Synthesis, crystal structure, spectroscopic and thermal analysis of hydrazinium hydrogen-3,5-pyrazoledicarboxylate monohydrate

A new hydrazinium(+1) salt of 3,5-pyrazoledicarboxylic acid (H₂Pzdca) of formula, (N₂H₅)HPzdca·H₂O (HPzdca is a hydrogen-3,5-pyrazoledicarboxylate anion), was prepared and characterized using elemental analysis, FT-IR spectroscopy, TG-DTA, and single-crystal X-ray diffraction analysis. The salt crystallized in the space group P2₁/c. The monoclinic unit cell of the title compound, N₂H₅⁺·C₅H₄N₂O₄⁻·H₂O, is a molecular salt consisting of one mono-protonated hydrazine cation [hydrazinium(+1) cation], one singly deprotonated hydrogen-3,5-pyrazoledicarboxylate anion, and one solvent water molecule. The overall structure is a 3-D supramolecular network formed through extensive O–H…O and N–H…O hydrogen-bonding interactions. Although the H₂Pzdca monohydrate exhibited a two-step (110 and 298 °C) decomposition, its salt had multiple-step decompositions. The free acid underwent only endothermic decomposition, while its hydrazinium salt underwent an endothermic decomposition, followed by sharp exothermic degradation to form a black residue as the end product.