Triniobium Polytungstophosphates. Syntheses, Structures, Clarification of Isomerism and Reactivity in the Presence of H₂O₂

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Abstract

The reaction of K(7)[HNb(6)O(19)], H(2)O(2) and A-Na(9)[PW(9)O(34)] in water followed by treatment with Cs(+) or (n-Bu(4)N)(+) (TBA) affords the corresponding salts of the tris(peroxoniobium) heteropolyanion A,beta-[(NbO(2))(3)PW(9)O(37)](6)(-) (1) in approximately 60% isolated yields. An X-ray structure of the Cs salt, Cs1 (monoclinic P2/c; a = 16.92360(10) Å, b = 13.5721(2) Å, c = 22.31890(10) Å, beta = 92.0460(10) degrees, and Z = 4) confirms the A-type substitution pattern of the three Nb atoms and clarifies the M(3) rotational (Baker-Figgis) isomerism in the Keggin unit as beta. The three terminal eta(2)-O(2)(2)(-) groups on the Nb atoms give 1 an overall symmetry approximating the chiral C(3). These terminal peroxo ligands, and these groups only, thermally decompose when either Cs1 or TBA1 is in solution unless additional H(2)O(2) is present. The peroxo groups can be titrated with triphenylphosphine (2.8 +/- 0.3 peroxide groups found per molecule). Refluxing TBA1 in acetonitrile for 24 h in the presence of base generates the parent heteropolyanion, [Nb(3)PW(9)O(40)](6)(-) (2) in 80% yield after isolation. Treatment of 2 with glacial acetic acid in acetonitrile converts it to [Nb(6)P(2)W(18)O(77)](6)(-) (3) in approximately 100% yield, while treatment of TBA3 with hydroxide converts it back to 2 in high yield. Spectroscopic (FTIR, Raman, (183)W NMR, and (31)P NMR), titrimetric, mass spectrometric (FABMS), and elemental analysis data are all consistent with these formulas. The addition of TBA1 to solutions of alkenes and 33% aqueous peroxide in acetonitrile at reflux results in the generation of the corresponding vicinal diols in high selectivity and yield at high conversion of substrate. Several spectroscopic and kinetics experiments, including a novel one correlating the incubation time of TBA1 under the reaction conditions with the rates of alkene oxidation, establish that TBA1 functions primarily as a catalyst precursor and that much of the catalytic activity is derived from generation of tungstate under the reaction conditions.

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