Multiple molecules in the crystallographic asymmetric unit. Self host-guest and doubly interpenetrated hydrogen bond networks in a pair of keto-bisphenols

The presence of two molecules in the crystallographic asymmetric unit in a pair of closely related keto-bisphenols that differ by a methyl substituent only, leads to open frameworks that fill space through self-inclusion in one case, and through interpenetration in the other

Interplay of strong and weak hydrogen bonding in molecular complexes of some 4,4'-disubstituted biphenyls with urea, thiourea and water

The crystal chemistry and engineering of a new family of host-guest complexes is described. 4,4'-Dicyanobiphenyl (DCBP) forms a 1:1 complex, 1 with urea wherein the DCBP host forms large hexagonal channels via C-H···N hydrogen bonds and the urea guest molecules are arranged in N-H···O ribbons which fit completely within the host channels. By analogy, 4,4'-bipyridine N,N'-dioxide (BPNO) was selected as a molecule that can form a corresponding C-H…O based channel. BPNO forms complexes with urea (2), thiourea (3) and water (4). Structures 2 and 3 provide some points of comparison with the structure of 1 but are not fully equivalent to it. In structure 4, the smaller guest water is able to fit neatly within the smaller hexagonal channel of BPNO and in this sense, the degree of structural predictability is satisfactory. To obtain another structure similar to that of 1, 4,4'-dinitrobiphenyl (DNBP) was identified as an alternative host compound. This choice was justified by the structure of its 1:1 complex, 5 with urea. In all cases, the guest molecules interact with each other via strong hydrogen bonds and form an essential template for the weak hydrogen bonded assembly of the host network structure but the latter is still of some significance. One finds consequently, in complexes 1-5, a constructive interplay of strong and weak hydrogen bonds

Molecular networks in the crystal structures of tetrakis(4-iodophenyl)methane and (4-iodophenyl)triphenylmethane

The crystal structure of tetrakis(4-iodophenyl)methane is analysed in terms of molecular networks wherein the tetraphenylmethane moieties and I4 synthons are considered as molecular and supramolecular nodes. This I4 cluster plays the same role in generating molecular networks as does the Br4 cluster in the isomorphous tetrakis(4-bromophenyl)methane derivative. (4-Iodophenyl)triphenylmethane crystallises in a lower symmetry space group but features an unusual I···Ph interaction. In this series of halo-substituted tetraphenylmethanes the molecules exhibit similar columnar packing in the solid state, accounting for their crystallisation in non-centrosymmetric space groups

Design of an SHG-active crystal, 4-iodo-4'-nitrobiphenyl: therole of supramolecular synthons

The crystal structure of 4-iodo-4'-nitrobiphenyl has been determined with packing calculations and the presence of polar and parallel iodo···nitro supramolecular synthons leads to non-centrosymmetry and measurable SHG activity

Supramolecular synthons mediated by weak hydrogen bonding: forming linear molecular arrays via C≡C-H···N≡C and C≡C-H···O₂N recognition

In crystalline 4-cyano-4'-ethynylbiphenyl and 4-ethynyl-4'-nitrobiphenyl, molecules are connected with the hitherto unreported Câ-·C-H···Nâ-·C and Câ-.C-H···O2N supramolecular synthons to form linear arrays

Molecular networks in the crystal structures of tetrakis(4-iodophenyl)methane and (4-iodophenyl)triphenylmethane

