Structural-chemical transformations during organic self-propagating high-temperature synthesis. Crystal structure of piperazine malonate and its crystal hydrate

An organic salt, piperazine malonate, was prepared by self-propagating high-temperature synthesis (SHS) in a solid-phase piperazine-malonic acid (P-M) system. Triclinic crystals of piperazine malonate (PM) hydrate (I) were isolated from an aqueous solution. Monoclinic crystals of anhydrous piperazine malonate (2) were isolated from a solution in DMSO. Crystals 1 and 2 were studied by X-ray structural analysis. Structures 1 and 2 consist of centrosymmetrical piperazinium cations, which are linked to anions of malonic acid through N−N…O hydrogen bonds. In structure 1, the anion is asymmetrical. The angle between the planes of the carboxyl groups and the plane of the CH2 groups are 29. 2° and 99.9°. In structure 2, the anion is located on a twofold axis, and the corresponding angles are 81.5°. The N…O distances are in the range of 2.691(4)–2.838(5) A.