Arylarsonate- and Phosphonate-Capped Polyoxomolybdates, [(RCHAs)MoO] and [(R'CHP)MoO].

We report on the synthesis and structural characterization of four arylarsonate- and phosphonate-capped polyoxomolybdates that exhibit different organic substituents in the para position of the phenyl group. The reaction of arylarsonates (RAsO, wherein R = 4-BrCH or 4-NCH) with molybdate in aqueous pH 3.5 media resulted in the cyclic hexamolybdates [(BrCHAs)MoO] () and [(NCHAs)MoO] (), whereas the reaction of arylphosphonates (R'PO, wherein R' = 4-OCCH or 4-OCCHCH) with molybdate in aqueous pH 3 media resulted in the cyclic pentamolybdates [(OCCHP)MoO] () and [(HOCCHCHP)MoO] (), respectively. Polyanions and comprise a ring of six MoO octahedra that is capped on either side by an organoarsonate group, whereas and consist of a ring of five MoO octahedra that is capped on either side by an organophosphonate group, with the organic arms protruding away from the metal-oxo core of the polyanions. All four polyanions , , , and have been characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analysis and in solution by multinuclear NMR (P, C, and H). The synthetic procedure of (4-bromophenyl)arsonic acid, BrCHAsOH, is reported here for the first time.