The interaction of appropriate metal ions (Pb(II), Zn(II)) with helical ligand strands, obtained by hydrazone polycondensation, generates polymetallic supramolecular architectures of rack and grid types, by uncoiling of the ligand. The interconversion between the helical free ligand and the linearly extended ligand in the complexes produces reversible ion-induced, nanomechanical molecular motions of large amplitude. It has been integrated in an acid-base neutralisation fuelled process, which links the extension/contraction of the ligand strands to alternating changes in pH.

As examples of supramolecular devices performing chemical (ionic, molecular) control of binding events and models of related natural systems, two molecular conformational switches are described, which display cation-controlled nanomechanical motion coupled to substrate binding and release. The substrate binding relies on donor/acceptor interactions, provided by intercalation between planar sites located at the extremities of the switching units, whereas cation complexation is responsible for conformational regulation. The terpyridine py-py-py-based receptor is activated toward substrate binding upon complexation of a zinc(II) cation and operates in a two-state process. The replacement of the central pyridine by a 4,6-disubstituted pyridimine as in py-pym-py induces a state reversal and yields a new receptor which binds a substrate in the absence of cation, and releases it when copper(I) is introduced, following a three-step process. These systems represent effector-triggered supramolecular switching devices leading toward multistate nanomechanical chemical systems. These two systems illustrate the use of simple conformational switches in the binding site and allosteric regulation of substrate affinity.

Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthday Control over the folding of molecular strands may be achieved by appropriate choice of the constituting subunits, in particular for chains of specific heterocycles such as sequences of directly connected pyridine (py) and pyrimidine (pym) rings, which are known to fold into extended helical structures. Since the hydrazone (hyz) group represents an isomorphic analogue of a py site, the condensation of hydrazine and carboxaldehyde derivatives of pym offers a very efficient approach to strands incorporating hyz instead of py units and constituted by sequences of alternating hyz and pym groups. A series of such strands of different lengths, up to ten hyz units, i.e., 1 ± 7, were synthesized. Their spectral properties indicate that they fold indeed into helical shapes. Extensive characterization was performed in solution by 1 H-NMR spectroscopy and in the solid state by determination of the crystal structures of eight such strands. They all display the expected helical geometry with up to 3 1 / 3 turns and direct stacking contacts. The efficiency and flexibility of the synthetic approach as well as its wide potential for generation of diversity through lateral decoration make the (hyzÀpym) subunit a particularly attractive helicity codon.Introduction. ± The design of molecular strands capable of taking up defined and predictable shapes is of much interest in view of its relation to biological folding processes in proteins as well as of the access it provides to the generation of welldefined geometries for functional devices and materials. Intense activity has recently been displayed in this area, implementing various types of supramolecular, noncovalent interactions (H-bonding, stacking and electrostatic interactions, metal-ion coordination) as well as medium effects, to induce the self-organization of a molecular strand into specific architectures [1]. A range of geometries from entirely helical to fully linear may thus be targetted.In our laboratories, we have made use of different interaction patterns and structuration subunits to generate single helices [2 ± 8]

Reaction of mercury(ti) acetate with the 1,3-aIternate conformer of ptert-butyltetramercaptocalix[4]arene, obtained by a highly efficient synthesis starting from p-tert-butylcalix[4]arene, leads to a binuclear complex in which both mercury ions are linearly coordinated to two arenethiolate groups.

Oligo-tridentate ligands based on alternating pyridines and pyrimidines were synthesised by Stille-type carbon–carbon bond-forming reactions. The terpyridine-like sites are designed to coalign upon metal complexation, giving rise to organized and rigidly spaced metal ions. Peripheral functionalization of the basic bis-tridentate framework was explored. The heterocycles in the ligands are in an all-trans conformation about the interannular bonds as indicated by comparison of their 1H NMR spectra. An X-ray crystal structure analysis of the nonchiral tris-tridentate ligand 2a reveals a helical structure in the solid state. The seven heterocycles form a helical structure with resulting overlap of the terminal pyridines. Their centroid-to-centroid distance is 4.523 Å with 38.8° between the planes. NMR investigations support a helical conformation in solution as well. Electrochemical and UV absorption measurements indicate that the LUMO resides on the pyrimidine moiety of the ligands. Keywords: nitrogen-containing ligands, Stille coupling, helical structure, supramolecular chemistry.