DAWSON TYPE HETEROPOLYANIONS .1. MULTINUCLEAR (P-31, V-51, W-183) NMR STRUCTURAL INVESTIGATIONS OF OCTADECA(MOLYBDOTUNGSTOVANADO)DIPHOSPHATES ALPHA-1,2,3-[P2MM'2W15O62]N- (M, M' = MO, V, W) - SYNTHESES OF NEW RELATED-COMPOUNDS

The mixed binary anions dimolybdopentadecatungstodiphosphate, 1,2-[P2Mo2W15O61]10-, hexadecatungstodivanadodiphosphate, 1,2-[P2V2W16O62]8-, and the ternary molybdotungstovanadophosphates 1,2-3-[P2Mo2VW15O62]7- and 1-2,3-[P2MoV2W15O62]8-, derived from the Dawson anion alpha-[P2W18O62]6- by formal substitution in one polar W3O13 group, have been synthesized. The lacunary dimolybdo anion P2Mo2W15, obtained by selective W elimination and then partial Mo addition, is the precursor for the saturated P2Mo2W16 and P2Mo2VW15. The divanado anions P2V2W16 and P2MoV2W15 were obtained by V substitution from P2W18 and P2Mo3W15, respectively. All the octadeca(molybdotungstovanado)diphosphates alpha-(1,2,3)-[P2MM'2W15O62]n- (M, M' = Mo, V, W) were characterized by electrochemistry and multinuclear (P-31, V-51, W-183) solution NMR techniques. An empirical incremental formula was established to calculate the P-31 chemical shift of the P(1) phosphorus atom in the half-anion PMM'2W6 substituted in the polar position. Moreover, a simple additive model allows one to predict the P-31 chemical shifts of Dawson polyanions substituted in the equatorial position. The reactivity of 4-P2MW17 toward alkaline degradation to get P2MW14 is discussed on this basis: vanadium(V) in the 4-location directs the departure of the W3O13 in the other half-anion (16-18-locations) whereas molybdenum(VI) has no directing effect. The W-183 chemical shifts are discussed also in relation with the nature and the location of the substituting atoms; although considerably remote from the polar substituted site, the tungsten atoms in the opposite polar group are generally as well affected (shielded) as those directly bound (mu-oxo junction in the same polar group) to the substituted atom (deshielded), even in the case of the isocharge W --> Mo replacement.