Self-Assembly Principles of Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 104

Supramolecular complexation reactions are governed by the equilibria between solv-
ated reactants, that is, metal ions and organic ligands. Assuming the Eigen-Wilkins mech-
anism (Equation 3.7, where K os is the equilibrium constant for the outer-sphere complex,
k ex is the solvent exchange rate, S and solv are solvent molecules) [32], the reaction rates
for the replacement of solvent molecules, which may be considered as monodentate lig-
ands, from the first coordination sphere of metal ions may vary extensively and depend
essentially on the nature of the metal ions.

K os k ex M

M ð solv Þ x þ L ð S Þ ) M ð solv Þ x ... L þ S ! ð solv Þ x 1 L þ S þ solv ð 3 : 7 Þ

These substitution reactions were classified by Langford and Grey in 1965 [33]. This
work represents a mechanistic support for interpreting elementary reaction steps in supra-
molecular chemistry. To give a short reminder, the ligand substitution reactions are
divided as associative, dissociative and interchange. In addition, we distinguish two kinds
of intimate mechanisms: associative and dissociative activation modes [34]. These ele-
mentary reactions may take place many times along the reaction pathway and are deter-
mining for the overall reaction rates.
The strict assembly of supramolecular compounds is supposed to spontaneously pro-
vide a single compound. The first evidence of a complex formation pathway was brought
by Lehn and coworkers in the study of pentanuclear helicates [35]. The proposed mecha-
nism identifies hairpin-type intermediates that slowly rearrange in the final helicates. Pio-
neering extensive kinetic studies (which are probably the only ones in the field of helicate
self-assembly) were carried out by Albrecht-Gary and coworkers [14], who launched
kinetic studies of different double- and triple-stranded helicates in order to identify the
reaction intermediates and the formation mechanisms in solution.
The collaboration of Albrecht-Gary with Lehn allowed a full characterization of the
formation of trinuclear double-stranded helicates [36]. The schematic representation of
the proposed mechanism consists of four elemental steps (Figure 3.9). The reaction is
initiated with a fast binding of two metal ions to the tritopic ligand in the terminal binding
sites. This can be explained by the prevention of some unfavourable electrostatic interac-
tions, if a copper ion would occupy the central position. Consequently, the binding of the
third metal ion occurs in the central site. However, the assembly is still not complete at
this point; the slow rearrangement processes (self-repairing steps) occur in the terminal
phase in order to adopt the helical structure with a minimum of potential energy.
The self-assembly of triple-stranded iron(II) helicates [Fe 2 L 3 ] 4þ with a ditopic ligand is
obviously favoured in ligand excess, where the ligand coordination sites are preferentially
bound to the iron(II) cation (Figure 3.10) [37]. This interesting feature found its origin in
the extra stabilization of low-spin tris-bipyridine complexes. In line with this interpreta-
tion, a hairpin complex [Fe 2 L 2 ] 4þ appears even in metal excess instead of a virtual
[Fe 2 L 2 ] 4þ helicate. The helicate assembly terminates by the metal binding to the second
already preorganized site, which reduces its effective charge by polarization.
Interesting features have been revealed during the study of the self-assembly of dinu-
clear triple-stranded helicates containing highly charged lanthanides [38,39]. The forma-
tion of a helicate [Eu 2 L 3 ] with a neutral ligand is favoured at stoichiometric conditions,
which leads to fast and quantitative self-assembly processes (Figure 3.11a). The binding