiometry, replacement of one or more addenda atoms with other transition or main group metals, and pH control. The structural variety available has enabled the use of polyoxometalates as catalysts for oxidation of hydrocarbons and functionalized organic substrates (alcohols, amines, sulfides, etc.) with a wide range of oxygen donors ranging from molecular oxygen, hydrogen peroxide, nitrous oxide, ozone, alkyl hydroperoxides, periodate, sulfoxide and others. The wide purview of oxidation reactions is enabled because the structural variety leads to oxidation through a number of different mechanistic motifs. From a synthetic organic point of view, the most applicable uses of polyoxometalates as catalysts involve the “green” oxygen donors – hydrogen peroxide and molecular oxygen. Since practical applications are in hand in this area, practical considerations concerned with catalyst recycle and/or recovery and the elimination of environmentally problematic solvents are also coming to the forefront. In this paper, we will present some of our activities in the area of “catalyst engineering” for catalytic synthetic applications by polyoxometalates including: (a) catalytic mesoporous solids from organic-polyoxometalate hybrid materials, (b) fluorous phase polyoxometalates with and without fluorous solvents and (c) the use of aqueous biphasic media for oxidation with hydrogen peroxide.

3D Supramolecular Array Assembled by Cross-like Arrangement of 1D Sandwich Mixed Molybdenum−Vanadium Polyoxometalate Bridged Coordination Polymer Chains: Hydrothermal Synthesis and Crystal Structure of {[MoVI5MoV3VIV8O40(PO4)][Ni(en)2]}[Ni(en)2]2·4H2O

Abstract: A novel organic−inorganic hybrid material constructed from mixed molybdenum−vanadium polyoxoanion clusters and [Ni(en)2]2+ complex groups {[MoVI5MoV3VIV8O40(PO4)][Ni(en)2]}[Ni(en)2]2·4H2O (1) (en = ethylenediamine), has been prepared under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. Complex 1 exhibits an interesting three-dimensional supramolecular array through cross-like arrangement of a one-dimensional coordination polymer anion chain {[MoVI5MoV3VIV8O40(PO4)][Ni(en)2]}4-, which is sandwiched by discrete cation complex fragments [Ni(en)2]2+. It is the first example of a three-dimensional organic/inorganic hybrid polyoxometalate formed by cross-like arrangement of a one-dimensional polyoxometalate bridged coordination polymer chain.

Directed Synthesis of a 1D Double-Chain Polyoxometalate Assembly: {[Ag2(bppy)3][Ag(bppy)2][Ag(bppy)]2PW11Co(bppy)O39}·2H2O

Abstract: A 1D double-chain assembly based on the monosubstituted Keggin polyoxometalate (POM) with pendant ligands and of formula {[Ag2(bppy)3][Ag(bppy)2][Ag(bppy)]2PW11Co(bppy)O39}·2H2O (1) [bppy = 5-(4-bromophenyl)-2-(4-pyridinyl)pyridine] has been synthesised and characterised. The crystals of 1 belong to the space group P , Mr = 5801.06, a = 18.720(4) A, b = 19.474(4) A, c = 19.815(4) A, α = 96.30(3), β = 96.07(3), γ = 90.12(3)°, V = 7139(2) A3, Z = 2, Dcalcd. = 2.699 Mg·m−3. The final statistics based on F2 are GOF = 1.007, R1 = 0.0620 and wR2 = 0.1346 for I > 2σ(I). X-ray diffraction analysis revealed that the structure of 1 contains a 1D chain constructed from a monosubstituted tungstophosphate with one pendant conjugated ligand, five silver−bppy groups and two water molecules. The periphery of the Keggin cluster consists of five Ag−bppy groups so that a bicapped trisupporting Keggin structure is obtained. It is rare that a Keggin anion cluster is simultaneously coordinated to five transition metal atoms. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Electrochromic multilayer films of tunable color by combination of copper or iron complex and monolacunary dawson-type polyoxometalate.

