4,4′-Bipyridine–5-fluoroisophthalic acid (1/1)

Co-crystallization of 5-fluoroisophthalic acid (H2fip) with 4,4′-bipyridine (bipy) leads to the formation of the title compound [(H2fip)(bipy)], C8H5FO4·C10H8N2, with an acid–base molar ratio of 1:1. The acid and base subunits are arrange alternately in the crystal structure, displaying a wave-like tape motif via intermolecular O—H⋯N and C—H⋯O hydrogen bonds [carboxyl–pyridine synthon of R 2 2(7) hydrogen-bond notation], which are further combined into a two-dimensional architecture through C—H⋯F interactions involving the bipy and H2fip molecules.


Comment
In light of the importance of hydrogen bonds in crystal engineering, the supramolecular synthon approach has been widely applied to adapt desired supramolecules by using identified robust intermolecular interactions (Desiraju, 1995;Nangia & Desiraju, 1998). Co-crystallization is a current theme in several research groups to study hydrogen bonding through X-ray diffraction technique (Aakeröy & Salmon, 2005), for the synthesis of interpenetrated networks (Sharma & Zaworotko, 1996), and especially in pharmaceutical developments (Schultheiss & Newman, 2009). At this stage, strong hydrogen bonds, such as O-H···N or charge-assisted N-H···O, are always essential in the co-crystallization of carboxylic acids with pyridyl bases, usually combining the auxiliary weak C-H···O interactions, lead to the familiar carboxyl/pyridyl heterosynthon [R 2 2 (7)] (Shan et al., 2002). Although aromatic dicarboxylic acids have been verified to be excellent building blocks in binary co-crystal assemblies with bipyridine-type components (Du et al., 2005), halogen substituented dicarboxylic acids have been seldom studied in this aspect (He, et al., 2009). Doubtless, substituents will profoundly influence the structural assemblies by demonstrating distinct hydrogen-bonding capability and potential steric/electronic effect. To further investigate the hydrogen-bonding networks involving halogen substituents, 5-fluoroisophthalic acid (H 2 fip) was chosen to construct binary cocrystal with familiar 4,4'-bipyridine (bipy) component as a hydrogen-bonding participant for the first time.
In this work, the reaction of 5-fluoroisophthalic acid (H 2 fip) with 4,4'-bipyridine (bipy) under ambient conditions and evaporation from the mixed CH 3 OH/H 2 O (2:1) solution of the reactants yields the crystalline binary adduct [(H 2 fip)(bipy)] (I). Single crystal X-ray diffraction reveals that compound (I) contains one-dimensional supramolecular tape via the connection of predictable carboxylate-bipyridine O-H···N/C-H···O interactions of R 2 2 (7) heterosynthon. Then, further C-H···F interactions extend the adjacent tape moieties into a two-dimensional (2-D) corrugated layer. The molecular structure contains one H 2 fip and one bipy molecule (Fig. 1). The two pyridyl rings within the basic unit form a dihedral angle of 30.9 (2)°.
The heterosynthon R 2 2 (7) ring pattern of O-H···N/C-H···O bonds (synthon I in Fig. 2, Table 1), connecting the base and acid moieties, is responsible for the formation of a 1-D wavelike tape structure. Analysis of the crystal packing of (I) suggests that a further C-H···F interaction (Table 1)  2). Within the 2-D layer, a new hydrogen-bonding pattern denoted as R 2 4 (14) (synthon II in Fig. 2, Etter, 1990) is found to link two pairs of centrosymmetry related carboxyl-bipyridine motifs from adjacent tape structures. By comparison, a closely related 1:1 binary cocrystal of isophthalic acid and bipy exhibits similar tapes of acid:base components formed via R 2 2 (7) synthons. But these tapes extend to form supramolecular sheets via additional C-H···O interactions (Shan et al., 2002).
In conclusion, this work demonstrates the first example for H 2 fip as a good participant in co-crystallization with basic modules. When co-crystallizing with rod-like 4,4'-bipyridine building block, the H 2 fip subunits fulfill the reliable carboxylic-pyridine synthon R 2 2 (7). Although the associated C-H···O bonds are not present between adjoining tape motifs, the introduction of fluorine substituents leads to a new hydrogen-bonding synthon R 2 4 (14). This result presents a new challenge in the exploration of crystalline products based on such halogen substituted benzene dicarboxylic acids.

Refinement
One restraint was applied to bonded N1 and C5 atoms to equalize each anisotropic vector component parallel to the bond (DELU command). H atoms bonded to C atoms were positioned geometrically (C-H = 0.93 Å for pyridyl and phenyl H atoms) and included in the refinement in the riding-model approximation, with U iso (H) = 1.2 U eq (C). O-bound H atoms were refined as rigid groups, allowed to rotate but not tip. Isotropic displacement parameters were derived from the parent atoms with U iso (H) = 1.5 U eq (O) and O-H distance of 0.82 Å. Fig. 1. The molecular structure of compound (I) drawn with 30% probability ellipsoids.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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 C1 0.2245 (