Crystal structures of three co-crystals of 4,4′-bipyridyl with 4-alkoxybenzoic acids: 4-ethoxybenzoic acid–4,4′-bipyridyl (2/1), 4-n-propoxybenzoic acid–4,4′-bipyridyl (2/1) and 4-n-butoxybenzoic acid–4,4′-bipyridyl (2/1)

Crystal structures of three co-crystals of bis(4-alkoxybenzoic acid) and 4,4′-bipyridyl have been determined at 93 K. The asymmetric unit of each compound comprises two crystallographically independent acid molecules and one base molecule, which are held together by O—H⋯N hydrogen bonds, forming a linear hydrogen-bonded 2:1 unit.


Structural commentary
The molecular structure of (I) is shown in Fig. 1. Compound (I) crystallizes in the space group P2 1 /n with Z = 4. For the structure (space group P2 1 ) previously determined by Lai et al. (2008), ADDSYM in PLATON (Spek, 2009) detected missed symmetry elements, viz. a centre of inversion and a glide plane. The molecular structures of (II) and (III) are shown in Figs. 2 and 3, respectively. The asymmetric units each comprise two crystallographically independent 4-alkoxybenzoic acid molecules and one 4,4 0 -bipyridyl molecule, and the two acids and the base are held together by O-HÁ Á ÁN hydrogen bonds (Tables 1, 2 and 3), forming a linear hydrogen-bonded 2:1 aggregate. Similar to the reported structure of the 2:1 unit of 4-methoxybenzoic acid-4,4 0 -bipyridyl (2/1) (Mukherjee & Desiraju, 2014;Ramon et al., 2014), the 2:1 unit of (I) also adopts nearly pseudo-C 2 symmetry, viz. twofold rotation around an axis passing through the mid-point of the central C21-C26 bond of the 4,4 0 -bipyridyl molecule. On the other hand, the 2:1 units of (II) and (III), except for the terminal alkyl chains, have pseudo-inversion symmetry.

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 drawn as circles of arbitrary size. The O-HÁ Á ÁN hydrogen bonds are indicated by dashed lines.

Figure 3
The molecular structure of compound (III), showing the atom-numbering scheme. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level and H atoms are drawn as circles of arbitrary size. The O-HÁ Á ÁN hydrogen bonds are indicated by dashed lines.

Figure 5
A partial packing diagram of compound (    3.7052 (5) and 3.7752 (6) Å , respectively. C-HÁ Á Á interactions (Table 1) are also observed between the columns and between the sheets. In the crystal of (II) and (III), the 2:1 units are linked by C-HÁ Á ÁO interactions (Tables 2 and 3), forming a doubletape structure along the a axis (Fig. 6) and a tape structure along the b axis ( Fig. 7), respectively. Between the tapes in (II) and (III) C-HÁ Á Á interactions are observed (Tables 2 and 3). A packing diagram of (III) viewed along the a axis, which is approximately perpendicular to the mean plane of the 2:1 unit, is shown in Fig. 8. The units are arranged into a layer parallel to the bc plane, which leads to a smectic structure. On the other hand, no such a layer structure is observed in compounds (I) and (II), which form nematic liquid phases.
Liquid crystalline phases of these compounds were confirmed by measurements of DSC (differential scanning calorimetry) and polarizing microscope. DSC measurements were performed by using Perkin Elmer Pyris 1 in the temperature range from 103 K to the melting temperature at a heating rate of 10 K min À1 . Phase transition temperatures (K) and enthalpies (kJ mol À1 ) determined by DSC are as follows: (I) 373 (2)   K i , S A , N and I denote crystal, smectic A, nematic and isotropic phases, respectively. The observed transition temperatures and enthalpies from the solid phase to the liquid crystalline phase are in good agreement with those reported Kato et al. (1990Kato et al. ( , 1993. Some unreported thermal anomalies, 373 (2) K for (I), 365 (1) and 369 (1) K for (II), and 358 (1) and 386 (1) K for (III), were also observed.