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)

The crystal structures of three hydrogen-bonded co-crystals of 4-alk­oxy­benzoic acid–4,4′-bipyridyl (2/1), namely, 2C(9)H(10)O(3)·C(10)H(8)N(2), (I), 2C(10)H(12)O(3)·C(10)H(8)N(2), (II) and 2C(11)H(14)O(3)·C(10)H(8)N(2), (III), have been determined at 93 K. Although the structure of (I) has been determined in the space group P2(1) with Z = 4 [Lai et al. (2008 ▸). J. Struct. Chem. 49, 1137–1140], the present study shows that the space group is P2(1)/n with Z = 4. In each crystal, the components are linked by O—H⋯N hydrogen bonds, forming a linear hydrogen-bonded 2:1 unit of the acid and the base. The 2:1 unit of (I) adopts nearly pseudo-C (2) symmetry, viz. twofold rotation around an axis passing through the mid-point of the central C—C bond of 4,4′-bipyridyl, while the units of (II) and (III), except for the terminal alkyl chains, have pseudo-inversion symmetry. The 2:1 units of (I), (II) and (III) are linked via C—H⋯O hydrogen bonds, forming sheet, double-tape and tape structures, respectively.