Seeking Rules Governing Mixed Molecular Crystallization

Mixed crystals result when components of the structure are randomly replaced by analogues in ratios that can be varied continuously over certain ranges. Mixed crystals are useful because their properties can be adjusted by increments, simply by altering the ratio of components. Unfortunately, no clear rules exist to predict when two compounds are similar enough to form mixed crystals containing substantial amounts of both. To gain further understanding, we have used single-crystal X-ray diffraction, computational methods, and other tools to study mixed crystallizations within a selected set of structurally related compounds. This work has allowed us to begin to clarify the rules governing the phenomenon by showing that mixed crystals can have compositions and properties that vary continuously over wide ranges, even when the individual components do not normally crystallize in the same way. Moreover, close agreement of the results of our experiments and computational modeling demonstrates that reliable predictions about mixed crystallization can be made, despite the complexity of the phenomenon.

Enantiotropic inconstancy, crystalline solid solutions and co-crystal in the salicylic acid–anthranilic acid system

T–X phase diagram of salicylic acid–anthranilic acid with three crystalline solid solution phases and a co-crystal, resulting in variable enantiotropic transition temperature and a polymorphic co-existence domain.

Polymorphism and cocrystal salt formation of 2-((2,6-dichlorophenyl)amino)benzoic acid, harvest of a second form of 2-((2,6-dimethylphenyl)amino)benzoic acid, and isomorphism between the two systems

Isomorphism and isostructurality were observed between form I of 2-((2,6-dimethylphenyl)amino)benzoic acid and its analog 2-((2,6-dichlorophenyl)amino)benzoic acid, which suggests double Cl–CH3 exchange also leads to structural similarity.

Isostructurality of quinoxaline crystal phases: the interplay of weak hydrogen bonds and halogen bonding

Creation of isostructural systems is a balance between thermodynamic and kinetic factors, shown by a set of substituted quinoxalines, where the lighter halogens form a set of metastable isostructural crystals, different to the heavy substitutions.

The impact of solid solution composition on kinetics and mechanism of [2 + 2] photodimerization of cinnamic acid derivatives

The effect of the solid solution composition on the kinetics of solid-state [2 + 2] photocycloadditions was evaluated via a combination of single crystal XRD, FTIR, Raman spectroscopy, and principal component analysis (PCA).

ROY Reclaims Its Crown: New Ways To Increase Polymorphic Diversity

Chemical compounds that exist in multiple crystalline forms are said to exhibit polymorphism. Polymorphs have the same composition, but their structures and properties can vary markedly. In many fields, conditions for crystallizing compounds of interest are screened exhaustively to generate as many polymorphs as possible, from which the most advantageous form can be selected. We report new ways to search for polymorphs and increase polymorphic diversity, based on crystallization induced by suitably designed mixed-crystal seeds. The potential of the strategy has been demonstrated by using it to produce new polymorphs of the benchmark compound ROY as single crystals structurally characterized by X-ray diffraction. This allows ROY to reclaim its crown as the most polymorphic compound in the Cambridge Structural Database. More generally, the methods promise to become valuable tools for polymorphic screening in all fields where crystalline solids are used.

Isomorphism: 'molecular similarity to crystal structure similarity' in multicomponent forms of analgesic drugs tolfenamic and mefenamic acid

The non-steroidal anti-inflammatory drugs mefenamic acid (MFA) and tolfenamic acid (TFA) have a close resemblance in their molecular scaffold, whereby a methyl group in MFA is substituted by a chloro group in TFA. The present study demonstrates the isomorphous nature of these compounds in a series of their multicomponent solids. Furthermore, the unique nature of MFA and TFA has been demonstrated while excavating their alternate solid forms in that, by varying the drug (MFA or TFA) to coformer [4-di-methyl-amino-pyridine (DMAP)] stoichiometric ratio, both drugs have produced three different types of multicomponent crystals, viz. salt (1:1; API to coformer ratio), salt hydrate (1:1:1) and cocrystal salt (2:1). Interestingly, as anticipated from the close similarity of TFA and MFA structures, these multicomponent solids have shown an isomorphous relation. A thorough characterization and structural investigation of the new multicomponent forms of MFA and TFA revealed their similarity in terms of space group and structural packing with isomorphic nature among the pairs. Herein, the experimental results are generalized in a broader perspective for predictably identifying any possible new forms of comparable compounds by mapping their crystal structure landscapes. The utility of such an approach is evident from the identification of polymorph VI of TFA from hetero-seeding with isomorphous MFA form I from acetone-methanol (1:1) solution. That aside, a pseudopolymorph of TFA with di-methyl-formamide (DMF) was obtained, which also has some structural similarity to that of the solvate MFA:DMF. These new isostructural pairs are discussed in the context of solid form screening using structural landscape similarity.

