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phen-ligand
6
, the helicate formation is not driven by positive cooperativity and the over-
all stability constant (logb
L
ΒΌ
16) reflects the relative difficulty in attaining a suitable
geometry. As a matter of fact even for the phen ligand, the mononuclear complex is rather
difficult to isolate and the final helicate is the sole compound which is easily isolated in
near quantitative yield. The conventional copper(I) bis-chelating N
py
,N
py
complex has
never been observed nor isolated. This is rather surprising considering the plethora of
such phenanthroline complexes described in the literature [44].
The systems described with imino-bpy and imino-phen are significantly more rigid
than those traditionally used to prepare metallohelicates and an important caveat
arising from this study is that positive cooperativity is not a prerequisite for helicate
formation. These hybrid ligands do not operate as single chelators towards Cu(I) cat-
ions but individual pyridine-N atoms act independently as secondary coordination
sites. Metal-induced self-organization of these ligands into helical structures is a
multistep process and with
6
a crucial intermediary complex in the formation of the
helicate has been isolated. With phenanthroline ligands, however, helicate formation
is restricted by thermodynamic considerations rather than by kinetic factors as in
certain pentanuclear helicates [45]. These metallohelicates displaying copper(I) cen-
ters in close proximity can be broken down under mild conditions by: (i) chemical
or electrochemical oxidation, (ii) competitive complexation, or (iii) addition of
excess ligand in the case of the phen ligand
6
.
The redox behavior of these unusual helical complexes is also very interesting but has
rarely been studied in detail. The copper(I) helicates exhibit four quasi-reversible reduc-
tion peaks corresponding to stepwise reduction of each coordinated imino group while in
the related free ligands only irreversible reductions are observed. This is interesting
because complexation of the imino ligand strongly stabilizes the imine function and
favors its reversible reduction. The oxidation of the copper(I) cations is irreversible and
results in the removal of one copper center. Poor thermodynamic stability of the resultant
copper(II) complex may be due to the fact that a tetrahedral coordination site is unsuitable
for binding of Cu(II). Indeed, chemical oxidation of the helicate with Ce(IV) results in the
formation of a stable complex with a lime-green color typical of five-coordinate mono-
nuclear copper(II) assembled via complexation to two bpy subunits and a single imino
group. Reformation of the dinuclear metallohelicate is quantitative when hydrazine is
used as a reducing agent.
Such structural reorganization due to preferential binding of cationic species to second-
ary complexation sites [translocation processes] have also been observed in related com-
plexes built from ligands possessing additional coordination sites such as the terpyridine
ligand
12
(Figure 7.5). The X-ray crystal structure shows that the helicate formed
between the imino-terpy
12
and copper(I) cations provides similar environments for the
metal centers lying 3.278 A
apart. Each segmented imino-terpy ligand is coordinated to
two copper(I) centers in a unsymmetrical fashion. Each ligand provides one bidentate site
involving a pyridine-N/imine-N combination and another involving two pyridine-N
donors to separate metal ions. Binding of the ligand strands in an antiparallel fashion in
the helicate results in each metal ion having an essentially tetrahedral (pyridine-
N)
3
(imine-N) coordination environment (Figures 7.5 and 7.6) [46].
It is noteworthy that the additional imine function is not coordinated (2.629 A
from the
copper center), but the fact that a
cis
-conformation of these imine is observed is
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