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] (1), [{Ru(C6H6)(OH)}3HMoO4][MoO4] (2), [{Ru(C6H6)}4H2Mo5O20] (3), [{Ru(C6H6)}4W2O10] (4), Na6[{Ru(C6H6)}2H2W8O30] (5), [{Ru(C6H6)}5H2W6O24] (6), and [{Ru(C6H6)(OH)}6W2O7][{Ru(C6H6)}2{Ru(C6H6)(OH2)}2H2W8O30]2 (7). They comprise small, reactive iso-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 1 contains extensive C-H···O hydrogen bonds, and it undergoes a reversible crystal-to-amorphous-to-crystal transition upon dehydration-hydration cycles.