Polymorphism and Solvatomorphism 2006

Vapochromic behaviour of a gold(I)-lead(II) complex as a VOC sensor

The reaction among [Au2Ag2(C6F5)4(OEt2)2]n, PbCl2 and terpyridine leads to the polymeric complex [{Au(C6F5)2}2{Pb(terpy)}]n (1). Its crystal structure reveals potential voids close to the lead centres large enough to hold different molecules. The availability of these free sites allows complex 1 to act as a VOC sensor. Thus, when 1 is exposed to different solvent vapours such as acetonitrile, toluene or THF, variations in its solid appearance and its photophysical properties are observed as a consequence of the formation of the new polymorphs [{Au(C6F5)2}2{Pb(terpy)(CH3CN)2}]n (2), [{Au(C6F5)2}2{Pb(terpy)}]n·Tol (3) and [{Au(C6F5)2}2{Pb(terpy)(THF)}]n·THF (4). Each polymorph displays a different emission energy depending on its structure and the presence of metallophilic interactions. In addition, the reversible solvent molecule exchange allows the tuning of the luminescence emissions in the greenish yellow-red range. DFT and TD-DFT calculations were performed to explain the origin of the luminescence of all these complexes.

The Lisbon Supramolecular Green Story: Mechanochemistry towards New Forms of Pharmaceuticals

This short review presents and highlights the work performed by the Lisbon Group on the mechanochemical synthesis of active pharmaceutical ingredients (APIs) multicomponent compounds. Here, we show some of our most relevant contributions on the synthesis of supramolecular derivatives of well-known commercial used drugs and the corresponding improvement on their physicochemical properties. The study reflects, not only our pursuit of using crystal engineering principles for the search of supramolecular entities, but also our aim to correlate them with the desired properties. The work also covers our results on polymorphic screening and describes our proposed alternatives to induce and maintain specific polymorphic forms, and our approach to avoid polymorphism using APIs as ionic liquids. We want to stress that all the work was performed using mechanochemistry, a green advantageous synthetic technique.

New crystal forms and amorphous phase of sophoricoside: X-ray structures and characterization

Sophoricoside, which is an isoflavone glycoside found in many plant species, has recently attracted attention because of its anti-fertility activity. One solvent-free form, two solvatomorphs and an amorphous phase of sophoricoside are reported for the first time. X-ray diffractometry, differential scanning calorimetry, thermal gravimetric analysis and Fourier-transform infrared spectroscopy were used to characterize the different forms. The results show that factors such as crystal symmetry, intermolecular arrangement, conformational flexibility, hydrogen-bonding interactions and solvent incorporation lead to different solid-state forms. An investigation of the transformations of the four forms showed that they can interconvert with each other under certain conditions. Amorphous phase and solvatomorphism were unstable but can improve the solubility of sophoricoside in water.

Recent progress of structural study of polymorphic pharmaceutical drugs

This review considers advances in the understanding of active pharmaceutical ingredient polymorphism since around 2010 mainly from a structural view point, with a focus on twelve model drugs. New polymorphs of most of these drugs have been identified despite that the polymorphism of these old drugs has been extensively studied so far. In addition to the conventional modifications of preparative solvents, temperatures, and pressure, more strategic structure-based methods have successfully yielded new polymorphs. The development of analytical techniques, including X-ray analyses, spectroscopy, and microscopy has facilitated the identification of unknown crystal structures and also the discovery of new polymorphs. Computational simulations have played an important role in explaining and predicting the stability order of polymorphs. Furthermore, these make significant contributions to the design of new polymorphs by considering structure and energy. The new technologies and insights discussed in this review will contribute to the control of polymorphic forms, both during manufacture and in the drug formulation.

A New, More Stable Polymorphic Form of Otilonium Bromide: Solubility, Crystal Structure, and Phase Transformation

A new polymorphic form of otilonium bromide is presented (Form I), and a thorough analysis of its crystal and molecular structure is performed. The compound suffers a temperature-driven first-order phase transition at about 396 K, which transforms it into the polymorph reported by Dapporto P and Sega A (Acta Cryst. 1986;C42:474-478) (Form II). Through thermal analysis and solubility experiments the relative stability of both crystal modifications were determined, confirming that at room temperature this new Form I is the more stable one, Form II existing just in a metastable state.

Gelation or crystallization? Subtle balance of structural factors for assembly of DBA derivatives with methyl esters

The aggregation of a series of strained dehydrobenzoannulene derivatives with methyl ester groups is revealed.

