Polymorphism in hydrazonium salt of 3,5-pyrazoledicarboxylic acid

Two polymorphic structures of hydrazonium monobasic salts of 3,5-pyrazoledicarboxylic acid are described. In both instances, one-dimensional infinite chains formed by acid molecules through O-H⋯O hydrogen bonding (1.68 Å, 177.1°; 1.71 Å 169°) are inter-linked by hydrazonium dications to form two-dimensional sheets. The subtle structural difference between the two forms arises in the packing of hydrazonium dications. In form A, the hydrazonium dications are relatively close packed compared to those in form B. The closest N.....N distance between two hydrazonium dications of inversion related layers in form A is 2.03 Å whereas the closest N.....N distance between two similar hydrazonium dications (disordered) is 4.89 Å for form B. This account for the higher packing fraction (78% vs 76.2%) of form A compared to form B.