The Characterization of a New Heteropolytungstoarsonate Anion, (7-). Topological Relationships among Ions Related to the Lindqvist Structure

Article abstract of DOI:10.1021/ja00303a016

Guanidinium salts of the anions (7-), where R=CH3, C6H5, and p-NH2-C6H4, have been isolated in crystalline form from weakly basic solutions (pH 7-8.5) of RAsO3(2-) and WO4(2-).Crystals of the methyl derivative as (CN3H6)7*3H2O are orthorhombic, space group Pnma, with a=17.605 (3) Angstroem, b=13.179 (1) Angstroem, and c=19.869 (3) Angstroem, Z=4, d_cacd=3.29 g cm^-3, and d_obsd=3.32 (2) g cm^-3.Mirror symmetry is imposed upon the anion and upon several cation sites.Least-squares refinement (2184 data with I>3?(I) and (sin θ)/λ<0-5958 Angstroem^-1) converged at values for R and R_W of 0.044 and 0.045.The CH3AsO3 moiety sits above one or the other of two chemically distinct triangular O3 sites in a 75:25 ratio.The W7O24 group comprises a horseshoe of four edge-shared WO6 octahedra linked at four vertices to a triangle of edge-shared octahedra.This group is topologically related to the Lindqvist M7O24(6-) structure.Some selected metrical details for the major isomer are given with the esd of an individual bond and the esd of the average, respectively, in parentheses; 13(W=O) 1.73 (2,3); 16(W-O_bridging) 1.93 (1,6); 2(W-O_{triply bridging}) 1.980 (13) and 2.213 (12); 3(W-O_As) 2.201 (12)-2.47 (2) Angstroem.The proton is almost certainly located for the major isomer on the doubly bridging oxygen atom to which the As' is attached in minor isomer (W-O=2.215 (12) Angstroem) and very likely for the minor isomer on the triply bridging oxygen atom to which As is attached for the major isomer, although in this case the expected extension of the W-O separation is masked.In the solid state the 13C CPMAS NMR spectrum shows the methyl resonance at 20.3 ppm with a shoulder at ca. 17 ppm attributed to the minor isomer.The heteropolyanion (1H NMR, 1.98 ppm, methyl) is rapidly converted into <(CH3As)2W6O25H)(5-) (2.1, 2.4 ppm) and/or CH3AsO3(2-) (1.8 ppm) in aqueous solution.Plausible mechanisms connecting W7O24(6-) (paratungstate-A, Lindqvist structure) and RAsO3(2-) with both isomers of the title heteropolyanion are described.