Crystal engineering and structural diversity of 2-aminopyridinium hypodiphosphates obtained by crystallization and dehydration

Abstract

Organic–inorganic salts and ionic co-crystals of hypodiphosphoric acid (H₄P₂O₆) with 2-aminopyridine (2ap) have been synthesized and characterized by X-ray crystallography, thermogravimetry, elemental analysis and Fourier-transform infrared spectroscopy. (2apH)₄(H₃P₂O₆)₂(H₂P₂O₆)·2H₂O (1), (2apH)₂(H₂P₂O₆)·2H₂O (4), (2apH)₂(H₂P₂O₆)·(H₄P₂O₆) (6) and (2apH)₂(H₂P₂O₆)·2(2ap) (7) were obtained in crystalline form in a typical solution-crystallization process. Compounds (2apH)₄(H₃P₂O₆)₂(H₂P₂O₆)·0.4H₂O (2), (2apH)₄(H₃P₂O₆)₂(H₂P₂O₆)·0.1H₂O (3) and (2apH)₂(H₂P₂O₆) (5) were formed during dehydration processes of (1) and (4), respectively, which took place as destructive transformations. Further annealing of compound (5) results in its partial decomposition into the anhydrous form of compound (3). Water removal is associated with a rearrangement of the crystal architecture and with the partial deformation of hypodiphosphate anions from the typical staggered conformation towards an eclipsed one.