A. Noman M. M. Rahman

Analysis of pi–halogen dimer interactions present in a family of staircase inclusion compounds

The family of staircase inclusion compounds formed by the tetrahalo aryl hosts 1 and 2 are re-evaluated in terms of the pi–halogen dimer interactions present in their structures.

Pi–halogen dimer interactions and the inclusion chemistry of a new tetrahalo aryl host

The preparation of 1,4,8,11-tetrabromo-5b[small alpha],6,12b[small alpha],13-tetrahydropentaleno[1,2-b:4,5-b[prime or minute]]diquinoline is described. This is a further member of the tetrahalo aryl host family, and forms crystalline lattice inclusion compounds with many guests. The X-ray structures of the allyl cyanide, 1,2,3-trichloropropane, chlorobenzene, toluene, benzene-water, methyl chloroform and carbon tetrachloride inclusion compounds are described, and compared with that of the solvent-free apohost. Although several different structural types are produced, the recently reported pi-halogen dimer (PHD) interaction plays an important role in all of these, except for that of pure (where the packing energy is the least favourable of the series).

The Pi – Halogen Dimer (PHD) Interaction: A Versatile New Construction Unit for Crystal Engineering

ABSTRACT The pi – halogen dimer (PHD) interaction is a new type of intermolecular packing motif discovered in certain crystalline diquinoline compounds. It consists of a combination of one aromatic offset face-face (OFF) interaction, and four aromatic pi – halogen interactions, and provides a compact building block for the self-assembly of lattice inclusion compounds. This article reviews the current state of knowledge about this new solid-state construction motif.

Synthesis of a new lattice inclusion host belonging to the tetrahalo aryl family

Abstract The tetrabromo diquinoline derivative 3 has been synthesised and its dichloromethane compound investigated by X-ray crystallography. This racemic host acts in an unusual manner by assembling into achiral molecular staircases and including the guests in parallel channels between these. Compound 3 belongs to a newly identified family of lattice inclusion hosts whose structural characteristics are described here for the first time.

Pi-halogen dimers and V-shaped tetrahalo aryl inclusion hosts

1,4,8,11-Tetrabromo-5bα,6,12bα,13-tetrahydropentaleno[1,2-b:4,5-b′]diquinoline 6 has been synthesised and found to be an efficient lattice inclusion host for a range of small solvent molecules. The compound is a new example of the tetrahalo aryl host family, a group of molecules characterised by having a combination of inclined aromatic planes and halogen substitution at their extremities. The X-ray structures of (6)·(carbon tetrachloride) and (6)2·(allyl cyanide) are presented and discussed in crystal engineering terms. Both structures contain a common motif (the pi-halogen dimer) where two inversion-related host molecules pack such that one bromine atom from each is situated in the cleft of its V-shaped partner. This efficient packing motif provides a combination of π⋯Br and OFF (offset face–face) interactions within one supramolecular building block.

Why a hexabromodiquinoline host preferentially includes small aromatic hydrocarbon guests

The preparation and properties of the new hexabromodiquinoline derivative 4 are described. This lattice inclusion host shows a strong preference for trapping small aromatic hydrocarbons. The X-ray crystal structures of the benzene, toluene, o-xylene, and p-xylene compounds are reported, and are analysed from a crystal engineering perspective. Crystallisation of 4 from the dual-nature solvent trifluoromethylbenzene yields the solvent-free material. Comparison of the parent crystal structure with those of its inclusion compounds reveals why inclusion of aromatic hydrocarbon guests is such a favoured process. The high concentration of Br...Br interactions in the structure of pure 4 is diluted and increasingly replaced by aromatic offset face-face (OFF) and aromatic edge-face (EF) interactions in the inclusion compounds, and this results in better lattice packing energies. For toluene, o-xylene, and p-xylene the host-guest ratio is 1:1. Inclusion of the smaller benzene molecule results in a change to 2:3 stoichiometry. This increase in guest content is assisted by replacement of host-host OFF and EF motifs with host-host pi-halogen dimer (PHD) interactions, which provide space for inclusion of the additional guest molecules. These changes result in the most efficient lattice packing of the series for compound (4)2.(benzene)3.

Preparation of solvent-free clathrand structures by the exclusion of an unwelcome guest

Crystallisation of clathrate host molecules, using an appropriate dual-nature solvent, can encourage both the growth of crystals and its exclusion as a guest. To achieve this end, the solvent should contain a part-structure that interacts favourably with the host molecules, thereby encouraging host association and crystal nucleation. It should also contain a part-structure that interacts unfavourably with the hosts, so that the solvent ultimately is excluded as the material crystallises. We report that trifluoromethylbenzene is an excellent solvent for obtaining crystalline solvent-free clathrands belonging to the diquinoline inclusion family. The X-ray crystal structures of one of these and its corresponding toluene inclusion compound are described and compared.

Interlocking molecular grid lattices involving weak assembly forces

The X-ray structures of the two diquinoline derivatives 6,13-dihydro-6,13-ethano-5,12-diazapentacene (1) and 6α,13α-dibromo-2,9-dichloro-5bα,6,12bα,13-tetrahydropentaleno[1,2-b:4,5-b′]diquinoline (2) reveal that both crystallise by forming network lattices involving only weak intermolecular forces. These very similar lattices contain no aryl edge–edge C–H⋯N R22(8) dimers or efficient aryl offset face–face interactions, commonly found in related diquinoline crystals. Both lattices arise from multiple interlocking, at an inclined angle, of two identical sets of parallel molecular grids. Each grid connector site comprises a centrosymmetric tetramer of molecules held together by weak intermolecular forces, unlike the metal co-ordination motifs normally used to generate grid structures. Edge–edge C–H⋯N weak hydrogen bonds of various types help stabilise these structures by linking molecules of opposite handedness in neighbouring grids.

Inclusion Properties of a Chlorinated Diquinoline Host

Abstract The new lattice inclusion host 6α,13α-dibromo-2,9-dichloro-5bα,6,12bα,13-tetrahydropentaleno[1,2-b:4,5-b′]diquinoline 8 was prepared, and found to include far fewer guests than its non-chlorinated analogue 2. Compounds (8)2·(ethyl acetate) and (8)2·(benzene) form molecular pen and staircase inclusion compounds respectively. Their X-ray structures are analysed and compared in crystal engineering terms.

Clathrate formation between halogenated species

Analysis of X-ray crystallographic data reveals how the exo,exo-dibromodiquinoline derivative 4 encloses small halogenated guests as clathrate structures.