Synthese et etude structurale par RMN ~(31)P et ~(183)W d'un octqadecatungstomonophosphate de type Dawson [H_4PW_(18)O_(62)]~(7-) et de deux de ses derives, [H_4PW_(17)O_(61)]~(11-) et [Zn(H_2O)(H_4PW_(17)O_(61))]~(9-)

摘要

sYNTHESES AND ~(31)P and ~(183)W NMR structural characterization of a novel Dawson-type tungstophosphate [H_4PW_(18)O_(62)]~(7-) and of its lacunary and zinc-substituted derivatives [H_4PW_(17)O_(61)]~(11-) and [Zn(H_2O)(H_4PW_(17)O_(61))]~(9-). Three novel polyoxotungstophosphates have been synthesized as potassium salts: a Dawson-type octadecatungstomonophosphate, K_7[H_4PW_(18)O_(62)] and its lacunary and zinc-substituted derivatives K_(11)[H_4PW_(17)O_(61)] and K_9[Zn(H_2O)(H_4PW_(17)O_(61))]. The saturated compound was obtained by refluxing for 60 h amixture of sodium tungstate and phosphoric acid(W/P = 20) at pH 2.2.~(31)P-HMR spectroscopy showed the formation of a single product only after prolonged heating, while a mixture of three isomers was initially obtained. The structure of [H_4PW_(18)O_(62)]~(7-) (abbreviated as PW_(18)) and of its derivatives was inferred from the comparison of their spectroscopic and electrochemical features with those of [P_2W_(18)O_(62)]~(6-)(P_2W_(18)). The ~(31)p-NMR spectrum of each of these species displays a unique signal at -6.7 ppm PW_(18), -O.25ppm for PW_(17) and -2.95 ppm for PW_(17)Zn (-8.6 ppm), thedata for PW_(17) and PW_(17)Zn are consistent with the removal or the replacement of one tungsten atom in a PW_9 half-unit. Moreover, in the region of the P-O stretching vibrations the IR spectra of PW_(17) and PW_(17)Zn exhibit only one band, which is in favour of the removal of a tungsten atom fromthe cap of thee PW_9 unit. The ~(183)W-NMR spectrum of PW_(18) exhibits four resonances at -181.3 (6W; d ~2J_w-o-p = 1.46 hz) and -144.9 ppm (3W; d, ~2J_(W-O-P) = 1.26 HZ) assigned t o the PW_I unit, and at -127.8 ppm (6W;s) and -135.7 ppm (3W;S) assigned to the H_nW_(18) anion, like P_2W_(18) consists of two A-type half-anions. Attempts t o locate the four protons by ~1H-NMR and to solve the crystal structure of PW_(18) by X-ray diffraction have been so far unsuccessful; however the isolated compound is probably the #alpha# isomer and the two minor products could be the #beta# or #gamma# isomers. The related tungstoarsenates were also obtained in similar conditions