183W INADEQUATE 2D NMR Spectroscopy of Hetero Arsenato-Phosphato-Tungstate PV/AsV Substitution in Dawson-Type α-[AsxP2-xW18O62]6- (x = 0-2) and α-[H4AsyP1-yW18O62]7- (y = 0, 1)

The Dawson-type arsenato-phosphato-tungstate α-[AsPW18O62]6- has been prepared and unambiguously identified for the first time. A comparative study including the four other already known compounds, the symmetric α-[X2W18O62]6- and unsymmetric α-[H4XW18O62]7- for X = PV, AsV, has been performed by spectroscopic 183W and 31P NMR. 2-D 183W INADEQUATE experiments were systematically employed to unequivocally verify structures, assign all resonances, and determine precisely 2JW-O-W scalar couplings. The effects of P/As substitutions, generating unsymmetric structures, on the NMR observables δW-183, δP-31, 2JW-O-W, and 2JW-O-P are discussed in relation to their bond length and bond angle alteration. General trends with respect to NMR parameter variations have been found when filling central cavities with P or As atoms, with less pronounced effects for As than for P. In addition, NMR characterization of three other isomers, i.e., β-[X2W18O62]6- and γ-[As2W18O62]6-, were also provided for comparison. The present NMR results could serve as representative reference data for understanding the relationship between structure and NMR observables in polyoxotungstates. [ABSTRACT FROM AUTHOR]