Abstract: Electrochromic multilayer films consisting of polyoxometalate (POM) cluster α-K10[P2W17O61]·17H2O (P2W17), copper(II) complex [CuII(phen)2](NO3)2 (phen = 1,10-phenithroline), and iron complex [FeII(phen)3](ClO4)2 were fabricated on silicon, quartz and ITO substrates by layer-by-layer self-assembly method. The multilayer films, PSS/CuII(phen)2/[(P2W17/CuII(phen)2)]n and PSS/FeII(phen)3/[(P2W17/FeII (phen)3)]n were characterized by UV−vis spectra, X-ray photoelectron spectra, cyclic voltammetry (CV), chronoamperometric (CA) and in-situ spectral electrochemical measurements. The interesting feature of the electrochromic film is its adjustable color by reduction of both transition metal complex and polyoxometalate at different potentials. The multilayer films also exhibit high optical contrast, suitable response time and low operation potential due to the presence of mono-lacunary-substituted polyoxometalate and transition metal complex. This is the first example that the color of electrochromic film can be a...

Selective precipitation of prions by polyoxometalate complexes.

Abstract: The efficacy of a variety of polyoxometalate anions in selectively precipitating the infectious isoform of the prion protein (PrPSc) over the normal, cellular protein (PrPC) was tested as a means of improving the sensitivity of a conformation-dependent immunoassay (CDI). Experiments utilizing neutralized aqueous solutions containing MgCl2.6H2O and Na2H[PW12O40] (1) indicated that use of the former is detrimental to selectivity, while use of the latter gives optimal selectivity at a concentration of 2.48% w/v, much higher than the 0.31% w/v employed in the current CDI protocol. Similar experiments using Na4[SiW12O40] (2), K5[BW12O40] (3), and Na6[H2W12O40] (4) were performed to probe the role of charge density in polyoxometalate anions possessing the Keggin structure. These, coupled with assignment of solution structures via NMR and infrared spectroscopy, revealed the following trend in optimal concentrations: [HBW11O39]8- < [SiW11O39]8- < [PW11O39]7- = [H2W12O40]6-. Interestingly, use of the salts K10[Zn4(H2O)2(PW9O34)2] (5), Na16[Zn4(H2O)2(P2W15O56)2] (6), and Na27[NaAs4W40O140] (7), containing larger polyoxometalate anions, was found to suppress PrPSc precipitation in a concentration-dependent manner. Taken together, the results suggest a precipitation mechanism wherein Keggin-type anions link multiple PrPSc moieties by binding a somewhat hindered cleft featuring one or more positively charged residues.

The High-Valent Iron−Oxo Species of Polyoxometalate, if It Can Be Made, Will Be a Highly Potent Catalyst for C−H Hydroxylation and Double-Bond Epoxidation

Abstract: This study uses density functional theory (DFT) calculations to explore the reactivity of the putative high-valent iron−oxo reagent of the iron-substituted polyoxometalate (POM−FeO4-), derived from the Keggin species, PW12O403-. It is shown that POM−FeO4- is in principle capable of C−H hydroxylation and CC epoxidation and that it should be a powerful oxidant, even more so than the Compound I species of cytochrome P450. The calculations indicate that in a solvent, the barriers, and especially those for epoxidation, become sufficiently small that one may expect an extremely fast reaction. An experimental investigation (by R.N .and A.M.K.) shows, however, that the formation of POM−FeO4- using the oxygen donor, F5PhI−O, leads to a persistent adduct, POM−FeO−I−PhF54-, which does not decompose to POM−FeO4- + F5Ph−I at the working temperature and exhibits sluggish reactivity, in accord with previous experimental results (Hill, C. L.; Brown, R. B., Jr. J. Am. Chem. Soc. 1986, 108, 536 and Mansuy, D.; Bartoli, J.-...

