Abstract
Partial hydration of organic compounds can be achieved by high-pressure crystallization. This has been demonstrated for the high-nitro-gen-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·xH2O, is isostructural with the ambient-pressure phase α of C4H2N5Cl, but the crystal volume is somewhat larger than that of the anhydrate. At 0.20 GPa, the α-C4H2N5Cl anhydrate phase transforms abruptly into a new higher-symmetry phase, α'; the transformation is clearly visible due to a strong contraction of the crystals. The hydrate α-C4H2N5Cl·xH2O can also be isothermally compressed up to 0.30 GPa before transforming to the α'-C4H2N5Cl·xH2O phase. The isochoric recrystallization of C4H2N5Cl above 0.18 GPa yields a new anhydrous phase β, which, on releasing pressure, transforms back to the α phase below 0.15 GPa. The structural transition from the α to the β 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⋯N and CH⋯Cl hydrogen bonds, as well as the N⋯N contacts. The α-to-α' phase transition increases the crystal symmetry in the subgroup relation; however, there are no structural nor symmetry relations between phases α and β.
Highlights
High-nitrogen organic compounds have relatively high density, but short intermolecular contacts are usually absent in their structures (Bernstein, 2002; Fabbiani & Pulham, 2006; Millar et al, 2010; Zakharov & Boldyreva, 2019)
We report a pressure and temperature dependence of the crystal structure of the pyridazine-based compound 6-chloro-1,2,3,4tetrazolo[1,5-b]pyridazine (C4H2N5Cl), hereafter CTP
With the exception of the three least-high-pressure phases of C4H2N5Cl we have revealed the different behavior of this compound compressed and recrystallized under high-pressure conditions
Summary
High-nitrogen organic compounds have relatively high density, but short intermolecular contacts are usually absent in their structures (Bernstein, 2002; Fabbiani & Pulham, 2006; Millar et al, 2010; Zakharov & Boldyreva, 2019). The strong interdependence of the density and properties generally involves intermolecular interactions (Gao & Shreeve, 2011; Nair et al, 2010) and thermodynamic conditions (Fabbiani & Pulham, 2006; Boldyreva, 2008, 2014; Resnati et al, 2015). We report a pressure and temperature dependence of the crystal structure of the pyridazine-based compound 6-chloro-1,2,3,4tetrazolo[1,5-b]pyridazine (C4H2N5Cl), hereafter CTP. It can transform between the azide and tetrazole forms in the gaseous and liquid states (Fig. 1).
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