Hydrogen-Bonding Motifs in Adducts of Allylamine with the 10 Simplest n-Alcohols: Single-Crystal X-ray Diffraction Studies and Computational Analysis

In this paper, we analyzed the homologous series of 10 allylamine adducts with n-alcohols from methanol to decanol. These are the first adduct structures containing aliphatic n-alcohols and an aliphatic amine as co-formers. While all of the ingredients are liquids under ambient conditions, the phases were synthesized with the use of the in situ crystallization technique assisted by IR laser-focused radiation at atmospheric pressure. The structures were characterized by single-crystal X-ray diffraction. All of the phases contain the amine and alcohol in a 1:1 ratio. The architecture of the structures, based on hydrogen-bonding interactions between NH₂ and OH moieties, depends on the size of the alcohol and changes in a systematic way. The three smallest alcohol adducts contain centrosymmetric layers of molecules of the L4(4)8(8) type. The next four alcohol adducts have the T4(2) topology. The structures with the biggest alcohols contain non-centrosymmetric L6(6) layers. The structural investigations were supported by periodic DFT calculations at the B3LYP/pobTZVP level. The cohesive and adhesive energies made up of layer (E _lbe) and ribbon (E _rbe) binding energies were used to predict which type of architecture can be formed. The thermal stabilities of the adducts correlate with the melting points of the co-forming alcohols, with no evident relation to the adduct architecture.

Molecular Crystal Forms of Antitubercular Ethionamide with Dicarboxylic Acids: Solid-State Properties and a Combined Structural and Spectroscopic Study

We report on the preparation, characterization, and bioavailability properties of three new crystal forms of ethionamide, an antitubercular agent used in the treatment of drug-resistant tuberculosis. The new adducts were obtained by combining the active pharmaceutical ingredient with three dicarboxylic acids, namely glutaric, malonic and tartaric acid, in equimolar ratios. Crystal structures were obtained for all three adducts and were compared with two previously reported multicomponent systems of ethionamide with maleic and fumaric acid. The ethionamide-glutaric acid and the ethionamide-malonic acid adducts were thoroughly characterized by means of solid-state NMR (13C and 15N Cross-Polarization Magic Angle Spinning or CPMAS) to confirm the position of the carboxylic proton, and they were found to be a cocrystal and a salt, respectively; they were compared with two previously reported multicomponent systems of ethionamide with maleic and fumaric acid. Ethionamide-tartaric acid was found to be a rare example of kryptoracemic cocrystal. In vitro bioavailability enhancements up to a factor 3 compared to pure ethionamide were assessed for all obtained adducts.

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.

Crystal structure of a 1:1 co-crystal of the anti-cancer drug gefitinib with azelaic acid

In the title co-crystal, C₂₂H₂₄ClFN₄O₃·C₉H₁₆O₄, gefitinib (GTB; systematic name: quinazolin-4-amine) co-crystallizes with azelaic acid (AA; systematic name: nona-nedioic acid). The co-crystal has the monoclinic P2₁/n centrosymmetric space group, containing one mol-ecule each of GTB and AA in the asymmetric unit. A structure overlay of the GTB mol-ecule in the co-crystal with that of its most stable polymorph revealed a significant difference in the conformation of the morpholine moiety. The significant deviation in the conformation of one of the acidic groups of azelaic acid from its usual linear chain structure could be due to the encapsulation of one acidic group in the pocket formed between the two pincers of GTB namely, the morpholine and phenyl moieties. Both GTB and AA mol-ecules form N-H⋯O, O-H⋯N, C-H⋯O hydrogen bonds with C-H⋯F close contacts along with off-stacked aromatic π-π inter-actions between the GTB mol-ecules.

Achievement of enhanced solubility and improved optics in molecular complexes based on a sulfonate–pyridinium supramolecular synthon

Complexes 1–3 aggregate as intriguing supramolecular assemblies and show enhanced solubility and optical properties vis-à-vis pristine components. 3 exhibits striking enhancement in solubility of 2350% via-a-vis 4-A-3-HNSA-2H.

Polymorphism of hydrogen-bonded star mesogens – a combinatorial DFT-D and FT-IR spectroscopy study

A comprehensive study combining detailed computational analyses with temperature-variable FT-IR experiments was performed in order to elucidate the structure of the hydrogen-bonded liquid crystals based on phloroglucinol and azopyridine in their mesophase. Conformational analysis revealed three relevant conformers: star, λ- and E-shape. The results demonstrate an entropy-driven unfolding mechanism of the assembly. The stability of the conformers is given by intermolecular π–π and dispersion interactions of the azopyridine side chains. Correlating the calculated vibrational frequency with experimental FT-IR spectra suggests a λ-folded conformation of the assemblies as the predominant species in the mesophase.

The structure of hydrogen-bonded star mesogens is investigated using modern quantum chemistry methods in combination with infrared spectroscopy.

