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 center dot center dot center dot 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 (ID) 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 center dot center dot center dot O interactions. While in 5, neutral acid and base are combined pair-wise by O-H center dot center dot center dot N and N-H center dot center dot center dot O bonds to form a ID tape and then the ID tapes are sequentially combined by water molecules to create a 3D network. Sonic interlayer or intralayer C-H center dot center dot center dot O, C-H center dot center dot center dot pi and pi center dot center dot center dot pi 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.