Fluorinated\nArylarsonate-Containing Polyoxomolybdates:\npH-Dependent Formation of Mo<sub>6</sub> vs Mo<sub>12</sub> Species\nand Their Solution Properties

We report on the\nsynthesis and structural characterization\nof six\nnovel arylarsonate-containing polyoxomolybdates with fluorinated-functionalities\nin the <i>para</i> position of the phenyl ring. The reaction\nof the various arylarsonic acids, RAsO<sub>3</sub>H<sub>2</sub> [<i>R</i> = 4-F–C<sub>6</sub>H<sub>4</sub> (H<sub>2</sub><b>L</b><sub><b>F</b></sub>), 4-F<sub>3</sub>C–C<sub>6</sub>H<sub>4</sub> (H<sub>2</sub><b>L</b><sub><b>CF3</b></sub>), 4-F<sub>3</sub>CO–C<sub>6</sub>H<sub>4</sub> (H<sub>2</sub><b>L</b><sub><b>OCF3</b></sub>)] with Na<sub>2</sub>MoO<sub>4</sub>·2H<sub>2</sub>O in aqueous pH 3 solution resulted\nin the heteropoly-6-molybdates [{(4-F–C<sub>6</sub>H<sub>4</sub>)As}<sub>2</sub>Mo<sub>6</sub>O<sub>24</sub>(H<sub>2</sub>O)]<sup>4–</sup> (<b>1</b>), [{(4-F<sub>3</sub>C–C<sub>6</sub>H<sub>4</sub>)As}<sub>2</sub>Mo<sub>6</sub>O<sub>24</sub>]<sup>4–</sup> (<b>2</b>) and [{(4-F<sub>3</sub>CO–C<sub>6</sub>H<sub>4</sub>)As}<sub>2</sub>Mo<sub>6</sub>O<sub>24</sub>(H<sub>2</sub>O)]<sup>4–</sup> (<b>3</b>), which were isolated\nas guanidinium salts. When the reaction was performed in aqueous pH\n1 solution the inverted-Keggin type heteropoly-12-molydates [{(4-F–C<sub>6</sub>H<sub>4</sub>)As}<sub>4</sub>Mo<sub>12</sub>O<sub>46</sub>]<sup>4–</sup> (<b>4</b>), [{(4-F<sub>3</sub>C–C<sub>6</sub>H<sub>4</sub>)As}<sub>4</sub>Mo<sub>12</sub>O<sub>46</sub>]<sup>4–</sup> (<b>5</b>) and [{(4-F<sub>3</sub>CO–C<sub>6</sub>H<sub>4</sub>)As}<sub>4</sub>Mo<sub>12</sub>O<sub>46</sub>]<sup>4–</sup> (<b>6</b>), were obtained and isolated\nas sodium salts. The 6-molybdates <b>1</b>–<b>3</b> and the 12-molybdates <b>4</b>–<b>6</b> can be\neasily interconverted reversibly in solution as a function of pH (3\nvs 1). Polyanions <b>1</b> and <b>3</b> are isostructural\nand they exhibit a bent hexamolybdate ring, whereas the ring is flat\nfor <b>2</b>. The inverted-Keggin polyanions <b>4</b>–<b>6</b> are isostructural and the metal-oxo core is capped by four\narylarsonate groups. All six polyanions have been characterized in\nthe solid state by single-crystal X-ray diffraction, Fourier transform\ninfrared spectroscopy, and hermogravimetric analysis as well as in\nsolution by multinuclear NMR (<sup>1</sup>H, <sup>19</sup>F). The\nsynthetic procedures for the arsonic acids (4-F<sub>3</sub>C–C<sub>6</sub>H<sub>4</sub>)AsO<sub>3</sub>H<sub>2</sub> (H<sub>2</sub><b>L</b><sub><b>CF3</b></sub>) and (4-F<sub>3</sub>CO–C<sub>6</sub>H<sub>4</sub>)AsO<sub>3</sub>H<sub>2</sub> (H<sub>2</sub><b>L</b><sub><b>OCF3</b></sub>) are reported for the first\ntime.