Helicates trinuclear

We present here a few examples of enantiomeric supramolecular absolute structures. In fact, many supramolecular diastereomers with chiral ligands have been isolated stereoselectively. For example, enantiomers of the chiral self-assembled triple helicates trinuclear [Ni3(L)3](C104)6 (L = tris(bpy)) and dinuclear [Co2(L)3]4 + (L = 5-bismpmb) were partially resolved by SP Sephadex chromatography.63,64 The ligands and structures are given in Figure 5.17a and b. 

Bridged binuclear, trinuclear and tetranuclear chelated octahedral structures were examined by Schaffer,17 who used the skew line helical definition for the chirality symbols A and A. The configurational isomers for a tetrakisbidentate, edge-fused-bis-octahedral structure are AA, AA, AA, and AA. 

Of course, it is quite possible to further extend these assembly processes to give doublehelical complexes with even more bond crossings. For example, a double-helical complex with three bond-crossings should result from the reaction of a molecular thread containing three metal-binding domains with three tetrahedral metal ions (Fig. 7-32). An example of the assembly of such a trinuclear double-helical complex is seen in the formation of 7.52 from the reaction of 7.51 with silver(i) salts (Fig. 7-33). 

Figure 7-32. The interaction of a molecular thread containing three didentate metal-binding domains with tetrahedral metal ions should give a trinuclear double-helical complex.

Figure 7-33. The interaction of the ligand 7.51, which contains three didentate metal-binding domains with copper(i) or silver(i) ions, results in the assembly of a trinuclear double-helical structure, 7.52.

Once again, it is possible to extend these ideas to the formation of complexes containing progressively more metal centres. As an example, consider the ligand 7.57. This contains a total of three didentate 2,2 -bipyridine-like domains. Upon reaction with nickel(n) salts, a trinuclear triple-helical complex, [Ni3(7.57)3]6+ 7.58, is formed, in which each of the six-co-ordinate nickel(n) centres is co-ordinated to a didentate metal-binding domain from each of three ligand threads. 

M. Lama et al., Lanthanide class of a trinuclear enantiopure helical architecture containing chiral ligands Synthesis, structure, and properties. Chem. Eur. J. 13, 7358-7373 (2007)... 

Shiga, T., Ohba, M., and Okawa, H. (2004) A series of trinuclear Cu Ln Cu complexes derived from 2,6-di(acetoacetyl)pyridine synthesis, structure, and magnetism. Inorganic Chemistry, 43, 4435 446. Albrecht, M., Schmid, S., Dehn, S., et al. (2007) Diastereoselective formation of luminescent dinuclear lan-thanide(III) helicates with enantiomerically pure tartaric acid derived bis(P-diketonate) ligands. New Journal of Chemistry, 31, 1755-1762. 

The Lehn group synthesised the trinuclear copper(I) complex of the substituted 2,2 -bipyridyl/ether derivative 29 and confirmed its helical structure by X-ray diffraction (Figure 6.15). A dinuclear copper(I) complex of 30 (Figure 6.15) was also synthesised and subsequently the corresponding dimeric ethane-bridged bipyridine and phenanthroline ligands were also demonstrated to yield similar di-helicate copper(I) species. These results thus indicate that helicate formation is tolerant towards minor modification of ligand structure. ... 

Figure 139. Formation of di- and trinuclear double-stranded helicates from a substituted sexipyridine ligand through supramolecular and electrochemical pathways.

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Fig. 2. Schematic views of copper and silver helical arrangements (a) infinite single strand (b) 3-D single helical copper(I) complex with mixed ligands (c) dinuclear double helicate (d) trinuclear double helicate (e) infinite double helicate (f) infinite chiral double helicate of copper(II) with arginine and m-phthalate (g) triple helicate of silver(I).

Their overall structural features are analogous to those of the trinuclear double helicates, but their total length is estimated to be as long as aroimd 22 A and 27 A, respectively. These self-organized nanostructures promise potential applications in the field of functional nanoscale species and molecular devices. 

FIGURE 3 (A) Self-assembly and (B) X-ray crystal structure of the trinuclear saturated homotopic double-stranded helicate [Cu3(Ll)2] (reproduced by permission from Hamacek et al., 2006, 2006 Royal Society of Chemistry Lehn et al., 1987, 1987 The National Academy of Sciences USA). 

FIGURE 7 Schematic representation of a trinuclear double-stranded helicate. 

Trinuclear unsaturated circular single-stranded helicate... 

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