Synthon evolution and unit cell evolution during crystallisation. A study of symmetry-independent molecules (Z' > 1) in crystals of some hydroxy compounds

Crystalline salts of a diuretic drug torasemide with improved solubility and dissolution properties

Two novel pharmaceutical crystalline salts of a diuretic drug torasemide with improved solubility and dissolution profiles are disclosed in this manuscript.

Crystals at a Carrefour on the Way through the Phase Space: A Middle Path

Multiple supramolecular functionalities of cyclic α-alkoxy tellurium-trihalides (including Te---O, Te---X (X = Br, I) and Te---π(C=C) supramolecular synthons) afford rich crystal packing possibilities, which consequently results in polymorphism or Z’ > 1 crystal structures. Example of three crystal forms of cyclohexyl-ethoxy-tellurium-trihalides, one of which combines the packing of two others, affords a unique model to observe the supramolecular synthon evolution at the early stages of crystallization, when crystals on the way find themself at a carrefour between the evolutionally close routes, but fail to choose between two energetically close packing patterns, so taking the “middle path”, which incorporates both of them (and results in two crystallographically independent molecules). In general, this allows a better understanding of the existing structures, and an instrument to search for the new polymorphic forms.

Can solvated intermediates inform us about nucleation pathways? The case of β-pABA

Using crystallography to search for nucleation pathways: α and β polymorphs of p-aminobenzoic acid.

Separation or combination of non-covalently linked partners provides polymorphs of N-(aryl)-2-(propan-2-ylidene)hydrazine carbothioamides

Polymorphs of N-(2-methoxyphenyl)-2-(propan-2ylidene)hydrazine carbothioamide and N-(4-nitrophenyl)-2-(propan-2-ylidene)hydrazine carbothioamide differing in homomeric assemblies are described.

Evaluating the importance of fractional Z′ polymorphs in a trifluoromethylated N,N′-diphenyloxalamide derivative

A curious case of crystal dimorphism reveals an adjusted fractional number of molecules in their respective crystallographic asymmetric units.

Conformational variation of ligands in mercury halide complexes; high and low Z′ structures

Small changes in the ligand resulted in a conformational variation of L^Py to L^Pz which led to high and low Z′ structures in the corresponding metal complexes.

A sensitive photoluminescent chemosensor for cyanide in water based on a zinc coordination polymer bearing ditert-butyl-bipyridine

Sensitive and direct sensing of cyanide in buffered aqueous solutions at pH = 7.0 by three new blue photoluminescent zinc-1,4-cyclohexanedicarboxylato coordination polymers bearing di-alkyl-2,2′-bipyridines has been achieved.

Energy frameworks and a topological analysis of the supramolecular features in in situ cryocrystallized liquids: tuning the weak interaction landscape via fluorination

Weak intermolecular interactions observed in crystalline materials are often influenced or forced by stronger interactions such as classical hydrogen bonds. Room temperature liquids offer a scenario where such strong interactions are absent so that the role and nature of the weak interactions can be studied more reliably. In this context, we have analyzed the common organic reagent benzoyl chloride (BC) and a series of its fluorinated derivatives using in situ cryocrystallography. The intermolecular interaction energies have been estimated and their topologies explored using energy framework analysis in a series of ten benzoyl chloride analogues, which reveal that the ππ stacking interactions serve as the primary building blocks in these crystal structures. The crystal packing is also stabilized by a variety of interaction motifs involving weak C-HO/F/Cl hydrogen bonds and FF, FCl, and ClCl interactions. It is found that fluorination alters the electrostatic nature of the benzoyl chlorides, with subsequent changes in the formation of different weak interaction motifs. The effects of fluorination on these weak intermolecular interactions have been systematically analyzed further via detailed inputs from a topological analysis of the electron density and Hirshfeld surface analysis.

Crystal nucleation and growth in a polymorphic system: Ostwald's rule, p-aminobenzoic acid and nucleation transition states

Water has a magic effect on the nucleation kinetics of an aminobenzoic acid – stacking interactions baffle Ostwald's rule and offer dual pathways.

Exploring the structural landscape with ‘partial’ fluoro-substitution as a probe

The landscape of β-cinnamic acid is explored by an extension of the previously reported fluorosubstitution protocol.

‘Quasi-isostructural polymorphism’ in molecular crystals: inputs from interaction hierarchy and energy frameworks

The polymorphs of (Z)-2-fluoro-N′-phenyl benzamidamide with multiple Z′ form “equi-energetic” crystal structures and exhibit “quasi-isostructurality”.

