Macrocyclic Ligands - Web del Profesor

mechanism is the assumption that coordination geometry
preferences can be suddenly changed at some point along
a series of ligands where gradual changes in the ligand
framework are made. Because these changes can occur at
different points for different metal ions, discrimination can
be achieved. A particularly appealing aspect of the mixed
donor aza–oxa systems is the lower ligand field which they
provide, which then tends to minimize spin state changes.
These systems are treated in a comprehensive review of
O3N2, O2N3, and other pentadentate macrocycles with N, O,
S heteroatoms. 104
Examples of the use of synthetic mixed donor macrocycles
in heavy metal ion separations are found in the discrimination
of silver from lead. A number of studies indicate that
the inclusion of sulfur in macrocyclic sequestering agents
shifts the discrimination to silver. 104 An example of this
is seen with (57) and (58). For the aza–oxa macrocycle
(57) the log K is 5.9 for both silver and lead ions,
while the thia-incorporated ligand (58) complexes silver
more efficiently (log K = 9.9) compared to lead (log
K = 5.7). 106,107
NH
O O
HN
NH
S S
O
O
(57) (58)
HN
The larger mixed aza–oxa, aza–thia, and aza–and
oxa–phospha macrocycles are noted for their ability to
complex more than one metal ion and to alter the magnetic
properties of bimetallic complexes. 108–110 An example of
tri-metal coordination is the tricopper complex of a 27member
ring system (59). 108 A classic series of dicopper
complexes which illustrates the influence of donor atoms
on magnetism are the dicopper structures (60)–(62). 110 The
magnetic properties were found to be extremely dependent
on the mode of azide coordination, which is thought to be
influenced by the orientation of the orbitals on the metal ions.
In complex (60), the two copper ions are ferromagnetically
coupled with a triplet ground state; in (61), the metal ions
are antiferromagnetically coupled; and in (62), the two copper
ions are not coupled.
4.4.2 Cryptands
Cryptands (15) are noted for their highly selective
complexation of alkaline earth metal ions, and for their
ring size–metal ion match ability. 5 The thermodynamic
properties of these macrocycles have been extensively
investigated, and results indicate that the high stability of the
N
H
N
H
O
Cu
HOH O
HN Cu Cu NH
HN NH
O
(59)
H
N
N3
O
N3
H
N
HN Cu Cu
N3
N
H
O
(61)
N
H
N 3
NH
MACROCYCLIC LIGANDS 15
HN
N 3
O
Cu
O
O
N
N
N
N
N
N
O
(60)
S
N3 N N N
S
O
Cu
O
S
S
N 3
NH
HN Cu Cu NH
N
N N N3
(62)
bicyclic macrocycles compared to their monocyclic analogs is
enthalpic in origin. 88
4.4.3 Compartmental Ligands
Compartmental ligands (16) provide extensive opportunities
for multiple metal ion complexation. An example of a
mixed donor ligand incorporating different metal ions is the
macrocyclic trinucleating ligand (63), which is capable of
complexing two ‘soft’ donor metal centers in addition to a
‘hard’ alkali or alkaline earth metal. 111
4.5 Oxa Macrocycles
4.5.1 Crown Ethers
In the crown ethers (18) the interactions between the ligand
and metal ion are considered to be more electrostatic in nature,
rather than the covalent binding observed for the transition
metal complexes of the aza, thia, and phospha macrocycles.
The thermodynamic properties of these macrocycles have
been extensively studied, with numerous reviews covering
complexation, selectivity, and structural aspects, some with
extensive tables of thermodynamic data. 69,70,112–119 Considerable
efforts have been made to correlate the interrelationship
between cavity size of the macrocycles and stability of alkali
and alkaline earth metal complexes. From X-ray and CPK
models, cavity radii are determined as 0.86–0.92 ˚A for 15crown-5
(64), 1.34–1.43 ˚A for 18-crown-6 (65), and about
1.7 ˚A for 21-crown-7 (66). 69 For complex formation between
the alkali metal ions and 18-crown-6, the maximum stability