Chemistry Reference
In-Depth Information
3.6 Secondary Structure and Stabilizing Interactions
Analogous to biological systems, the formation of metallo-organic helical structures is
often induced by intrinsic structural modifications resulting from the interactions of
“informed” components (helical conformational folding). This preorganization can be
described as a self-templating process for the propagation of self-assembly. In addition,
different weak interactions may favourably act in the assembly of helical structures. A
common way is templating by anions, which are present in the reaction mixture and which
are the key elements controlling the final helical structure. This effect is often encountered
with circular helicates and can be illustrated with Lehn's pentanuclear helicate templated
by Cl (Figure 3.20a) that transforms into a hexanuclear analogue in the presence of
sulfates [53]. The templating ions may also behave as switching components between
different supramolecular forms. For instance, the conversion of triple-stranded helicate
into 3D tetrahedral cages is induced by the presence of an alkylammonium guest [54].
In many cases, the “third” component in the self-assembly process contributes to the
stabilization of helicates, and several examples can be found in the literature. In the solid-
state structure of dinuclear triple-stranded helicates, a potassium cation coordinated to
internal oxygen atoms weakly interacts with the double bond [55]. Although the presence
of potassium is not required for the existence of this assembly in solution, this interaction
indicates a possible way of stabilizing it. In this context, the presence of hydrogen bond-
ing can also be crucial. This stabilizing effect was identified as the origin of positive coop-
erativity detected with thermodynamic modelling in triple-stranded dinuclear lanthanide
helicates (Figure 3.11b) [15a].
A “miraculous” effect of secondary interactions is shown with the isolation of neutral
amphiphilic lanthanide helicates [56]. While the usual self-assembly process gives only
an insoluble mixture of oligomers instead of the expected [Ln 2 L 3 ] helicate, the addition
of two equivalents of Ag þ leads: (i) to a complete dissolution and (ii) to the almost quan-
titative formation of the dinuclear lanthanide helicate, where Ag þ cations sit at its extrem-
ities and interact with the ligand chains (Figure 3.20b).
As shown in the above examples, the secondary interactions may delicately influence
the self-assembly process and bring a decisive energetic contribution, which will direct
the reaction toward a specific product. The stabilization may possibly come from a
change of the overall solvation energy, or from partial compensation of a low effective
molarity, which is commonly detected in both double- and triple-stranded helicates. The
programming of secondary weak interactions would be beneficial for the designing of
new helicates. However, their thermodynamic modelling was overlooked until now due to
the lack of reliable experimental data, but their effect is hidden in different cooperativity
factors. In this context, molecular dynamics simulations are proving to be a powerful tool
for optimizing supramolecular interactions, if we refer to their recent application to differ-
ent heteronuclear metallo-helicates [56].
3.7 Conclusions
The present chapter describes physico-chemical principles that govern coordination pro-
cesses in multicomponent self-assembly. The basic concepts of coordination chemistry
Search WWH ::




Custom Search