Anion-π stacks of Lindqvist superoctahedra [Mo6O19]2− supported by caffeinium and theophyllinium cations

[Display omitted] •Two new caffeinium and theophyllinium Lindqvist hexamolybdates reveal significance of anion⋅⋅⋅π interactions.•Electron deficient aromatic cations generate anion⋅⋅⋅π⋅⋅⋅anion links when stacked to the oxide surfaces of POM anions.•Additional CH3 group at the cation frame productively contributes to the total interaction energies.•Hirshfeld surface analysis detects enhancement of anion⋅⋅⋅π bonding in the case of caffeinium vs. theophyllinium. Ionic lattices of two new Lindqvist polyoxometalates (HCaf)2[Mo6O19]⋅2EtOH (1) and (HThe)2[Mo6O19]⋅H2O (2) [Caf = caffeine, The = theophylline] reveal extensive anion⋅⋅⋅π interactions, which result in the generation of dense stacks of the electron deficient aromatic cations to the faces of metal-oxide polyhedra with interplanar distances of 2.989(5)-3.048(4) Å and interplanar angles of 2.60(13)-7.35(13)o. These interactions are applicable to the construction of purine/ polyoxometalate solids as special kinds of supramolecular synthons providing linkage of metal-oxide polyhedra through anion⋅⋅⋅π⋅⋅⋅anion links. Hirshfeld surface analysis for the individual anions in 1 and 2 as well as for related structures in CSD supports the relevance of this new interaction for crystal chemistry of polyoxometalates. The total interaction energies of the {(Cation)4[Mo6O19]}2+ synthons are evaluated and compared by using DFT calculations, which suggest larger stability of the (HCaf)+ complexes (–290.3 kcal/mol for 1 and –250.9 for 2) conditioned by additional CH···O contacts involving the methyl groups and the O-atoms of the [Mo6O19]2− anions. Interesting supramolecular assemblies in compounds 1 and 2 have been also rationalized by using molecular electrostatic potential (MEP) surface plots and the noncovalent interaction plot (NCIplot) index. The present structures provide reliable molecular models for geometries and energetics of the stacks subtended by purine bases and polyoxometalate anions that could be relevant for specific targeting of POMs in biomedical systems.