Structure and electrostatic properties of the pyrimethamine-3,5-dihydroxybenzoic acid cocrystal in water solvent studied using transferred electron-density parameters

Pyrimethamine is an antimalarial drug. The cocrystal salt form of pyrimethamine with 3,5-dihydroxybenzoic acid in water solvent has been synthesized, namely 2,4-diamino-5-(4-chlorophenyl)-6-ethylpyrimidin-1-ium 3,5-dihydroxybenzoate hemihydrate, C₁₂H₁₄ClN₄⁺·C₇H₅O₄^-·0.5H₂O. X-ray diffraction data were collected at room temperature. Refinement of the crystal structure was carried out using the classical Independent Atom Model (IAM), while the electrostatic properties were studied by transferring electron-density parameters from an electron-density database. The Cl atom was refined anharmonically. The results of both refinement methods were compared. Topological analyses were carried out using Bader's theory of Atoms in Molecules (AIM). The three-dimensional Hirshfeld surface analysis and the two-dimensional fingerprint maps of individual molecules revealed that the crystal structures are dominated by H...O/O...H and H...H contacts. Other close contacts are also present, including weak C...H/H...C contacts. Charge transfer between the pyrimethamine and 3,5-dihydroxybenzoic acid molecules results in a molecular assembly based on strong intermolecular hydrogen bonds. This is further validated by the calculation of the electrostatic potential based on transferred electron-density parameters. The current work proves the significance of the transferability principle in studying the electron-density-derived properties of molecules in cases where high-resolution diffraction data at low temperature are not available.

Modulating the physical properties of solid forms of urea using co-crystallization technology

The solid-form landscape of urea was explored using full interaction maps (FIMs) and data from the CSD to develop optimum protocols for synthesizing co-crystals of urea. As a result, 49 of the 60 attempted reactions produced new co-crystals, and the crystal structures of four of these are presented. Moreover, the goal of reducing the solubility and lowering the hygroscopicity of the parent compound was achieved, which in turn offers new opportunities for application as a slow-release fertilizer with limited hygroscopicity, thereby reducing many current problems of transport, handling, and storage of urea.

Electrostatic properties of the pyrimethamine–2,4-dihydroxybenzoic acid cocrystal in methanol studied using transferred electron-density parameters

The crystal structure of the cocrystal salt form of the antimalarial drug pyrimethamine with 2,4-dihydroxybenzoic acid in methanol [systematic name: 2,4-diamino-5-(4-chlorophenyl)-6-ethylpyrimidin-1-ium 2,4-dihydroxybenzoate methanol monosolvate, C12H14ClN4 +·C7H5O4 −·CH3OH] has been studied using X-ray diffraction data collected at room temperature. The crystal structure was refined using the classical Independent Atom Model (IAM) and the Multipolar Atom Model by transferring electron-density parameters from the ELMAM2 database. The Cl atom was refined anharmonically. The results of both refinement methods have been compared. The intermolecular interactions have been characterized on the basis of Hirshfeld surface analysis and topological analysis using Bader's theory of Atoms in Molecules. The results show that the molecular assembly is built primarily on the basis of charge transfer between 2,4-dihydroxybenzoic acid and pyrimethamine, which results in strong intermolecular hydrogen bonds. This fact is further validated by the calculation of the electrostatic potential based on transferred electron-density parameters.

Organic–inorganic ionic co-crystals: a new class of multipurpose compounds

Reacting molecular organic solids with inorganic salts gives access to novel properties via ionic co-crystal formation.

Making crystals with a purpose; a journey in crystal engineering at the University of Bologna

The conceptual relationship between crystal reactivity, stability and meta-stability, solubility and morphology on the one hand and shape, charge distribution, chirality and distribution of functional groups over the molecular surfaces on the other hand is discussed, via a number of examples coming from three decades of research in the field of crystal engineering at the University of Bologna. The bottom-up preparation of mixed crystals, co-crystals and photoreactive materials starting from molecular building blocks across the borders of organic, organometallic and metalorganic chemistry is recounted.

Correlating the melting point alteration with the supramolecular structure in aripiprazole drug cocrystals

Structural reasons for the melting point variations in isostructural cocrystals of the aripiprazole drug are investigated through combined spectroscopic and diffraction studies.

Isoniazid cocrystallisation with dicarboxylic acids: vapochemical, mechanochemical and thermal methods

A systematic study on mechanochemical, thermal and vapochemical cocrystallisation demonstrates the effect of compound properties on the outcome of the reaction.

