László Fábián

Rapid Room‐Temperature Synthesis of Zeolitic Imidazolate Frameworks by Using Mechanochemistry

Freshly ground: Improved mechanochemical methodologies, such as liquid-assisted grinding and ion- and liquid-assisted grinding enable the rapid and topologically selective synthesis of porous and nonporous zeolitic imidazolate frameworks with diverse topologies, at room temperature and directly from zinc oxide.

Volumetric measure of isostructurality

The numerical descriptors of isostructurality are critically reviewed and to overcome their limitations a new volumetric measure is proposed. It is defined as the percentage ratio of the overlapping volume of molecules in the analyzed structures to the average of the corresponding molecular volumes. For the calculation of this index a numerical approach is presented, which is also capable of treating disordered structures. The use of isostructurality calculations is demonstrated using a series of examples. Homologous 9-alkylthiophenanthrenes are used as simple illustrations of the necessity and applicability of the new descriptor. The structural changes in the inclusion compounds of 5-methoxysulfadiazine with chloroform, dioxane and tetrahydrofuran are analyzed and rationalized with the aid of isostructurality indices. The diverse relationships among a series of helical tubuland diols cocrystallized with simple phenols are also characterized. The modifications in molecular and crystal structures are correlated with the calculated degree of isostructurality.

Mechanochemical conversion of a metal oxide into coordination polymers and porous frameworks using liquid-assisted grinding (LAG)

Liquid-assisted grinding enabled the rapid and environmentally-friendly conversion of a simple and inexpensive metal oxide precursor into a variety of metal–organic materials, demonstrated by the quantitative construction of six coordination polymers and porous frameworks from a mixture of ZnO and fumaric acid.

Organic crystal hydrates: what are the important factors for formation

A database study of 34 770 accurate organic crystal structures reveals that the most important factor determining a higher frequency of hydrates is the sum of the average donor and acceptor counts for the functional groups. Different parameter forms for donor/acceptor properties were investigated for correlation with formation of hydrates, and are discussed. We did not find that the donor/acceptor ratio or the molecular weight significantly affects the hydrate formation. We also examined a wide range of calculated molecular properties and found that increasing polar surface is correlated with increasing hydrate formation. The water environment pattern of donor and acceptor hydrogen bonds in the crystal is influenced by the donor/acceptor ratio of the molecule.

Knowledge-based model of hydrogen-bonding propensity in organic crystals

A new method is presented to predict which donors and acceptors form hydrogen bonds in a crystal structure, based on the statistical analysis of hydrogen bonds in the Cambridge Structural Database (CSD). The method is named the logit hydrogen-bonding propensity (LHP) model. The approach has a potential application in identifying both likely and unusual hydrogen bonding, which can help to rationalize stable and metastable crystalline forms, of relevance to drug development in the pharmaceutical industry. Whilst polymorph prediction techniques are widely used, the LHP model is knowledge-based and is not restricted by the computational issues of polymorph prediction, and as such may form a valuable precursor to polymorph screening. Model construction applies logistic regression, using training data obtained with a new survey method based on the CSD system. The survey categorizes the hydrogen bonds and extracts model parameter values using descriptive structural and chemical properties from three-dimensional organic crystal structures. LHP predictions from a fitted model are made using two-dimensional observables alone. In the initial cases analysed, the model is highly accurate, achieving approximately 90% correct classification of both observed hydrogen bonds and non-interacting donor-acceptor pairs. Extensive statistical validation shows the LHP model to be robust across a range of small-molecule organic crystal structures.

Exploring cocrystal–cocrystal reactivity via liquid-assisted grinding: the assembling of racemic and dismantling of enantiomeric cocrystals

The reaction between pairs of enantiomeric cocrystals involving caffeine or theophylline and a chiral cocrystal former has been investigated by liquid-assisted grinding: we demonstrate two different outcomes for such cocrystal-cocrystal reactions.

Towards an environmentally-friendly laboratory: dimensionality and reactivity in the mechanosynthesis of metal–organic compounds

We present a proof-of-principle study of an environmentally-friendly approach to laboratory research, in which the synthesis and structural characterisation of metal-organic complexes and frameworks are achieved without using bulk solvents; our study addresses the use of heteroditopic ligands for manipulating the dimensionality of metal-organic materials and describes how kinetic obstacles in such mechanosynthesis can be overcome.

Isostructurality in one and two dimensions: Isostructurality of polymorphs

A set of polymorphic crystal structures was retrieved from the Cambridge Structural Database in order to estimate the frequency of isostructurality among polymorphs. Altogether, 50 structures, the polymorphs of 22 compounds, were investigated. It was found that one-, two- or three-dimensional isostructurality is exhibited by approximately half of the compounds analyzed. Among the isostructural polymorphs, the frequency of one-, two- and three-dimensional isostructurality is similar. From the examples, it appears that three-dimensional isostructurality is connected to the gradual ordering of crystal structures, while one- and two-dimensional isostructurality can often be related to specific packing interactions. The possibility of many similar interactions seems to decrease the probability of the occurrence of isostructural polymorphs. Conformational polymorphs do not exhibit isostructurality.

Protein-based integrated optical switching and modulation

The static and dynamic response of optical waveguides coated with a thin protein film of bacteriorhodopsin was investigated. The size and kinetics of the light-induced refractive index changes of the adlayer were determined under different conditions of illumination. The results demonstrate the applicability of this protein as an active, programmable nonlinear optical material in all-optical integrated circuits.

A model for a solvent-free synthetic organic research laboratory: click-mechanosynthesis and structural characterization of thioureas without bulk solvents

The mechanochemical click coupling of isothiocyanates and amines has been used as a model reaction to demonstrate that the concept of a solvent-free research laboratory, which eliminates the use of bulk solvents for either chemical synthesis or structural characterization, is applicable to the synthesis of small organic molecules. Whereas the click coupling is achieved in high yields by simple manual grinding of reactants, the use of an electrical, digitally controllable laboratory mill provides a rapid, quantitative and general route to symmetrical and non-symmetrical aromatic or aromatic–aliphatic thioureas. The enhanced efficiency of electrical ball milling techniques, neat grinding or liquid-assisted grinding, over manual mortar-and-pestle synthesis is demonstrated in the synthesis of 49 different thiourea derivatives. Comparison of powder X-ray diffraction data of mechanochemical products with structural information found in the Cambridge Structural Database (CSD), or obtained herein through single crystal X-ray diffraction, indicates that the mechanochemically obtained thiourea derivatives are pure in a chemical sense, but can also demonstrate purity in a supramolecular sense, i.e. in all structurally explored cases the product consisted of a single polymorph. As an extension of our previous work on solvent-free synthesis of coordination polymers, it is now demonstrated that such polymorphic and chemical purity of selected thiourea derivatives, the latter being evidenced through quantitative reaction yields, can enable the direct solvent-free structural characterization of mechanochemical products through powder X-ray diffraction aided by solid-state NMR spectroscopy.