Crystal structures of hydrogen-bonded co-crystals as liquid crystal precursors: 4-(n-pentyloxy)benzoic acid–(E)-1,2-bis(pyridin-4-yl)ethene (2/1) and 4-(n-hexyloxy)benzoic acid–(E)-1,2-bis(pyridin-4-yl)ethene (2/1)

The two title compounds comprise two acid molecules and one base molecule linked by O—H⋯N hydrogen bonds, forming a linear hydrogen-bonded 2:1 unit.


Structural commentary
The molecular structures of compounds (I) and (II) are shown in Figs. 1 and 2, respectively. The asymmetric unit of (I) consists of one 4-pentyloxybenzoic acid molecule and one half-molecule of (E)-1,2-bis(pyridin-4-yl)ethene, which lies about an inversion centre. The two acid molecules and the base molecule are linked via O-HÁ Á ÁN hydrogen bonds (Table 1) to afford a centrosymmetric linear 2:1 unit. The hydrogen-bonded asymmetric unit is essentially planar with dihedral angles of 1.98 (10), 2.00 (10) and 3.69 (4) , respectively, between the pyridine N1/C13-C17 and carboxyl O1/C7/ O2 planes, the carboxyl and benzene C1-C6 planes, and the pyridine and benzene rings, respectively. On the other hand, the terminal alkyl C9-C12 chain deviates from the benzoic acid plane and adopts a gauche conformation with a C9-C10-C11-C12 torsion angle of À65.22 (10) .
The asymmetric unit of (II) is composed of two crystallographically independent 4-hexyloxybenzoic acid molecules and one (E)-1,2-bis(pyridin-4-yl)ethene molecule, and the two acids and the base are linked by O-HÁ Á ÁN hydrogen bonds (Table 2), forming a linear hydrogen-bonded 2:1 aggregate with trans-zigzag alkyl chains. The base molecule is orientationally disordered over two sets of sites approximately around the NÁ Á ÁN long axis of the molecule (Fig. 3), as also observed in the co-crystal of 4,4 0 -sulfonyldiphenol-(E)-1,2bis(pyridin-4-yl)ethene (1/1) (Ferguson et al., 1999). Similar orientational disorder has been observed in the crystals of stilbene and azobenzene (Harada & Ogawa, 2004). The occupancy of the two components was refined to 0.647 (4) and 0.353 (4). Both the major and minor components of the base molecule are approximately planar with dihedral angles of 8.0 (2) and 7.0 (5) , respectively, between the two pyridine rings in each component. The two independent acid molecules are also approximately planar. The maximum deviation from the mean plane of O1-O3/C1-C13 is 0.1530 (9) Å at atom O2, and that from the plane of O4-O6/C14-C26 is 0.1336 (9) Å at atom O4. The dihedral angles between the O1/C7/O2 and C1-C6 planes and between the O4/C20/O5 and C14-C19 planes are 8.57 (14)  The molecular structure of compound (I), showing the atom-numbering scheme. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level and H atoms are shown as circles of arbitrary size. O-HÁ Á ÁN hydrogen bonds are indicated by dashed lines. [Symmetry code: (ii) Àx + 2, Ày À 1, Àz.]

Figure 2
The molecular structure of compound (II), showing the atom-numbering scheme. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level and H atoms are shown as circles of arbitrary size. O-HÁ Á ÁN hydrogen bonds are indicated by dashed lines. For the disordered base molecule, only the major component is shown. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Figure 3
The disordered structure of the (E)-1,2-bis(pyridin-4-yl)ethene molecule in compound (I). The major and minor components are shown as solid and open bonds, respectively.

Figure 4
A partial packing diagram of compound (I), showing a column structure formed by C-HÁ Á Á andstacking interactions (dashed lines). H atoms except for the hydroxy and methyl groups have been omitted.

Phase transitions
Phase transitions for compounds (I) and (II) were observed by DSC and the liquid crystal phases were confirmed by polarizing microscope. DSC measurements were performed by using a Perkin Elmer Pyris 1 in the temperature range from 110 K to the melting temperature at a heating rate of 10 K min À1 . In addition, for compound (I) DSC was carried out in the range of 420-450 K at a rate of 0.5 K min À1 to determine the transition temperatures and enthalpies of two successive phase transitions. Phase transition temperatures (K) and enthalpies (kJ mol À1 ) obtained by DSC are as follows: K, S A , N and I denote crystal, smectic A, nematic and isotropic phases, respectively. The observed transition temperatures and enthalpies are good agreement with the reported values (Kato et al., 1993).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds, C-bound H atoms were positioned geometrically with C-H = 0.95-0.99 Å and were refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). The O-bound H atoms were located in a difference Fourier map and refined freely [refined O-H = 1.02 (2) Å for (I), and 0.99 (2) and 1.09 (2) Å for (II)]. In (II), the 1,2-bis(pyridin-4-yl)ethene molecule was found to be disordered over two sets of sites in the difference Fourier map and the occupancy of the two components was refined to 0.647 (4) and 0.353 (4). For the minor component, C and N atoms were refined isotropically to avoid undesirable displacement ellipsoids. The geometry of the pyridine rings of the minor component was restrained to be similar to that of the major one using a SAME instruction.

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. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement. _reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.

(II) 4-(n-Hexyloxy)benzoic acid-(E)-1,2-bis(pyridin-4-yl)ethene
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.29 e Å −3 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. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement. _reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.