February 2014 issue
Special issue on Crystal engineering
Guest Editor: Professor Andrew D. Bond (University of Copenhagen, Denmark)
This special issue on crystal engineering is part of a renewed commitment from IUCr for Acta Crystallographica Section B to publish high-quality results in the area of crystal engineering. The contents of the issue emphasise the fundamentals and two main areas where crystal engineering is now commonly applied, namely pharmaceutical solids and metal-organic frameworks (MOFs).
This article highlights general and modern synthetic strategies to synthesize metal-organic frameworks (MOFs), and discusses their structural diversity and properties with respect to application perspectives.
Solvothermal crystallization of cobalt(II) 1,4-benzenedicarboxylates in the presence of substituted pyridine-N-oxides as co-ligands yields four new materials whose structures have been solved and refined from single-crystal X-ray diffraction data. The phases all have structures related to the metal-organic framework MIL-53 with the pyridine-N-oxide acting as a pendant ligand, projecting into the pores of the structure giving new distortions of the well known metal-organic framework (MOF) structure.
Three new metal-organic framework structures containing EuIII and the little explored methanetriacetate ligand (mta3−) have been synthesized. Gel synthesis yields a two-dimensional framework with the formula [Eu(mta)(H2O)3]n·2nH2O, while two polymorphs of the three-dimensional framework [Eu(mta)(H2O)]n·nH2O are obtained through hydrothermal synthesis at either 423 or 443 K.
Three new crystalline uranyl polyphosphonate metal-organic frameworks have been prepared by hydrothermal synthesis. Two of these compounds are truly porous with cavities accounting for 23.6–26.9% of the total volume of the unit cell.
Newly designed alicyclic compounds that bear an enediyne functional group have been employed as ligands for coordination and supramolecular network assembly with silver(I) trifluoroacetate.
The crystal structures of three new drug-like bicyclo derivatives are correlated with measured thermodynamic quantities for sublimation and melting processes. Results from calculations of the crystal lattice energies are discussed.
Four solvates of an antifungal drug, griseofulvin, were identified. All the solvates were fully characterized and their thermal and solution stability was evaluated. Desolvation of solvates resulted in form (I) of the griseofulvin.
Co-crystals comprising the active pharmaceutical ingredient 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene and camphoric acid in 1:1 and 3:2 stoichiometries have been prepared and characterized by single-crystal X-ray diffraction and other physicochemical techniques.
A rich variety of polymorphic and solvated forms of the 1:1 co-crystal system of caffeine and anthranilic acid is described. The unprecedented range of forms for a co-crystal system is discussed in terms of the supramolecular hydrogen-bonded units present.
Six new co-crystals of the diuretic drug hydrochlorothiazide have been prepared. The crystal structures of four of them are discussed. The co-crystal with p-aminobenzoic acid shows a sixfold increase in solubility compared with the pure drug compound.
Statistical models to predict hydrogen bond coordination in organic crystals have been derived using the Cambridge Structural Database, leading to development of a methodology to construct hypothetical hydrogen-bond arrangements. We investigate an initial application towards the assessment of structural stability of industrially relevant polymorphs, co-crystals and hydrates.
The isostructurality of solid solutions of N-phenylformamides and thioamides is influenced by N—H⋯O=C or N—H⋯S=C hydrogen-bonded chains. While the molecular conformation may be cis or trans in the reactant crystals and both conformations exist in the solutions, only the trans conformation is observed in the solid solutions.
Three nickel(II) isothiocyanato complexes of the formula trans-[NiL4(NCS)2] (L = ethylisonicotinate, methylisonicotinate and 4-benzoylpyridine) have similar molecular structures but were found to have quite different crystal densities, prompting a comparative study of their crystal packing.
Depending on the solvent mixture employed for crystallization, racemic 2,4,6,8-tetracarbomethoxybicyclo[3.3.0]octa-2,6-diene-3,7-diol yields two inclusion compounds containing tetrahydrofuran (THF), with essentially identical host arrangements, but different proportions, orientations and host–guest interactions of the included THF molecules.
Inclusion of 18-crown-6 into the co-crystallization of 3,5-dinitrobenzoic acid and 4-aminobenzoic acid results in the creation of a ternary complex. The enhanced match between the crown and the protonated form of 4-aminobenzoic acid directs the formation of the complex system.
Two polymorphs of the 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4) illustrate the two main modes of solid-state organization of π-type radical species, either in extended delocalized chains with the possibility for conductivity, or into dimeric or tetrameric units with localized bonding interactions.
Three halogen-bonded co-crystals containing the azobenzene-based difunctional halogen-bond donor (E)-bis(4-iodo-2,3,5,6-tetrafluorophenyl)diazene, C12F8I2N2, demonstrate that the molecule is a reliable tecton for assembling halogen-bonded co-crystals with potential photoresponsive behaviour.
Competition between hydrogen (O—H⋯O, O—H⋯F) and halogen (Cl⋯Cl, O⋯Br) bonding interactions influences the molecular arrangement in the crystal structures of a series of boranthrenes. Substitution with a halogen atom enables fine-tuning of the strength of the interactions and occurrence of either halogen or hydrogen bonds.
The most important non-covalent interactions determining a crystal structure can be inferred objectively by using the observed crystal morphology to define the preferential lines of crystal growth. This top–down approach to crystal engineering is applied to the structure of a racemic Δ2-isoxazoline.
A crystal engineering approach is used to stabilize the radical anion in a series of crystalline alkali metal salts of 2,3-dicyano-5,6-dichlorosemiquinone (DDQ). The radical anions form close pairs connected by π–π interactions, and the short interplanar distances are correlated with observed spin coupling.