Macrocyclic Ligands - Web del Profesor

to six, the complex stability decreases. A detailed discussion
of the thermodynamics of changing chelate sizes for tetraaza
macrocycles can be found. 17,78
M N
N
H H
H
H
H
H N
M
N N
N
(31) (32)
N
N
H
H
An elegant example of the importance of conformational
changes in tetradentate macrocycles is the blue to yellow
conversion observed for nickel(II) complexes. The yellow
form is the low-spin square planar complex NiL 2+ , while the
blue form is high-spin pseudooctahedral [NiL(H2O)2] 2+ .In
the blue to yellow conversion the Ni–N bonds contract, which
compensates for the breaking of the axial Ni–OH2 bonds. The
reaction is controlled by entropy, and the addition of an inert
salt is such as to favor the dissociation of the water molecules.
At equilibrium in aqueous solution, both [12]aneN4 (cyclam)
(1) and the 15-membered analog, [15]aneN4, have 99% of
the high-spin form present, while the 13- and 14-membered
macrocycles exist in predominantly the low-spin square planar
form (87 and 71%, respectively). 79
For the nonplanar octahedral cis-coordinated macrocycles
[n]aneN4, changes in the ligand field correlate well with the
analogous ligand field strengths for nonmacrocyclic analogs,
specifically as related to the chelate ring size. A general rule
of thumb is that increasing the chelate ring size from five
to six increases the stability of complexes of smaller metals
compared to larger metal ions. The origin of this effect can be
attributed to increases in ring strain energy when metal ions
larger than tetrahedral carbon are part of the ring. 78 For the
planar-coordinated macrocycles, the equatorial ligand field,
as anticipated, is dependent on the ring size. These findings
have been related to the calculation of the optimum hole size
permitting the macrocycle to adopt its most preferable endo
configuration. Thus, it has been found that the macrocyclic
hole size increases by 10–15 pm for each increment in n for
[n]aneN4. 79
In order to introduce a greater rigidity into the
flexible polyaza macrocycles and to implement greater
hole size–metal ion size match correlations, reinforced
macrocycles such as (33) have been created, which contain
fused diaza rings. 80 Crystallographic results for the nickel(II)
complex indicate that the Ni–N bonds are shortened from
the strain-free value of 1.91 ˚A for diamagnetic nickel to
1.86 ˚A. Ligand field strength is also found to increase, and
this has been suggested as being due to the compression
of the bond lengths 80 as well as the presence of tertiary
nitrogen donors. 78 In a more recent comparative study of
nickel macrocycles with two fused 1,3-diazacyclohexane rings
(35) compared to two fused 1,3-diazacyclopentane rings (34),
MACROCYCLIC LIGANDS 11
structural results revealed weaker ligand field strengths for the
1,3-diazacyclohexane compared to 1,3-diazacyclopentane. 54
N
N
NH HN
(33)
N
NH
N
N
(34)
HN
N
N
NH
N
N
(35)
HN
Attempts to achieve macrocycles that are capable of
stabilizing highly oxidized transition metal complexes has
led to the design of ‘noninnocent’ ligands. 81 The structures of
high-valent chromium(V) oxo species with the two tetraamido
N ligands (36)and(37) were determined. Both structures were
found to contain distinctly nonplanar amide groups, and in
(36) all four amides are nonplanar.
O
NH
NH
O
HN
HN
O
Cl Cl
NH
N
O NH HN O
(36) (37)
HN
Polyaza macrocycles with pendant arms have been studied
extensively, in particular with respect to protonation and
complexation as well as to the kinetics of metal complex
formation. These aspects are treated in a review by Kaden. 11
Of particular interest is the fact that metal complex formation
constants of macrocycles with pendant carboxylates can be
10 3 to 10 4 times higher than for the unsubstituted analogs.
4.2.3 Higher Polyaza Macrocycles
Transition metal complexes of the larger polyaza
macrocyclic ligands have been less extensively studied than
for the smaller ring systems. For the pentaaza macrocycles,
[15]aneN5 with ethylene bridges appears to form the most
stable complexes with most metal ions. 17 Structural data
for a variety of pentaaza macrocyclic complexes have
been reviewed. 16 The N–H bonds as well as the different
sized chelate rings must be considered in calculating the