Knots dicopper

Figure 16. Precursor and reaction scheme leading to the dicopper(I) trefoil knot (Cu2(K-86)2+). Formation of the double helix is essential this step is the weak point of the present synthesis.

Besides the knot, the major cyclization product (24%) obtained in the latter reaction could be identified as a dicopper complex consisting of two 43-membered rings arranged around the metallic centers in an approximate face-to-face geometry [94]. This unknotted compound originates from a non-helical precursor which is in equilibrium with the expected double helix. Figure 18 describes in a schematic way the alternative cyclization reaction leading to the unknotted face-to-face complex and the equilibrium which interconverts the helical and the non-helical precursors. 

Figure 18. Scheme of the two main cyclization reactions leading to the unknotted complex and the dicopper(I) trefoil knot. 

Before we obtained the X-ray structure determinations of both the knotted and unknotted dicopper(I) complexes, we could identify them after careful comparative H NMR and mass spectroscopy studies performed not only on the dinuclear complexes but also on their respective free ligands afforded by demetalation. Treatment of Cii2(m-43) + by a large excess of potassium cyanide led indeed quantitatively to a plain 43-membered macrocycle whereas an analogous treatment of Cii2(K-86r+ led to the free knot K-86 whose topological chirality could be demonstrated by NMR and mass spectroscopy (Figure 20). 

Figure 22. Reaction scheme leading to the dicopper(I) trefoil knot Cn2(K-84) f.

Analogous to the previously synthesized dicopper(I) knots [97], Cu2(K-84)p+ gives characteristic 1H NMR and FABMS spectroscopy data [102], In particular, some of the aromatic protons of the bis-chelates are strongly shielded, which indicates that Cu2(K-84)p is compact and that its helical core is geometrically very similar to that observed in 32. This was fully confirmed by subsequent X-ray structure determination. 

High-Yield Synthesis of a Dicopper(I) Trefoil Knot using Ring-Closing Metathesis Methodology... 

The dicopper(I) trefoil knot 36 was first obtained as a mixture of isomers (cis-cis, cis-trans and tra.ns-tra.ns products), because of the cis or trans nature of the two cyclic olefins formed. These olefins can be easily and quantitatively reduced at room temperature by catalytic hydrogenation yielding the knot 37. 

Although the preparative yields for most of the dicopper(I) trefoil knots described above were modest (0.5 to 8% for the methylene-bridged knots, 30% for the phe-nylene-bridged knot Cu2(K-84)p+) the amounts obtained were sufficient to undertake the study of their specific properties. 

Table 1. Kinetic, electrochemical, and photophysical properties of dicopper(I) knots containing polymethylene bridges.

Figure 27. Schematic representation of the two steps in the demetalation of the dicopper(I) knot Cu2(K-84)p+.

Very different is the situation as far as the kinetic parameters of the complexes are concerned. The surprising inertness of the various dicopper(I) knots to de-metalation is clearly due to the knotted nature of their ligand. This is obvious from the data of Table 1. As far as (K-84)p is concerned, the amazing kinetic stability of its mono- and di-copper(I) complexes arises from topological and geome-... 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

Figure 28. Principle of the resolution of the dicopper(I) molecular trefoil knot Cu2(K-84) +. The chiral auxiliary used is S-(+)-l,l -binaphthyl-2,2 -diyl phosphate (BNP-).

Synthetic scheme for dicopper and free trefoil knots. From C. O. Dietrich-Buchecker, J. Guilhem, C. Pascard and J.-P. Sauvage, Angew. Chem. Int. Ed. 29, 1154-6 (1990). 

Fig. 33. a) Demetallation of the dicopper(I) trefoil knot 852 + leading to the trefoil knot 86. b) Two topological stereoisomers trefoil knot 86 and unraveled macrocycle 87... 

Fig. 35 a, b. Crystal structure of the dicopper trefoil knot 852 + a) stereo view of a single molecule (the circles representing the atoms are given in black for oxygen, dotted for nitrogen and crosshatched for copper) b) stereoview of the unit cell. It contains two homochiral molecules, four PFg anions and two benzene molecules... 

These results were very recently confirmed by the X-ray crystal structure analysis of 852+ (Fig. 35 a) [95]. Interestingly, this dicopper(I) knot crystallizes as a conglomerate of enantiomers. This spontaneous resolution is easily recognized by noticing that the unit cell contains only a single enantiomer (Fig. 35 b). 

The low yield achieved for the first dicopper knotted 86-membered macrocycle was improved significantly by changing the length of the linker connecting the two chelates and the long functionalised linker used in the cyclisation step [31-33]. The best yields for the cyclisation reaction were around 8%. The use of a 1,3-phenylene spacer between the coordinating units caused a great improvement [34]. 

Related to its overall kinetic inertness, the above dicopper(l) complex of the 1,3-phenylene-containing knot was able to be demetallated in a two-step sequence. Such behaviour has been used synthetically to produce hetero-binuclear species. 

Figure 11. Dicopper(I) trefoil knots and related acyclic monocopper(I) complex.

The early methylenic bridged symmetrical dicopper knots... 

Figure 20. The dicopper(I) knots made of two identical chromophoric centers linked by two threads (X and Z), each of which can be of different length.

Table 12. Selected photophysical properties for the dicopper(I) knots with the interchromophoric polymethylene spacer ...

In summary, for this family of dicopper(I) dinuclear knots the photophysical and electrochemical properties allow them to be ordered in a series where the two extremes are CuCu.12 and CuCu.l , the former being the most rigid and the one in which Cu is best shielded. This shows that the length of the shorter Z connection plays an important role in determining the fine structure of the geometric arrangement. 

The possibility of observing photoinduced intercomponent processes in hetero-dinuclear knotted complexes was made possible with the knots bearing a phenylene unit, as interchromophoric spacer [52b] (Figure 21). In fact, in this case the synthetic yield of the parent dicopper complex is high enough to allow further decomplexation recomplexation steps, as required. 

Demetallation of the dicopper knotted complex 5.72 occurs by treatment with KCN to give the trefoil 5.73 which acquires a topological chirality. The CD curve of this species was recorded and shows a pronounced signal in the range 280-340 nm. Unfortunately no X-ray strucmres could be determined for any of three compounds 5.71-5.73. 

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