Stochastic hydration of a high-nitrogen-content molecular compound recrystallized under pressure

Partial hydration of organic compounds can be achieved by high-pressure crystallization. This has been demonstrated for the high-nitrogen-content compound 6-chloro-1,2,3,4-tetrazolo[1,5-b]pyridazine (C4H2N5Cl), which becomes partly hydrated by isochoric crystallizations below 0.15 GPa. This hydrate, C4H2N5Cl center dot xH(2)O, is isostructural with the ambient-pressure phase alpha of C4H2N5Cl, but the crystal volume is somewhat larger than that of the anhydrate. At 0.20 GPa, the alpha-C4H2N5Cl anhydrate phase transforms abruptly into a new higher-symmetry phase, alpha'; the transformation is clearly visible due to a strong contraction of the crystals. The hydrate alpha-C4H2N5Cl center dot xH(2)O can also be isothermally compressed up to 0.30 GPa before transforming to the alpha'-C4H2N5Cl center dot xH(2)O phase. The isochoric recrystallization of C4H2N5Cl above 0.18 GPa yields a new anhydrous phase beta, which, on releasing pressure, transforms back to the alpha phase below 0.15 GPa. The structural transition from the alpha to the beta phase is destructive for the single crystal and involves a large volume drop and significant elongation of all the shortest intermolecular distances which are the CH center dot center dot center dot N and CH center dot center dot center dot Cl hydrogen bonds, as well as the N center dot center dot center dot N contacts. The alpha-to-alpha' phase transition increases the crystal symmetry in the subgroup relation; however, there are no structural nor symmetry relations between phases alpha and beta.