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Encyclopedia of Physical Science and Technology EN011A-543 February 12, 2002 12:40

518 Organic Macrocycles

is surrounded by the macrocycle structure, it is partly or lectivity of 18-crown-6 for K . Likewise, among alkaline
+
completely isolated from the solvent. By this means, it earth cations, the Ba 2+ ion both fits best in the 18-crown-6
is possible to solubilize bound substrates into solvents or ligand cavity and is bound most strongly. Table I, which
membranes in which the unbound substrate is not solu- lists the ionic radii of a number of metal cations, shows
+
ble. Furthermore, the change in chemical reactivity of the that K and Ba 2+ are of almost equal size.
bound substrate may be exploited to yield catalytic and Figure 2 shows that 21-crown-7, like 18-crown-6, binds
biomimetic substrate transformation. the monovalent cation whose size matches that of the
ligand cavity, that is, Cs . However, the selectivity of
+
15-crown-5 is not easily explained on the basis of relative
+
I. MACROCYCLIC STRUCTURE AND size. While the cation Na best matches the cavity in size,
+
METAL CATION COMPLEXATION K is bound slightly more strongly. This case illustrates
that size is not the only factor and is often not the deter-
Macrocycles, also called macrocyclic compounds, macro- mining factor that controls cation selectivity. The problem
+
cyclicligands,macropolycycles,andsoforth,includeava- arises from the fact that Na ion is slightly too large to fit
riety of basic structures. The major headings below group into the 15-crown-5 cavity. For such a case, cation solva-
currently known macrocycles into broad, general classes. tion energies dominate in the free energy cycle
However, the term macrocycle is not confined to the lim-
ited number of representative compounds presented in this M g + L g −→ ML g

article.

A. Crown Ethers M s + L s −→ ML s
The name crown ether was applied to cyclic polyether for complex formation. (The subscripts g and s indicate
molecules such as compounds 1–3 in Fig.1, by Pederson, species in the gas and solvent phases, respectively.) Com-
+
who first reported their preparation in 1967. A trivial non- pared to Na , the larger K is less strongly solvated be-
+
rigorous nomenclature is commonly used to streamline cause of its lower charge-to-radius ratio, so less energy is
naming of these complex molecules. Names are structured expended in removing solvent molecules in the complex-
as follows: (1) principal ring substituents, (2) heteroatoms ation process. Because the range of macrocycle sizes is
substituted for oxygen, (3) number of atoms in the princi- much larger than the range of cation sizes, it is relatively
pal ring, (4) the name crown, and (5) the number of het- rare that selectivity is governed by size predominantly.
eroatoms in the principal ring. Thus, compound 1 is named It is more often the case that solvation, ligand flexibility,
15-crown-5, compound 2 is 1,10-dithia-18-crown-6, and the effective charge on the binding sites play the
and compound 3 is dibenzo-30-crown-10. dominant roles.
Crown ethers are particularly interesting ligands for two Crown ethers have affinity for metal ions besides those
reasons: They measurably bind alkali metal cations in wa- of the alkali and alkaline earth series. Figure 4 shows the
ter solution, and they demonstrate size-based selectivity binding constants of 18-crown-6 with the series of triva-
of metal ions. These features are illustrated for the lig- lent lanthanide cations, which decrease in size across the
ands 15-crown-5, 18-crown-6, and 21-crown-7 in Fig. 2, series. Table II shows the binding constants of several sim-
where the thermodynamic equilibrium constant K for the ple crown ethers with Pb , Ag , Tl , and Hg .
+
+
2+
2+
reaction in methanol When the oxygen heteroatoms of crown ethers are re-
placed by nitrogen or sulfur, the selectivity of the ligands
M n + + 18-crown-6 = (M − 18-crown-6) n +
changesmarkedly.Forexample,sulfur-containinganalogs
is plotted versus cation radius. Of the monovalent metal of 18-crown-6 have lower affinity for alkali and alka-
+
cations, K is bound most strongly by 18-crown-6. line earth cations and greater affinity for more polarizable
+
2+
X-ray crystallographic determination of the structure of cations such as Tl and Hg . When nitrogen is substi-
+
the K –18-crown-6 complex shows that the K ion sits at tuted, the affinity for alkali and alkaline earth cations also
+
the center of the ligand cavity surrounded by the six ligand drops, while that for Pb 2+ and Ag 2+ increases.
oxygen atoms as shown schematically in Fig. 3. The K + Substitution of aliphatic or aromatic groups onto the
ion is nearly the correct size to fill the ligand cavity and heterocyclic backbone of crown ethers has a destabilizing
+
is bound most strongly. The Na ion is smaller than the effect on complex stability. Table III shows that the stabili-
18-crown-6 ligand cavity, so the ligand must fold slightly ties of complexes of dicyclohexano-18-crown-6 are much
to permit all six oxygens to associate with the cation. Both more like those of 18-crown-6 than are those of dibenzo-
+
+
Rb andCs aretoolargetofitintotheligandcavity.Thus, 18-crown-6. In the latter case, the benzene rings with-
the relative sizes of cation and ligand cavity explain the se- draw electron density from the oxygen atoms, lowering the