Synthesis and crystal structure of a molybdovanadate with a Keggin-type anion

Institute of Inorganic Chemistry, Academy of Sciences of the USSR, Siberian Branch. Institute of Chemical Sciences, Academy of Sciences of the Kazakh SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 6, pp. 97–103, November–December, 1990.     

Thermal decomposition of 10-molybdo-2-vanadosilicic heteropolyacid:29Si and51V-NMR study

Thermal decomposition of 10-molybdo-2-vanadosilicic heteropolyacid has been studied by²⁹Si and⁵¹V-NMR method.²⁹Si-NMR MAS spectra of solid samples indicate that the HPA begins to decompose at T<300°C and the decomposition is complete at 300–400°C.

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²⁹Si ⁵¹V Si–Mo-V () 12- . ²⁹Si MAS , 300°C 300–400°C.

     

Oxidation of methyl phenyl sulfide with hydrogen peroxide catalyzed by Ti(IV)-substituted heteropolytungstate

Alkylammonium salts of Ti(IV)-substituted heteropolytungstate, PW₁₁TiO ₁₀ ⁵⁻ , catalyze the oxidation of methyl phenyl sulfide with hydrogen peroxide. The yield of the corresponding sulfoxide and sulfone is practically quantitative. A³¹P NMR study confirms the formation and reactivity of the PW₁₁O₃₉TiO ₂ ⁵⁻ peroxo complex in organic media.     

Kinetics of reduction of tungstovanadophosphoric heteropoly anions with VO2+ ions

The rate constants of interactions of PW_12-nV_nO₄₀ ^(3+n)– (n=1–4) heteropoly anions (HPAs) and various PW₁₀V₂O₄₀ ^–5 isomers with VO²⁺ were measured. The reactions occurred via formation of intermediate active complexes with V(IV) ions incorporated into the coordination sphere of HPA.

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VO²⁺ PW_12-nV_nO₄₀ ^–(3+n) (n=1–4) PW₁₀V₂O₄₀ ^–5. , V(IV) .

     

95Mo and 17O NMR studies of aqueous molybdate solutions

Aqueous molybdate solutions with molybdenum concentrations of [Mo]=1, 0.4, and 0.2 M have been studied by NMR at pH 7-1 and in 0.3-6 M HClO4. The 95Mo NMR spectrum of isopolyanion (IPA) Mo7O24(6-) (I) at pH=5 consists of a signal at 210 ppm and two overlapping peaks at 32 and approximately 15 ppm with the intensity ratio approximately 1:4:2, and that of beta-Mo8O26(4-) (II) consists of two signals at approximately 100 and 10 ppm with the intensity ratio approximately 1:3. A broad 95Mo NMR line at around 0 ppm was observed in the pH range of IPA Mo36O112(8-) (III), and a signal of cationic oxospecies including MoO2(2+) (IV) was observed from -62 to -69 ppm. Two protonation sites of IPA I have been identified from 17O NMR spectra, which suggests binding of up to two protons. The distribution diagram, derived from the 95Mo NMR spectra, is given for [Mo]=0.4 M. The 95Mo NMR signals shift to lower frequencies with increasing number and strength of the Mo-O terminal bonds.     

Zr(IV)-monosubstituted Keggin-type dimeric polyoxometalates: synthesis, characterization, catalysis of H2O2-based oxidations, and theoretical study

The previously unknown Zr(IV)-monosubstituted Keggin-type polyoxometalates (Zr-POMs), (n-Bu4N)7H[{PW11O39Zr(mu-OH)}2] (1), (n-Bu4N)8[{PW11O39Zr(mu-OH)}2] (2), and (n-Bu4N)9[{PW11O39Zr}2(mu-OH)(mu-O)] (3) differing in their protonation state, have been prepared starting from heteropolyacid H5PW11ZrO40.14H2O. The compounds were characterized by elemental analysis, potentiometric titration, X-ray single-crystal structure, and IR, Raman, and 31P and 183W NMR spectroscopy. The single-crystal X-ray analysis of 2 reveals that two Keggin structural units [PW11O39Zr]3- are linked through two hydroxo bridges Zr-(OH)-Zr with Zr(IV) in 7-fold coordination. The IR spectra of 1 and 2 show a characteristic band at 772 cm(-1), which moves to 767 cm(-1) for 3, reflecting deprotonation of the Zr-(OH)-Zr bond. Potentiometric titration with methanolic Bu4NOH indicates that 1-3 contain 2, 1, and 0 acid protons, respectively. (83W NMR reveals Cs symmetry of 2 and 3 in dry MeCN, while for 1, it discovers nonequivalence of its two subunits and their distortion resulting from localization of the acidic proton on one of the Zr-O-W bridging O atoms. The (31)P NMR spectra of 2 and 3 differ insignificantly in dry MeCN, showing only signals at delta -12.46 and -12.44 ppm, respectively, while the spectrum of 1 displays two resonances at delta -12.3 (narrow) and -13.2 (broad) ppm, indicating slow proton exchange on the (31)P NMR time scale. The theoretical calculations carried out at the density functional theory level on the dimeric species 1-3 propose that protonation at the Zr-O-Zr bridging site is more favorable than protonation at Zr-O-W sites. Calculations also revealed that the doubly bridged hydroxo structure is thermodynamically more stable than the singly bridged oxo structure, in marked contrast with analogous Ti- and Nb-monosubstituted polyoxometalates. The interaction of 1-3 with H(2)O and H(2)O(2) in MeCN has been studied by both (31)P and (183)W NMR. The stability of the [PW(11)O(39)ZrOH](4-) structural unit toward at least 100-fold excess of H2O2 in MeCN was confirmed by both NMR and Raman spectroscopy. The interaction of 1 and 2 with H2O in MeCN produces most likely monomeric species (n-Bu4N)3+n[PW11O39Zr(OH)(n(H2O)(3-n)] (n = 0 and 1) showing a broad 31P NMR signal at delta -13.2 ppm, while interaction with H2O2 leads to the formation of an unstable peroxo species (delta -12.3 ppm), which reacts rapidly with cyclohexene, producing 2-cyclohexen-1-one and trans-cyclohexane-1,2-diol. Both 1 and 2 show a pronounced catalytic activity in H2O2 decomposition and H2O2-based oxidation of organic substrates, including cyclohexene, alpha-pinene, and 2,3,6-trimethylphenol. The oxidation products are consistent with those of a homolytic oxidation mechanism. On the contrary, 3 containing no acid protons reacts with neither H2O nor H2O2 and shows negligible catalytic activity. The Zr-monosubstituted polyoxometalates can be used as tractable homogeneous probes of Zr single-site heterogeneous catalysts in studying mechanisms of H2O2-based oxidations.     

