Double helical copper® complexes

Figure 7-30. The reaction of 7.48 with copper(i) gives the desired double-helical dinuclear complex 7.49. The two molecular threads have been shaded differently in 7.49 to emphasise the helical structure.

Terpyridine is the simplest oligopyridine capable of forming a double-stranded helicate. Copper complexes with terpyridine derivatives have been reported. In [Cu2(L17)2](PF6)2 (45) and [Cu2(Li8)2](PF6)2 (46), the ligands have essentially distributed themselves to present bidentate domains to one metal center and a single pyridine donor to the other, to give a [4 + 2] double helicate. The two copper atoms involve different coordination environments, one with a distorted tetrahedral geometry and the second in a approximately linear two-coordinate environment (Fig. 10). 

To make a knot is a task easily accomplished by a child playing with a rope, or even by a careless adult with shoelaces. For a chemist playing with molecules this is an intrinsically difficult problem and so far only a few of these topologically interesting compounds have been synthesized. A strategy that finally led to success is based on the self-assembly of double-helical copper phenanthroline complexes. The helix represents the core structure from which the trefoil knot 30 in Fig. 15 is obtained in a final cyclization step. It is important to note that a trefoil knot is chiral. The resolution of the two enantiomers was recently accomplished by fractional crystallization of the diastereoisomers obtained with a chiral counterion. 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

The occurrence of the redox-driven reversible assembling-disassembling process involving copper complexes of 16 has been verified through cyclic voltammetry experiments at a platinum electrode in a MeCN solution. Figure 2.17 shows the CV profile obtained with a solution of the double-strand helicate complex [ Cu 21 (16)212 +. 

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

However, it has been recently demonstrated by Pallavicini et al. that the lifetime of the dicopper(II) double-strand helicate [ 2 (16)]4 + can be significantly increased by introducing hindering substituents on the framework of 16. In particular, this was shown to occur with the copper complexes of the bis-bidentate ligand 17.21... 

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]. 

Similarly, when a mixture of the two tris-bipyridine ligands 129 and 148 is allowed to react simultaneously with copper(l) and nickel(il) ions, only the double helicate 132 and the triple helicate 149 are formed (Figure 49). Thus, parallel operation of two programmed molecular systems leads to the clean self-assembly of two well-defined helical complexes from a mixture of their four components in a process involving the assembly of altogether 11 particles of four different types into two supramolecular species. 

When 7.48 reacts with copper(i), which usually forms four-co-ordinate tetrahedral complexes, a double-helical species, 7.49, is indeed formed (Fig. 7-30). This is a genuine self-assembly process - simply treating the ligand with the appropriate metal ion leads to the desired structure. A wide variety of other spacer groups have been incorporated between didentate domains. In practice, some consideration needs to be given to the nature of the spacer group that is selected. If it is too long, or too flexible, other co-ordination possibilities can occur. 

The introduction of bis(p-chlorophenyl) and bis(methylthio) substituents in the 4 - and 4"-positions of quinquepyridine (35) (that is, on the backbone of the second and fourth pyridyl rings in the string) was reported to have little effect on the coordination behaviour of these ligands relative to that of the unsubstituted analogue (see earlier). Namely, with nickel(II) and copper(I) and/or copper(II), doublehelical 2 2 complexes were obtained, while cobalt(II) forms both a 1 1 and a 2 2 complex in the solid state (although, in solution only a mononuclear seven-co-ordinate complex occurs - with the solid state, double-helical structure interconverting to this form upon dissolution). 

Figure 6.38 Structure of the double-helical, dinuclear copper(I) complex, 99 ...

Lehn and coworkers have designed many double-helical compoimds. Copper(I), cobalt(II) and iron(II) were allowed to complex with the ethylene-bridged bis(bipyridine) (10) and bis(phenanthroline) (11) to examine the stereochemical preferences of the metal ions. When ligand (10) was treated with copper(I), a complex of 2 2 stoichiometry (molecular weight confirmed by... 

FABMS) with a double-helical conformation was formed (Figure 4). Each copper atom possesses a distorted tetrahedral geometry, and the complex was shown to persist in solution by H NMR and cyclic voltammetry. In contrast, when hgand (11) was treated with cobalt(II), a 1 1 complex was formed. The crystal structure reveals that the aggregate is not a double helix, and the cobalt possesses a distorted octahedral geometry [23],... 

Figure 8 Schematic structure of the double-helical complex formed from (20) and copper(I)...

The final system in which racemization has been studied in detail concerns the ligands 23 and 24. The complex [Cu2(23)2] has a double-helical structure in which the pyridine binds weakly to both copper ions [49], whereas the complex [Cu2(24)2] adopts a side-by-side meso- structure [50]. Figure 14 shows the two structures. 

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