Triple-helical metal complexes

Self-Assembly of Double-Helical and Triple-Helical Metal Complexes The Helicates... 

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]). 

It should be stressed that the coding for the formation of these topologically complex molecules needs to be carefully controlled in order to obtain the desired structures. To illustrate this, consider ligand 7.59, which contains two didentate metal-binding domains. This might be expected to react with octahedral metal ions to give a triple-helical dinuclear complex. Reaction with iron(n) does indeed give a species of stoichiometry [Fe2(7.59)3]4+ however, the crystal structure reveals that an untwisted complex, 7.60, has been formed. 

Figure 7-36. The ligand 7.56 contains two didentate metal-binding domains. On reaction with cobalt(n), a triple-helical dinuclear complex [Co2(7.56)3]4+ is formed, in which each six-co-ordinate cobalt(n) centre is co-ordinated to a didentate metal-binding domain from each of three ligand threads.

Figure 26 Diagrammatic representations of helical metal complexes. The circles are metal atoms and each long block represents a rigid chelate unit with connectors Y or a single bond distinct ligands are shaded differently (a) the dihelicate where each chelate can be bidentate (tetrahedral metal coordination) or tridentate (octahedral metal a coordination) (b) die triple dihelicate and (c) the triple trihelicate, in which each chelate is bidentate and the metal coordination is octahedral...

Spontaneous self assembly of a dinuclear triple helical complex is observed with linked bis-[4,5]-pineno-2,2 -bipyridines. Studies by electrospray mass spectrometry, CD and NMR determined that the major species in solution was a complex of Zn L = 2 3 stoichiometry with a triple helical structure and an enantiomerically pure homochiral configuration at the metal centers. The preference for the formation of one of the possible stereoisomers over the other is of interest.265 Another binuclear triple helical complex is formed from zinc addition to bis[5-(l-methyl-2-(6-methyl-2 -pyridyl)benzimidazolyl)]methane. Spectrophotometric titrations with a zinc solution... 

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. 

Triple Helicates. The steric information contained in the oligo-bipy strands based on bipy units connected in the 6,6 positions is designed to yield double helices on complexation of metal ions undergoing tetrahedral coordination. Steric effects due to the 6,6 -disubstitution hinder the binding of metal ions of octahedral coordination geometry, which would be expected to lead to triple helical complexes. 

Further developments involve the investigation of the mechanism of formation of double- and triple helicates and of the effect of variations in ligand structure on their features, the determination of their physico-chemical (thermodynamic, kinetic, electrochemical, photochemical) properties, the exploration of the coordination chemistry of the ligand strands. For instance, it may be possible to obtain quadruple helical complexes with ions of high coordination number such as the lanthanides and linear ligands containing bipy or terpy units. Using cubic metal ions would also be of interest. 

There are many examples of compounds, usually short polymers, that spontaneously form double helical structures. A classic example involves a number of bipyridyl ligands joined by short, flexible spacers. In the presence of Cu(I), which has both an affinity for nitrogen-containing ligands and a preference for a tetrahedral disposition of donor atoms, discrete double helical complexes are formed [23], A similar phenomenon gives rise to a triple helical motif when a metal that preferentially adopts an octahedral coordination geometry is used [24],... 

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