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

Organic-inorganic salts and ionic co-crystals of hypodiphosphoric acid (H 4 P 2 O 6 ) with 2-aminopyridine (2ap) have been synthesized and characterized by X-ray crystallography, thermogravimetry, elemental analysis and Fourier-transform infrared spectroscopy. (2apH) 4 (H 3 P 2 O 6 ) 2 (H 2 P 2 O 6 )·2H 2 O ( 1 ), (2apH) 2 (H 2 P 2 O 6 )·2H 2 O ( 4 ), (2apH) 2 (H 2 P 2 O 6 )·(H 4 P 2 O 6 ) ( 6 ) and (2apH) 2 (H 2 P 2 O 6 )·2(2ap) ( 7 ) were obtained in crystalline form in a typical solution-crystallization process. Compounds (2apH) 4 (H 3 P 2 O 6 ) 2 (H 2 P 2 O 6 )·0.4H 2 O ( 2 ), (2apH) 4 (H 3 P 2 O 6 ) 2 (H 2 P 2 O 6 )·0.1H 2 O ( 3 ) and (2apH) 2 (H 2 P 2 O 6 ) ( 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. Organic-inorganic salts and ionic co-crystals of hypodiphosphoric acid (H 4 P 2 O 6 ) with 2-aminopyridine (2ap) are reported and the dehydration of water-containing substances to produce new crystal forms otherwise inaccessible is described.