Self-Assembly Principles of Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 93

and the energetic gain cannot be further increased except by using a ligand with a stronger
affinity. This classical principle was reformulated in supramolecular chemistry as the
principle of maximum occupancy [9], implying that all receptor binding sites are involved
in the assembly.
Of course, the reaction can be driven to a predicted completion only at specific exoge-
nous conditions ( T , pH, solvent, etc.). Among these, the stoichiometry of the reactants is
probably the most important. An optimal set of external conditions can be referred as
experimental matching .

3.2.1.1 Chelate Effect

Coordination reactions often occur with x -dentate ligands (Figure 3.1). The formation of
multiple binding interactions (multivalency) is energetically favourable (decrease of D G ):
(i) due to a positive entropy change in comparison with the binding of the equivalent
number of monodentate ligands and (ii) due to a favourable enthalpy resulting from the
preorganization of donor atoms [2]. This behaviour is well known in coordination chemis-
try as the chelate effect. A closer inspection of the chelate binding shows that the reaction
of a multidentate ligand consists of an intermolecular binding of the first donor atom to a
metal ion. This initial step is followed by one or more intramolecular connections with
other donors in such a way as to incorporate the cation into the ring. A schematic illustra-
tion of the chelate effect in Figure 3.2 shows the reaction of two ligands L , each possess-
ing two donor atoms, with a divalent metallic receptor M . The receptor and two ligands
may mutually interact to give three possible complexes. The intermolecular binding of the
first donor atom in L occurs with the microscopic affinity and provides a partially bound

1 : 1 open complex o -M L . The latter species may be further transformed by an inter-
molecular reaction with the second ligand to form a M L 2 complex, or by an intra-
molecular reaction to form the cyclic complex c -M L with the successive binding constant
k

k

. EM , where EM is the microscopic effective molarity and essentially parameterizes the
difference between intra- and intermolecular processes with respect to the entropy and
enthalpy [11-13]. Since EM has a concentration unit, the corresponding free energy

4 κ

+ 2 +

κ

M L L

o -ML

1
2 κ · EM

1
2 EM

+

ML 2

c -ML L

Figure 3.2 Chelate effect in coordination chemistry with competing intramolecular and
intermolecular reactions. Adapted with permission from [11]. Copyright Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim