Vi T. Nguyen

Molecular Solids Formed by the Self‐Organisation of Dialcohols into Hydrogen‐Bonded Ladders

The ability of certain dialcohols to form solid-state structures containing unidirectional hydrogen-bonded ladders has been investigated. Two double-stranded structures, staircase-ladders and step-ladders, have been identified. In each, dialcohol molecules are hydrogen-bonded into linear strands with two parallel strands cross-linked through additional hydrogen bonding. Staircase-ladders are made up of (O−H)n chains of hydrogen bonds, with the molecules in the two strands out of phase with each other. Step-ladders are formed from (O−H)4 cycles of hydrogen bonds, with the molecules of the two strands in phase. Sixteen examples of staircase-ladder structures and twelve cases of step-ladder structures were identified by use of the Cambridge Structural Database. A further three examples, all shown to be staircase-ladders by single-crystal X-ray analysis, were synthesised. Distinct structural preferences in ladder formation can be identified. Nearly all staircase-ladders contain only one type of enantiomer, with the dialcohol building blocks arranged around a twofold screw axis. This type of ladder is thus favoured for enantiomerically pure compounds. The preferred step-ladder construction contains (+)-enantiomers in one strand and (−)-enantiomers in the other, giving two repeating centres of symmetry along the ladder axis. There are, however, many exceptions to this norm. These two ladder types are compared with each other and with those formed by organic molecules containing other hydrogen-bonding functionalities.

Crystallisation of C2-symmetric endo,endo-bicyclo[3.3.1]nonane-2-6-diols: supramolecular synthons and concomitant degrees of enantiomer separation

Each of the three racemic C2-symmetric endo,endo-bicyclo[3.3.1]nonane-2,6-diols 6–8 shows a dominant, but different, mode of crystallisation across a wide range of solvents. These outcomes depend on the type of supramolecular synthon employed and the resulting degree of enantiomer separation that takes place. Diol 6 utilises the centrosymmetric hydrogen bonded (O–H)6 cycle and forms racemic crystals containing an intimate mix of the two enantiomers. In contrast, diol 7 uses (O–H)4 cycles with concomitant formation of homochiral layers, but racemic crystals still result since adjacent layers have opposite handedness. Diol 8 forms a hydrogen bonded network by using cross-linked (O–H)8 repeat helices and self-resolution yields a mixture of pure (+)- and pure (−)-crystals (a conglomerate). Since conglomerates may be separated into their pure enantiomers, the latter discovery offers a new preparative approach for obtaining chirally pure bicyclo[3.3.1]nonane compounds of value in organic synthesis.

Nodal equivalence of (O–H)6 and aromatic rings: a supramolecular cousin of Dianin's compound and β-hydroquinone

The simple dialcohol endo-4,endo-8-dimethylbicyclo[3.3.1]nonane-endo-2,endo-6-diol 2 yields inclusion compounds (diol)3.(guest) from nearly all recrystallisation experiments. These crystals have a hydrogen bonded network structure in cubic space group Ia, in which the disordered guest molecule occupies a spherical cage enclosed by eight (O–H)6 cycles positioned at the corners of a cube. Comparison of this structure with the inclusion crystals formed by Dianins compound 1, and the β-form of hydroquinone 3, reveals familial supramolecular behaviour between the three cases. The nodal relationship of their (O–H)6 cycles or aromatic rings creates allied crystal engineering assemblies despite their dissimilar molecular structures.

Alternative crystal forms produced by a dialcohol inclusion host : Resolutions and polymorphs. Part 3

The racemic C2-symmetric dialcohol endo-4,endo-8-dimethylbicyclo[3.3.1]nonane-endo-2,endo-6-diol 2 yields isostructural inclusion compounds in cubic space group Ia from nearly all recrystallisation experiments, but it has a different apohost structure. The X-ray structures of the alternative inclusion crystal forms, (2)6·(p-xylene) and (2)2·(water), are described here. Both are formed in space groupP21/c but their structures are entirely dissimilar. The p-xylene inclusion compound is constructed from a repeat of four different layers that are cross-linked by means of hydrogen bonding, whereas the hemihydrate co-crystal has a network structure involving two different hydrogen bonded layers that are cross-linked through the water molecules. The structures of these alternative crystal forms are discussed and contrasted in the context of crystal design. These help define the limits of solvents that can be incorporated into the baseline Ia structure in terms of size, awkwardness and hydrogen bonding capability.