Molecular structure investigation of organic cocrystals of 1,10-phenanthroline-5,6-dione with aryloxyacetic acid: a combined experimental and theoretical study

Two organic cocrystals namely, 1,10-phenanthroline-5,6-dione:2-naphthoxyacetic acid [(phendione)(2-naa)] (1) and 1,10-phenanthroline-5,6-dione:2-formylphenoxyacetic acid [(phendione)(2-fpaa)] (2) were synthesized and studied by single crystal XRD, FT-IR, NMR, thermogravimetric, and powder X-ray diffraction analysis. The molecular properties of cocrystals were studied using density functional theory (DFT), basis set B3LYP/6-31G(d,p). Both cocrystals are stabilized through intermolecular hydrogen bonding (OH⋯N). The total electron density and molecular electrostatic potential surfaces of the cocrystals were constructed by NBO analysis using B3LYP/6-31G(d,p) method to display the electrostatic potential (electron+nuclei) distribution. The energy gap between HOMO and LUMO was measured for both cocrystals.

Improvement of the water solubility of tolfenamic acid by new multiple-component crystals produced by mechanochemical methods

Tolfenamic acid (HTA) is a drug characterized by very poor solubility in water. By mechanochemical methods, new solid-state forms of HTA were obtained, showing better thermal stability than pure HTA and an improved dissolution rate.

Rationale of using Vinca minor Linne dry extract phytocomplex as a vincamine's oral bioavailability enhancer

Vincamine is a poorly soluble potent neuroprotector and cerebral vasodilator, used for the treatment for CNS disorders. In some cases, the bioavailability of pure compounds is strongly influenced by the co-administration of other constituents, and in some cases, the so called 'phytocomplex' may act as enhancer of absorption of selected phytochemicals. In this paper, the oral bioavailability of vincamine when administered as a standardised Vinca minor L. leaf dry extract rather than pure indole alkaloid is demonstrated to be higher. The chosen alkaloid-enriched and standardised dry extract was widely characterised by means of HPLC-MS, PXRD, DSC, XPS, (13)C and (15)N solid-state NMR (SSNMR) using pure vincamine as a matter of comparison. Then, the in vitro dissolution performances of the two products and their in vivo bioavailability in rats were evaluated. The sevenfold improvement in oral bioavailability of the dry extract with respect to the pure vincamine was ascribed to interactions between the indole alkaloid and the corollary of ingredients of the dry extract, giving rise to the protonation of the alkaloid vincamine, thus enhancing its dissolution in physiological fluids. Present data demonstrate that alkaloid vincamine administered as a whole plant extract has a higher bioavailability compared to the pure chemical compound.

Quantifying homo- and heteromolecular hydrogen bonds as a guide for adduct formation

An investigation into the predictability of molecular adduct formation is presented by using the approach of hydrogen bond propensity. Along with the predictions, crystallisation reactions (1a-1j) were carried out between the anti-malarial drug pyrimethamine (1) and the acids oxalic (a), malonic (b), acetylenedicarboxylic (c), adipic (d), pimelic (e), suberic (f), azelaic acids (g), as well as hexachlorobenzene (h), 1,4-diiodobenzene (i), and 1,4-diiodotetrafluorobenzene (j); seven (1a to 1g) of these successfully formed salts. Five of these seven salts were found to be either hydrated or solvated. Hydrogen bond propensity calculations predict that hydrogen bonds between 1 and acids a-g are more likely to form rather than the H bonds involved in self-association, providing a rationale for the observation of the seven new salts. In contrast, propensity of hydrogen bonds between 1 and h-j is much smaller as compared to other bonds predicted for self-association/solvate formation, in agreement with the observed unsuccessful reactions.

Applications of high-resolution 1H solid-state NMR

This article reviews the large increase in applications of high-resolution (1)H magic-angle spinning (MAS) solid-state NMR, in particular two-dimensional heteronuclear and homonuclear (double-quantum and spin-diffusion NOESY-like exchange) experiments, in the last five years. These applications benefit from faster MAS frequencies (up to 80 kHz), higher magnetic fields (up to 1 GHz) and pulse sequence developments (e.g., homonuclear decoupling sequences applicable under moderate and fast MAS). (1)H solid-state NMR techniques are shown to provide unique structural insight for a diverse range of systems including pharmaceuticals, self-assembled supramolecular structures and silica-based inorganic-organic materials, such as microporous and mesoporous materials and heterogeneous organometallic catalysts, for which single-crystal diffraction structures cannot be obtained. The power of NMR crystallography approaches that combine experiment with first-principles calculations of NMR parameters (notably using the GIPAW approach) are demonstrated, e.g., to yield quantitative insight into hydrogen-bonding and aromatic CH-π interactions, as well as to generate trial three-dimensional packing arrangements. It is shown how temperature-dependent changes in the (1)H chemical shift, linewidth and DQ-filtered signal intensity can be analysed to determine the thermodynamics and kinetics of molecular level processes, such as the making and breaking of hydrogen bonds, with particular application to proton-conducting materials. Other applications to polymers and biopolymers, inorganic compounds and bioinorganic systems, paramagnetic compounds and proteins are presented. The potential of new technological advances such as DNP methods and new microcoil designs is described.