Tobias Gruber

Fine tuning of crystal architecture by intermolecular interactions: synthon engineering

There has been a long time effort to influence or favourably fine tune structural properties by introduction of substituents or guest molecules of different sizes, shapes and chemical composition to consequently alter physico-chemical properties of the respective crystals. These attempts require the recognition, understanding and application of intermolecular interactions, crystallographic and, in case of occurrence, non-crystallographic symmetries. It brings us to the field of crystal engineering, which aims to produce new substances with required properties based on the knowledge of the structural properties of already characterised solids. A series of calixarene crystal structures are presented where the crystal packing is determined by spatial or by electrostatic effects. A series of laterally substituted calixarenes where both steric requirements and electrostatic forces play a role in the crystal architecture shows how the supramolecular synthon can be engineered.

Conformational behaviour and first crystal structures of a calix[4]arene featuring a laterally positioned carboxylic acid function in unsolvated and solvent-complexed forms

A detailed conformational analysis of a rarely investigated type of compound, a laterally monosubstituted calix[4]arene (1, which has a carboxylic acid function in the lateral position), is reported. 2D solution NMR techniques at various temperatures and in different solvents have been used, showing interesting aggregation behaviour for the different conformers. The first illustrations of crystal structures of this compound type are given, including the unsolvated carboxylic calix[4]arene and two mixed solvent complexes containing EtOH–H2O and EtOH–THF, respectively. Isostructurality calculations have been carried out, allowing detailed comparison of the investigated structures, and an unusual conformational chirality isomerism of the calixarene molecule is demonstrated.

Influence of laterally attached alkyl groups on the conformational behaviour of a basic calix[4]arene: combined NMR, molecular mechanics and X-ray study

Three new calixarenes 3–5 featuring an alkyl residue of different chain lengths attached to one of the central ring methylene groups of the basic calix[4]arene 1 have been prepared. A systematic study that includes also the lower homologous compound 2 showing the effect of the alkyl substitution on the conformational behaviour of the calixarene framework in comparison with the unsubstituted parent compound 1 is reported. The application of special 2D NMR techniques, 2D-EXSY and ROESY methods at various temperatures establishes that calixarenes 2–5 adopt the partial cone conformation of lower symmetry and far less the symmetric cone and 1,2-alternate conformations. In solution, they undergo a fast interconversion with relatively low activation energies of about 15 kcal/mol at room temperature. The conformer distribution is well reproduced by molecular mechanistic calculations (MMFF94), indicating the present conformers to assume the lowest steric energies. A single-crystal X-ray structure of the lateral ethyl derivative 2 corroborates these results, showing the molecule in a sterically favourable partial cone conformation.

Versatile Inclusion Behaviour of a Dinitrocalix[4]arene Having Two Ester Pendants – Preparation and X-ray Crystal Structures of Complexes

An upper-rim dinitro-substituted calix[4]arene possessing two lower-rim ethyl ester pendant groups (1) has been shown to form solid inclusion compounds with acetone (1:1) (1a), DMF (1:1) (1b), DMSO (1:1) (1c) and n-BuOH (2:1) (1d). X-ray crystal structures of the four complexes 1a–d are reported and comparatively discussed, including isostructurality calculations. Although the solid-state conformation of the dinitrocalix[4]arene moiety, stabilized by two intramolecular O–H…O bonds, is maintained in the four inclusion compounds, and all four co-crystals have similar unit cell dimensions and identical space group symmetries, only three of them (1a–c) are homostructural. Depending on the nature of the guest molecule, either the upper or the lower rim site of the calixarene is involved in the complexation, demonstrating either cavitate- or clathrate-type of supramolecular interactions, respectively. Moreover, due to the different guest recognition modes, the calixarene host in 1d is rotated through a non-crystallographic virtual rotation of 180° within the unit cell, in relation to the host molecules in each of the other three homostructural compounds 1a–c, thus giving rise to supramolecular morphotropism – to our knowledge the first case ever described.

