Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 129

Fig. 3 Evolution of ligands containing sp 3 nitrogen donor atoms: from ammonia, to ethylenediamine,
to cyclam, to sarcophagine
Fig. 4 Thermodynamic macrocyclic effect: complexation equilibria involving the Ni II aqua.ion
and the champions of non-cyclic tetra-amines 1 (2.3.2-tet) and of the tetra-amine macrocycles
2 (cyclam)
champion of tetramine macrocycles. The macrocyclic complex is ca. 4,000-fold
more stable than that of the open-chain ligand. Figure 5 shows some structural
details of the low-spin Ni II complexes of 2.3.2-tet [ 5 ], and cyclam [ 6 ], as obtained
from X-ray diffraction studies on crystalline salts. Both complexes exhibit a square
planar coordination geometry. Ni II -N distances and N-Ni II -N angles are nearly the
same for both complexes, which indicates the establishing of metal-ligand
interactions of comparable intensity. Thus, the thermodynamic macrocyclic effect
does not depend upon a different strength of coordinative bond, and, in particular,
it has to be ascribed to the fact that cyclam is perfectly preorganised for metal
coordination, whereas the open-chain ligand must spend energy (both enthalpic and
entropic in nature) in folding itself in order to fulfil the geometrical requirements of