Hydrogen-bonding directed cocrystallization of flexible piperazine with hydroxybenzoic acid derivatives： Structural diversity and synthon prediction

Four hydroxybenzoic acid building blocks, m-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, 2,5-dihydroxyterephthalic acid, and 5-hydroxyisophthalic acid, have been synthesized as robust cocrystallizing agents and employed in reactions with piperazine, including [（C4H12N2 2＋）.（C7H5O3-）2] （l）, [（CaH12N2 2＋）.（C7H5O4-）2] （2）, [（C4H12N2 2＋）.（C8H5O6 2- ）] （3）, and [（C4H12N2 2＋）1/2. （C8H5O5）] . 2H2O （4）. Hydrogen-bonded directed assemblies of four salts were validated by single-crystal X-ray diffraction analysis. In compounds 1-4, hydroxybenzoic acids are all deprotonated and piperazine molecules are all protonated to form piperazine dications and keep the chair conformation. Thermal stability of these compounds has been investigated.