Chemistry Reference
In-Depth Information
Mixtures between cosan and neutral thiacalixarene CAc9 (cone conformer) are
also characterized by a strong synergistic effect in the extraction of Eu
3+
,
D
Eu
increas-
ing from below detection limits to log
D
= 1.9 (0.3 M HNO
3
, 0.05 M of each synergist
in chlorobenzene).
194
The effect of the anion on selectivity over alkali ions and on
D
Eu
is most pronounced at ratios CAc9:cosan 1:1 and 0.6:0.4, respectively. Various com-
plexes are extracted, depending on the phase compositions, with CAc9 Eu(cosan)
3
existing at high metal loading. The rates of forward and backward phase transfer can
be accelerated by pre-equilibrating the mixture in the absence of metal ions.
The combination of cosan and CMPO groups on the calix[4]arene skeleton turned
out to produce a strong intramolecular allosteric effect.
195
For example, the extract-
ability of Am(III) and Ln(III) increases in the order Cos3, CPn1 < Cos6 mixed with
Cos7 < Cos5 (1,3-alternate) < Cos4 (cone). With 1 mM Cos4
in 1,2-dichloroethane,
log
D
> 2 from 1 M HNO
3
. The synergistic enhancement factor for Cos6 + Cos7, 1
mM each, equals 10,000 at pH 1. The efficiency of Cos5 is also surprising, taking
into account the charge separation, but the flexible spacers may contribute to it. The
diluent promotes extraction in the order of nitrobenzene < dichloroethane < octan-
2-one:TPH (1:1 mixture).
The ligands Cos8-Cos13 show strongly enhanced extraction abilities for trivalent
actinides and lanthanides in comparison to the previously reported simple covalent
combination of one Cosan
−
with one CMPO
group or to various synergistic mixtures
of calixarenes substituted exclusively with two Cosan
−
or two CMPO moieties. These
compounds are the first examples of molecules bearing combinations of hydrophobic
anionic groups, Cosan
−
and chelating functions for trivalent lanthanides and actinides
CMPO
in a geometrically predefined manner. The combination of the two groups
on a calix[4]arene platform affords extractants with strongly increased extraction
efficiency for europium and americium from acidic aqueous phases. However, as
expected from the behavior of Cosan, distribution ratios decrease strongly as the
HNO
3
concentration increases.
196
Distribution ratios of more than 100 for a 10
−5
M
extractant concentration are obtained with Cos10. They
are unprecedented and sub-
stantially higher than those reported previously for the covalent combination of one
Cosan
(−)
with one CMPO
group.
187
Comparison of derivatives in the
cone
and
1,3-alternate
conformations
shows that the four groups must be attached to the same side of the molecule.
Modification of the spacer length for the CMPO groups (with constant spacer
of the Cosan groups) shows that the distances from the platform must be simi-
lar to allow a cooperative action. A similar cooperative effect is observed when
the same concentration of CMPO/Cosan groups is offered by a 1:1 mixture of
the tetra-substituted calix[4]arenes Cos3 and CPn4. Complexation studies with
Cos10 for La
3+
, Eu
3+
, and Yb
3+
in methanol carried out by UV spectrophotometry
and microcalorimetry revealed the formation of ML and ML
2
complexes.
196
This
formation of ML
2
complexes is quite unusual for narrow-rim CMPO calixarenes.
The log β values of the complexes of La
3+
, Eu
3+
, and Yb
3+
with Cos10 range
from 9.6 to 11.7 and from 7.2 to 7.5 with Cos7 corresponding to a low selectiv-
ity between lanthanides. Similarly, the discrimination between europium and
americium is not marked.
