UV, IR, 71Ga and 17O NMR Spectroscopic studies of the 12-Molybdogallate Heteropolyanion

Abstract

Abstract UV, IR, 7Ga and 17O NMR spectra clearly demonstrate the Keggin structure for the new heteropolyanion—12-molybdogallate (12-MGA). The UV spectrum of the 12-MGA solution has charge-transfer bands at 49,000, 40,000 and 32,500 cm−1, which have been attributed to oxygen-to-molybdenum(VI) transitions. The IR spectra of the synthesized samples of 12-MGA contain the expected triplet of bands, which could be assigned to molybdenum-oxygen stretching vibrations. The data obtained from the 71Ga and 17O NMR spectroscopy confirmed the tetrahedral oxygen environment of the gallium atoms and the Keggin structure of 12-MGA. The 71Ga NMR chemical shifts of crystalline (TEBA)5GaMo12O40 (TEBA—triethylbenzyl ammonium) as well as of aqueous solutions of 12-MGA are situated in the range characteristic of a tetrahedral oxygen environment of gallium (223 ppm relative to Ga(H2O)63+). In these solutions GaMo6O24H63− polyanions can co-exist (52 ppm). The 17O NMR spectrum of Na5GaMo12O40 has three resonances with chemical shifts of 923, 575 and 568 ppm (relative to H2O) and an intensity ratio 1:1:1, which is similar to the spectra of other Keggin heteropolyanions. The changes of the nature and length of the MoO bonds in the 12-molybdogallate relative to the parent 12- molybdophosphate are discussed.