Copper complexes molecular knots

After many attempts with various linkers, we found that 1,10-phenanthroline nuclei connected via their 2-positions by a -(CH2)4- linking unit will indeed form a double helix when complexed to two copper(I) centers. In addition, by introducing appropriate functions at the 9-positions, the strategy of Figure 14 could be followed to achieve the synthesis of a molecular knot of the (D) type. The precursors used and the reactions performed are represented in Figure 16. 

Sauvage [36] succeeded in synthesizing the molecular knot 30 by cyclization of the binuclear helical phenanthroline-copper(I)-complex 29 with hexaethylene glycol diiodide in DMF under extreme dilution and addition of cesium salt. However, the yield amounted to less than 3%. 

This bis-copper complex 252+.(BF4 )2 is a dark red crystalline solid (needles). The high resolution mass spectrum (FAB) of 252+.(BF4 )2 shows a molecular peak at 1816,74 (calculated molecular weight for 25 + = 1817.10). The NMR spectrum of 25 + is in complete agreement with its structure. Since 25 + contains a double helix on two copper(I) atoms, it is chiral. This was demonstrated in the presence of Pirkle s reagent.[33] In order to prove the knotted topology of 25 +, we had to consider the various possible compounds obtained in the cyclisation reaction. From a tetrafunctional double helicoidal precursor, several 2+2 connections are possible, with the most probable ones being indicated in Figure 11. 

We realized that improvement in the yields of trefoil knots would be determined by (i) the proportion of double helix precursor formed compared with face-to-face open chain complex (Figure 18) (ii) the spatial arrangement of the four reacting ends of the helical dinuclear complex. This latter factor will reflect the extent of winding of the two molecular strings interlaced around the copper atoms. The various complexes synthesized and studied are depicted in Figure 21 [96]. 

To our knowledge, topologically chiral molecules have not yet been resolved into enantiomers. However, we may anticipate that their energy barrier to racemization will be extremely high, compared to Euclidean chiral molecules. Therefore they are expected to be useful in enantioselective interactions or reactions. For example, it has been shown that tetrahedral copper(I) bis-2,9-diphenyl-l,10-phenanthroline complexes (which form the catenate subunits) are good reductants in the excited state [97] therefore the chiral Cu(I) catenates could be used for enantioselective electron-transfer reactions. Alternatively, the resolution of topologically chiral molecules would allow to answer fundamental questions, such as what are the chiroptical properties of molecular trefoil knots ... 

The effect of interlocking on the properties of molecules is dramatic. For example, the basicity of the 2,9-diphenyl-1.10-phenanthroline unit is enhanced by several orders of magnitude when it is present in a [2]catenand. The proton catenate displays a similar molecular structure to that of the corresponding copper(I) catenate, whereas that of the catenand is completely different. The special topology of the catenands and knots makes them unique ligands, with strong complexes being formed with a variety of metal ions. ° ... 

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