Organic–Inorganic Hybrids Based on Novel Bimolecular [Si2W22Cu2O78(H2O)]12− Polyoxometalates and the Polynuclear Complex Cations [Cu(ac)(phen)(H2O)]nn+ (n=2, 3)

Abstract: The reaction of a monosubstituted Keggin polyoxometalate (POM) generated in situ with copper-phenanthroline complexes in excess ammonium or rubidium acetate led to the formation of the hybrid metal organic-inorganic compounds A7[Cu2(ac)2(phen)2(H2O)2][Cu3(ac)3(phen)3(H2O)3][Si2W22Cu2O78(H2O)].approximately 18 H2O (A=NH4+ (1), Rb+ (2); ac=acetate; phen=1,10-phenanthroline). These compounds are constructed from inorganic and metalorganic interpenetrated sublattices containing the novel bimolecular Keggin POM, [Si2W22Cu2O78(H2O)]12-, and Cu-ac-phen complexes, [Cu(ac)(phen)(H2O)]n n+ (n=2, 3). The packing of compound 1 can be viewed as a stacking of open-framework layers parallel to the xy plane built of hydrogen-bonded POMs, and zigzag columns of pi-stacked Cu-ac-phen complex cations running along the [111] direction. Magnetic and EPR results are discussed with respect to the crystal structure of the compounds. DFT calculations on [Cu(ac)(phen)(H2O)]n n+ cationic complexes have been performed, to check the influence of packing in the complex geometry and determine the magnetic exchange pathways.

Selective Oxidations on Recoverable Catalysts Assembled in Emulsions

Abstract: Catalysts assembled in emulsions are found to be potentially recoverable and efficient for a number of catalytic reactions. The catalysts composed of polyoxometalate anions and quaternary ammonium cations have been designed and synthesized according to the catalytic reactions and by optimizing the structures of cations and anions. The catalysts act essentially as surfactants, which are uniformly distributed in the interface of the emulsion droplets, and accordingly behave like homogeneous catalysts. The catalysts show remarkable selectivity and activity in the oxidation of sulfur-containing molecules to sulfones in diesel and the selective oxidation of alcohols to ketones, using H2O2 as oxidant. For an example, the catalyst demonstrated over 96% efficiency of H2O2 and ˜100% selectivity to sulfones for the selective oxidation of sulfur-containing molecules in real diesel. Moreover, the catalysts can be separated and recycled by a simple demulsification and re-emulsification.

A series of new polyoxoanion-based inorganic-organic hybrids: (C6NO2H5)[(H2O)4(C6NO2H5)Ln(CrMo6H6O24)]·4H2O (Ln = Ce, Pr, La and Nd) with a chiral layer structure

Abstract: Four new polyoxoanion-based hybrids with a chiral layer structure, (C6NO2H5)[(H2O)4(C6NO2H5)Ln(CrMo6H6O24)]·4H2O (Ln = Ce, 1; Pr, 2; La, 3 and Nd, 4), have been synthesized and characterized by elemental analysis, IR spectroscopy, TG analysis and single crystal X-ray diffraction. All the compounds are isostructural and crystallize in the monoclinic space group C2/c. Their crystal structures consist of two kinds of chiral layers, one left-handed and the other right-handed, each built up from the same-handed rare earth polyoxometalate helical chains, which leads to mesomeric solid state compounds. To our knowledge, this represents the first example of a 2D chiral layer framework consisting of rare earth polyoxometalate helical chains and organic bridging ligands. The magnetic properties of compounds 1, 2 and 4 have been studied by measuring their magnetic susceptibility over the temperature range of 2–300 K.

The role of protons in cyclohexene oxidation with H2O2 catalysed by Ti(IV)-monosubstituted Keggin polyoxometalate

