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

Abstract

Oligomerization of monomeric molybdate and tungstate oxyanions in the presence of organometallic cations produces a group of organometallicpolyoxometalate clusters, which have diverse structures based on cubane-like {M 4 O 4 } units resembling the structure of oxide surfaces. This work investigated the oligomerization of [MoO 4 ] 2− and [WO 4 ] 2− oxyanions in aqueous solutions in the presence of {Ru(C 6 H 6 )} 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(C 6 H 6 )} 4 Mo 4 O 16 ] (1), [{Ru(C 6 H 6 )(OH)} 3 HMoO 4 ][MoO 4 ] (2), [{Ru(C 6 H 6 )} 4 H 2 Mo 5 O 20 ] (3), [{Ru(C 6 H 6 )} 4 W 2 O 10 ] (4), Na 6 [{Ru(C 6 H 6 )} 2 H 2 W 8 O 30 ] (5), [{Ru(C 6 H 6 )} 5 H 2 W 6 O 24 ] (6), and [{Ru(C 6 H 6 )(OH)} 6 W 2 O 7 ][{Ru(C 6 H 6 )} 2 {Ru(C 6 H 6 )(OH 2 )} 2 H 2 W 8 O 30 ] 2 (7). They comprise small, reactive iso-polyoxometalate building blocks that are capped by {Ru(C 6 H 6 )} 2+ cations, resulting in cubane-like {Ru 3 MO 4 }, {Ru 2 M 2 O 4 }, or {RuM 3 O 4 } (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.