Chemistry Reference
In-Depth Information
six different acidic functions located over the crown cavity were synthesized. These
include a carboxylic acid group, four N-(
X
) sulfonyl carboxamide groups in which
the acidity can be “tuned” by variation of
X
, and a very weakly acidic fluorinated
alcohol group. For comparison of the proton-ionizable ligands with an analogous
nonionizable extractant (MC36-MC41) with
R
= H was also prepared.
For evaluation of the influence of proton-ionizable group positioning relative to
the crown cavity, extractant series (MC42, MC43) and (MC44, MC45) were realized.
In (MC42, MC43), the proton-ionizable group is located directly over the crown
cavity. In (MC44, MC45), the proton-ionizable group points away from the crown
cavity. For extractant series (MC42, MC43) and (MC44, MC45), the proton-ionizable
groups are of the N-(
X
) sulfonyl carboxamide variety with
X
= CH
3
and CF
3
to pro-
vide for a substantial acidity variation. For this comparison, the 2-ethylhexyl groups
were not needed, as the
n
-octyl chains confer sufficient solubility.
The proton-ionizable families (BC21-BC23) and (MC36-MC41) are not only
strong extractants for cesium under alkaline conditions, but they also possess a strik-
ing switching-off effect under acidic conditions;
D
Cs
swings as much as six orders
of magnitude between alkaline and acidic conditions. Under alkaline conditions,
the value of
D
Cs
increases to an expected plateau (except for
R
= CHCF
3
OH, whose
presumed plateau occurs at unreachable alkalinity), where the ionizable protons
are presumed to be quantitatively exchanged for sodium ions. At the plateau, the
effective extraction reaction is thus expected to be an exchange of sodium for more
strongly favored cesium, a pH-independent process. A constant plateau value of
approximately 200 is reached for all N-sulfonylcarboxamide compounds. It may be
seen that the pH effect is governed by the nature of the ionizable group. Among the
N-sulfonylcarboxamides tested, the strength of extraction at a given pH value below
the range of plateau values follows the order:
X
= trifluoromethyl >
p
-nitrophenyl
> methyl ~ phenyl. Results for the mono crowns in chloroform were similar to the
results of bis-crowns in toluene, though the magnitude of change in
D
Cs
values with
pH swing was not as large. Bis-crowns exhibited less change in
D
Cs
, as the curves
tend to level off at a higher value of
D
Cs
in the low-pH region, undoubtedly owing to
the fact that the second crown ring is capable of extracting cesium nitrate unencum-
bered by the presence of the ionizable group. The authors make the important con-
clusion that the pH-switching effect is enhanced by a blocking effect of the ionizable
group upon cesium binding. That is, the neutral ionizable group must make cesium
binding unfavorable by steric or hydrogen-bonding interactions.
71
4.3.8 P
H o t o S e n S i t i v e
C
a l i X a r e n e S
Often, decomplexation of cations from extractants is difficult when strong binding
ligands are used. As shown in this review, the binding ability and selectivity of most
macrocyclic compounds are mainly governed by the size and shape of the cavity.
34
Many systems are described in which changing the cavity shape by allosteric effects
allows the cation binding ability and the selectivity of the receptor to be modified and
controlled. For instance a photo-responsive
cis/trans
isomerizable azobenzene unit
has been introduced in macrocyclic structures in order to change the receptors cavity
shape, leading to a photo-control of ion extraction.
