Bis(2,6-diaminopyridinium)–adipate–adipic acid–water (2/1/1/2)

The crystal structure of the title compound, 2C5H8N3 +·C6H8O4 2−·C6H10O4·2H2O, consists of aminopyridinium cations, adipate dianions, adipic acid molecules and disordered solvent water molecules [occupancies 0.50 (4) and 0.50 (4)]. Both the adipate and adipic acid are located across inversion centres. Eight-membered hydrogen-bonded rings exist involving aminopyridinium and adipate ions. Adipic acid molecules and adipate anions are linked into zigzag supramolecular chains by O—H⋯O hydrogen bonds.

The author acknowledges the Faculty of Arts and Sciences, Ondokuz Mayıs University, Turkey, for the use of the Stoe IPDS II diffractometer (purchased under grant F.279 of the University Research Fund). The author also thanks Professor Dr O. Bü yü kgü ngö r for collection of the X-ray data and Professor Dr M. Odabaşog lu for the synthesis of the title compound.

S1. Comment
The title crystal is obtained with the reaction of 2-aminopyridine (used in hair and other dyes) and adipic acid (used to esters for plasticizers and as food additive). In addition, The cupper(II) complexes of 2-aminopyridinium carboxylates have important properties in the applications of pharmaceuticals, fungicides, oxygen transfer, oxidative addition, homogenous hydrogenation, gas occlusion compounds, and solvent extractions processes (Lah et al., 2001;Yang et al., 1995). Hydrogen bonding plays a key role in molecular recognition (Goswami & Ghosh, 1997). The design of highly specific solid-state structure is of considerable significance in organic chemistry due to their important applications in the development of new optical, magnetic and electronic systems (Lehn, 1990). This report concerns the x-ray structure analysis of the title complex. In the structure investigations show that the 2-aminopyridinium ions are linked to the adipate ions by N1-H1···O1 and N3-H3···O2 hydrogen bonds through the formation of cyclic eight-membered hydrogen bonded rings.
2-Aminopyridine and derivatives, are protonated in acidic solution. As for mono-aminopyridinium adipate-adipic acid and in some 2-aminopyridine-containing molecules, the bonding of the H atom to the ring N atom of 2-aminopyridine rather than the amine N atom gives an ion for which an additional resonance structure can be written.
The H atoms between the water molecules are distorted because of the mutual repulsive interactions. So, H5B and H5C atoms are the same H-atom in the water molecule, and this H-atom makes a flip-flop motion between the two positions with site occupation factors of 0.50 (4) and 0.50 (4), respectively. The other H-atom (H5A) of the water molecule is localized due to the O5-H5A···O4 intermolecular hydrogen bond between adipic acid molecules (Fig. 3).

S2. Experimental
The title compound was prepared by as described by Odabaşoğlu & Büyükgüngör (2006), using 2,6-diaminopyridine and adipic acid as starting materials. Crystals were obtained by slow evaporation from an aqueous solution.

S3. Refinement
The carboxy H atom was located in a difference Fourier map and refined isotropically. Water H atoms were located in a difference map and refined isotropically with O-H and H···H distances restrained to 0.82 (2) and 1.55 (4) Å, respectively. The site occupancies for disordered H5B and H5C was refined to 0.50 (4) and 0.50 (4), respectively. Other H atoms were refined using the riding model approximation, with C-H = 0.93 (aromatic), 0.97 Å (methylene) and N-H = 0.86 Å, U iso (H) = 1.2U eq (C,N).  A perspective view of the packing in (I).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (