Polynuclear complexes helicates

Simple mixed donor ligands are commonly anticipated to form mononuclear complexes. However, many examples of polynuclear complexes that form readily through prior ligand design or via spontaneous self-assembly processes have appeared. A range of shapes, including dendrimers and helical systems, have been identified. One of the... 

Our intention in this chapter is to draw attention to the chiral aspects of self-assembled polynuclear complexes, and to show how the study of properties related to chirality can afford useful information. After a brief review of the nature of chiral centres in these complexes and the experimental methods used for studying them, we will discuss a number of structure types which are currently attracting interest, such as helicates, dendrimers, molecular boxes, and topologically complex molecules. 

Dinuclear double helicates are too numerous to be fully discussed here. In contrast, tri, tetra, penta-, and polynuclear double-helical complexes of copper and silver appeared only in a limited num-... 

The elegant self-assembly of rm-bipyridine ligands with iron(ll) afforded novel polynuclear complexes possessing cyclic double-helix character, termed circular helicates 20 <97JA10956>. 

A strategy of using polynuclear double helices as precursors to more complex interlocked and knotted systems should allow the carrying out of template synthesis of trefoil, pentafoil, heptafoil... knots containing a set of two, four, six. .. metals, respectively. 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2 -diimine systems with their substitutionally inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates is believed to entail a kinetically favored triple helicate intermediate. Self-assembly of chiral dinuclear binaphthol-linked iron(III) porphyrin complexes into extended polynuclear species takes place through the intermediacy of fi-oxo dimers. Predetermined //-oxo-di-iron-dimers may be used in this type of synthesis. 

This discussion shows that polynuclear systems can contain a very large number of chiral centres. Consider, for example, a triple helical dinuclear complex shown in Figure 6. 

Self-assembly of mono- and polynuclear helical complexes... 

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