Poly-stoichiometry of hydrated phases is relatively uncommon for organic materials and extended libraries of such species adopting different aqua-to-substrate ratios are still rare. The kinetically controlled higher hydrates could be particularly interesting for their structural relationships, which presumably may imprint some features of the substrate/substrate and aqua/substrate bonding in solutions, and provide insights into the nucleation stage. Two metastable high hydrates are prepared by crash crystallization. The crystal structures of 3,3',5,5'-tetranitro-4,4'-bipyrazole tetrahydrate, C6H2N8O8·4H2O, (1), and 3,3',5,5'-tetranitro-4,4'-bipyrazole pentahydrate, C6H2N8O8·5H2O, (2), are intrinsically related to the previously reported anhydrate and monohydrate, while displaying natural evolution of the patterns upon progressive watering. The accumulation of the water molecules causes their clustering, with the generation of one-dimensional tapes and two-dimensional layers in the genuine channel hydrates (1) and (2), respectively, versus the pocket hydrate structure of C6H2N8O8·H2O. The hydration primarily affects the pyrazole sites. It conditions the emergence of N-H...O and O-H...N hydrogen bonds, which is a destructive factor for pyrazole/pyrazole N-H...N hydrogen bonding. At the same time, extensive noncovalent interactions of the organic molecules, namely, lone pair-π-hole O...N interactions of the NO2/NO2 and NO2/pyrazole types, are more competitive to the hydrogen bonding and the motifs of mutual organic/organic stacks remain intact with the increase in hydration. These trends agree with the results of Hirshfeld surface analysis. The contributions of the contacts involving H atoms are increased in line with the growing number of water molecules, while the fraction of O...N/N...O (NO2) contacts is nearly invariant. One may postulate the significance of the lone pair-π-hole interactions to the aggregation of nitro species in solutions and their relevance for the sebsequent development of the solid-state patterns through nucleation.
Keywords: N-heterocyclic aromatic compound; Ostwald rule; crystal structure; hydrogen bonding; lone pair–π-hole interaction; molecular hydrate; nitro compounds.