Syntheses, characterization and catalytic activity of peroxo tantalum polyoxotungstates

Peroxo-tantalum and tantalum mono-substituted tungstosilicates with the Keggin structure, K-5[alpha-SiW11Ta(O-2) O-39] 18H(2)O ( 1) and K-5[alpha-SiW11TaO40] 14H(2)O ( 2) respectively, and peroxo-tantalum and tantalum monosubstituted tungstophosphates with the Wells - Dawson structure K-7[alpha(2)- P2W17Ta(O-2) O-61] 16H(2)O ( 3) and K-7[alpha(2)-P2W17TaO62] 14H(2)O (4), respectively, were prepared by stereoselective synthesis and characterized by elemental analysis, i.r. and u. v. spectroscopy, XPS, P-31 and W-183 n.m.r. spectroscopy, and polarography, as well as cyclic voltammetry. The presence of a peroxo group in the complexes ( 1) and ( 3) was supported by observation of an i.r. band due to the -O-O- bond at 881 cm(-1), the charge transfer bands of (eta(2)-O-2) --> Ta at ca. 316 and 389 nm, the O-1s binding energy at 532.0 eV and a new irreversible reduction polarographic wave at E-1/2 approximate to 0.1 V, assigned to the reduction of peroxo group and which enhances the oxidant properties of the heteropoly anions. There are six lines for complex ( 1) and nine lines for complex ( 3) in the W-183 n. m. r. spectra, showing that the complex ( 1) has a mono-substituted Keggin structure and complex ( 3) has a mono-substituted a2- Wells - Dawson structure. The catalytic activities of the complexes with respect to epoxidation of maleic acid with H2O2 as oxidant were examined and the best result, 83.5% yield, was obtained for [(C4H9)(4)N](5)[alpha-SiW11Ta(O)(2)O-39] center dot 18H(2)O.