Solhe F. Alshahateet

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.

Dimeric C–H⋯N interactions and the crystal engineering of new inclusion host molecules

Single crystal X-ray studies of (6)·(tetrahydrofuran)2 and (6)·(benzene)1.5 reveal the presence of two distinct types of dimeric edge–edge C–H⋯N packing motifs. All of the racemic dibromides 2–4 and 6 form clathrate compounds where their opposite enantiomers are joined edge–edge by means of C–H⋯N dimers. Hence this interaction is robust and of considerable value in crystal engineering. Modification of the benchmark centrosymmetric aryl–H⋯N dimer can result, however, in more complex modifications which better suit a given host–guest combination. Therefore a range of different C–H⋯N dimers is observed, with the new motif present in (6)·(benzene)1.5 completing a sub-set of these variants. This structural adaptability increases the likelihood of inclusion because competition between many weak intermolecular contacts is a central part of the host design philosophy.

New edge–edge packing motifs present in the crystal structures of a thia-bridged tetrabromo aryl host

Thia-bridged diquinoline tetrabromide 7 has been synthesised and found to be a further example of the tetrahalo aryl inclusion host family. The X-ray crystal structures of the (7)2·(benzene) and (7)2 (chloroform) inclusion compounds are described and analysed. In both, the hosts assemble by means of two new types of centrosymmetric aryl edge–edge packing motifs. The first type is a C–Br⋯N dimer which generates chains of 7 molecules that are cross-linked by the second interaction, a double bifurcated Br⋯Br⋯H motif. The included guest molecules are enclosed within host molecular pens in both structures. Novel S⋯Br interactions also play an important role in linking these building blocks.

The ether–1,3-peri aromatic hydrogen interaction

Single crystal X-ray structures of the oxygenated diquinoline derivatives 3 and 4 show that, in both cases, the ether oxygen atom interacts very effectively with two 1,3-peri aromatic hydrogens of an adjacent molecule. A six-membered cycle involving two C–H⋯O weak hydrogen bonds results and in structure 4 this plays a key role in preventing inclusion host behaviour. The occurrence of this motif has been investigated using the Cambridge Structural Database and is compared with the behaviour more commonly observed between ethers and aromatic hydrogens.

An oxa-bridged tetrahalo aryl lattice inclusion host

The oxygen-substituted diquinoline tetrabromide 11 has been synthesised and found to be a further example of the tetrahalo aryl inclusion host family. This host forms achiral molecular staircases by means of aromatic offset face–face (OFF) and centrosymmetric pi–halogen dimer (PHD) interactions. The X-ray crystal structures of the compounds (11)2·(chloroform) and (11)2·(dichloromethane) show that the included guest molecules occupy channels between the parallel staircases, and that host inter-staircase O⋯Br association is also significant.

Pseudopolymorphic Clathrate Structures Formed by an Alicyclic Dialcohol Inclusion Host

Dialcohol host 2,7-dimethyltricyclo[4.3.1.03,8]undecane-syn-2,syn-7-diol 1 can form either ellipsoidal clathrate or helical tubulate inclusion compounds where only dispersion forces operate between the hosts and guests. The former (tetragonal space group I41/acd), built from two interpenetrating sublattices containing both diol enantiomers, encloses the guests in rugby ball-shaped cavities. The latter (trigonal space group P3121 or P3221), containing only one diol enantiomer, traps the guests within parallel tubes. Which inclusion type is produced is determined by the guest size and shape and, hence, control is possible over these structures. At room temperature, cyclohexane gives the tetragonal structure, but fluorocyclohexane yields the trigonal structure. Chloroform produces both pseudopolymorphs: the tetragonal form at higher and the trigonal form at lower temperatures. Powder and single-crystal structural X-ray data are reported for these clathrate compounds.

Co-crystalline hydrogen bonded solids based on the alcohol-carboxylic acid-alcohol supramolecular motif

The helical tubuland diol 4 forms 1∶1 hydrogen bonded co-crystalline adducts with 2-propanol and propanoic acid in space group P21/c, and with benzoic acid in C2/c. In the first of these compounds, the 2-propanol molecules contribute one hydroxy group to the intermolecular O–HO–HO–H hydrogen bond repeat unit. The co-crystals formed with propanoic acid and benzoic acid both contain a O–HOC(R)–O–HO–H repeat unit with the hydrogen bonds surrounding a pseudo-threefold screw axis, and where the carboxylic acid group functions like an extended alcohol hydroxy group. Aromatic offset face–face associations also play a significant role in the crystal structure of the benzoic acid compound.

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.

Clathrate inclusion behaviour of thia-substituted diquinoline host molecules

Continuing our search for new compounds which function as lattice inclusion hosts, the thia-substituted diquinoline derivatives 5 and 6 have been synthesised and the X-ray structures of (5)6·(CH3OH) and (6)·(benzene) determined. A literature survey on the thioether–1,3-peri aromatic hydrogen interaction reveals that this is reasonably common, but neither crystal structure contains this motif despite expectations from our earlier work on the oxygenated analogues 3 and 4. It is also significant that the commonly observed edge–edge aryl C–H⋯N dimer motif is absent in (5)6·(CH3OH) and only present in (6)·(benzene) as an ineffectual, long-range contact. Instead, opposite host enantiomers in (5)6·(CH3OH) are linked through aza–1,3-peri aromatic hydrogen interactions, and the prevalence of this motif in the literature is also surveyed. Six molecules of 5 surround the methanol guest which is disordered on a site. The benzene molecules in (6)·(benzene) assemble by means of aryl edge–face interactions to produce parallel columns. This arrangement is the same as part of the lattice structure in solid benzene.

Host pre-resolution versus self-resolution in the formation of helical tubulate inclusion compounds

Racemic samples of the dialcohol 2 are known to yield two distinct types of clathrate inclusion compounds on crystallisation. Smaller guests (such as dichloromethane or cyclohexane) are included in cages within the ellipsoidal clathrate lattice (constructed from both enantiomers of the host 2). Larger guests (such as carbon tetrachloride or fluorocyclohexane) are enclosed in parallel tubes as part of the helical tubulate lattice. This structure contains only one enantiomer of 2 as a consequence of a spontaneous self-resolution process. Enantiomerically pure samples of 2 have now been prepared and the inclusion type found to switch to the helical tubulate for the smaller guests. Some much larger guest molecules (such as t-butylcyclohexane), which were not included at all by racemic 2, now also form helical tubulate inclusion structures when the resolved dialcohol 2 is used. Thus pre-resolution of the host allows extension of helical tubulate guest inclusion in both size directions.