As an exercise in crystal engineering, low-temperature X-ray structures were determined for six rationally designed ionic solids of general formula BH+(MeSO2)2N−, where BH+ is 2-aminopyridinium (2, monoclinic, space group P21/c, Z = 4), 2-aminopyrimidinium (3, orthorhombic, Pbca, Z = 8), 2-aminothiazolium (4, orthorhombic, Pbcn, Z = 8), 2-amino-6-methylpyridinium (5, solvated with 0.5 H20, monoclinic, C2/c, Z = 8), 2-amino-1,3,4-thiadiazolium (6, triclinic, P1̄, Z = 2), or 2-amino-4,6-dimethylpyrimidinium (7, orthorhombic. Fdd2, Z = 16). The onium cations in question exhibit a trifunctional hydrogen-bond donor sequence H − N (H*)-C (sp2) − N − H , which is complementary to an O − S (sp3)−N fragment of the anion and simultaneously expected to form a third hydrogen bond via the exocyclic N − H* donor. Consequently, all the crystal packings contain cation-anion pairs assembled by an N − H ∙∙∙ N and an N −H ∙∙∙ O hydrogen bond, these substructures being mutually associated through an N − H* ∙∙∙ O bond. For the robust eight-membered ring synthon within the ion pairs [graph set N2 = R22(8), antidromic], two supramolecular isomers were observed: In 2 and 3, N − H ∙∙∙ N originates from the ring NH donor and N − H ∙∙∙ O from the exocyclic amino group, whereas in 4-7 these connectivities are reversed. The third hydrogen bond, N − H*∙∙∙ O , leads either to chains of ion pairs (generated by a 21 transformation in 2-4 or by a glide plane in 5) or to cyclic dimers of ion pairs (Ci symmetric in 6, C2-symmetric in 7). The overall variety of motifs observed in a small number of structures reflects the limits imposed on the prediction of hydrogen bonding patterns. Owing to the excess of potential acceptors over traditional hydrogen-bond donors, several of the structures display prominent non-classical secondary bonding. Thus, the cyclodimeric units of 6 are associated into strands through short antiparallel O ∙∙∙ S(cation) interactions. In the hemihydrate 5, two independent C-H(cation) ∙∙∙ O bonds generate a second antidromic R22(8) pattern, leading to sheets composed of N − H ∙∙∙ N/O connected catemers; the water molecules are alternately sandwiched between and O - H ∙∙∙ O bonded to the sheets to form bilayers, which are cross-linked by a third C − H (cation ) ∙∙∙ O contact. The roof-shaped cyclodimers occurring in 7 occupy the polar C2 axes parallel to z and build up hollow Car− H ∙∙∙ O bonded tetrahedral lattices; in order to fill their large empty cavities, five translationally equivalent lattices mutually interpenetrate.
