Syntheses and 31P and 183W NMR structural characterization of a novel Dawson-type tungstophosphate [H4PW18O62]7- and of its lacunary and zinc-substituted derivatives [H4PW17O61]11- and [Zn(H2O)(H4PW17O61)]19-

Three novel polyoxotungstophosphates have been synthesized as potassium salts: a Dawson-type octadec-atungstomonophosphate, K-7[H4PW18O62] and its lacunary and zinc-substituted derivatives K-11[H4PW17O61,] an K-9[Zn(H2O)(H4PW17O61)]. The saturated compound was obtained by refluxing for 60 h a mixture of sodium tungstate and phosphoric acid (W/P = 20) at pH 2.2. P-31-NMR spectroscopy showed the formation of a single product only after prolonged heating, while a mixture of three isomers was initially obtained. The derivatives were obtained by controlled alkaline degradation as usual for Dawson-type compounds. The structure of [H4PW18O62](7-) (abbreviated as PW18) and of its derivatives was inferred from the comparison of their spectroscopic and electrochemical features with those of [P2W18O62](6-) (P2W18). The P-31-NMR spectrum of each of these species displays a unique signal at - 6.7 ppm for PW18, - 2.05 ppm for PW17 and - 2.95 ppm for PW17Zn. When compared with P2W18 ( - 12.45 ppm), with the PW8 unit of alpha(2)-P2W17 (- 7.1 ppm) and the PW8Zn unit of alpha(2)-P2W17Zn (- 8.6 ppm), the data for PW17 and PW17Zn are consistent with the removal or the replacement of one tungsten atom in a PW9 half-unit. Moreover, in the region of the P-O stretching vibrations the IR spectra of PW17 and PV17Zn exhibit only one band, which is in favour of the removal of a tungsten atom from the cap of the PW9 unit. The W-183-NMR spectrum of PW18 exhibits four resonances at - 181.3 (6W; d, (2)J(W-O-P) = 1.46 Hz) and -144.9 ppm (3W; d, (2)J(W-O-P) = 1.26 Hz) assigned to the PW9 unit, and at -127.8 ppm (6W; s) and -135.7 ppm (3W; s) assigned to the HnW9 unit. The three homonuclear 2J(W-O-W) coupling constants are in the range 20-27 Hz, which further confirms that the PW18 anion, like P2W18 consists of two A-type half-anions. Attempts to locate the four protons by H-1-NMR and to solve the crystal structure of PW,, by X-ray diffraction have been so far unsuccessful; however the isolated compound is probably the ct isomer and the two minor products could be the beta or gamma isomers. The I elated tungstoarsenates were also obtained in similar conditions. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.