Seed-triggered solid-to-solid transformation between color polymorphs: striking differences between quasi-isomorphous crystals of dichloro-substituted salicylideneaniline regioisomers

The title compound exhibited two color polymorphs, of which the yellow form transformed to the orange form during heating, and this peculiar phase transition behavior was explained in relation to the pseudo-symmetry of its molecular structure.

Systematic synthesis of a 6-component organic-salt alloy of naftopidil, and pentanary, quaternary and ternary multicomponent crystals

The single-crystal X-ray structure of a 6-component organic-salt alloy (hexanary) of naftopidil (1) (an active pharmaceutical ingredient) with benzoic acid (2) and four different hydroxy-substituted benzoic acids, i.e. salicylic acid (3), 2,3-di-hydroxybenzoic acid (4), 2,4-di-hydroxybenzoic acid (5) and 2,6-di-hydroxybenzoic acid (6), is reported. The hexanary assembly originates from the observation that the binary salts of naftopidil with the above acids are isostructural. In addition to the 6-component solid, we also describe five 5-component, ten 4-component, and ten 3-component organic-salt alloys of naftopidil (1) with carboxylic acids (2)-(6). These alloys were obtained from different combinations of the acids with the drug. The synthetic design of the multicomponent organic alloys is based on the rationale of geometrical factors (shape and size) and chemical interactions (hydrogen bonds). The common supramolecular synthon in all these crystal structures was the cyclic N+-H⋯O- and O-H⋯O hydrogen-bonded motif of (9) graph set between the 2-hy-droxyammonium group of naftopidil and the carboxyl-ate anion. This ionic synthon is strong and robust, directing the isostructural assembly of naftopidil with up to five different carboxylic acids in the crystal structure together with the lower-level multicomponent adducts. Solution crystallization by slow evaporation provided the multicomponent organic salts and alloys which were characterized by a combination of single-crystal X-ray diffraction, powder X-ray diffraction, NMR and differential scanning calorimetry techniques.

Precessional Motion in Crystalline Solid Solutions of Ionic Rotors

The order-disorder phase transition associated with the uprise of reorientational motion in (DABCOH2)²⁺ , in the supramolecular salts of general formula [1⋅(DABCOH₂ )]X₂ (where 1=12-crown-4, DABCO=1,4-diazabicyclo[2.2.2]octane, and X=Cl^- or Br^- ), has been investigated by variable temperature X-ray diffraction on single crystals and powder samples, as well as by DSC and solid-state NMR spectroscopy (SSNMR). The two compounds undergo a reversible phase change at 292 and 290 K, respectively. The two crystalline materials form solid solutions [1⋅(DABCOH₂ )]Cl_2x Br_2(1-x) in the whole composition range (0 < x<1), with a decrease in the temperature of transition to a minimum of ca 280 K, corresponding to x=0.5. Activation energy values for the dynamic processes, evaluated by variable-temperature ¹³ C magic-angle spinning (MAS) SSNMR and line-shape analysis are ca. 50 kJ mol^-1 in all cases. Combined diffraction and spectroscopic evidence has allowed the detection of a novel dynamic process for the (DABCOH₂ )²⁺ dications, based on a room temperature precessional motion that is frozen out below the disorder-order transition; to the best of the authors' knowledge this phenomenon has never been observed before.

Co-crystal synthesis: fact, fancy, and great expectations

Some strategies for driving co-crystal synthesis using a variety of competing non-covalent interactions are presented.

A chiral spiroborate anion from diphenyl-l-tartramide [B{l-Tar(NHPh)2}2]−applied to some challenging resolutions

A chiral spiroborate anion [B{l-Tar(NHPh)₂}₂]⁻is effective in challenging high yield, 1-pot resolutions, as for the S-2-phenylpropylammonium salt shown.

Prediction and experimental validation of solid solutions and isopolymorphs of cytosine/5-flucytosine

A computational search for polymorphs of cytosine, 5-flucytosine and a 1 : 1 mixture of the two substances not only rationalised the preferred packing arrangements but also enabled the finding and characterisation of cytosine/5-flucytosine solid solutions. The structures of the new solid forms were determined by combining laboratory powder X-ray diffraction data and computational modelling.

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.