Crystal structure and pseudosymmetry analysis of the triclinic prodrug cloxazolam (Z' = 4).

The prodrug cloxazolam [systematic name: 13-chloro-2-(2-chlorophenyl)-3-oxa-6,9-diazatricyclo[8.4.0.02,6]tetradeca-1(10),11,13-trien-8-one], C17H14Cl2N2O2, crystallizes in the triclinic space group P-1, with four chemically identical independent molecules in the asymmetric unit. However, in order to facilitate the analysis of the striking pseudosymmetry relating the four independent molecules, the structure has been analysed and reported in the nonconventional centred B-1 space-group setting. Pseudosymmetry is an eminently local property, valid only in the realm of the unit-cell boundary and not propagating to the whole crystal structure. It has been analyzed using the MP procedure described in the preceding article [Baggio (2019). Acta Cryst. C75, 837-850]. The molecules consist of a rigid core made up of three rings (five-, six- and seven-membered) and an extra six-membered ring joined to the latter group by a single C-C bond, together with a clamping intramolecular C-H...O interaction preventing free rotation and providing additional rigidity. The four molecules in the asymmetric unit pair into dimers with almost exact twofold pseudosymmetry, further linked into (001) slabs as the building bricks of the structure. Interpenetration of slabs finally leads to a three-dimensional structure of unusual compactness for an organic structure, with a Kitaigorodskii packing index of ca 0.71.

Polymorphic forms of bendamustine hydrochloride: crystal structure, thermal properties and stability at ambient conditions.

Crystallographic, thermal and stability analyses are presented of three different anhydrated forms of bendamustine hydrochloride [(I), (III) and (IV)] and a fourth, monohydrated one (II). Since form (I) presents the higher melting point and the higher heat of fusion, according to the `heat of fusion' rule it should be the most stable in thermodynamic terms [Burger & Ramberger (1979). Mikrochim. Acta, 72, 259-271], though it is unstable in high-humidity conditions. The monohydrate structure (II), in turn, dehydrates by heating and topotactically transform into anhydrate (III). This latter form appears as less stable than anhydrate (I), to which it is linked via a monotropic relationship. For these three different forms, the crystal structure has been determined by single crystal X-ray diffraction. The crystal structures and molecular conformations of forms (II) and (III) are quite similar, as expected from the topotactic transformation linking them; furthermore, under high-humidity conditions, form (III) shows changes compatible with a transformation into form (II) within 24 h. The crystal structure of form (I) is different from the other two. The remaining polymorphic form (IV) could only be obtained as a powder, from which its crystalline structure could not be determined. The relative thermodynamic stability of the different crystalline forms was determined by differential scanning calorimetry and thermogravimetrical studies, and their stability under different humidity conditions analysed.

A crystallographic and thermal study of pridinol mesylate and its monohydrated solvate.

Herein are reported the crystal and molecular structures of the pridinol mesylate salt (C20H25NO+·CH3O3S-) (I) and its monohydrated solvate form (C20H25NO+·CH3O3S-·H2O) (II). A comparison of both with the already reported structure of pure pridinol [1,1-diphenyl-3-piperidino-1-propanol, C20H25NO; Tacke et al. (1980). Chem. Ber. 113, 1962-1980] is made. Molecular structures (I) and (II) are alike in bond distances and bond angles, but differ in their spatial conformation, and, more relevant still, in their hydrogen-bonding motifs. This gives rise to quite different packing schemes, in the form of simple dimers in (I) but water-mediated hydrogen-bonded chains in (II). The dehydration behaviour of form (II) is highly dependent on the heating rate, with slow rates leading to a clear endothermic dehydration step, towards anhydrous (I), with subsequent melting of this latter phase. Increased heating rates result in a more unclear behaviour ending in a structural collapse (melting of the hydrated phase), at temperatures significantly lower than the melting point of the anhydrous phase. The eventual relevance of the water link in the structure of (II) is discussed in regard to this behaviour.