4,4′-Bipyridine–butane-1,2,3,4-tetracarboxylic acid (1/1)

The title compound, C10H8N2·C8H10O8, is an example of a system with a short O⋯H⋯N hydrogen bond [O⋯N = 2.565 (3) Å]. The crystal structure comprises a 1:1 adduct between 4,4′-bipyridine and butane-1,2,3,4-tetracarboxylic acid, where both components are centrosymmetric. The component molecules are linked through strong O⋯H⋯N hydrogen bonds, forming chains extending approximately along [11]. The chains are interconnected by O⋯H⋯O hydrogen bonds and weak stacking interactions involving the pyridyl rings of the 4,4′-bipyridine molecules [centroid–centroid distance = 3.73 (2) Å and interplanar distance = 3.35 (1) Å]. The H atom of the short O⋯H⋯N hydrogen bond is disordered over two positions with site occupancy factors of ca 0.6 and 0.4. One methylene group is disordered over two positions; the site occupancy factors are ca 0.9 and 0.1.


D-HÁ
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: TK2261).

Comment
Systems with short O···H···N hydrogen bonds have been widely studied. A correlation of the geometric parameters defining the O···H···N bridge for amine -phenol complexes and the pK a values has been established (Majerz et al., 1997 and references therein). It was shown that the shortest O···N distances of about 2.52 Å are realised when the proton is near the centre of the O···H···N bridge. The first example of a crystal structure of this type to be investigated using neutron diffraction was the adduct of 2-methylpyridine and pentachlorophenol (Steiner et al., 2000). Also, temperature-dependent neutron diffraction studies have been performed, for example, on the 1:2 co-crystal of benzene-1,2,4,5-tetracarboxylic acid and 4,4'-bipyridine (Cowan et al., 2003). One of the shortest known O···H···N hydrogen bonds was observed in the crystal structure of 4-methylpyridine and pentachlorophenol (Steiner et al., 2001) with the O···N distance of 2.506 (3) Å at 20 K and the H atom essentially at the centre of the O and N atoms.
The title crystal is an example of a system with short O···H···N hydrogen bonds with the O···N distance being 2.565 (3) Å. It contains butane-1,2,3,4-tetracarboxylic acid (BTCA), which has been widely used as a cross-linking agent for cotton fabrics (Wang & Chen, 2005) and also in crystal engineering studies of hydrogen bonding arrays (Barnes & Barnes, 1996).
Attempts to obtain the acid in crystalline form have so far been unsuccessful (Barnes & Barnes, 1996).
The centrosymmetric 4,4'-bipyridine molecule is planar and its geometric parameters are comparable to other reported cases of planar molecule of this formula (e.g. Wang & Wei, 2006).
The butane-1,2,3,4-tetracarboxylic acid and 4,4'-bipyridine molecules are connected by short O···H···N hydrogen bonds (with H atom disordered over two positions -nearer to the O and nearer to the N atom with the occupancy factors of 0.59 (3) and 0.41, respectively) to form chains extending approximately along [311] (Fig. 2). In each such hydrogen bond the O···N distance is 2.565 (3) Å ( Table 2). The chains are interconnected by the O-H···O hydrogen bonds where the O12 atom from one of the carboxylic groups participates as the donor and the O21 atom from the other carboxylic group as acceptor (Table   2). Thus, R 2 2 (14) motifs are formed (Etter et al., 1990). The chains also interact through weak stacking interactions between supplementary materials sup-2 the pyridyl rings ( Fig. 2) with the distance between the rings centroids of 3.73 (2) Å. The interplanar distance between the planes of interacting rings is 3.35 (1) Å.
Experimental 4,4'-Bipyridine and butane-1,2,3,4-tetracarboxylic acid were purchased from Merck. A solution of butane-1,2,3,4-tetracarboxylic acid (1.5 mmol) in hot water (250 ml) was added dropwise to a vigorously stirred suspension of 4,4'-bipyridine (2.5 mmol) in water (25 ml) over a period of 5 min. and was heated to obtain a homogeneous solution. The solution was slowly cooled to room temperature. The resulting crystals in form of colourless plates were filtered and recrystallized from water.
The presence of short O···H···N hydrogen bond is confirmed by the IR spectrum collected in KBr pellet, which shows presence of broad bands ascribed to O···H···N vibrations.

Refinement
All H atoms were localized from difference Fourier maps. Subsequently, the H atoms bonded to C atoms were included in the model using the riding model approximation with U iso set at 1.2 U eq (parent atom). The H12 atom bonded to the O12 atom was kept using AFIX 147 restraint with U iso set at 1.2 U eq (parent atom). The H1 atom (participating in strong O···H···N hydrogen bond) U eq was refined isotropically and was localized at the centre of the O···N distance. On an examination of a difference Fourier map, the structure was re-refined with this H1 atom disordered over two   (Etter et al., 1990) is given for the described ring motif. Symmetry codes: [iii] -x + 1, -y, -z + 2; [iv] -x + 2, -y, -z + 1; [v] x + 1, y, z; [vi] -x + 3, -y, -z + 1. The disordered part of the BTCA acid molecule as well as H atoms not involved in hydrogen bonds are omitted for clarity.