Fluorinated Arylarsonate-Containing Polyoxomolybdates: pH-Dependent Formation of Mo₆ vs Mo₁₂ Species and Their...

We report on the synthesis and structural characterization of six novel arylarsonate-containing polyoxomolybdates with fluorinated-functionalities in the para position of the phenyl ring. The reaction of the various arylarsonic acids, RAsO₃H₂ [R = 4-F-C₆H₄ (H₂L), 4-F₃C-C₆H₄ (H₂L), 4-F₃CO-C₆H₄ (H₂L)] with Na₂MoO₄·2H₂O in aqueous pH 3 solution resulted in the heteropoly-6-molybdates [{(4-F-C₆H₄)As}₂Mo₆O₂₄(H₂O)]^4- (1), [{(4-F₃C-C₆H₄)As}₂Mo₆O₂₄]^4- (2) and [{(4-F₃CO-C₆H₄)As}₂Mo₆O₂₄(H₂O)]^4- (3), which were isolated as guanidinium salts. When the reaction was performed in aqueous pH 1 solution the inverted-Keggin type heteropoly-12-molydates [{(4-F-C₆H₄)As}₄Mo₁₂O₄₆]^4- (4), [{(4-F₃C-C₆H₄)As}₄Mo₁₂O₄₆]^4- (5) and [{(4-F₃CO-C₆H₄)As}₄Mo₁₂O₄₆]^4- (6), were obtained and isolated as sodium salts. The 6-molybdates 1-3 and the 12-molybdates 4-6 can be easily interconverted reversibly in solution as a function of pH (3 vs 1). Polyanions 1 and 3 are isostructural and they exhibit a bent hexamolybdate ring, whereas the ring is flat for 2. The inverted-Keggin polyanions 4-6 are isostructural and the metal-oxo core is capped by four arylarsonate groups. All six polyanions have been characterized in the solid state by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, and hermogravimetric analysis as well as in solution by multinuclear NMR (¹H, ¹⁹F). The synthetic procedures for the arsonic acids (4-F₃C-C₆H₄)AsO₃H₂ (H₂L) and (4-F₃CO-C₆H₄)AsO₃H₂ (H₂L) are reported for the first time.