Helical metal complexes

Kramer, R. Lehn, J.-M. Marquis-Rigault, A. Self-recognition in heUcate self-assembly Spontaneous formation of helical metal complexes from mixtures of ligands and metal ions. Proc. Natl. Acad. Sci. USA 1993, 90, 5394-5398. 

Section 9.7). Our work involved first the self-assembly of double-helical metal complexes. This particularly interesting case of self-assembly of a coordination compound represented a first step in our studies on the design of inorganic programmed systems. 

Self-Assembly of Double-Helical and Triple-Helical Metal Complexes The Helicates... 

This radical change in outlook builds on the richness of constitutional diversity and the benefits of variability. It stresses the virtues of instructed mixtures [3, 31], such as was revealed in the self-selection processes occurring in the side-by-side self-assembly of double helical metal complexes (helicates), whereby only the correctly paired double helicates were produced from a mixture of ligands and metal ions in dynamic coordination equilibrium [31, 37c]. It is this work that first led us in the early 1990s to envisage a dynamic chemistry bringing into play the constitution of chemical species. 

A number of semi-flexible, large-ring macrocyclic ligands have been demonstrated to yield helical metal complexes. For example, the Schiff base derivative 61 was postulated to adopt helical co-ordination geometries with a number of metal ions. The X-ray structure of [PbL] + (L = 61) is shown in Figure 6.27. The doublehelical array is stabilised in this case by no less than five aromatic 7i-interactions. NMR evidence suggests that the helical arrangement persists in solution. 

The first double-helical metal complex described was the tetrahedral bis(zinc formylbiliverdinate) (Figure 5.34). The helical dimer forms spontaneously, if the planar monomeric zinc complex, which carries an axial water ligand, is dehydrated by HCl" . 

Related double-helical metal complexes have been used to investigate the role of cooperativity in self-assembly. Ligands (14a) and (14b) were allowed to react with CUBF4 to form the corresponding helicates (15a) and (15b). The formation of helicates (15a) and (15b) was confirmed in solution by spectrophotometric titration. Two sharp isosbestic points are seen in the spectrum, and the excess absorbance diagram is linear even at a very tow... 

Helicate self-organization-positive cooperativity in the self assembly of double-helical metal-complexes, A. Pfeil and J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1992, 838. 

Figure 26 Diagrammatic representations of helical metal complexes. The circles are metal atoms and each long block represents a rigid chelate unit with connectors Y or a single bond distinct ligands are shaded differently (a) the dihelicate where each chelate can be bidentate (tetrahedral metal coordination) or tridentate (octahedral metal a coordination) (b) die triple dihelicate and (c) the triple trihelicate, in which each chelate is bidentate and the metal coordination is octahedral...

Like all simple triazolopyridines, bitriazolopyridines 76 react with electrophiles to produce 2,2 -bipyridines 118 (Scheme 24). With these reactions a general route to 2,2 -bipyridines has been discovered with a variety of substituents in the 6 and 6 positions (97T8257). These compounds have use in supramolecular chemistry because of their great complexing power for metal ions and, in particular, 2,2 -disubstituted-6,6 -bipyridines are useful building blocks for oligobipyridines, which spontaneously form helical metal complexes (92T8451). 

Kano K, Hasegawa H, Chiral recognition of helical metal complexes by modified cyclodextrins, J. Am. Chem. Soc. 2001 123 10616-10627. 

Helicates are helical metal complexes that contain organic strands intertwined around the metal centers. Compounds... 

Helicates Metal complexes that possess a helical and therefore chiral structure geometry akin to a screw thread. The ligands are called helicands. 

Lehn, J-M. (1990) Perspectives in supramolecular chemistry, Angew. Chem. Int. Ed. Engl. 29, 1304-1319. Koert, U., Harding, M.M., and Lehn, J.-M. (1990) DNH deoxyribonucleo-helicates Self-assembly of oligonucleosidic double-helical metal complexes. Nature 346, 339-342. 

Double-stranded and triple-stranded helicates as well as double helical and triple helical metal complexes, are formed by the spontaneous organisation of two or three linear polybipyridine ligands of suitable structure into a double or a triple helix by binding of specific metal ions displaying respectively tetrahedral (Cu ) and octahedral (Ni ) coordination geometry. These species are illustrated by the trinuclear double helicate 1 [27] and triple helicate 2 [28] (see also [29]). 

A variety of supramolecular architectures have also been created by the use of metal coordination. Synthesis of catenanes, knots [88], and double-helical metal complexes (linear or circular heli-cates) [4] are few of the prominent examples. Nanostructured supramolecular squares and capsules are also formed by self-assembly (Figure 13) [89]. 

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