The crystal structure of tetrakis(4-iodophenyl)methane is analysed in terms of molecular networks wherein the tetraphenylmethane moieties and synthons are considered as molecular and supramolecular nodes. This I 4 I 4 cluster plays the same role in generating molecular networks as does the cluster in the isomorphous Br 4 tetrakis(4-bromophenyl)methane derivative. (4-Iodophenyl)triphenylmethane crystallises in a lower symmetry space group but features an unusual IÉ É ÉPh interaction. In this series of halo-substituted tetraphenylmethanes the molecules exhibit similar columnar packing in the solid state, accounting for their crystallisation in non-centrosymmetric space groups. A network depiction of an organic crystal structure is intuitive in those cases where strong and directional interactions are responsible for crystal architecture.1 Thus it comes as no surprise that the crystal structures of hydrogen-bonded solids2 and those that are held together by coordination bonds3 may be easily represented in this way. Such a depiction of an organic crystal is not new. In Powell and PalinÏs classical description of the hydroquinone clathrates, the molecules are shown as points and the network structure is highlighted.4 The advantage of such an analysis is that it facilitates our understanding of complex packing arrangements. For example, further simpliÐcation of the hydroquinone clathrate structure reveals its topological similarity to the easily understood b-polonium structure.5 In molecular crystals that are held together by forces weaker and less directional than conventional hydrogen bonds, the relevance of the network depiction is not so obvious.6 However, in any organic crystal structure, the molecules may be reduced to points and the points joined according to established protocols and criteria to obtain a network

Inclusion compounds of tetrakis(4-nitrophenyl)methane: C-H···O networks, pseudopolymorphism, and structural transformations

Tetrakis(4-nitrophenyl)methane is a new host material with considerable structural adaptability over a range of solvents. The crystal structures of 14 of these solvates have been determined and classified into three groups. The diamondoid group, wherein the host molecules form a 2-fold interpenetrated diamondoid network structure, is unprecedented in that network connections are made exclusively with weak C-H···O and ∏ ···∏ interactions. This group consists of the solvates of THF, dioxane, nitrobenzene, 4-bromoanisole, anisole, phenetole, p-xylene, and chlorobenzene. The rhombohedral group, which is characterized by specific host···guest interactions of the C-H···O and halogen···O2N type, consists of the solvates of CHCl3 and CHBr3 and somewhat surprisingly DMF, which shows an unusual 3-fold disorder mimicking in part the shape and size of the haloform molecules though not their orientation. The third group comprises solvent-rich solvates of the host with mesitylene, collidine, and o-xylene with quite different crystal structures. The THF solvate was found to lose solvent over limited temperature ranges transforming reversibly from the diamondoid structure to the rhombohedral structure. A mechanism for this process is outlined. Material from which solvent has been removed by heating was also found to resolvate upon soaking in appropriate solvents. In summary, the title compound forms a host network that is partially robust and in part flexible. It is possible that this fluxional nature of the host network derives from the weakness of the connecting interactions

Crystal engineering: some further strategies

Structural studies currently underway in our group are reported in this paper. Molecular symmetry is rarely carried over into the crystal, posing problems when high-symmetry networks are desired. This is illustrated by the low-symmetry structure of 2,4,6-trinitromesitylene. However, the involvement of the Cl<SUB>3</SUB> supramolecular synthon ensures a hexagonal network structure for 2,4,6-tris-(4-chlorophenoxy)-1,3,5-triazine. Arguments following from the equivalence between molecular and supramolecular synthons lead to the tetragonal network structure of the 1:1 complex of tetraphenylmethane and CCl<SUB>4</SUB>. With a similar reasoning, 4-(triphenylmethyl)benzoic acid is identified as a precursor of a supramolecular wheel-and-axle host substance. The study of novel and weaker intermolecular interactions is often useful. In N,N'-dibenzyl-1,4-cubanedicarboxamide, the acidity of the cubyl C-H groups leads to the formation of C-H&#183;&#183;&#183;O hydrogen bonds. Polymorphism is a difficult challenge for the crystal engineer and, in its most intriguing manifestation, two crystalline forms of a substance appear in the same crystallization batch. This is observed for 4,4-diphenyl cyclohexadienone. The ultimate frontier in the subject is an understanding of the phenomenon of crystallization, and the unexpected crystal structure of quinoxaline, with five symmetry-independent molecules, could possibly represent a case of arrested crystallization