Themed collection New Trends in Crystal Engineering

The articles and highlights presented at the second CrystEngComm meeting, held in Nottingham in September 2004 are reviewed.

Contributions of crystal engineering to the development of industrially relevant materials (e.g. pigments, dyes, agrochemicals and drugs) are described. Future needs and possible development trends in crystal engineering are discussed with a special emphasis on novel innovative materials.

The aim of this highlight is to provoke interest in the similarities (and differences) between molecular crystals and other forms of soft matter, with particular attention to the relationships between length scale, energy, and structure.

A discussion is presented on the rational synthesis of linear, zigzag or helical chains when two vertices of an octahedron, which are either adjacent (cis) or opposite (trans), are linked.

Crystal structure prediction studies of organic packing are highlighted, with particular emphasis on what they reveal about the transferability assumptions that underpin the use of empirical atom–atom intermolecular potentials and intuitive crystal design.

Two solid-state coordination reactions for the polymers g[N(CH₂CH₂)₃N]₂[CH₃COO]·5H₂O and Zn[N(CH₂CH₂)₃N] and their solution counterparts, as well as two discussions of X-ray results are presented.

¹H NMR complexation-induced changes in chemical shift can be used to determine high resolution three-dimensional structures of complexes in solution, as well as for probing the organisation of molecules in the solid state.

Recrystallisation of organic compounds from solution at high pressure is shown to be a powerful technique for the discovery of new polymorphs and solvates. A new polymorph of phenanthrene and a new dihydrate of paracetamol have been discovered and structurally characterised.

Small differences in methylene C–H⋯O intermolecular contacts are analyzed by X-ray crystallography and by CSD data.

The validity of the so-called strength–length correlation (“the shorter the contact, the stronger it is”) has been evaluated for short O–H⋯O contacts by correlating the MP2 interaction energy with the O–H⋯O distance.

Examination is performed of a series of salts of square planar metallate anions involving a range of ligands. The hydrogen bonding capacity of these anions and the manner in which such interactions dictate the motifs and the resulting crystal structures is examined. This study explores the prospects for the planned elaboration of molecular species, tectons, as programmed components of crystal structures.

The synthesis and crystal structure of [Co(Hbiim)₂(H₂biim)]₂(p-OOCC₆H₄COOH)₂·H₂O is presented. Quantum chemical calculations (DFT and semi-empirical methods) were used by estimation of the directional forces in the structure.

Dimers formed by self-association of a novel Horning-crown macrocycle appear in a number of inclusion complexes/solvates. These dimers, stabilised by π–π interactions, are conserved, but on crystallisation diverse packing motifs and inclusion occur.

Three new compounds [Cu(Hmal)₂ and Cu(H₂O)(H₂mal)(mal)], in which three species of the malonate (H₂mal, Hmal⁻, and mal²⁻) are present, have been prepared and structurally characterised. The effectiveness of the magnetic exchange coupling was investigated as related to the degree of protonation of the malonate group.

Crystallographic study is reported on complexes of metal chloride and amines forming layered inorganic–organic hybrid structures. The structures are based on anionic metal halide hydrogen bond acceptors and cationic phenylammonium N–H donors.

The crystal structures of three peptide–biphenyl hybrids are reported. These compounds crystallize in helical structures maintained by extensive H-bond arrays. The results show the mutual influences of the molecular and supramolecular structures in the solid state.

A surface film that acts as a molecular mangle is described that delivers continuous ribbons of selected crystals with nm thickness. Initially, this has been evaluated for BaSO₄. The approach is shown to be successful at preparing metastable nanoribbons of barite. The opportunities that this provides for crystal engineering more generally are discussed.

The occurrence of interpenetration in metal–organic and inorganic networks has been investigated by a systematic analysis of the CSD and ICSD structural databases. For this purpose, a novel version of TOPOS (a program package for multipurpose crystallochemical analysis) has been employed, where the recognition of interpenetrating nets is based on the representation of a crystal structure as a finite reduced graph.

A supramolecular assembly strategy for coordination networks is described, which utilizes cis-protecting ligand that generate neutral tectons with two available trans positions where a supramolecular reagent can bind to generate extended hydrogen bonded networks.

Two compounds [(L³)MnCl₂(EtOH)] and [(L⁶)FeCl₃] were synthesized and structurally characterized in network design. It was demonstrated that these coordination units can be used effectively as supramolecular synthons in crystal engineering to construct neutral networks.

The paper is a detailed analysis of acetylene∶pyridine type co-crystals characterized by low-temperature X-ray diffraction where intermolecular interactions are evaluated in their importance for the packing.

The methyl orange anion has been incorporated into hydrogen bonding networks through use of guanidinium and substituted guanidinium cations. [C(NH₂)₃][O₃SC₆H₄NNC₆H₄NMe₂] reacts with hydrochloric acid to give the zwitterionic compound O₃SC₆H₄NHNC₆H₄NMe₂ whereas solid samples reversibly absorb HCl gas.

Even in the solid state, crystal-to-crystal transformation for three diol-BQ adducts, rac-BN-BQ, 2BP-BQ and 4BP-BQ, occurs without going through an amorphous intermediate. This indicates that molecules can diffuse substantially, sever and form hydrogen bonds, and rearrange themselves to form a 3-D periodicity in the solid state.

The synthesis and structure of 2-D silver(I) coordination polymers of triphosphines with heterocyclic cores are described. They are formally related by ring-opening polymerisation to a porous ‘chickenwire’ coordination polymer, obtained with a the benzene-cored triphosphine 1,3,5-tris(diphenylphosphino)benzene.