Trinuclear double-helical complexes

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.

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).

Double-stranded and triple-stranded helicates as well as double helical and triple helical metal complexes, are formed by the spontaneous organisation of two or three linear polybipyridine ligands of suitable structure into a double or a triple helix by binding of specific metal ions displaying respectively tetrahedral (Cu ) and octahedral (Ni ) coordination geometry. These species are illustrated by the trinuclear double helicate 1 [27] and triple helicate 2 [28] (see also [29]). 

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