Supramolecular networks supported by the anion...π linkage of Keggin-type heteropolyoxotungstates.

Anion...π interactions are newly recognized weak supramolecular forces which are relevant to many types of electron-deficient aromatic substrates. Being less competitive with respect to conventional hydrogen bonding, anion...π interactions are only rarely considered as a crystal-structure-defining factor. Their significance dramatically increases for polyoxometalate (POM) species, which offer extended oxide surfaces for maintaining dense aromatic/inorganic stacks. The structures of tetrakis(caffeinium) μ-silicato-tetracosa-μ-oxido-dodecaoxidododecatungsten trihydrate, (CHNO)[SiWO]·3HO, (1), and tris(theobrominium) μ-phosphato-tetracosa-μ-oxido-dodecaoxidododecatungsten ethanol sesquisolvate, (CHNO)[PWO]·1.5CHOH, (2), support the utility of anion...π interactions as a special kind of supramolecular synthon controlling the structures of ionic lattices. Both caffeinium [(HCaf) in (1)] and theobrominium cations [(HTbr) in (2)] reveal double stacking patterns at both axial sides of the aromatic frameworks, leading to the generation of anion...π...anion bridges. The latter provide the rare face-to-face linkage of the anions. In (1), every square face of the metal-oxide cuboctahedra accepts the interaction and the above bridges yield flat square nets, i.e. {(HCaf)[SiWO]}. Two additional cations afford single stacks only and they terminate the connectivity. Salt (2) retains a two-dimensional (2D) motif of square nets, with anion...π...anion bridges involving two of the three (HTbr) cations. The remaining cations complete a fivefold anion...π environment of [PWO], acting as terminal groups. This single anion...π interaction is influenced by the specific pairing of (HTbr) cations by double amide-to-amide hydrogen bonding. Nevertheless, invariable 2D patterns in (1) and (2) suggest the dominant role of anion...π interactions as the structure-governing factor, which is applicable to the construction of noncovalent linkages involving Keggin-type oxometalates.