Structural variety and dehydration in 3-aminopyridine–hypodiphosphoric acid–water system

Abstract

Organic–inorganic salts and ionic co-crystals of hypodiphosphoric acid (H₄P₂O₆, PP) with 3-aminopyridine (3ap) have been synthesized and characterized by X-ray diffraction (single-crystal and micro-powder), thermogravimetry and Fourier-transform infrared spectroscopy. (3apH)₂(H₂P₂O₆)·0.5H₄P₂O₆·H₂O (1), (3apH)₆(H₂P₂O₆)(P₂O₆)·2H₂O (3) and (3apH₂)(H₂P₂O₆) (5) were obtained in crystalline form in typical solution-crystallization processes, using the molar ratio of PP acid (water solution) : 3ap (solid) 1 : 1, 1 : 3 or 2 : 1, respectively. Compounds (3apH)₄(H₃P₂O₆)₂(H₂P₂O₆) (2) and (3apH)₂(H₂P₂O₆)·3ap (4) were formed during dehydration processes of (1) and (3), respectively, which took place as destructive transformations. Compound (2) was also obtained by crystallization from a molten salt. The removal of water results in rearrangement of the crystal architecture (both the hypodiphosphate substructure and the organic part), and formation of polar structure in (4).