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2 The C3-symmetrical tris-ACE-6,6’-bipyridyl--CyD tripod 9
14 was synthesized from 6 A ,6 C ,6 E -triazido-6 A ,6 C ,6 E -trideoxy-
6 B ,6 D ,6 F -tri-O-acetyl-hexakis-2,3-di-O-acetyl-cyclomaltohexaose
8 8 (scheme 1) by the tandem Staudinger-Aza-Wittig (SAW) key
reaction in 50 % yield 9 (Scheme 1). The monoaminomethyl-6methyl-6’-2,2’-bipyridin
4 was obtained in three steps from 6,6’dimethyl-2,2’-bipyridin
1. 10 Compounds 6-8 were prepared by
known methods. 11 All compounds 2-9 were characterized by
current spectroscopic methods. The spectroscopic data are in
agreement with the assigned structures. (see experimental section
). 12 As expected, the IR spectrum of 9 exhibited characteristic
frequencies of the carbonyl functions: urea CO-NH at 1640 cm -1 ;
ester CO-O at 1750cm -1 (acetates) and aromatic double bonds of
bipyridyl groups at 1560 cm -1 . The 13 C-NMR spectrum in
solution (DMSO) indicate that the ligand have a C3-symmetry.
The signals corresponding to the NH-CO-NH carbonyl carbons at
158.1 ppm, the CO-O carbonyl carbons at 171.4 to 162.9 ppm
and the signals corresponding to the bipyridyl carbons from
138.1 to 118.5 ppm were detected.
Metal ion complexation
The electronic spectrum of the free tripod 9 recorded in
MeOH shows two maxima at max = 244 nm (14700 mol -1 .dm 3
.cm -1 ) and max = 290 nm (32000 mol -1 .dm 3 .cm -1 ). These
absorptions correspond to the -* transitions of bipyridyls and
to the n-* urea and carboxylate carbonyl double bonds.
Figure 1. Schematic representation of the -CyD tripod 9 and its potent
complexation sites.
Figure 2. Postulated folding pathway for the metal ion-assisted selfassembly
of Cu II and Fe III /Cu II left-handed (M) triple-helices of 9.
The extinction value calculated from the molecular extinction
coefficient in 9 is c.a. 10700 mol -1 .dm 3 .cm -1 per bipyridine unit ;
this value is within the range found in the literature. 15 As
illustrated in Figure 1, the tripod 9 represents the first term of a
new set of heterotritopic molecular receptor having two potent
metal coordination sites: one formed by the bipyridyl units and
the second by ureas and /or carboxylates, the third site is formed
by the CyD torus cavity, dedicated to the formation of host-guest
complexes via inclusion of small organic hydrophobic molecules.
Tetrahedron Letters
The titrations of 9 with “hard” to “borderline” (HSAB theory)
cations were monitored by Uv-Vis spectroscopy. As expected
and based on results previously established in our research group,
“borderline” cations as Cu II and Ni II were complexed at the
bipyridyls while “hard” cations as Fe III or Ru III were coordinated
to the carbonyle urea/ester functions (Figure 2.) according to the
HSAB theory. The titration (Figure 3.) and the ligand speciation
curves (Figure 4.) showed examples of mononuclear podandate
formation of [1:1] stoichiometry [9/Cu II ] and [1:1:1] dinuclear
podandate [9/Fe III /Cu II ].
(A) (dinuclear complex)
(B)
Figure 3. Spectrophotometric titration of ligand 9 (c = 1.0 × 10 -5 mol.L -1 )
(A) in MeOH with CuCl2.2H2O; (B) in MeOH with Fe2(SO4)3 1.0 equiv.
(black lines) then CuCl2. 2H2O 1.0 equiv.(colored lines); (C) in MeOH
with RuCl3 .6H2O.
The complexation constants log( 11) have been determined for
the four cations in MeOH as solvent. 19 It should be assumed that
the Fe III cation in 9 is coordinated and localized at the urea
functions (“hard” carbonyls centres) with a log ( 11) constant
value of 14.9 ± 0.31 whereas e.g. Cu II and Ni II “borderline”
cations are localized and coordinated in a different site, likely to
the nitrogens of the three bipyridyl units giving hexa-coordinated
octahedral complexes with constant values of 10.1 ± 0.13 and
8.8 ± 0.06 respectively.
(A) (B)
(C)
Figure 4. Ligand speciation curves for (A) Fe III , (B) Cu II and (C) Ru III
with podand 9. Free metal (dashed line), complexes (solid line), free
ligand (dashed – dotted line).
(C)