Modelling of crystal structure of cis-1,2,3,6 and 3,4,5,6-tetrahydrophthalic anhydrides using lattice energy calculations

Lattice energy calculations using a model potential were performed to model the crystal structures of cis-1,2,3,6- and 3,4,5,6-tetrahydrophthalic (THP) anhydrides. The optimized molecular models using the DFT method at the B3LYP/6-31G** level were found consistent with the available experimental evidence and allowed all differences observed in crystal packing between cis-1,2,3,6- and 3,4,5,6-THP anhydrides to be reproduced. Calculations provide evidence for the presence of dipole-dipole C=O⋯C=O intermolecular interactions and support the idea that the molecules distort from their ideal geometries, improving packing in both crystals. The search for minima in the lattice energy of both crystals amongst the more common space groups with Z' = 1, using a simulated annealing crystal structure prediction procedure followed by lattice energy minimization showed that the observed structure of 3,4,5,6-THP anhydride (Z' = 2) is the thermodynamically most stable, and allowed us to justify why 3,4,5,6-THP anhydride crystallizes in such a complex structure with 16 molecules in the unit cell. The computational model was successful in predicting the second observed form at 173 K for cis-1,2,3,6-THP anhydride as a polymorph, and could predict several hypothetical structures with Z' = 1 that appear competitive with the observed structures. The results of phonon estimates of zero point intermolecular vibrational energy and entropy suggest that crystal structures of cis-1,2,3,6-THP anhydride cannot be predicted solely on the basis of lattice energy; factors other than thermodynamics favor the observed structures.

A spontaneous single-crystal-to-single-crystal polymorphic transition involving major packing changes

4,6-O-Benzylidene-α-d-galactosyl azide crystallizes into two morphologically distinct polymorphs depending on the solvent. While the α form appeared as thick rods and crystallized in P21 space group (monoclinic) with a single molecule in the asymmetric unit, the β form appeared as thin fibers and crystallized in P1 space group (triclinic) with six molecules in the asymmetric unit. Both the polymorphs appeared to melt at the same temperature. Differential scanning calorimetry analysis revealed that polymorph α irreversibly undergoes endothermic transition to polymorph β much before its melting point, which accounts for their apparently same melting points. Variable temperature powder X-ray diffraction (PXRD) experiments provided additional proof for the polymorphic transition. Single-crystal XRD analyses revealed that α to β transition occurs in a single-crystal-to-single-crystal (SCSC) fashion not only under thermal activation but also spontaneously at room temperature. The SCSC nature of this transition is surprising in light of the large structural differences between these polymorphs. Polarized light microscopy experiments not only proved the SCSC nature of the transition but also suggested nucleation and growth mechanism for the transition.

Unusual co-crystal of isonicotinamide: the structural landscape in crystal engineering

The idea of a structural landscape is based on the fact that a large number of crystal structures can be associated with a particular organic molecule. Taken together, all these structures constitute the landscape. The landscape includes polymorphs, pseudopolymorphs and solvates. Under certain circumstances, it may also include multi-component crystals (or co-crystals) that contain the reference molecule as one of the components. Under still other circumstances, the landscape may include the crystal structures of molecules that are closely related to the reference molecule. The idea of a landscape is to facilitate the understanding of the process of crystallization. It includes all minima that can, in principle, be accessed by the molecule in question as it traverses the path from solution to the crystal. Isonicotinamide is a molecule that is known to form many co-crystals. We report here a 2:1 co-crystal of this amide with 3,5-dinitrobenzoic acid, wherein an unusual N-H···N hydrogen-bonded pattern is observed. This crystal structure offers some hints about the recognition processes between molecules that might be implicated during crystallization. Also included is a review of other recent results that illustrate the concept of the structural landscape.

Inversion symmetry and local vs. dispersive interactions in the nucleation of hydrogen bonded cyclic n-mer and tape of imidazolecarboxamidines

Substitutional changes to imidazolecarboxamidine that preserved intermolecular hydrogen bonding in the solid state were used to study the relationship between packing and the hydrogen bond motif. Various motifs competed, but the most common imidazolecarboxamidine crystalline phase was a C_i symmetric dimer that established inversion centers by associating enantiomeric tautomers. Counter to intuition, the calculated gas-phase energies per molecule of the solid state atomic coordinates of the C_i dimer motifs were higher than those of the C₁ dimer, trimer, tetramer and tape motifs, while the packing densities of C_i dimers were found to be higher. This result was interpreted as an enhanced ability of the C_i dimers to pack. If other motifs competed, the hydrogen bonds and conformations should be lower in energy than the C_i dimer. The results detail the effect of packing on the conformation in these molecules. The results are interpreted as a rough measure of the energetic compromise between packing and the energies related to the coordinates involving one dihedral angle and hydrogen bonding. The results establish a connection between solution and solid phase conformation.

