Band 229, Heft 9 - Special Issue: In Situ Crystallization / Edited by Roland Boese

In situ cryo-crystallization was used to obtain single crystals of the low-melting tetrafluoropyrimidine and trifluorotriazine in order to explore the molecular aggregation of nitrogen-containing aromatic compounds with weak directing substituents in the solid state. A comparative crystallographic study of the series hexafluorobenzene, pentafluoropyridine, tetrafluoropyrimidine and trifluorotriazine was performed to clarify an influence of a successive substitution of C-F groups by a further nitrogen atom on the crystal architecture. Due to the absence of the hydrogen atoms different supramolecular motifs based on short F···F contacts were observed in all crystal structures.

Crystal structures of a series of isomers of chlorofluorobenzene, bromofluorobenzene and iodofluorobenzene, all of which are liquids under ambient conditions, are determined by a technique of in situ cryocrystallography. These simple dihalo substituted benzenes provide clear insights into subtle interplay of packing interactions preferred by fluorine and heavier halogens for example, C–H···X hydrogen bonds vs. X···X halogen bonds (X=F, Cl, Br, I). The interaction patterns noted here are purely characteristic of halogens, having not been influenced by other stronger interactions. Variability of principal supramolecular synthons among the isomers highlights the importance of molecular shape and relative position of interacting atoms while preserving the basic intermolecular bonds. Mutually exclusive occurrence of homo (I···I) and hetero (I···F) halogen bonds in polymorphs of 4-iodofluorobenzene questions the robustness and reliability of these interactions.

“In situ crystallization ” is a technique, which is used to crystallize the materials, which are liquid at ambient conditions, on the single crystal X-ray diffractometer. This technique has been utilized to study crystal structures of a number of liquids, gases and mixtures of liquids and gases in the past. In this manuscript, we describe the formation of cocrystals of a few cresols with aniline and fluoroanilines using this technique. Different pairs of cresol and aniline/fluoroaniline resulted into various cocrystals, which display different crystalline motifs involving strong O–H···N, N–H···O hydrogen bonds along with weak C–H···O and C–H···F hydrogen bonds. The observed differences in crystal packing indicate that the role of weak hydrogen bonds in these systems is significant and need to be considered.

In situ crystallization technique at low temperature was used to obtain single crystals of the low melting compounds N(SiMe 3 ) 3 1 and As(SiMe 3 ) 3 3. N(SiMe 3 ) 3 has a waxy consistence at room temperature. The solid-state structure of both compounds has been determined by single-crystal X-ray diffraction analysis. N(SiMe 3 ) 3 is distorted and shows nearly a trigonal planar coordination of the central nitrogen atom. In contrast, the heavier homologue As(SiMe 3 ) 3 shows a trigonal pyramidal coordination with Si-As-Si 103.4(6)°–104.9(6)°.

The molecular complexes of tetrahydrofuran ( THF ) and diethyl ether ( DEE ) with chloroform ( CF ), dichlodromethane ( DCM ) and bromoform ( BF ) have been synthesised by in situ cryo-crystallization and their structures have been determined by single-crystal X-ray diffraction. CF forms 2:1 co-crystals with THF and DEE , where the components are linked in isolated 2:1 complexes by Cl 3 C-H…O interactions. Molecules of components in THF-BF complex form halogen-bonded chains, while the complexes DCM and BF with DEE contain two independent pairs of the components linked by C-H…O contacts. Supramolecular architecture of the molecular complexes is analysed using energy-based topology of intermolecular interactions.

MeNSOF 2 was crystallised by laser-assisted zone refinement at 150 K. The structure obtained by X-ray diffraction initially on cooling to 140 K was triclinic, P -̄, but this transformed slowly to a second phase, which was monoclinic C 2/ c . Bond distances and angles are similar to equivalent interactions in (MeN) 2 SF 2 and SO 2 F 2 . Intermolecular interactions in both phases are dominated by dispersion, though electrostatics are also important in all the most energetic contacts. Though no interatomic contacts fall within the sums of van der Waals radii of the contacting atoms, PIXEL calculations indicate that some intermolecular energies are similar to medium-strength hydrogen bonds. The monoclinic phase is denser than the triclinic phase, having stronger dispersion interactions. PIXEL and DFT calculations indicate that the two phases are energetically very finely balanced, but phase I becomes competitive at higher temperature on account of the entropy advantage of its ‘looser’ structure. DFT phonon energy calculations suggest that the mechanism of the transition may involve coupling between acoustic and low energy optical phonons.

A manual in situ crystallization technique is described, for application on a κ-geometry area-detector instrument. The technique has been applied to grow crystals of some linear alkynes: 1-heptyne, 1-octyne, 1-nonyne and 1-decyne, C n H 2 n – 2 ( n =7, 8, 9, 10). The structures with odd n (1-heptyne and 1-nonyne) crystallize in space group Pca 2 1 . The structures with even n (1-octyne and 1-decyne) crystallize in space group P 2 1 / c , with two molecules in the asymmetric unit ( Z′= 2) and an approximately orthorhombic metric. All of the structures contain layers of molecules with a square unit mesh of dimensions 5.7×7.2 Å, identical to the layers present in the previously published structures of 1,7-octadiyne and 1,9-decadiyne. The structures differ where the methyl groups meet, giving systematically greater packing efficiency for 1-heptyne and 1-nonyne, compared to 1-octyne and 1-decyne. This systematic alternation in crystal density is correlated with an alternation in melting points.

The structure of a high-pressure polymorph of propionamide is reported from synchrotron single-crystal X-ray diffraction data. The polymorph was obtained by in situ high-pressure crystallisation from solution in a diamond-anvil cell at 0.9 GPa; its structure is 23% denser than that of the ambient-pressure polymorph at STP. PIXEL lattice and dimer energies calculations indicate that the ambient- and high-pressure polymorphs are energetically competitive. Together with acetamide and formamide, this is the third high-pressure polymorph of an aliphatic amide crystallising as a Z′ =2 structure; the H-bonded network is however significantly different from the ones reported for the other two compounds. This study highlights the power of in situ high-pressure crystallisation techniques for accessing novel polymorphic forms of molecular compounds.