TRINIOBIUM POLYTUNGSTOPHOSPHATES - SYNTHESES, STRUCTURES, CLARIFICATION OF ISOMERISM AND REACTIVITY IN THE PRESENCE OF H2O2

The reaction of K-7[HNb6O19], H2O2 and A-Na-9[PW9O34] in water followe d by treatment with Cs+ or (n-Bu4N)(+) (TBA) affords the corresponding salts of the tris(peroxoniobium) heteropolyanion A,beta-[(NbO2)(3)PW9 O37](6-) (1) in similar to 60% isolated yields. An X-ray structure of the Cs salt, Cs1 (monoclinic P2/c; a = 16.92360(10) Angstrom, b =13.57 21-(2) Angstrom, c = 22.31890(10) Angstrom, beta = 92.0460(10)degrees; and Z = 4) confirms the A-type substitution pattern of the three Nb a toms 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 N b atoms give 1 an overall symmetry approximating the chiral C-3. These terminal peroxo ligands, and these groups only, thermally decompose w hen either Cs1 or TBA1 is in solution unless additional H2O2 is presen t. The peroxo groups can be titrated with triphenylphosphine (2.8 +/- 0.3 peroxide groups found per molecule). Refluxing TBA1 in acetonitril e far 24 h in the presence of base generates the parent heteropolyanio n, [Nb3PW9O40](6-) (2) in 80% yield after isolation. Treatment of 2 wi th glacial acetic acid in acetonitrile converts it to [Nb6P2W18O77](6- ) (3) in similar to 100% yield, while treatment of TBA3 with hydroxide converts it back to 2 in high yield. Spectroscopic (FTIR, Raman, W-18 3 NMR, and P-31 NMR), titrimetric, mass spectrometric (FABMS), and ele mental analysis data are all consistent with these formulas. The addit ion of TBA1 to solutions of alkenes and 33% aqueous peroxide in aceton itrile at reflux results in the generation of the corresponding vicina l 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 pri marily as a catalyst precursor and that much of the catalytic activity is derived from generation of tungstate under the reaction conditions .