Page 234 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GNH Final Pages
Encyclopedia of Physical Science and Technology EN011A-543 February 12, 2002 12:40

Organic Macrocycles 521

TABLE III Effect of Substituent Groups on Cation Com-
plex Stability (log K ) with 18-Crown-6 and Its Analogs in
◦
Methanol at 25 C
Stability constant
Crown ether Na + + K + + Ba 2+ +

18-Crown-6 4.36 6.06 7.04
Cyclohexano-18-crown-6 4.09 5.89 —
Dicyclohexano-18-crown-6 3.68 5.38 —
(cis-anti-cis)
Benzo-18-crown-6 4.35 5.05 5.35
Dibenzo-18-crown-6 4.36 5.00 4.28

in lipid membranes. Careful kinetic studies demonstrate
+
that the selectivity of the ligand for K is a function of the
rate of cation release from the complex, there being little
difference in the rate of cation uptake into the ligand.

D. Tetraaza Macrocycles

Macrocyclic ligands containing four nitrogen heteroatoms
separated by various organic bridges have been studied

3+
FIGURE 4 Log K for the reaction M 3+ + L = ML (L = 18-crown-
6) in methanol at 25 C versus reciprocal of cation radius. No re-
◦
action was observed with Tb 3+ ,Dy 3+ ,Ho 3+ ,Er 3+ ,Tm 3+ ,Yb 3+ ,
or Lu 3+ .

Valinomycin (compound 5, Fig. 1) is similar to a cyclic
protein in structure. In the unfolded configuration, it is too
large to accommodate metal ions. However, intramolecu-
lar hydrogen bonds cause the ring to tighten in on itself,
providing a nearly octahedral arrangement of the oxygen
atoms of the correct size to accommodate a K ion. The
+
exterior of the ligand is hydrophobic, making it soluble

TABLE II Stability Constants (log K ) of Crown Ethers with
Heavy Metal Ions in Water at 25 C
◦
Stability constant
Ligand Pb 2+ + Hg 2+ + Tl + + Ag + +
15-Crown-6 1.85 1.68 1.23 0.94
18-Crown-6 4.27 2.42 2.27 1.50
Dibenzo-18-crown-6 1.89 — 1.50 1.41
Dicyclohexano-18-crown-6 4.43 2.60 1.83 1.59
(cis-anti-cis) FIGURE 5 Variation of equilibrium constant K in 95 volume per-
1,10-Dithia-18-crown-6 3.13 >5 0.93 4.34 cent methanol for the reaction of several cryptands (see com-
+
pound 4, Fig. 1) with the alkali metal ions Li –Cs + (plotted ac-
1,10-Diaza-18-crown-6 6.90 17.85 — 7.8
cording to increasing metal ion radius).