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Formation of a triple helix by coordination with metal cations
was ascertained by Circular Dichroïsm. As expected but
differently of 5,5’analogues 5 , coordination of ureido- 6,6’bipyridyls
arms with e.g. Cu II cation spontaneously generates a
strong exciton coupling-type positive Cotton effect ( = + 33
mol -1 .cm -1 ) between the bipyridine units (i.e. a negative band at
290nm, a positive band at 315 nm and an isodichroic point at
approximatively 302 nm) proving a high helicity induction
(Figure 5.).
(A)
(B)
Figure 5. Circular Dichroïsm titration of ligand 9 (c = 5.0 × 10 -5 mol L -1 )
(A) in MeOH with CuCl2,2H2O; (B) in MeOH with Fe2(SO4)3 [1:1]
complex, followed by CuCl2,2H2O 1 equiv.
The same phenomenon was observed by adding Cu II cation to
the [Fe III /9] mononuclear complex, giving a Fe III /Cu II dinuclear
complex. In this case a similar positive Cotton effect of ( = +
16 mol -1 .cm -1 ) was observed. It should be noted (Figure 5.) that
coordination of Fe III at the “urea” site, didn’t induce any superior
helicity into the podandate. The difference in helicity (almost the
half) between the mononuclear and the dinuclear species was
attributed to a less degree of flexibility of ureido-bipyridyl arms
when urea-carbonyles are first involved in coordination with the
Fe III cation. Nevertheless, a helix is characterized by an axis, a
screw sense and a pitch. This helicity is a special case of chirality
as defined earlier by Cahn, Ingold and Prelog. 16 It may be righthanded
(P) or left-handed (M). The remaining question is
whether this induced helicity was right-handed or left-handed in
the present case.
The large exciton coupling-type positive Cotton effects of e.g.
[Cu II /9] and [Cu II /Fe III /9] CD spectra, are pointed in favor of a
single diastereoisomer as indicated earlier by Lehn and coworkers.
17 Previous examinations on the chirality of C 3symmetric
Fe III tripodal--cyclodextrin derivatives 18,19 in
different solvents, showed ligand having short spacers (between
the cyclodextrin torus and the metal coordination site) exhibited
negative exciton coupling and -helicity in all solvents, while
those having long spacers formed a helicate and a positive
exciton coupling. 20 It is currently, admitted that a positive
coupling-type exciton implies a complexation with a -helix
(same handedness as for the naturally ferrichrome 21 ). Looking at
the [Cu II /9] and [Cu II /Fe III /9] CD titration spectra, (Figure 5) one
can see the free ligand 9 in MeOH displayed itself a weak exciton
coupling-type positive Cotton effect. The addition of Cu II
aliquots to 9 strongly enhances this effect in the same sense, so it
may be concluded reasonably to the preferential formation of a
-helix with a left-handed screw propeller (M).
Coordination of Ni II cation to the bipyridyl nitrogens induced
an unexpectedly low kinetic to complete the [1:1] mononuclear
complex formation of 9 (3hours) compared to the immediacy
with Cu II . Circular dichroïsm titration of 9 with Ni II also
generates a strong exciton coupling-type positive Cotton effect
( = + 91 mol -1 .cm -1 ) between the bipyridine units (Figure 7.).
This confirms the preferential formation of an analogous lefthanded
(M) triple -helix.
Figure 6. Spectrophotometric titration of ligand 9 (c = 1.0 × 10 -5 mol L -1 )
(A ) in MeOH with NiCl2, 6H2O; (a delay of 30min. was respected
between each curve to reach equilibrium) (B) Kinetic of the [Ni II /9]
mononuclear complex formation (c = 1.0 × 10 -5 mol L -1 ) with NiCl2.6H2O
(1.0 equiv.).
Figure 7. Circular Dichroïsm titration of ligand 9 (c = 5.0 × 10 -5 mol L -1 )
in MeOH with NiCl2.6H2O.
Furthermore, it has been demonstrated that Cu II and Ni II
complexation to the bipyridyl units of 9 in a protic solvent gave
the same helicity with an apparent higher rotational strength in
the case of Ni II , but a large difference was observed in kinetics
between the two cations. The low kinetic of the Ni II could be
explain in terms of equilibration reactions, by which a bipy
ligand moves from one Ni II complex to another proceed much
slower. 22a-b
In summary, a novel C 3-symmetrical tris-ACE-6,6’-bisheterocylic--CyD
was prepared. Its spectroscopic behavior was
examined to investigate the formation of tripodal mononuclear
and dinuclear metal complexes. It was observed the 2,2’bipyridine
units anchored at their 6-position, induce formation of
preferentially -CyD-based tripod helicates with a high
chirality. Further experiments are under progress to examine
complexation behavior with other metal cations notably with
cations having a tetrahedral coordination geometry. In extension,
the synthesis and characterization of new -CyD-based tripods
with different bis-heterocyclic units is in progress and
complexation behavior of these new compounds will be explored
in detail.
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