Crystal engineered acid–base complexes with 2D and 3D hydrogen bonding systems using p-hydroxybenzoic acid as the building block

p-Hydroxybenzoic acid (p-HOBA) was selected as the building block for s elf-assembly with five bas es, i.e., diethy lamine, tert-butylamine, cy clohexylamine, i midazole and pi perazine, and generation of t he correspon ding acid–base c omplexes 1–5. Cry stal structure analy ses suggest that proton -transfer from the carboxyl hydrogen to the nitrogen atom of the b ases can be observed in 1–4, while only in 5 does a solvent water molecule co-exist with p-HOBA and piperazine. With the presence of O–H ⋅⋅⋅O hydrogen bonds in 1–4, the de protonated p-hydroxybenzoate anion s (p-HOBAA–) are si mply connected each other in a head-totail motif to form on e-dimensional (1D) array s, which are further extend ed to distinct two-dimensional (2D) (for 1 and 4) and three-dimensional (3D) (for 2 and 3 ) networks via N–H⋅⋅⋅O interactions. While in 5, neutral ac id and ba se are co mbined p air-wise by O–H ⋅⋅⋅N and N–H ⋅⋅⋅O bonds to for m a 1D tape and t hen the 1D tap es are seq uentially combined by water molecules to create a 3D network. Some interlayer or intralayer C–H⋅⋅⋅O, C–H⋅⋅⋅π an d π⋅⋅⋅π int eractions help to stabili ze the supra molecular buildi ngs. Melting point determination analyses indica te that the five acid–ba se co mplexes are n ot the ordinary superposition of t he reactant s an d they are more stable than the original reactants.