Arylarsonate- and Phosphonate-Capped Polyoxomolybdates, [(RC₆H₄As)₂Mo₆O₂₄] and [(R'C₆H₄P)₂Mo₅O₂₁]

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 (RAsO₃, wherein R = 4-BrC₆H₄ or 4-N₃C₆H₄) with molybdate in aqueous pH 3.5 media resulted in the cyclic hexamolybdates [(BrC₆H₄As)₂Mo₆O₂₄]^4- (MoAsL) and [(N₃C₆H₄As)₂Mo₆O₂₄]^4- (MoAsL), whereas the reaction of arylphosphonates (R'PO₃, wherein R' = 4-O₂CC₆H₄ or 4-O₂CC₆H₄CH₂) with molybdate in aqueous pH 3 media resulted in the cyclic pentamolybdates [(O₂CC₆H₄P)₂Mo₅O₂₁]^6- (MoPL) and [(HO₂CC₆H₄CH₂P)₂Mo₅O₂₁]^4- (MoPL), respectively. Polyanions MoAsL and MoAsL comprise a ring of six MoO₆ octahedra that is capped on either side by an organoarsonate group, whereas MoPL and MoPL consist of a ring of five MoO₆ 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 MoAsL, MoAsL, MoPL, and MoPL 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 (³¹P, ¹³C, and ¹H). The synthetic procedure of (4-bromophenyl)arsonic acid, BrC₆H₄AsO₃H₂, is reported here for the first time.