2-Aminopyridinium–succinate– succinic acid (2/1/1) Orhan Bu ¨yu ¨kgu ¨ngo ¨r a * and Mustafa Odabas ˛og ˇlu b a Department of Physics, Ondokuz Mayıs University, TR-55139, Samsun, Turkey, and b Department of Chemistry, Ondokuz Mayıs University, TR-55139, Samsun, Turkey Correspondence e-mail: orhanb@omu.edu.tr Received 19 July 2002 Accepted 30 September 2002 Online 31 October 2002 In the title compound, 2C 5 H 7 N 2 + C 4 H 4 O 4 2 C 4 H 6 O 4 , cyclic eight-membered hydrogen-bonded rings exist involving 2-aminopyridinium and succinate ions. The succinic acid and succinate moieties lie on inversion centres. Succinic acid molecules and succinate ions are linked into zigzag chains by O—HO hydrogen bonds, with OO distances of 2.6005 (16) A ˚ . Comment Hydrogen bonding plays a key role in molecular recognition (Goswami & Ghosh, 1997) and crystal engineering research (Goswami et al., 1998). The design of highly specific solid-state structures is of considerable significance in organic chemistry, due to their important applications in the development of new optical, magnetic and electronic systems (Lehn, 1990). Our investigation of the title compound, (I), shows that the 2-aminopyridinium ions are linked to the succinate ions through N1—HO3 and N2—H2NAO4 hydrogen bonds, resulting in the formation of cyclic eight-membered hydrogen- bonded rings. Each amino H atom in (I) is involved in a single hydrogen bond, one in the cyclic eight-membered hydrogen-bonded ring system and the other to another succinate ion (Fig. 1). The 2-aminopyridinium–succinate–succinic acid units are arranged so that a two-dimensional network of intermolecular N2— H2NBO4 and O2—H6O3 hydrogen bonds exists on each nearly perpendicular 2-aminopyridinium–succinate– succinic acid unit [dihedral angle = 84.34 (8)  ] (Fig. 2). 2-Aminopyridine, like other organic bases, is protonated in acidic solutions. The bonding of the H atom to the ring N atom of 2-aminopyridine, but not to the amino N atom, gives an ion for which an additional resonance structure can be written. As this monocation has more resonance energy (additional ionic resonance) than 2-aminopyridine itself, 2-aminopyridine is a strong base, like amidines (Acheson, 1967). The present investigation clearly shows that the positive charge in (I) is on the amino group in the crystal. The C1—N2 bond length is 1.3272 (19) A ˚ , and this is approximately equal to a previously reported C N double-bond length (Shanmuga Sundara Raj, Fun, Lu et al., 2000). The location of the positive charge on the amino group is also supported by the C1—N2—H2NA angle of 118.4 (11)  and by the fact that atoms C1, N2, H2NA and H2NB lie in the pyridine plane, with a maximum deviation of 0.04 (2) A ˚ for atom H2NB . Simi- lar C1—N2 bond lengths have also been observed in other 2-aminopyridine-containing molecules (Yang et al., 1995; Grobelny et al. , 1995; Shanmuga Sundara Raj, Fun, Zhao et al. , 2000). Acta Cryst. (2002). C58, o691–o692 DOI: 10.1107/S0108270102017808 # 2002 International Union of Crystallography o691 organic compounds Acta Crystallographica Section C Crystal Structure Communications ISSN 0108-2701 Figure 1 A view of the three moieties of (I), with the atom-numbering scheme and 50% probability displacement ellipsoids. H atoms are shown as small spheres of arbitrary radii [symmetry codes: (i) x; y; z]. Figure 2 A packing diagram for (I), viewed along the b axis. electronic reprint