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Figure 7.4 ORTEP representations of the [Cu 2 (L) 2 ](ClO 4 ) 2 complexes, showing 50% proba-
bility ellipsoids, L ¼7 (left) and L ¼6 (right). The hydrogen atoms are omitted for clarity.
the bipyridine units being twisted by 33
C bond. Interest-
ingly, the copper(I) center is significantly closer to the imino N atom than to the pyridine-N.
In the phen case, the twist is also clearly evident in the solid state but significantly less
pronounced (about 6 ; see Figure 7.4 right). The severe internal distortion apparent in the
X-ray structures of these helicates is strong testimony to the coordinative ability of the imino
function [33].
These crystallographic structures clearly show that, at least in the solid state, individual
pyridine groups within a given ligand can be bound to separate cations. Each copper(I)
ion is coordinated to four nitrogen atoms, provided by the imine and pyridine functions of
two distinct ligands, to give a distorted tetrahedron with a bite angle of 80 . It appears,
therefore, that incorporation of imine groups at the a,a 0 positions of a bipyridine or phe-
nanthroline ligand can cause the central unit to become non-chelating and, instead, to
adopt a bridging role. This is not too surprising for 7 , where individual pyridines can
rotate freely, but is unparalleled in the case of the phenanthroline-based ligand 6 where
the pyridino N atoms are well sited to form a chelate.
These two unique complexes were the first examples to unveil the ability of bipyridine
and phenanthroline framework to act selectively as bridging ligands rather than biden-
tates. This unusual mode of coordination has further been confirmed in many cases in
combinatorial chemistry and dynamic systems [42].
The use of spectrophotometric titrations enabled the evaluation of the stability con-
stants for each step in the helicate formation. These data, as well as the molecular
mechanics simulations, support a mechanism in which the first step involves attachment
of a copper(I) fragment to a single N donor atom belonging to the imino function. There is
apparently positive cooperativity in the binding of bpy-ligand 7 to Cu(I) and the overall
stability constant for the metallohelicate is logb L ¼
about the exocyclic central C
26 and compares quite well with the
only available literature data for a somewhat comparable process [43]. In the case of the
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