Polymorphism of monotropic forms: relationships between thermochemical and structural characteristics

New Findings on the Crystal Polymorphism of Imepitoin

Scientific and industrial reasons dictate the study of the solid state of imepitoin, a highly safe and tolerable anticonvulsant drug used in the therapy of epileptic dogs that was approved in the Europe Union in 2013. Our investigations allowed us to discover the existence of a new polymorph of imepitoin, which finds itself in a monotropic relationship with the crystalline form (polymorph I) already known and present on the market. This form (polymorph II), obtained by crystallization from xylene, remains metastable under ambient conditions for at least 1 year. Both solid forms were characterized by thermal (DSC and TGA), spectroscopic (FT-IR and Raman), microscopic (SEM and HSM), and diffractometric techniques. The thermodynamic relationship between the two polymorphs (monotropic) is such that it is not possible to study the melting of polymorph II, not even by adopting appropriate experimental strategies. Our measurements highlighted that the melting peak of imepitoin actually also includes an onset of melt decomposition. The ab initio structure solution, obtained from synchrotron X-ray powder diffraction data collected at room temperature, allowed us to determine the crystal structure of the new polymorph (II). It crystallizes in the monoclinic crystal structure, P21/c space group (#14), with a = 14.8687(6) Å, b = 7.2434(2) Å, c = 12.5592(4) Å, β = 107.5586(8)°, V = 1289.61(8) Å3, and Z = 4.

Thermal and Structural Characterization of Two Crystalline Polymorphs of Tafamidis Free Acid

Tafamidis, chemical formula C14H7Cl2NO3, is a drug used to delay disease progression in adults suffering from transthyretin amyloidosis, and is marketed worldwide under different tradenames as a free acid or in the form of its meglumine salt. The free acid (CAS no. 594839-88-0) is reported to crystallize as distinct (polymorphic) crystal forms, the thermal stability and structural features of which remained thus far undisclosed. In this paper, we present-by selectively isolating highly pure batches of Tafamidis Form 1 and Tafamidis Form 4-the full characterization of these solids, in terms of crystal structures (determined using state-of-the-art structural powder diffraction methods) and spectroscopic and thermal properties. Beyond conventional thermogravimetric and calorimetric analyses, variable-temperature X-ray diffraction was employed to measure the highly anisotropic response of these (poly)crystalline materials to thermal stimuli and enabled the determination of the linear and volumetric thermal expansion coefficients and of the related indicatrix. Both crystal phases are monoclinic and contain substantially flat and π-π stacked Tafamidis molecules, arranged as centrosymmetric dimers by strong O-H···O bonds; weaker C-H···N contacts give rise, in both polymorphs, to infinite ribbons, which guarantee the substantial stiffness of the crystals in the direction of their elongation. Complete knowledge of the structural models will foster the usage of full-pattern quantitative phase analyses of Tafamidis in drug and polymorphic mixtures, an important aspect in both the forensic and the industrial sectors.

Factors Controlling Persistent Needle Crystal Growth: The Importance of Dominant One-Dimensional Secondary Bonding, Stacked Structures, and van der Waals Contact

Needle crystals can cause filtering and handling problems in industrial settings, and the factors leading to a needle crystal morphology have been investigated. The crystal growth of the amide and methyl, ethyl, isopropyl, and t-butyl esters of diflunisal have been examined, and needle growth has been observed for all except the t-butyl ester. Their crystal structures show that the t-butyl ester is the only structure that does not contain molecular stacking. A second polymorph of a persistent needle forming phenylsulfonamide with a block like habit has been isolated. The structure analysis has been extended to known needle forming systems from the literature. The intermolecular interactions in needle forming structures have been analyzed using the PIXEL program, and the properties driving needle crystal growth were found to include a 1D motif with interaction energy greater than -30 kJ/mol, at least 50% vdW contact between the motif neighbors, and a filled unit cell which is a monolayer. Crystal structures are classified into persistent and controllable needle formers. Needle growth in the latter class can be controlled by choice of solvent. The factors shown here to be drivers of needle growth will help in the design of processes for the production of less problematic crystal products.

Proton sponge lead halides containing 1D polyoctahedral chains

Hybrid organic/inorganic lead halides with the proton sponge moiety show face-sharing [PbX₆] octahedra forming linear 1D chains. These species exhibit complete (Br, I) miscibility and exceptional anisotropic thermal expansion.

Melting points of one- and two-component molecular crystals as effective characteristics for rational design of pharmaceutical systems

Based on the review of the literature results the database of the fusion temperatures of two-component molecular crystals (1947 co-crystals) and individual components thereof was built up. To improve the design of co-crystals with predictable melting temperatures, the correlation equations connecting co-crystals and individual components melting points were deduced. These correlations were discovered for 18 co-crystals of different stoichiometric compositions. The correlation coefficients were analysed, and the conclusions about the main/determinative and slave components of a co-crystal were made. The comparative analysis of the melting points of co-crystals composed from the same components but with different stoichiometry showed a co-crystal melting temperature growth when increasing the content of a high-melting component. The differences in the melting temperatures were determined and discussed for the following: (a) monotropic polymorphic forms, (b) two-component crystals with the same composition and different stoichiometry, and (c) two-component crystals based on racemates and enantiomers. The database analysis revealed the active pharmaceutical ingredients (APIs) and co-formers (CFs) more particularly used for co-crystal design. The approach based on an efficacy parameter allowing the prediction of co-crystals with melting points lower than those of individual compounds was developed.