Hydrogen bonding, pi-pi stacking and van der Waals forces-dominated layered regions in the crystal structure of 4-aminopyridinium hydrogen (9-phosphonononyl) phosphonate

The asymmetric unit of the title molecular salt, [C5H7N2+][(HO)(2)OP(CH2)(9)PO2(OH)(-)], consists of one 4-aminopyridinium cation and one hydrogen (9-phosphonononyl) phosphonate anion, both in general positions. As expected, the 4-aminopyridinium moieties are protonated exclusively at their endocyclic nitrogen atom due to a mesomeric stabilization by the imine form which would not be given in the corresponding double-protonated dicationic species. In the crystal, the phosphonyl (-PO3H2) and hydrogen phosphonate (-PO3H) groups of the anions form two-dimensional O-H center dot center dot center dot O hydrogen-bonded networks in the ab plane built from 24-membered hydrogen-bonded ring motifs with the graph-set descriptor R-6(6)(24). These networks are pairwise linked by the anions' alkylene chains. The 4-aminopyridinium cations are stacked in parallel displaced face-to-face arrangements and connect neighboring anionic substructures via medium-strong charge-supported N-H center dot center dot center dot hydrogen bonds along the c axis. The resulting three-dimensional hydrogen-bonded network shows clearly separated hydrophilic and hydrophobic structural domains.