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) μ12-silicato-tetracosa-μ2-oxido-dodecaoxidododecatungsten trihydrate, (C8H11N4O2)4[SiW12O40]·3H2O, (1), and tris(theobrominium) μ12-phosphato-tetracosa-μ2-oxido-dodecaoxidododecatungsten ethanol sesquisolvate, (C7H9N4O2)3[PW12O40]·1.5C2H5OH, (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+)2[SiW12O40]4-}n. 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 [PW12O40]3-, 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.
Keywords: Hirshfeld surface; Keggin anion; POM; anion...π interactions; caffeine; crystal structure; heteropolyoxotungstate; hydrogen bonding; theobromine.