Two new metastable forms of 6-chloroquinolin-2(1H)-one: crystal structure, Hirshfeld surfaces and spectroscopic studies

Two new metastable crystalline forms of 6-chloroquinolin-2(1H)-one (forms II and III), which were induced by two kinds of 3D inorganic anions (ClO4- and BF4-), have been prepared and characterized in this work. We performed single-crystal diffraction, X-ray powder diffraction (XRPD), Hirshfeld surfaces, solid-state vibrational spectroscopy (IR) and thermal analysis (DSC, TGA) to these two new forms as well as the original one (form I). Form I featured with 1D chain, while form II and III featured with dimeric unit with stronger π⋯π and hydrogen bonds interactions. The 3D inorganic anions force the self-assemble behavior of 6-chloroquinolin-2(1H)-one molecules from 1D chain structure (form I) to dimeric unit (forms II and III) due to their large steric hindrance, and different anions induced different orientations for dimeric unit. These transformations lead to the increase of N-H⋯O, π⋯π, and Cl-Cl intermolecular interactions, which lead to blue shift of IR spectra in 1700-1500 cm(-1) region as well as luminescence peaks, while red shift of IR spectra in 650-550 cm(-1) region.

Pharmaceutical cocrystals: the coming wave of new drug substances

Solid crystalline phases containing two cocrystallized components offer a new development pathway whereby one can potentially improve the physical characteristics (i.e., equilibrium solubility, dissolution rate, solid-state stability, etc.) of a drug substance that exhibits a profile that is less than desirable. In this commentary, the topic of pharmaceutical cocrystals will be briefly explored, and a short exposition of the solubility and dissolution rate advantages that have been realized in various systems will be provided. The Guidance for Industry document recently proposed by United States Food and Drug Administration will be outlined, and its requirements explained. Finally, the subset of pharmaceutical cocrystals that consist of a drug substance and a salt of that substance (termed a salt cocrystal) will be examined to illustrate this additional class of pharmaceutical cocrystals that may offer significant scientific and regulatory advantages.

Stabilization of quinapril by incorporating hydrogen bonding interactions

In the present study stability of various known solvates of quinapril hydrochloride has been compared with nitromethane solvate. Nitromethane solvate was found to be more stable compared to other known solvates. Single crystal X-ray diffraction analysis of quinapril nitromethane solvate shows intermolecular hydrogen bonding between quinapril molecule and nitromethane. Stabilization of quinapril by forming strong hydrogen bonding network as in case of co-crystals was further studied by forming co-crystal with tris(hydroxymethyl)amino methane. Quinapril free base forms a stable salt with tris(hydroxymethyl)amino methane not reported earlier. Quinapril tris(hydroxymethyl)amino methane salt found to be stable even at 80° for 72 h i.e. hardly any formation of diketopiperazine and diacid impurity. As expected single crystal X-ray diffraction analysis reveals tris(hydroxymethyl)amino methane salt of quinapril shows complex hydrogen bonding network between the two entities along with ionic bond. The properties of this stable salt - stable in solid as well as solution phase, might lead to an alternate highly stable formulation.

Integrated approach to study the dehydration kinetics of nitrofurantoin monohydrate

There is a need for thorough knowledge of solid-state transformations in order to implement quality by design (QbD) methodology in drug development. The present study was aimed at gaining a mechanistic understanding of the dehydration of nitrofurantoin monohydrate II (NF-MH). The dehydration was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hot-stage microscopy (HSM), and variable temperature X-ray powder diffraction (VT-XRPD). Isothermal TGA data were used to study dehydration kinetics using model-fitting and model-free approaches. Model-fitting analysis indicated a good fit for several models derived from nucleation-growth and/or geometric contraction mechanisms. However, based on visual observations during HSM, Avrami-Erofeyev equations A3 and A4, indicating nucleation-growth phenomenon, were found to be the most suitable kinetic models. HSM showed initiation of dehydration with random nucleation, and nuclei coalesced with the progress of dehydration reaction. VT-XRPD revealed formation of anhydrate beta form on dehydration of NF-MH. The phenomenon of random nucleation is justified based on the crystal structure of NF-MH, which showed presence of water molecules in an isolated manner, prohibiting directional dehydration. It was found that supplementary information from HSM and VT-XRPD can be valuable to gain a better understanding of dehydration from formal solid-state kinetics analysis.

Powder NMR crystallography of thymol

A protocol for the structure determination of powdered solids at natural abundance by NMR is presented and illustrated for the case of the small drug molecule thymol. The procedure uses proton spin-diffusion data from two-dimensional NMR experiments in combination with periodic DFT refinements incorporating (1)H and (13)C NMR chemical shifts. For thymol, the method yields a crystal structure for the powdered sample, which differs by an atomic root-mean-square-deviation (all atoms except methyl group protons) of only 0.07 A from the single crystal X-ray diffraction structure with DFT-optimized proton positions.