Two acyclic and three macrocyclic polyethers, three [2]catenanes, and one [2]rotaxane, each containing one 4,4'-azobiphenoxy unit, have been synthesized. In solution, the azobenzene-based acyclic polyethers are bound by cyclobis(paraquat-p-phenylene)Ða tetracationic cyclophaneÐin their trans forms only. On irradiation (l 360 nm) of an equimolar solution of the tetracationic cyclophane host and one of the guests containing a trans-4,4'-azobiphenoxy unit, the trans double bond isomerizes to its cis form and the supramolecular complex dissociates into its molecular components. The trans isomer of the guest and, as a result, the complex are reformed, either by irradiation (l 440 nm) or by warming the solution in the dark. Variable temperature 1 H NMR spectroscopic investigations of the [2]catenanes and the [2]rotaxane revealed that, in all cases, the 4,4'-azobiphenoxy unit resides preferentially alongside the cavities of their tetracationic cyclophane components, which are occupied either by a 1,4-dioxybenzene or by a 1,5-dioxynaphthalene unit. In the acyclic and macrocyclic polyethers containing 1,4-dioxybenzene or 1,5-dioxynaphthalene chromophoric groups and a 4,4'-azobiphenoxy moiety, the fluorescence of the former units is quenched by the latter. Fluorescence quenching is accompanied by photosensitization of the isomerization. The rate of the energy-transfer process is different for trans and cis isomers. In the [2]rotaxane and the [2]catenanes, the photoisomerization is quenched to an extent that depends on the specific structure of the compound. Only in one of the three [2]catenanes and in the [2]rotaxane was an efficient photoisomerization (l 360 nm) from the trans to the cis isomer of the 4,4'-azobiphenoxy unit observed. Single crystal X-ray structural analysis of one of the [2]catenanes showed that, in the solid state, the 4,4'-azobiphenoxy unit in the macrocyclic polyether component also resides exclusively alongside. The cavity of the tetracationic cyclophane component of the [2]catenane is filled by a 1,5-dioxynaphthalene unit, and infinite donor ± acceptor stacks between adjacent [2]catenanes are formed in the crystal. These supramolecular complexes and their mechanically interlocked molecular counterparts can be regarded as potential photoactive nanoscale devices.

Crystal and MolecularStructure of Parthenolide [4,5-€poxygermacra-I (1 0),11(13)-dien-12,6-olactone] By Andrew Quick and Donald Rogers," Chemical Crystallography Laboratory, Imperial College, London SW7 2AYThe crystal structure of the title compound has been determined from single-crystal X-ray data by direct methods.Crystals are orthorhombic, a = 7 1.929(1), b = 12.267(2), c = 18.761 (2) 8, space group P212121, Z = 8, i.e. two independent niolecules per asymmetric unit. Full-matrix least-squares refinement based on 2 330 independent reflections has reached R 0.047. Thedouble bond C(l)-C(lO) and the equivalent double bond C(4)-C(5) are both rrans, both methyl groups are @, and the ten-membered ring has a conformation typical of several germacranolides.THE sesquiterpenic lactone, parthenolide, isolated from Chrysanthemum 9arthenizm (L) , was originally assigned the structure (I).l The same compound was isolated from the trunk bark of Michelia chaqbaca and its structure revised to (11) 2 from a study of its n.m.r. spectrum and chemical reactions. The absolute configuration of the molecule has been determined and it was shown that parthenolide can be represented by the stereoformula (111) .3The present crystal structure, part of a series of studies of ten-membered ring compounds, was undertaken to show the conformation of the epoxide ring and also the orientation of the methyls at C(4) and C(10). E SPERIMENTAL The crystals were colourless plates. Crystal Data.-C,,H,,O,, n/I = 248.3, Orthorhombic. a = 11.929(1), b = 12.267(2), c = 18.761(2) A, U = 2 745.4 A3, t Reprints are not available.

The crystal structure of the title compound has been determined by the heavy-atom method from a three-dimensional X-ray diffraction study. Least-squares refinement, based on 1 1 1 9 visually estimated independent reflections, has reduced R to 0.1 09. Crystals are orthorhombic, a = 7.74(4), b = 13.78(3), c = 14.81 (3) 8, space group PZ12,2, and Z = 4.In the complex the peptide molecule behaves as a terdentate chelate via its terminal amino-group, peptide nitrogen atom, and the carboxylate ion. The single peptide nitrogen is deprotonated. The Curl atom has co-ordination number 4, the peptide atoms and one water molecule forming an approximate square.TRYPTOPHAN is one of the naturally occurring aminoacids. It is a precursor of neurochemically important compounds and chemically related to a group of psychopharmaca. Several structure determinations of tryptophan derivatives have been carriedThe present X-ray analysis was undertaken since the title complex is the first-reported metal-peptide structure in which one of the peptide residues is tryptophan. A preliminary account has been published.6 EXPERIMENTAL A deep blue solution of the title complex was prepared by reaction of equimolar amounts of either (a) freshly precipi-

The crystal structure of imidazole imidazolium perchlorate has been determined at room temperature from three-dimensional diffractometer data, and refined by full-matrix least-squares to a final R = 0.051. The planar imidazole rings are linked by a [Formula: see text] hydrogen bond of length 2.73 Å and rotated with respect to each other about this bond by an angle of 50°. The analysis has failed to give conclusive evidence as to the position of the hydrogen atom between the two imidazoles.

The crystal and molecular structures of the title compound have been determined from single-crystal X-ray diffractometer data. The crystals are monoclinic with a = 15.61 0, b = 18.51 0, c = 10.650 8, p = 11 4.68". space group P2,/a, and Z = 4. The structure has been solved by Patterson and Fourier methods, and refined by least squares, using 2 678 independent reflections, to an R value of 0.052. The monomeric molecules have distorted octahedral co-ordination. The benzoylhydrazonide chelate adopts a predominantly enolic structure and there is no evidence for the incorporation of the metal into any delocalised system. The Re-0 distance [2.013(7) 81 is shorter than the Re-N distance [2.127(10) A] and does not appear to have been influenced by the multiply bonded oxo-function [ R e r O 1.685( 8) A] trans to it. The remaining metal-ligand distances are Re-CI 2.388(3) (trans to P) and 2.344(4) A (trans to N) and Re-P 2.472(2) A.