Syntheses and low-temperature X-ray structures are reported for three ionic solids of general formula BH+(MeSO2)2N-, where BH+ is 3,4,6,7,8,9-hexahydro-2H-pyrimido[1,2-a]pyrimidinium (1, monoclinic, space group P21/c), N,N′-diphenylformamidinium (2, monoclinic, P21/c), or 2-phenylaminopyridinium (3, triclinic, P1̄̄, two independent formula units). As a common feature, the onium cations in question exhibit a difunctional hydrogen-bond donor sequence H -N -C (sp2) - N - H geometrically complementary to an O - S - N fragment of the anion. Consequently, the three crystal packings consist of discrete cation-anion pairs (formula units) assembled by an N -H ··· N and an N - H O hydrogen bond. In both independent ion pairs of 3, the N -H ··· N bond originates from the pyridinium centre and the N - H - O bond from the amino group. The eight-membered ring [graph set N2 = R22(8), antidromic] that results in all these prototypical structures has potential as a new and robust supramolecular synthon. In each case, short C - H ··· O contacts with H ··· O ≤ 260 pm and C - H ··· O ≥ 140° are present; in 2, two Car - H ··· O hydrogen bonds are assisted by a C(formyl)- H ··· O contact and a π - π interaction to create a centrosymmetric formula unit dimer.
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