Knots metal-templated

The functionalisation also allows extending the complexity of intertwined molecular assemblies involving molecular catenanes, rotaxanes and knots. Elaborate interlocked assemblies constructed by means of metal-templation techniques and ji-ji-stacking preorganisation were reviewed [3, 11], Our last survey was devoted to the hydrogen bond templated synthesis of amide-based catenanes and rotaxanes [32], Since then a considerable advancement in elucidation of mechanisms of templation and derivatisation of the amide-based interlocked structures has been reached. Moreover, in 2000 we reported a one pot synthesis of amide-based knots such as 8 [21], which is so far the easiest preparation of molecular knots. In the following, specific possibilities of functionalisation of amide-based catenanes, rotaxanes and knots will be discussed. 

Fig. 12 Examples of the use of color in depictions of various types of MIMs. Note how the colors and positions of constituent parts in the three-dimensional structures reflect those in the structural drawings to enhance clarity between representations of (a) a donor-acceptor [2]catenane [75] and (b) an ammonium-binding [2]rotaxane [76] from our group, (c) a transition metal-templated Solomon Knot from the Sauvage and Fujita groups [77], and (d) a benzylic amide [2]catenane from the Leigh group [78]. Reproduced with permission from [75] (copyright 1991 Royal Society of Chemistry), [76] (copyright 2000 Wiley-VCH), [77] (copyright 1999 Royal Society of Chemistry), [78] (copyright 1995 Wiley-VCH)...

Figure 3.46 (a) The helicand precursor to a metal-templated knot, (b) the X-ray crystal structure of its methoxy-derivatised helicate (X = m-phenyl) and (c) the X-ray crystal structure of the metal-templated trefoil knot with polyethyleneglycol bridges. 

Figure 11 A general strategy to knot synthesis based on the combination of metal templating and the ring-closing metathesis reaction.

The knots based on neutral, purely organic molecules are obviously not prone to classical diastereomer resolution, and, while chromatographic methods were not suitable for the separation of the two enantiomers of the metal-templated trefoil knot, they have been proved successful in the amide-containing knots. As far as these knotted molecules are concerned, it must be noted that they incorporate classical stereogenic centers (carbon atoms), which makes them very different from the copper-based systems in terms of chirality. In the first instance, the separation of the two enantiomers of six different knots was achieved with a colunm that was not conunercially available (chiral-AD type). Trichloromethane was needed to obtain an optimal separation. The silica gel and the chiral stationary phase were covalently bound so that the material did not bleed out when the lipophilic eluent was used. Moreover, comparison of the experimental CD of the pure enantiomers of a knot with a theoretically calculated CD (based on X-ray structure and a fiiUy optimized AMI geometry) permitted assignment of the absolute configuration of this knot. The latter preparation of soluble knots based on substitution of the 5-position of the pyridine moieties in 13 afforded molecules that were soluble in solvents which could be used in commercially available chiral columns." On the other hand, the substitution of a racemic mixture of knots with chiral auxiliaries allows the separation of the diastereomeric product." ... 

The use of metal coordination in the template-directed synthesis of knots and links is one of the earliest successful strategies employed by chemists. The first example of metal-templated synthesis of a [2]catenane was reported in Strasbourg by Sauvage et al. who first used the method to assemble two phenanthroline-based strands around a tetrahedral copper(I) cation (Scheme 17.1). 

SCH EME 17.2 Tris-bidentate ligand developed by Hunter et al and x-ray crystal structure. H atoms are omitted for clarity. Redrawn from CCDC163073) of its Zn complex obtained by metal templation and forming a trefoil knot. 

Recently, we reported the synthesis of a metal-templated trefoil knot using dynamic covalent chemistry of imine bonds. Inspired by the work of Stoddart on the synthesis of Borromean rings and with the aid of DFT calculations, we anticipated that the combination of a 5,5 -dimethyl-2,2 -bipyridine functionalized with para-aminomethyl-phenyl ether (Scheme 17.6) would provide adequate geometry for the bidentate coordination to a Zinc(II) cation. 

Sauvage, J.-P. Dietrich-Buchecker, C. O. Chambron, J.-C. Transition metals as assembling and templating species synthesis of catenanes and molecular knots. In Comprehensive Supramolecular Chemistry, Sauvage, J.-P. Hosseini, M.W., Ed., Publisher Elsevier, Oxford, UK 1996 Vol. 2, p 43. 

Figure 14. Template synthesis of a trefoil knot (two metal centers involved).

Fig. 29. Template synthesis of the trefoil knot. The molecular thread is a metal-ligand that contains two chelating sites and the black dot represents a transition-metal ion in a pseudo-tetrahedral environment...

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