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, 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.