The CCG session on Supramolecular Structure began with Jeremy Sanders (Cambridge) speaking about his "Adventures in Molecular Recognition", which covered two aspects of molecular recognition in supramolecular chemistry: (i) the assembly and properties of metalloporphyrin oligomers using metal-ligand interactions, and (ii) the construction of catenanes via donor-acceptor interactions. The first part showed how the combination of reversible zinc-amine interactions with irreversible carbon-carbon bond formation led to a rich variety of new receptors which could be prepared using templated chemistry. His cyclic porphyrin trimers are able to influence the rate and stereochemical outcome of reactions within their cavities. The primary structural tools used in this work are solution state NMR and UV/visible spectroscopies: solubility considerations dictate the use of long floppy sidechains and large amounts of solvent are occluded so crystal structures are rare: they are useful for rationalising events after they have happened, but are rarely in time to have predictive value. Good structures from powder diffraction are urgently needed! By contrast the design of successive generations of catenane in the second half of the talk was driven at every stage by single crystal structures that gave key information about the geometry of intermolecular (or intercomponent) non-bonded interactions. Structures obtained on Station 9.8 of the Daresbury SRS were central to this work as the crystals of such molecules are so tiny.
Neil Champness (Nottingham) described the synthesis of Cu(I) and Ag(I) inorganic supramolecular networks, with emphasis on the use of building-block design for achieving control of network structure. Examples were provided of how ligand, anion and metal fragment functionality has been used to control the construction of inorganic arrays. For example, in some Cu(I) complexes the degree of interpenetration can be varied. Anion functionality has also been studied in related Ag(I)-bipyridyl systems, one result being the synthesis of the first example of a polymeric helical staircase structure. Copper(I) halides have been used to construct inorganic arrays incorporating a range of motifs, including Cu2X2 dimers, Cu4X4 cubanes and Cu4X4 ladders. This has allowed the construction of a range of uncharged inorganic networks that contain channels and cavities for solvent inclusion, some of which maintain their structural integrity even upon desolvation.
Guy Orpen (Bristol) spoke on "Intermolecular Interactions involving Metal Complexes", comparing the intermolecular contacts seen in organic moieties to those in metal complexes. The systems studies included M-Cl, Cl- and C-Cl; M-NO2, NO2- and C-NO2; and M-OC(O)R, RCO2 - and R'OC(O)R and the Cambridge Structural Database provided data on the frequency and geometry of the hydrogen bonds they form, which allows assessments to be made of their usefulness as hydrogen bond acceptors. Ab inito calculations were carried out on some representative examples. The metal-bound ligands appear to be stronger acceptors than their carbon-bound analogues, although still weaker than the free anions. Such studies are being applied to supramolecular engineering, bio-inorganic chemistry and crystal engineering, with exceptional examples exploited as design leads.
Jacqui Cole (Kent), in a lecture entitled "Designing Non-linear Optical Materials: Supramolecular Requirements", described the essential structure/property relationships for non-linear optical materials. In particular, the inherent link between the second harmonic generation (SHG) effect and supramolecular structure was detailed, including examples where the magnitude of the SHG output can only be explained when one considers intermolecular interactions, packing effects and the phase-matching ability of a material. The intramolecular and intermolecular structures therefore have a synergic relationship.
Under the title "TTF Derivatives and Charge-Transfer Complexes: Molecular Flexibility and Packing" Andrei Batsanov (Durham) reviewed the crystal packing and conformational flexibility of structures containing tetrathiafulvalene (TTF) and suggested that molecular planarity may be the result rather than the cause of stack formation. The folding of the dithiole rings, giving a boat conformation, offers a route to increased dimensionality for organic metals: substituents increase the folding while oxidation reduces it. The overlap of TTF moieties in a stack was rationalised in orbital terms, and it was shown that this could explain the change in the nature of the overlap upon oxidation. Hydrogen bonds and other specific interactions between substituents have been used (with mixed success) to influence the crystal packing, but directed synthesis of organic conductors remains beyond reach.
The CCG Oral Poster session was orchestrated with great efficiency by
Bob Gould and there were almost forty CCG posters exhibited
in the main CCG poster session.
The judges awarded the poster prize to
P. Guionneau (Durham) and the runner-up was P.A. Thomas
(Warwick). Also commended were posters by A. Fitch (Keele) and R.Gregson
(St Andrews).
With the advent of more powerful radiation sources and more sensitive detectors for diffracted X-ray photons, it is becoming possible to obtain viable datasets from previously intractable crystals. However, many of these marginal samples will be affected by problems such as twinning and the Chemical Crystallography Group (CCG) therefore invited Dr Regine Herbst-Irmer of the University of Göttingen, an acknowledged expert in this area, to present a Workshop. Approximately 40 people attended.
Regine began by defining twins and giving simple examples of twinning, twin laws and fractional contributions before detailing the four different types of twinning: merohedral, pseudo-merohedral, reticular merohedral and non-merohedral. She then presented a number of detailed examples of the different types, their diagnosis and their treatment in refinement. She compared the use of twin laws to disorder modelling in twinned crystals and showed the former not only gave superior results but did so at the expense of only one additional refinement variable.
Dr Eric Hovestreydt of Bruker AXS contributed a short presentation on software being developed by Bob Sparks for automatic treatment of non-merohedral twins on both area detector and four-circle diffractometers. The basic algorithm involves indexing groups of reflections from different twin components and extracting the twin law relating them.
Regine wound up the Workshop by providing some characteristic warning signs for twinning. These included metric symmetry higher than Laue symmetry, similar Rint values for higher and lower Laue groups, values of |E2-1| << 0.736 and unusually long cell axes.
A.J. Blake
University of Nottingham
Last update: 7 Jul 98
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