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

We report on the synthesis and structural characterization of six novel arylarsonate-containing polyoxomolybdates with fluorinated-functionalities in the <i>para</i> position of the phenyl ring. The reaction of the various arylarsonic acids, RAsO₃H₂ [<i>R</i> = 4-F-C₆H₄ (H₂<b>L</b>_<b>F</b>), 4-F₃C-C₆H₄ (H₂<b>L</b>_<b>CF3</b>), 4-F₃CO-C₆H₄ (H₂<b>L</b>_<b>OCF3</b>)] with Na₂MoO₄·2H₂O in aqueous pH 3 solution resulted in the heteropoly-6-molybdates [{(4-F-C₆H₄)As}₂Mo₆O₂₄(H₂O)]^4- (<b>1</b>), [{(4-F₃C-C₆H₄)As}₂Mo₆O₂₄]^4- (<b>2</b>) and [{(4-F₃CO-C₆H₄)As}₂Mo₆O₂₄(H₂O)]^4- (<b>3</b>), 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- (<b>4</b>), [{(4-F₃C-C₆H₄)As}₄Mo₁₂O₄₆]^4- (<b>5</b>) and [{(4-F₃CO-C₆H₄)As}₄Mo₁₂O₄₆]^4- (<b>6</b>), were obtained and isolated as sodium salts. The 6-molybdates <b>1</b>-<b>3</b> and the 12-molybdates <b>4</b>-<b>6</b> can be easily interconverted reversibly in solution as a function of pH (3 vs 1). Polyanions <b>1</b> and <b>3</b> are isostructural and they exhibit a bent hexamolybdate ring, whereas the ring is flat for <b>2</b>. The inverted-Keggin polyanions <b>4</b>-<b>6</b> 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 (H₂<b>L</b>_<b>CF3</b>) and (4-F₃CO-C₆H₄)AsO₃H₂ (H₂<b>L</b>_<b>OCF3</b>) are reported for the first time.