Directing the Linkage of Small Polyoxometalate Building\nBlocks Using (Benzene)ruthenium Cations

Oligomerization of\nmonomeric molybdate and tungstate oxyanions\nin the presence of organometallic cations produces a group of organometallic-polyoxometalate\nclusters, which have diverse structures based on cubane-like {M<sub>4</sub>O<sub>4</sub>} units resembling the structure of oxide surfaces.\nThis work investigated the oligomerization of [MoO<sub>4</sub>]<sup>2–</sup> and [WO<sub>4</sub>]<sup>2–</sup> oxyanions\nin aqueous solutions in the presence of {Ru(C<sub>6</sub>H<sub>6</sub>)}<sup>2+</sup> as the organometallic structure-directing agents.\nThe reactions produced a mixture of several species, and fractional\ncrystallization by adjusting crystallization temperature and molar\nratios of Ru:Mo or Ru:W allowed the isolation of seven types of (benzene)ruthenium–polyoxometalate\ncomplexes: [{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] (<b>1</b>), [{Ru(C<sub>6</sub>H<sub>6</sub>)(OH)}<sub>3</sub>HMoO<sub>4</sub>][MoO<sub>4</sub>] (<b>2</b>), [{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>4</sub>H<sub>2</sub>Mo<sub>5</sub>O<sub>20</sub>] (<b>3</b>), [{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>4</sub>W<sub>2</sub>O<sub>10</sub>] (<b>4</b>), Na<sub>6</sub>[{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>2</sub>H<sub>2</sub>W<sub>8</sub>O<sub>30</sub>] (<b>5</b>), [{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>5</sub>H<sub>2</sub>W<sub>6</sub>O<sub>24</sub>] (<b>6</b>), and [{Ru(C<sub>6</sub>H<sub>6</sub>)(OH)}<sub>6</sub>W<sub>2</sub>O<sub>7</sub>][{Ru(C<sub>6</sub>H<sub>6</sub>)}<sub>2</sub>{Ru(C<sub>6</sub>H<sub>6</sub>)(OH<sub>2</sub>)}<sub>2</sub>H<sub>2</sub>W<sub>8</sub>O<sub>30</sub>]<sub>2</sub> (<b>7</b>). They comprise small, reactive <i>iso</i>-polyoxometalate\nbuilding blocks that are capped by {Ru(C<sub>6</sub>H<sub>6</sub>)}<sup>2+</sup> cations, resulting in cubane-like {Ru<sub>3</sub>MO<sub>4</sub>}, {Ru<sub>2</sub>M<sub>2</sub>O<sub>4</sub>}, or {RuM<sub>3</sub>O<sub>4</sub>} (M = Mo or W) structural motifs. Moreover,\nthe molecular crystal of <b>1</b> contains extensive C–H···O\nhydrogen bonds, and it undergoes a reversible crystal-to-amorphous-to-crystal\ntransition upon dehydration–hydration cycles.