Crystal structures of three co-crystals of 4,4'-bipyridyl with 4-alk-oxy-benzoic acids: 4-eth-oxy-benzoic acid-4,4'-bipyridyl (2/1), 4-n-propoxybenzoic acid-4,4'-bipyridyl (2/1) and 4-n-but-oxy-benzoic acid-4,4'-bipyridyl (2/1).

The crystal structures of three hydrogen-bonded co-crystals of 4-alk-oxy-benzoic acid-4,4'-bipyridyl (2/1), namely, 2C9H10O3·C10H8N2, (I), 2C10H12O3·C10H8N2, (II) and 2C11H14O3·C10H8N2, (III), have been determined at 93 K. Although the structure of (I) has been determined in the space group P21 with Z = 4 [Lai et al. (2008 ▸). J. Struct. Chem. 49, 1137-1140], the present study shows that the space group is P21/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.