Self-assembly of a molecular figure-of-eight strip† Kai P. Strotmeyer, a Igor O. Fritsky, b Hans Pritzkow a and Roland Krämer* a a Anorganisch-Chemisches Institut der Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. E-mail: roland.kraemer@urz.uni-heidelberg.de; Fax: (+49) 6221 548599; Tel: (+49) 6221 548438 b Department of Chemistry, National Taras Shevchenko University, 01033 Kiev, Ukraine. E-mail: ifritsky@ukr.ne; Fax: (+380) 44 2278982; Tel: (+380) 44 2393393 Received (in Cambridge, UK) 4th August 2003, Accepted 30th October 2003 First published as an Advance Article on the web 26th November 2003 A hexanuclear copper(II) complex with a figure-of-eight strip topology is formed by metal-directed self-assembly of tritopic ligand L, bis-bidentate glycine hydroxamic acid and Cu(II) ions in a 2:2:6 ratio. A figure-of-eight strip is produced by twisting a ribbon through 360° and joining its two ends. Here we describe the unprecedented formation of a molecule with this exotic topology 1 by self- assembly. On the molecular level, figure-of-eight topologies have been observed, for instance, for covalent macrocyclic ring systems (such as expanded porphyrins) 2 and their metal complexes. 3 Such compounds are highly dynamic due to rotation around single bonds, while a strip-like topology does not allow interconversion of the helical enantiomers without bond disruption. Directed self-assembly of “programmed” organic ligands with transition metal ions is a powerful method for the construction of nanometer-sized, complex supramolecular architectures; 4 exam- ples include polyhedra, helicates, grids, bowls, capsules or polymeric networks. The tritopic ligand L (Scheme 1) was prepared in a four step reaction from ethyl picolinate and 1,3-diaminopro- pane. The coordination chemistry of a related ligand has been investigated in detail in a different context: 5 in a 1:1 copper complex the metal ion is bound to the tetradentate site and the complex has a helical structure with tetrahedrally distorted N 4 Cu coordination plane. When L is mixed with 3 equiv. CuCl 2 and 1 equiv. glycine hydroxamic acid (GHA) (Scheme 2) in the presence of base in MeOH/water, green crystals of [(L-2H) 2 Cu 6 (GHA-2H) 2 (OH- ) 0.5 (ClO 4 ) 1.5 (Cl,ClO 4 )(H 2 O) 4.32 ](ClO 4 )·10.64 H 2 O (1) formed on addition of NaClO 4 and were investigated by X-ray crystallog- raphy.‡ The unit cell contains a hexanuclear complex cation (Scheme 2) composed of two doubly-deprotonated L (L-2H), two GHA dianions and 6 Cu 2+ ions as well as disordered coligands OH 2 , H 2 O, ClO 4 2 , Cl 2 . One Cu 2+ ion is tightly bound to the tetradentate site of L by two deprotonated amide nitrogen atoms and two pyrimidine N-Atoms. As a consequence of pyrimidine C2-H repulsion, the LCu moiety is not planar. Its intrinsic helicity is crucial for the assembly of the figure-of-eight topology, and the helicity of the two subunits is essentially the same. The remaining two Cu ions of an LCu 3 unit coordinate to the bidentate pyridyl-pyrimidinyl sites. All Cu ions form 4 short in- plane bonds (distances 1.89–2.02 Å), and the coordination spheres are completed to square-pyramidal or octahedral by weak axial interactions with highly disordered coligands. The two LCu 3 fragments are linked by two GHA dianions which coordinate to the cis-oriented vacant sites of the peripheral Cu ions and bridge the Cu ions in a bis-bidentate O,O- and N,N chelating fashion. Previous examples for the use of a-amino hydroxamic acids in metal- directed self-assembly reactions are the synthesis of “metal- lacrowns”, e.g. from GHA and Cu 2+ in combination with a heterometal ion, 6 and of helicates. 7 Fig. 1 illustrates the figure-of-eight strip topology of the hexanuclear complex cation which has a length of about 1.5 nm and a width of about 1.0 nm. The racemic crystal contains both helical enantiomers in a 1:1 ratio. Atom connectivities in 1 do not allow the interconversion of the helical isomers by rotation around single bonds. A crystalline sample of 1 is homogeneous: five crystals selected from two independent synthetic crops had identical unit cell parameters, and the presence of only one solid phase was confirmed by powder diffraction. Attempts to identify the intact complex cation of 1 in methanolic solution by electrospray ionisation MS failed, only signals of Na + adducts of the mononuclear complex (L- 2H)Cu or of its oligomers were found, with loss of the peripheral Cu ions under electrospray MS conditions. † Dedicated to Prof. Dr. Bernt Krebs on the occasion of his 65th birthday. Scheme 1 Scheme 2 Formation of the complex cation of 1 by self-assembly from L, GHA and CuCl 2 . Axial Cu-ligands are omitted for clarity. This journal is © The Royal Society of Chemistry 2004 28 Chem. Commun., 2004, 28–29 DOI: 10.1039/b309215a