supplementary materials

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 crystal structure comprises 4,4'-bipyridine and butane-1,2,3,4-tetracarboxylic acid in a 1:1 ratio (Fig. 1 & Table 1). Both components are centrosymmetric. The carboxylic acid groups with C31 and C12 atoms are gauche with the C12—C11—C21A—C31 torsion angle being -48.6 (3)°. These groups are mutually twisted with the interplanar angle between the planes defined by O12, O22, C12, C11 and O11, O21, C31, C21A, respectively, being 73.7 (1)°.

There are only two reports of crystal structures containing anions of butane-1,2,3,4-tetracarboxylic acid: the structure of its ammonium (Barnes & Barnes, 1996) and guanidinium (McKee & Najafpour, 2007) salts. In both of these structures, the anions are centrosymmetric and not protonated. However, the conformation of the anion resembles that reported here with the torsion angles equivalent to C31—C21A—C11—C12 being -61.1 (2)° in the guanidinium salt (McKee & Najafpour, 2007), and -55.2 (2)° and -60.9 (2)° for the two symmetry independent anions in the ammonium salt (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, R²₂(14) motifs are formed (Etter et al., 1990). The chains also interact through weak stacking interactions between 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.