Macrocyclic Ligands - Web del Profesor

NH
NH
NH
NH
NH
NH
(46)
HN
HN
HN
HN
the capability of these macrocycles beyond simple oxygen
binding to oxygenase activity observed for the cytochrome
P-450s. This has been achieved by adding piperazine ‘risers’
to increase the cavity size (9 ˚A high) as well as increasing the
hydrophobicity of the molecules, and by adding anthracene
and durene ‘roofs’. The crystal structure of the anthracenebridged
derivative shows that the macrocycle is indeed capable
of hosting an acetonitrile molecule.
4.2.6 Sepulchrates
Sepulchrates (7) are the most noted of the caged
macrocyclic ligands and are the nitrogen analogs of the
cryptands. The Co–N distances are 1.99 ˚AforCo III and 2.16 ˚A
for Co II from crystallographic data, and do not vary greatly
from other cobalt amines. 20
4.2.7 Expanded Porphyrins
A review of expanded porphyrin ligands can be found. 92
The texaphyrins (8) can be considered as 22-π-electron
benzannulene systems with an 18-π-electron delocalization
path, based on crystal structure data as well as NMR.
The cadmium complex of the macrocycle is found to be
planar with pentadentate coordination of the macrocycle to
cadmium, which becomes seven-coordinate as a result of
axial coordination to two pyridine molecules. The cavity is
nearly circular with a center-to-nitrogen distance of 2.39 ˚A.
Because of the larger size of this macrocycle, metal ion
coordination is generally seen with the larger transition metals
and lanthanides.
A more flexible expanded porphyrin is the ‘accordion’
porphyrin (9). 22 The structural aspects of this macrocycle
illustrate the importance of flexibility in achieving unanticipated
structures. The free-base macrocycle is elliptical with
the inclusion of two water molecules (47), while the dicopper(II)
complex is highly distorted by means of exo and endo
orientations of the imine groups (48).
N 3
N
N N
Cu
MACROCYCLIC LIGANDS 13
Ph
NHNH
O
N
H HN
N H H N
O
HN HN
N
Ph
(47)
(48)
N
N
Cu
N
N3
4.3 Polythia and Polyphospha Macrocycles
4.3.1 Polythia Macrocycles
The coordination chemistry of thioether macrocycles has
expanded greatly only since the mid-1980s, as seen by a number
of reviews. 55,93–95 The macrocyclic effect is also noted for
thioethers, but to a lesser extent than some of the other macrocyclic
ligands. This is due primarily to the reorganizational
energy requirements, since a number of the free-ligand thia
macrocycles have a tendency to adopt ‘exodentate’ conformations
in the uncomplexed form, where the sulfurs are pointed
out of the macrocycle (49). Macrocyclic thioethers must then
undergo a reorganization of their exo lone pairs in order to
incorporate metal ions within the cavity. It was found in a study
of the complexation of a number of open-chain thia ligands and
thia macrocycles that the enthalpy changes were essentially
identical for both macrocyclic and nonmacrocyclic ligands.
Hence, the favorable macrocyclic effect is more attributable to
the entropy changes in the sulfur macrocycles. 96 The smaller
trithia analog of the extensively studied nitrogen donor triazacyclononane
does not require such organization and, as such,
has been extensively studied itself. 55 Because of the preference
for exodentate sulfurs, metal ion coordination in many
cases is external to the cavity (50). 97 A comprehensive review
of the structural aspects of thia macrocycles can be found. 55
Considerable effort has been made with regard to
conformation analysis of crown thioethers. It has been found
that ligand strain is most evident in torsion angles, whereby
an examination of the deviations from the optimum values of
N