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

Oligomerization of monomeric molybdate and tungstate oxyanions in the presence of organometallic cations produces a group of organometallic-polyoxometalate clusters, which have diverse structures based on cubane-like {M₄O₄} units resembling the structure of oxide surfaces. This work investigated the oligomerization of [MoO₄]^2- and [WO₄]^2- oxyanions in aqueous solutions in the presence of {Ru(C₆H₆)}²⁺ as the organometallic structure-directing agents. The reactions produced a mixture of several species, and fractional crystallization by adjusting crystallization temperature and molar ratios of Ru:Mo or Ru:W allowed the isolation of seven types of (benzene)ruthenium-polyoxometalate complexes: [{Ru(C₆H₆)}₄Mo₄O₁₆] (<b>1</b>), [{Ru(C₆H₆)(OH)}₃HMoO₄][MoO₄] (<b>2</b>), [{Ru(C₆H₆)}₄H₂Mo₅O₂₀] (<b>3</b>), [{Ru(C₆H₆)}₄W₂O₁₀] (<b>4</b>), Na₆[{Ru(C₆H₆)}₂H₂W₈O₃₀] (<b>5</b>), [{Ru(C₆H₆)}₅H₂W₆O₂₄] (<b>6</b>), and [{Ru(C₆H₆)(OH)}₆W₂O₇][{Ru(C₆H₆)}₂{Ru(C₆H₆)(OH₂)}₂H₂W₈O₃₀]₂ (<b>7</b>). They comprise small, reactive <i>iso</i>-polyoxometalate building blocks that are capped by {Ru(C₆H₆)}²⁺ cations, resulting in cubane-like {Ru₃MO₄}, {Ru₂M₂O₄}, or {RuM₃O₄} (M = Mo or W) structural motifs. Moreover, the molecular crystal of <b>1</b> contains extensive C-H···O hydrogen bonds, and it undergoes a reversible crystal-to-amorphous-to-crystal transition upon dehydration-hydration cycles.