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

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Abstract
The reaction of K7[HNb6O19], H2O2 and A-Na9[PW9O34] in water followed by treatment with Cs+ or (n-Bu4N)+ (TBA) affords the corresponding salts of the tris(peroxoniobium) heteropolyanion A,β-[(NbO2)3PW9O37]6- (1) in ∼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, β = 92.0460(10)°, and Z = 4) confirms the A-type substitution pattern of the three Nb atoms and clarifies the M3 rotational (Baker−Figgis) isomerism in the Keggin unit as β. The three terminal η2-O22- groups on the Nb atoms give 1 an overall symmetry approximating the chiral C3. These terminal peroxo ligands, and these groups only, thermally decompose when either Cs1 or TBA1 is in solution unless additional H2O2 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, [Nb3PW9O40]6- (2) in 80% yield after isolation. Treatment of 2 with glacial acetic acid in acetonitrile converts it to [Nb6P2W18O77]6- (3) in ∼100% yield, while treatment of TBA3 with hydroxide converts it back to 2 in high yield. Spectroscopic (FTIR, Raman, 183W NMR, and 31P 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.