Abstract: The effect of the number of protons in the Ti(IV)-monosubstituted Keggin polyoxometalate Na 5 − n H n PTiW 11 O 40 ( n = 1–5; Ti-POM) on its catalytic behaviour in cyclohexene (CyH) oxidation with aqueous H 2 O 2 in MeCN is reported. It has been found that Ti-POMs with n = 2–5 catalyse efficiently CyH oxidation to yield trans -cyclohexane-1,2-diol as the main reaction product, while Ti-POM containing only one proton shows lower activity in CyH oxidation and produces allylic oxidation products, 2-cyclohexene-1-ol and 2-cyclohexene-1-one, along with comparable amounts of the corresponding epoxide and diol. The obtained results strongly support homolytic oxidation mechanism for CyH oxidation in the presence of the monoprotonated Ti-POM and heterolytic oxygen-transfer mechanism in the presence of Ti-POMs having two and more protons. The 31 P and 183 W NMR studies revealed that Ti-POMs are stable towards at least 100-fold excess of H 2 O 2 and the high catalytic activity of Ti-POMs with n = 2–5 is not due to the formation of lower nuclearity species. The addition of one equivalent of H + to the monoprotonated peroxo complex [Bu 4 N] 4 [HPTi(O 2 )W 11 O 39 ] ( I , 31 P NMR in MeCN: −12.40 ppm) results in the formation of the diprotonated titanium peroxo species [H 2 PTi(O 2 )W 11 O 39 ] 3− ( II , 31 P NMR in MeCN: −12.14 ppm). This peroxo species can also be obtained by adding an excess of H 2 O 2 to Na 5 − n H n PTiW 11 O 40 ( n = 2–5) in MeCN. The presence of the second proton in the peroxo species is a crucial factor determining the capability of II to oxidise alkenes via heterolytic oxygen transfer mechanism. Both 31 P NMR and GC–MS studies corroborated that II reacts with CyH producing trans -cyclohexane-1,2-diol as the main reaction product, whereas I is not reactive towards CyH under stoichiometric conditions.

Selective ortho hydroxylation of nitrobenzene with molecular oxygen catalyzed by the H5PV2Mo10O40 polyoxometalate.

Abstract: Nitrobenzene was regioselectively oxidized to 2-nitrophenol with oxygen in a reaction catalyzed by the H5PV2Mo10O40 polyoxometalate. The reaction was first order in oxygen and catalyst. 15N NMR showed the interaction between nitrobenzene and the polyoxometalate. Use of labeled 18O2, H218O, a competitive kinetic isotope experiment, and use of phenyl-tert-butylnitrone as a spin-trap and identification by EPR provided evidence for formation of a radical intermediate involving a selective intramolecular interaction at the ortho position due to formation of a H5PV2Mo10O40−nitrobenzene complex.

Association of Keggin-type anions with cationic meso-substituted porphyrins : synthesis, characterization and oxidative catalytic studies

Abstract: New compounds based on the association of polyoxometalates and cationic porphyrins or metalloporphyrins were prepared with the aim of obtaining new bi-functional catalysts. A combined study on the syntheses, characterization and catalytic activity of these compounds was performed. The new compounds have the general formula (porphyrin) x H y [XM 12 O 40 ]· z solv, x = 0.75 or 1, X = P or Si and M = W or Mo. The porphyrins used were 5,10,15,20-tetrakis(4-pyridyl)porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin, 5,10,15-tris(2,6-dichorophenyl)-20-(4-pyridyl)porphyrin, and 5,10,15-tris(2,6-dichlorophenyl)-20-(1-methyl-4-pyridinio)porphyrin, either as free bases or complexed with Mn III (one case with Zn II ). A few compounds with metal substituted Keggin anions ([PW 11 Y(H 2 O)O 39 ] n − , Y = Mn, Fe, Ni, Zn) were also prepared. All compounds were characterized by spectroscopic and analytical techniques. The oxidation of cis -cyclooctene, geraniol and (+)-3-carene by hydrogen peroxide catalysed by the polyoxometalate/Mn III porphyrin associations and co-catalysed by ammonium acetate was examined. Some of the associations significantly increased the percentage of conversion of the substrates relatively to the respective metalloporphyrin alone. However, the regio-, chemo- and stereoselectivity of the oxidation reactions catalysed by metalloporphyrins were globally preserved: c is -cyclooctene was selectively epoxidised to epoxycyclooctane; geraniol gave 6,7-epoxygeraniol as the major product and (+)-3-carene was preferably oxidised to the α-3,4-epoxycarane. In the conditions studied, the polyoxoanions seem to contribute mainly to stabilise the metalloporphyrin against deactivation during catalytic cycles.