Chemistry Reference
In-Depth Information
BC20. Stripping under acidic conditions gives approximately the same value of
D
Cs
as extraction under nitric acid conditions, confirming that back-extraction is
enhanced. A combination of high extraction ability under basic conditions and high
stripping efficiency under acidic conditions make compound BC20
an attractive
extractant candidate. For BC7, at low concentrations of cesium, the stoichiometry of
the calix:cesium complex is 1:1. A further loading of BC7 can be achieved to give a
calix:cesium ratio of 1:2, in agreement with the number of crown rings available to
complex cesium. In contrast, the stoichiometry between BC20 and cesium remained
unchanged, suggesting that one of the two crown cavities of this molecule is unsuit-
able for binding cesium, evidence that the nonfunctionalized crown ring contains the
cesium. The implication is thus that positioning the methylamino group on one of
the phenyl groups on the calixarene “belt” destabilizes cesium binding, although the
amino group itself is ordinarily capable of acting as an electron-pair donor.
68-71
The goal of further work was to evaluate the role of the amino group of amino
methyl calix[4]arene-[bis-4-(2-ethylhexyl)benzo-crown-6] (BC20) in the extraction
of cesium from acidic and basic mixtures of sodium nitrate and other concentrated
salts. The extraction of cesium from nitrate media was measured as a function of
extractant concentration, nitrate concentration, cesium concentration, and pH over the
range 1-13. Rather than the nitrobenzene diluent used in previous studies, an alcohol-
modified alkane was employed. The initial studies showed a moderate decrease in the
extraction of cesium in acidic media, which indicated the binding of cesium by the
calixarene-crown was weakened by the protonation of the amine group. The results
also indicated that a 1:1:1 Cs-ligand-nitrate complex is formed in the organic phase.
The formation constants of the complexes formed in the organic phase computed
from empirical data showed that the attachment of the amine group to the calixarene-
crown molecule reduced the binding stability for the cesium ion upon contact with
an acidic solution. The small magnitude of the charge-charge repulsion effect likely
implies that the cesium binding in BC20 occurs in the cavity opposite that of the pen-
dent amino group, such that the positive charges are not in close proximity.
72
A series of calix[4]arene-bis(crown-6) ligands BC21, BC22, and BC23 and three
series of calix[4]arene-monocrown-6 ligands (MC36-MC41), (MC42-MC43), and
(MC44-MC45) have been synthesized.
70
The lipophilic calix[4]arene-bis(crown-6)
extractant series BC21, BC22, and BC23 has the same general structure as nonion-
izable BC6 with the exception that the lipophilic groups are 2-ethylhexyl instead
of
tert
-octyl and the presence over the face of one crown ring of a proton-ionizable
group. In addition to a carboxylic acid group, the ionizable groups include two
N-(
X
) sulfonyl carboxamide groups in which the acidity of the function is “tuned”
by varying the electron withdrawing ability of
X
. The change from
X
= CH
3
to CF
3
is expected to increase the ligand acidity by about three p
K
a units. Since calix[4]
arene-biscrown-6 extractants have two crown units, there is a possibility that in addi-
tion to the complexation of cesium ion by one crown unit, the second crown unit
could complex cesium, thereby escaping the switching mechanism. Additionally,
the possibility of sodium or potassium ion binding by the second crown unit could
reduce the cesium extraction selectivity. To eliminate these unwanted effects, the
lipophilic calix[4]arene-monocrown-6 series (MC36-MC41) was prepared. To allow
the effect of varying the proton-ionizable group's acidity to be assessed, ligands with