Upper rim site lipophilic calix[4]arenes as receptors for natural terpenes and functionally related solvent molecules: combined crystal structure and QMB sensor study

Three upper rim site lipophilic calix[4]arenes 1–3 featuring different para-alkanoyl substituents at the phenolic moieties, i.e.n-hexanoyl, n-octanoyl and 3-cyclohexylpropanoyl groups, are reported to complex, aside from the common solvent molecule n-butanol, different keto and hydroxylic terpene guest molecules such as (−)-menthone, (−)-menthol or (+)-carvone in the solid state. Although for all six inclusion compounds described here, a strict 1 : 1 host : guest stoichiometry is observed, complementary size and polarity relationships between the cone shaped calixarene cavity and the guest molecule emerge from single crystal X-ray structural study. Dependent on the structure of the guest molecule, the calixarenes are arranged in a head-to-head or head-to-tail orientation in the crystalline packing, giving rise to capsular or otherwise deeply enclosed inclusion mode of the guest molecules, respectively. Furthermore, the O-acyl atoms of the calixarenes were found to be directly involved in host–guest interaction. In order to estimate the degree of isostructurality of the host frameworks and to examine the influence of the guest molecules on the solid-state conformation of these calixarene molecules, isostructurality comparison, cell similarity and molecular isometricity calculations were carried out revealing a rare case of supramolecular morphotropism for the pair of (−)-menthone and (−)-menthol inclusions of the n-hexanoyl substituted calixarene. Structure–inclusion property relationship was examined by QMB measurements of thin layers of the calixarenes. Vapor studies with different terpenes and common organic solvents show an increased affinity towards guest molecules with polar functionalities, whereas small solvent molecules like acetone are bound considerably better than the more bulky terpene molecules.

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetra­methoxy­calix[4]arene dichloro­methane hemisolvate

In the title compound, C48H64O4·0.5CH2Cl2, both crystallographically independent calixarene molecules display a partial cone conformation. Their crystal packing is stabilized by C—H⋯π contacts involving the methoxy groups. The solvent molecule is located interstitially between two calixarene units with C—H⋯Cl contacts to methoxy and tert-butyl groups. One tert-butyl residue of each calixarene molecule is disordered over two positions (occupancies 0.60/0.40 and 0.63/0.37), resulting in bond distances that deviate from ideal values. The tetramer calixarene molecules present models with approximate non-crystallographic Cs symmetry.

Crystal structures of a calix[4]arene controlled by two affixed pyrene units

The synthesis and crystal structures of a calix[4]arene (1) with two affixed pyrene units and its corresponding inclusion compound with chloroform (1a) are reported. In both cases, stacking structures resulting from the influencing control of the pyrene units are observed. The occurrence of infinite or dimeric stack motifs of the pyrene units is dependent on the absence or presence of the included guest solvent.

Simple dinitro substituted calix[4]arene forming a honeycomb-like architecture with hydrophobic channels

A rather easily structured permethylated dinitro calix[4]arene was found to exhibit large, stable and rigid channels in the solid state. These were obtained as guest free as well as solvent filled species and proved to reversibly adsorb selected organic solvents. Combined use of QMB measurements and X-ray powder diffraction revealed the predominantly reversible interaction of dichloromethane vapour with the channel structure as well as the integrity of the nanopores during adsorption and desorption. Examination of the flexible host component by NMR spectroscopy revealed a mixture of interchanging conformational isomers which could explain the high sensitivity of the crystallization process from the employed solvents.

1-(Hydroxy­meth­yl)pyrene

The asymmetric unit of the title compound, C17H12O, contains two molecules, in which the fused aromatic ring systems are almost planar [maximum deviations = 0.0529 (9) and 0.0256 (9) Å]. In the crystal, aromatic π–π stacking interactions (perpendicular distance of centroids of about 3.4 Å) and strong O—H⋯O hydrogen bonds result in a helical arrangement of pyrenyl dimers.

1-Methyl-4-(4-nitro­benzo­yl)pyridinium perchlorate

In the main molecule of the title compound, C13H11N2O3 +·ClO4 −, the two aromatic rings are twisted by 56.19 (3)° relative to each other and the nitro group is not coplanar with the benzene ring [36.43 (4)°]. The crystal packing is dominated by infinite aromatic stacks in the a-axis direction. These are formed by the benzene units of the molecule featuring an alternating arrangement, which explains the two different distances of 3.3860 (4) and 3.4907 (4) Å for the aromatic units (these are the perpendicular distances of the centroid of one aromatic ring on the mean plane of the other other aromatic ring). Adjacent stacks are connected by π–π stacking between two pyridinium units [3.5949 (4) Å] and weak C—H⋯O interactions. The perchlorate anions are accomodated in the lattice voids connected to the cation via weak C—H⋯O contacts between the O atoms of the anion and various aromatic as well as methyl H atoms.