1,3-Di-4-pyridylpropane–4,4′-oxydibenzoic acid (1/1)

In the title compound, C13H14N2·C14H10O5, a 1:1 cocrystal of 1,3-di-4-pyridylpropane (bpp) and 4,4′-oxydibenzoic acid (H2oba), the dihedral angle between the two benzene rings of the flexible H2oba molecule is 57.07 (1)°; the two pyridine rings of bpp make a dihedral angle of 27.52 (1)°. Strong intermolecular O—H⋯N hydrogen bonds link the molecules into chains, which are then linked into a three-dimensional network through intermolecular C—H⋯O and π–π stacking interactions [centroid–centroid distance = 3.7838 (3) Å].

Data collection: SMART (Bruker, 1997); cell refinement: SAINT (Bruker, 1997); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. Co-crystallization reactions provide helpful means for probing the importance and balance between different intermolecular interactions, and thus offer practical guidelines for developing new methodologies in supramolecular synthesis (Desiraju,2003;Shan et al., 2002). The role of hydrogen bonding and π-π stacking for these purposes is well established (Shattock et al.,2005). We attempted to synthesize a Cd II complex with the mixed ligand using hydrothermal synthesis conditions. However, we were not successful and a new co-crystal, (bpp)(H 2 oba)(I), was isolated instead and its structure is reported here. A similar structure has been reported (Ma et al., 2006) and an independent determination of the structure of (I) is reported in the preceeding paper (Li et al., 2008).
The asymmetric unit consists of one bpp and one H 2 oba as shown in Fig 1. The dihedral angle between the two phenyl rings of the flexible H 2 oba molecule is 57.07°, while it is 27.52° for the two phenyl rings of the bpp. The COOH group(O4-C27-O5) is co-planar with the phenyl ring and the other COOH group(O2-C20-O3) is slightly twisted with a the twist angle is 10.507 (8)°. In (I), the protonated carboxylate O2 of the flexible H 2 oba molecule forms two kinds of strong intermolecular hydrogen bonds with atoms N1 and N2 of the bpp molecule (Table 1), linking the molecules into one-dimensional chains. C-H···O hydrogen bonds involving the bpp carbon atoms (C4,C5 and C12) and uncoordinated carboxy oxygen atoms (O3 and O4) provide additional attractive forces between adjacent chains. Furthermore, there are π-π aromatic stacking interactions involving bpp ligands of adjacent units [centroid-centroid distance = 3.7838 (3) Å] that taken together with the C-H···O interactions form a three-dimensional supramolecular motif (Fig. 2).

Experimental
All chemicals were of reagent grade quality obtained from commercial sources and used without further purification. H 2 oba (0.5 mmol, 0.129 g), Cd(CH 3 COO) 2 .2H 2 O(1.5 mmol, 0.400 g), bpp(0.5 mmol, 0.099 g) and water (15 ml) were placed in a 25 ml Teflon-lined stainless steel reactor and heated at 453 K for five days, and then cooled slowly to 298 K at which time colourless crystals were obtained.The crystal used for data collection was obtained directly from the reaction mixture on cooling without further re-crystallization.

Refinement
All H atoms were positioned geometrically (C-H = 0.93 ?and O-H = 0.82 ?) and allowed to ride on their parent atoms, with U iso (H)values equal to 1.2U eq (C) or 1.5U eq (O).
supplementary materials sup-2 Figures Fig. 1. The structure of (I), with the atom-numbering scheme for the asymmetric unit, showing displacement ellipsoids at the 30% probability level.  Atomic displacement parameters (Å 2 )