The potentially hexadentate ligand L, which contains two terminal coumarin fluorophores, forms dinuclear double-stranded helicates with dicationic metal ions, giving species of the generic form [M(2)L(2)](4+). In solution the free ligand was fluorescent with emission attributed to the coumarin fluorophores (lambda(em) = 437 nm). The luminescent properties of the corresponding dimetallic helicates complexes were examined and revealed that the Zn(2+) complex demonstrates enhanced emission when compared to the parent ligand, whereas Co(2+), Cu(2+), Cd(2+) and Hg(2+) induce varying degrees of fluorescence quenching. In particular, comparative luminescence measurements at 77 K and room temperature showed that the quenching mechanism for [Cu(2)L(2)](4+) can be attributed to a photoinduced electron transfer. ESI-MS selectivity studies showed that in the presence of a mixture of metal dications no preference for any one metal ion was observed.

The formation of either dinuclear double-stranded or pentanuclear circular helicates from a ligand containing two tridentate domains separated by a phenylene unit can be controlled by inter-ligand steric interactions which themselves are governed by the size of the metal ion.Controlling the structure of multi-component assemblies is one of the leading challenges for the supramolecular chemist. One of the simplest assemblies is the dinuclear doublestranded helicate, and the rules that govern the formation of this species are largely established. [1][2][3][4][5][6][7] The formation of the helicates' higher nuclearity cousin, the cyclic helicate, is conversely less well understood. One of the major problems in the formation of these higher nuclearity assemblies is that the design principles that apply to helicate formation, i.e. using a ligand that contains two binding domains that coordinate different metal ions, equally apply to the formation of cyclic helicates. For the larger cyclic species to preside in solution, the formation of the entropically favoured dimer has to be prevented and this can be achieved by intermolecular interactions (e.g. templation by anions) 8 or by intramolecular interactions which stabilise the formation of the cyclic species relative to its double-stranded alternative. As an example of the first of these approaches, in the work carried out by Ward et al., a ligand with two bidentate domains separated by a 1,8-naphthalenediyl spacer was reported to form a simple mononuclear species with Cu(CF 3 SO 3 ), but in the presence of tetrafluoroborate, a tetranuclear cyclic helicate [Cu 4 L 4 ] 4+ was observed. 9 Hannon et al., on the other hand, demonstrated that a metal ion's preference for different coordination geometries could affect the self-assembly outcome. In this case a bis-bidentate ligand containing a 1,3-bis(aminomethyl)phenyl spacer formed linear dimers with tetrahedral metal ions and trinuclear circular helicates with octahedral metal ions. 10 Other reports have cited inter-strand CHÁ Á Áp interactions as the principal driving force for the preferential formation of high complexity cyclic assemblies over their dimeric In this communication we describe how the formation of either dinuclear double-stranded or pentanuclear circular helicates can be controlled by inter-ligand steric interactions which, in turn, are governed by the size of the metal ion. This approach allows for the specific formation of either of the two structures and gives valuable insight into some of the factors which control the formation of cyclic helicates.The ligand L 1 , which was prepared by the reaction of 2,2 0 -bipyridine-6-thioamide with 1,3-di(a-bromoacetyl)benzene, contains two tridentate binding domains separated by a phenylene ring (Fig. 1) was confirmed by a single crystal X-ray diffraction study (Fig. 2).z In the solid-state the ligand partitions into two tridentate domains, each comprising a thiazole-pyridyl-pyridyl

A new route to luminescent derivatives of androgenic steroids containing a ketone group in the 3-or 17-position has been developed. Reaction with the fac-Re(CO) 3 Cl complex of 3,3'-diamino-2,2'-bipyridine (complex 1) afforded a cyclic aminal product with different steroids. The rate of reaction and yield varies according to the conjugation or steric hindrance around the ketone group.

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. 1They 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 respective M + and M 2+ ions are accommodated according to their respective geometry preferences.1,2 The second strategynamely, 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. 3They 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 heterobimetallic triple-stranded helicate. 5In 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. Li...