Role of the (O‒H)6 Synthon in the Construction of Organic Inclusion Compounds

Abstract The racemic diol 5 crystallises from ethanol in the orthorhombic space group Cmca (a 19.05, b 14.93, c 10.12 Å) as the cocrystalline solid 5°C2H5OH. In contrast, the racemic diol 6 crystallises from benzene in the cubic space group Ia3 (a 19.85 Å) as a clathrate compound. The principal supramolecular synthon involved in both cases is a cyclic hexamer of hydrogen bonded hydroxy groups, (O-H)6. While the former cycle is almost planar, rather uncommonly the latter is cyclohexane-like in geometry. A major function of the (O-H)6 cycle is to allow efficient packing between opposite enantiomers within the crystal lattice.

Hydrogen-Bonded Ladders formed by Crystalline Diols

Certain diol molecules crystallize from solution as unidirectional double-stranded ladders. When these two strands assemble in-phase, a step-ladder arrangement results with the diols linked by (O–H)4 cycles of hydrogen bonds. Using racemic diol samples, the preferred arrangement has two enantiomerically pure strands of opposite handedness. If the two strands assemble out-of-phase, a staircase-ladder results where the molecules are connected by a hydrogen-bonded (O–H)n chain. The preferred arrangement contains enantiomerically pure diol molecules surrounding a 21 screw axis. If a racemic diol is used then either the crystals contain both (+)- and (–)-ladders or, alternatively, self-resolution occurs to yield a conglomerate. New diols (7)–(9) were synthesized and an X-ray investigation confirmed the prediction that these would produce further double-stranded ladders. These structures revealed new variants of the staircase-ladder where both enantiomers are present, inclusion compounds are formed, or crystallographically independent diol molecules are involved. Manuscript received: 27 August 2001. Final version: 5 November 2001.

A New Lattice Inclusion Host Involving Double-stranded Columns of Diol Molecules [1]

Abstract Crystallization of dialcohol 4 from benzene yields an inclusion compound whose crystal structure [(C13H24O2)2·(C6H6), P21/c, a 7.918(2), b 13.505(2), c 14.924(5) Å, β 109.30(1)°, Z 2, R 0.069] shows that the host molecules are present as parallel doubly-stranded columns. Each column is constructed from one strand of (+)-, and a second of (-)-, enantiomers of 4. These two chirally pure strands are linked through a continuous chain of hydrogen bonding …O—H…O—H…O—H… to complete the column, and the benzene guests occupy interstitial sites between the parallel columns.

A versatile but unexpected new clathrate inclusion host

Racemic endo-4,endo-8-dimethylbicyclo[3.3.1]nonane-endo-2,endo-6-diol 1 forms clathrate inclusion compounds when crystallised from many common solvents. From diethyl ether solution crystals of (1)3.(C4H10O) are produced in the cubic space group Ia3. Here the diol hosts assemble by means of hydrogen bonded (O-H)6 cycles into a network structure. The guests are enclosed in cages with only dispersion forces operating between the two molecular components. Crystals of solvent-free 1, produced by sublimation of (1)3.(C4H10O) under reduced pressure, occupy the monoclinic space group P21/c. Here the diols are linked by means of hydrogen bonded (O-H)4 cycles producing layers. This structure provides no indication of packing difficulties which might have allowed the versatile inclusion behaviour of 1 to be predicted

(1R*,2R*,4S*,5R*,6R*,8S*)-4,8-Dimethyl-2,6-diphenyl­bicyclo­[3.3.1]nonane-2,6-diol

The racemic title compound, C23H28O2, crystallizes in the space group C2/c as a layered structure in which a centrosymmetric three hydrogen bond sequence links four molecules. Both hydroxy groups are involved in this arrangement, but they differ in that one participates in two hydrogen bonds while the other takes part in only one. Between layers, the aromatic rings take part in edge-face interactions [shortest C—H⋯C distances 3.04, 3.10 and 3.12 Å and angle between normal to planes 86.7(2)°], forming a centrosymmetric dimer. The lattice is further stabilized by C—H⋯π interactions involving both methyl (shortest C⋯C 3.82 and 3.97 Å) and methylene (shortest C⋯C 3.60 Å) groups.

Dialcohol Hydrogen Bonded Ladder Structures

Appropriate dialcohols have the ability to form unidirectional ladder structures on crystallisation from solution. These assemblies are built from two internally hydrogen bonded molecular strands which are cross-linked by further hydroxy group hydrogen bonding. In staircase-ladder structures the molecules of the two strands are out of phase and linked by (O-H) n chains. Alternatively, if the molecules of the two strands are in phase, a step-ladder structure results where the dialcohol units are linked by (O-H) 4 cycles. Both ladder types show strong enantiomer ordering and symmetry preferences. For example staircase-ladders normally comprise dialcohol molecules of one handedness surrounding a 2 1 screw axis. New racemic dialcohols are described which form staircase-ladders. Self-resolution, guest inclusion, enantiomer ordering, and asymmetric unit phenomena are described which result in previously unobserved types of staircase-ladder assembly.