2,3,4,5,6-Penta-bromo-phenol

The title compound, C₆HBr₅O, is the perbrominated derivative of phenol. The mol­ecule shows non-crystallographic mirror symmetry. Bond lengths between the C and Br atoms are normal. In the crystal structure, O—H⋯O hydrogen bonds connect the mol­ecules into infinite strands. Dispersive Br⋯Br contacts are observed. No significant π–π stacking is obvious.

Conformational polymorphism in organic crystals

Polymorphs are different crystalline modifications of the same chemical substance. When different conformers of the same molecule occur in different crystal forms, the phenomenon is termed conformational polymorphism. Occasionally, more than one conformer is present in the same crystal structure. The influence of molecular conformation changes on the formation and stability of polymorphs is the focus of this Account. X-ray crystal structures of conformational polymorphs were analyzed to understand the interplay of intramolecular (conformer) and intermolecular (lattice) energy in the crystallization and stability of polymorphs. Polymorphic structures stabilized by strong O-H...O/N-H...O hydrogen bonds, weak C-H...O interactions, and close packing were considered. 4,4-Diphenyl-2,5-cyclohexadienone (1) and bis(p-tolyl) ketone p-tosylhydrazone (3) are prototypes of C-H...O and N-H...O hydrogen-bonded structures. Distance-angle scatter plots of O-H...O and C-H...O hydrogen bonds extracted from the Cambridge Structural Database indicate that polymorphs with a larger number of symmetry-independent molecules (high Z') generally have better interactions when compared with the polymorphs with lower Z' values, with the implication that these symmetry-independent molecules have different conformations. Since molecular conformer (E(conf)) and crystal lattice (U(latt)) energy differences are of the same magnitude in organic crystals (typically <5 kcal mol(-1)), situations wherein these two factors compensate or cancel one another are illustrative. Calculation of conformer and lattice energies using Gaussian 03 and Cerius(2) in 23 recently published polymorph sets shows that a strained conformer (higher E(conf)) is stabilized by stronger interactions or better crystal packing (lower U(latt)) in two-thirds of the cases, whereas there is no energy balance in the remaining structures. Organic molecules with flexible torsions and low-energy conformers have a greater likelihood of exhibiting polymorphism because (1) different conformations lead to new hydrogen-bonding and close-packing modes and (2) the tradeoff reduces the total energy difference between alternative crystal structures. As a test case, polymorph promiscuity in fuchsones (6) is related to the conformational diversity at the exo-methylene phenyl rings and the small energy difference computed for dimethyl fuchsone polymorphs. These ideas find application in the design of putative pharmaceutical polymorphs and crystal structure prediction.

Crystal engineering: a holistic view

Crystal engineering, the design of molecular solids, is the synthesis of functional solid-state structures from neutral or ionic building blocks, using intermolecular interactions in the design strategy. Hydrogen bonds, coordination bonds, and other less directed interactions define substructural patterns, referred to in the literature as supramolecular synthons and secondary building units. Crystal engineering has considerable overlap with supramolecular chemistry, X-ray crystallography, materials science, and solid-state chemistry and yet it is a distinct discipline in itself. The subject goes beyond the traditional divisions of organic, inorganic, and physical chemistry, and this makes for a very eclectic blend of ideas and techniques. The purpose of this Review is to highlight some current challenges in this rapidly evolving subject. Among the topics discussed are the nature of intermolecular interactions and their role in crystal design, the sometimes diverging perceptions of the geometrical and chemical models for a molecular crystal, the relationship of these models to polymorphism, knowledge-based computational prediction of crystal structures, and efforts at mapping the pathway of the crystallization reaction.

Polymorphs of 1,1-bis(4-hydroxyphenyl)cyclohexane and multiple Z? crystal structures by melt and sublimation crystallization

Close packing conflict in a metastable polymorph of the pure title host (Z' = 2, melt crystal m) is resolved in the stable form (Z' = 1, sublimed crystal s) as O-H...O hydrogen bond changes to O-H...pi interaction. Melt crystallization and sublimation show a greater percentage of high Z' structures in CSD statistics.

Unusual variations in the incidence of Z? > 1 in oxo-anion structures

A series of Cambridge Structural Database studies show that ionic species generally form low Z' structures, even in those cases where charge assisted hydrogen bonding is a key feature, e.g. oxo-anion complexes. By introducing a competing pi-pi stacking interaction, two oxo-anion compounds are shown to crystallise with more than one molecule in the asymmetric unit, including the first hydrogen phosphate containing structure to have Z' > 2.