Oxidative ammonolysis of methylpyrazine over binary catalytic systems: III. Phosphorus-molybdenum system: Catalytic properties and the active component

The phase composition of the binary phosphorus-molybdenum system with a Mo/P ratio of 3-24.5 and its catalytic properties in the reaction of oxidative ammonolysis of methylpyrazine are studied. X-ray amorphous phases of molybdenyl phosphate and phosphorus modified molybdenum trioxide are active in the formation of pyrazinonitrilee.     

Esterification of n-Butanol with Acetic Acid in the Presence of Heteropoly Acids with Different Structures and Compositions

The esterification reaction of n-butanol with acetic acid ([BuOH] : [HOAc] = 1 : 15 mol/mol; 55°C, 5% H₂O) was studied in the presence of tungsten heteropoly acids of the Keggin (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₅PW₁₁TiO₄₀, H₅PW₁₁ZrO₄₀, and H₃PW₁₁ThO₃₉) and Dawson structure (-H₆P₂W₁₈O₆₂, H₆P₂W₂₁O₇₁(H₂O)₃, H₆As₂W₂₁O₆₉(H₂O), and H₂₁B₃W₃₉O₁₃₂). The reaction orders with respect to H₆P₂W₂₁O₇₁(H₂O)₃, H₃PW₁₂O₄₀, and H₆P₂W₁₈O₆₉are equal to 0.78, 1.00, and 0.97, respectively. It was found that the reaction rate depends on the acidity, as well as on the structure and composition of heteropoly acids. The H₂₁B₃W₃₉O₁₃₂heteropoly acid is most active, whereas the Keggin-structure heteropoly acids exhibit the lowest activities. Of the Keggin structure heteropoly acids, H₅PW₁₁ZrO₄₀exhibits the highest activity because of the presence of a Lewis acid site in its structure.     

Synthesis, characterization, and reactivity of Ti(IV)-monosubstituted Keggin polyoxometalates

Ti(IV)-monosubstituted Keggin-type polyoxometalates (Ti-POMs), mu-oxo dimer [Bu4N]8[(PTiW11O39)2O] (1), and three monomers [Bu4N]4[PTi(L)W11O39], where L = OH (2), OMe (3), and OAr (4, ArOH = 2,3,6-trimethylphenol (TMP)), have been prepared starting from mu-hydroxo dimer [Bu4N]7[(PTiW11O39)2OH] (5) or heteropolyacid H5PW11TiO40 or both. The compounds have been characterized by elemental analysis, IR, UV-vis, and multinuclear (31P, 1H, 183W) NMR. The interaction of 1 and 3-5 with H2O in MeCN produces 2. The hydrolysis constants, estimated from 31P and 1H NMR data, are 0.006 and 0.04 for 1 and 3, respectively. Studies by 31P NMR, IR, potentiometric titration, and cyclic voltammetry revealed that 1-3 and 5 afford the same protonated titanium peroxo complex [Bu4N]4[HPTi(O2)W11O39] (I) upon interaction with aqueous H2O2 in MeCN. The rates of formation of I correlate with the rates of hydrolysis of the Ti-POMs and follow the order of 5 > 1 > 3. A two-step mechanism of the reaction of Ti-POMs with H2O2, which involves hydrolysis of the Ti-L bonds to yield 2 followed by fast interaction of 2 with hydrogen peroxide producing I, is suggested. The equilibrium constant for the reaction of 2 with H2O2 to yield I and H2O, estimated using 31P NMR, is 10. The interaction of the Ti-POMs with TMP follows the trends similar to their interaction with H2O) and requires preliminary hydrolysis of the Ti-L bonds. All of the Ti-POMs catalyze the oxidation of TMP with H2O2 in MeCN to give 2,3,5-trimethyl-p-benzoquinone and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol. The product distribution is similar for all of the Ti-POMs. The catalytic activities of the Ti-POMs correlate with the rates of formation of I and follow the order of 2 > 5 > 1 > 3. The findings lay a basis for a better understanding of the nature of the reactivity of titanium in Ti-catalyzed oxidations.