Isomeric pyridyl-thiazole donor units for metal ion recognition in bi- and tri-metallic helicatesw Sam Bullock, a Lisa J. Gillie, a Lindsay P. Harding, a Craig R. Rice,* a Thomas Riis-Johannessen b and Martina Whitehead a Received (in Cambridge, UK) 5th May 2009, Accepted 12th June 2009 First published as an Advance Article on the web 2nd July 2009 DOI: 10.1039/b908911j Linking two isomeric tridentate N-chelates together produces a hetero-ditopic ligand capable of selectively binding Hg 2+ and Zn 2+ ions in a double-stranded helicate. Underlying recognition phenomena in multi-component self-assembly processes can be exploited for the site-specific inclusion of different metal ions into polymetallic arrays. In the field of metallosupramolecular chemistry this is typically accomplished by programming either (i) the denticity of or (ii) the nature of the donor atoms in the binding sites of a polytopic ligand, prior to self-assembly with target metal ions. 1 Segmental oligo-N-heterocyclic ligands that contain both tridentate and bidentate chelates, or the ability to partition as such, exemplify the first of these approaches. 1 They form double-stranded hetero-bimetallic helicates with metal mono- and dications due to the preference of the former (e.g. Ag + , Cu + ) for tetrahedral coordination geometry and that of the latter (e.g. Cu 2+ , Co 2+ , Fe 2+ ) for octahedral coordination geometry. 1 When two such ligands are arranged appropriately, e.g. a co-aligned head-to-head (HH-) fashion in the case of a simple ditopic ligand, the resulting assembly features four- and six-coordinate sites into which the respec- tive M + and M 2+ ions are accommodated according to their respective geometry preferences. 1,2 The second strategy— namely, the use of a polytopic ligand whose binding sites vary with regards to the nature of their donor atoms—has been applied with considerable success by Piguet and Bunzli et al. for the challenging task of selectively incorporating different lanthanide (Ln) trications into triple-stranded helicate arrays. 3 They demonstrated that the self-assembly of ditopic ligands comprised of a triimine N 3 - and a diimine/amide N 2 O-domain, with various Ln–Ln 0 pairs, can produce up to 90% of the desired hetero-bimetallic helicate. Indeed, extensions of this work further lead to the isolation of tri- and tetranuclear hetero-bimetallic complexes in much higher yield than predicted from a purely statistical standpoint. 4 A related approach saw the use of a ditopic catechol/thiocatechol ligand which, upon reaction with Ti 3+ and Mo 3+ , formed a hetero- bimetallic triple-stranded helicate. 5 In this paper we use a new class of ditopic segmental pyridyl-thiazole (py-tz) N-donor ligand (Scheme 1) to demonstrate an alternative strategy for selectively introducing different metals into polynuclear arrays. The simplest of these ligands, L 1 , contains two tridentate N 3 binding domains which are structural isomers of one another. Self-assembly with Hg 2+ or Zn 2+ ions gives various isomers of a dinuclear double-stranded complex in solution. In the presence of both ions, however, only one species is formed in which the py-py-tz sequences bind to Zn 2+ and the py-tz-py sequences bind to Hg 2+ . The metal/site specificity is attributed to the divergent nature of the three N-donors in each tridentate domain, which varies according to the order in which the heterocycles appear in the sequence. When combined with a simple bidentate chelate for binding tetrahedral Cu + , the Zn 2+ /Hg 2+ recognition effects displayed by the two tridentate units can be exploited for the self-assembly of a Zn 2+ /Hg 2+ /Cu + hetero-trimetallic helicate. Ligand L 1 (Scheme 1) was prepared by reaction of its methylene hydroxy- and chloro-substituted py-py-tz and py-tz-py constituents, respectively, in a Williamson ether synthesis. Purification of the reaction mixture gave L 1 as a colourless solid (see ESIw). 6 Reaction of L 1 with one equivalent of Zn(ClO 4 ) 2 6H 2 O in MeCN gives a colourless solution for which ESI mass spectro- scopy showed an intense peak at m/z 1469 corresponding to the dizinc(II) species [Zn 2 (L 1 ) 2 (ClO 4 ) 3 ] + . The 1 H NMR (CD 3 CN) spectrum features two major sets of resonances, in addition to a third minor set which accounts for o5% of the total ligand (peak integration also suggests that one of the Scheme 1 a Department of Chemical and Biological Sciences, University of Huddersfield, Huddersfield, UK HD1 3DH. E-mail: c.r.rice@hud.ac.uk; Fax: +44 (0)148-447-2182 b EPFL SB ISIC LCS, BCH 3307 (Ba ˆtiment de chimie UNIL), CH-1015 Lausanne, Switzerland w Electronic supplementary information (ESI) available: Details of synthesis and characterisation of ligands L 1 –L 4 and [HgZn(L 1 ) 2 ] 4+ ; UV-vis spectrophotometric titration of L 2 with Zn 2+ and Hg 2+ . Full crystallographic refinement details. CCDC 730541 and 730542. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b908911j 4856 | Chem. Commun., 2009, 4856–4858 This journal is  c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm Downloaded by ECOLE POLYTECHNIC FED DE LAUSANNE on 11 June 2012 Published on 02 July 2009 on http://pubs.rsc.org | doi:10.1039/B908911J View Online / Journal Homepage / Table of Contents for this issue