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 self-assembly with five bases, i.e., diethylamine, tert-butylamine, cyclohexylamine, imidazole and piperazine, and generation of the corresponding acid-base complexes 1-5. Crystal structure analyses suggest that proton- -transfer from the carboxyl hydrogen to the nitrogen atom of the bases 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 deprotonated p-hydroxybenzoate anions (p-HOBAA-) are simply connected each other in a head-to-tail motif to form one-dimensional (1D) arrays, which are further extended 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 acid and base are combined pair-wise by O-H…N and N-H…O bonds to form a 1D tape and then the 1D tapes are sequentially combined by water molecules to create a 3D network. Some interlayer or intralayer C-H…O, C-H…π and π …π interactions help to stabilize the supramolecular buildings. Melting point determination analyses indicate that the five acid-base complexes are not the ordinary superposition of the reactants and they are more stable than the original reactants. [ABSTRACT FROM AUTHOR]