Page 235 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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

522 Organic Macrocycles

for many decades. Examples are porphyrin (compound 6,
Fig. 1) and its analogs, which are the metal-binding sites
in many metalloenzymes, hemoglobin, and other natu-
rally occurring compounds. There are also many synthetic
tetraaza macrocycles that have affinity for divalent (and
other) transition metal ions. Their binding constants with
these cations are generally much higher than those typical
of crown ethers and cryptands, so the metal ion is held
virtually irreversibly. For example, the binding constant
2+
(log K) of compound 7, Fig. 1, with Hg is 23 and with
2+ FIGURE 6 Diagrammatic representation of mode of binding of
Ni is 23.5. The importance of these complexes lies in
(a) anilinium and (b) benzenediazonium cations to 18-crown-6.
the binding of additional ligands at the axial sites of the
bound ion, which serves as the enzyme’s active site. The
degree and type of aromaticity in the tetraaza ligand struc- Unlike ammonium cations, diazonium cations complex
ture has a profound influence on the electronic properties to crown ethers by insertion of the positive moiety into the
of the bound metal ion, which in turn affects the strength cavity, as in Fig. 6b. Table V shows that complex stability
and nature of binding to additional ligands. deteriorates markedly with ortho substitution in benzene-
diazonium cation because of steric hindrance. Figure 7
E. Other Macrocycles
TABLE IV Stability Constants (log K ) for Reac-
A wide variety of macrocycle types has been reported in tion of 18-Crown-6 and with Several Organic Am-
◦
addition to the general categories discussed above. The monium Cations in Methanol at 25 C
calixarene ligands (compound 8, Fig. 1), which are water Cation Log K
+
insoluble, have a strong, selective affinity for Cs . They
+
form neutral complexes through loss of a proton. The lariat RNH cations
3
ethers (compound 9, Fig. 1) form neutral complexes by NH + 4 4.27 ± 0.02
the same mechanism, resembling a crown ether with an HONH + 3.99 ± 0.03
3
arm that can reach around to provide ligation at the axial NH 2 NH + 3 4.21 ± 0.02
position. Macrotricyclic cryptands (compound 10, Fig. 1) CH 3 NHNH + 3 3.41 ± 0.02
provide essentially spherical or cylindrical neutral traps CH 3 NH + 4.25 ± 0.04
3
for metal ions. Spherands (compound 11, Fig. 1) likewise CH 3 CH 2 NH + 3 3.99 ± 0.03
offer elegant binding geometries in which metal ions are CH 3 CH 2 OC(O)CH 2 NH + 3.84 ± 0.04
3
bound. The list of macrocycles is far greater than can be CH 3 (CH 2 ) 2 NH + 3.97 ± 0.07
3
presented in this limited space. CH 3 (CH 2 ) 2 NH + 3.90 ± 0.04
3
CH 2 CHCH 2 NH + 4.02 ± 0.03
3
CHCCH 2 NH + 4.13 ± 0.02
3
(CH 3 ) 2 CHNH + 3.56 ± 0.03
II. COMPLEXATION OF ORGANIC CATIONS 3
CH 3 CH 2 OC(O)CH(CH 3 )NH + 3 3.28 ± 0.02
(CH 3 ) 3 CNH + 2.90 ± 0.03
Ammonium and organosubstituted ammonium cations 3
PhCH(CH 3 )NH + 3.84 ± 0.01
bind to crown ethers and other macrocycles by the for- 3
PhNH + 3.80 ± 0.03
mation of hydrogen bonds to the ligand heteroatoms. An 3
2-CH 3 C 6 H 4 NH + 2.86 ± 0.03
example is the complex of an alkylammonium cation with 3
4-CH 3 C 6 H 4 NH + 3.82 ± 0.04
18-crown-6 shown in Fig. 6a. The stability of such com- 3
2,6-(CH 3 ) 2 C 6 H 3 NH + 2.00 ± 0.05
plexes is influenced by the number of hydrogen bonds 3
3,5-(CH 3 ) 2 C 6 H 3 NH + 3.74 ± 0.02
that can form and by the degree of steric hindrance for 3
R 2 NH +
approach of the substrate to the ligand. Table IV lists the 2
NH 2 C(NH 2 )NH + 21.7 ± 0.02
binding constants for a number of ammonium cations with 2
(CH 3 ) 2 NH + 1.76 ± 0.02
18-crown-6. The stability drops dramatically as the num- 2
(CH 3 CH 2 ) 2 NH +
ber of available hydrogen bonds is reduced from 3 to 2 2
+
R 3 NH cations
to 1. Furthermore, anilinium ions, which contain ortho
(CH 3 ) 3 NH + No complex
substituents, form weak or no complexes because the sub-
R 4 N cations
+
+
stituents sterically hinder approach of the −NH group to
3 (CH 3 ) 4 N + No complex
the ligand.