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crown ethers, but sometimes enhance selectivity, Casnati et al. proposed to improve
the selectivity for cesium over potassium of these compounds by enlarging the crown
of calix[4]arene crown-6 by the introduction of a propylene unit in place of the cen-
tral ethylene unit. The authors modeled the structure of the complexes of “enlarged
calix[4]arene crown-6” with potassium and cesium in the gas phase. The main con-
clusions of the modeling are the following. The calix[4]arene basket undergoes a
significant conformational reorganization upon complexation, the opposite phenolic
units are forced to rotate toward the exterior of the macrocycle to favor the binding
of the metal ion, and the rotations increase as the size of the cation increases. The
analysis of the interatomic M
+
-O distances shows that the K
+
ion is tetra-coordinated
(the K
+
-O bond distances range from 2.64 to 2.89 Å) in contrast with cesium, which
is hexa-coordinated (the Cs
+
-O bond distances range from 3.00 to 3.43 Å). It is, how-
ever, surprising that a small shortening of the Cs
+
-O distance is observed in spite of
an enlargement of the size of the crown. Therefore, these data, which show a relevant
difference in the coordination numbers of the two cations, suggesting an improved
selectivity for cesium over potassium, prompted the authors to synthesize two
calix[4]arene-propylene-crown-6 compounds. The binding properties of the two new
ligands, MC23 and MC24, with propylene units were determined by Cram's method
and compared to those of their counterparts with ethylene units. A strong decrease in
cation-binding properties of propylene calix[4]arene crown-6 is observed. This effect
is more significant for larger cations and with the dibenzo-crown-6 derivative. This
discrepancy is ascribed to the fact that effects due to solvation and interaction with
the counterion are not taken into account in the modeling.
63
4.3.6.2 thiacalix[4]arene
Another way to act on the selectivity of calix[4]arene-crown is to modify the size
of the cavity of calixarene by replacing methylene by sulfur bridges. Crystal struc-
tures and extraction data obtained with the thiacalix[4]arene-bis(crown-5) and the
thiacalix[4]arene-bis(crown-6) lead to the conclusion that these compounds are less
interesting than calix[4]arene-crown-6 for the extraction of alkali cations (Table 4.6).
In agreement with crystal structures, MD simulations showed that the thiacalix[4]
arene cavity size was approximately 0.05 nm larger than that of calix[4]arene. MD
simulations show that the cations are located close to the thiacalix[4]arene cavity
with the possibility of migration through the latter and that the crown does not
correctly fulfill its role of ligand.
64
Complexation studies confirm these extraction
results: the replacement of the bridging CH
2
groups of calix[4]arene-bis(crown-
n
) by
sulfur atoms of thiacalix[4]arene-bis(crown-
n
) leads to a strong decrease in complex-
ation levels of alkali metal ions, but does not affect the selectivity within the series of
crown ethers. No clear-cut conclusions about the possible interactions between these
cations and the sulfur atoms can be drawn.
65
In conclusion, calix[4]arene-bis(crown-6) and dialkoxy calix[4]arene crown-6,
and especially their dibenzo derivatives, which display an important hydrophobic-
ity, seem best suited for the extraction of cesium from very acidic media or media
containing large amounts of sodium. In 2006, Mohapatra confirmed the high effi-
ciency and selectivity of BC5 and BC10 for the extraction of cesium from simulated
acidic high-activity level waste. The Cs distribution ratio follows the trend of diluent
