J Incl Phenom Macrocycl Chem (2010) 66:223–230
229
Acknowledgements The authors would like to thank the Russian
Foundation for Basic Research (Grant 07-08-00246) and the Acad-
emy of Science of Finland for their financial support of the work. We
¨
also wish to express our gratitude to Paivi Joensuu for assistance in
sample analysis.
CsL in both molecular solvents and RTIL. At the same
time the corresponding entropy change for this solvent is
also the highest, diminishing the enthalpy contribution to
the log K1. The data listed in Table 2 obviously indicate,
that both cation and anion of RTIL affect the complex
formation stability and thermodynamic functions change.
This is not simply explained in terms of the solvation of
the cesium ion and 18C6. The tentative scheme of complex
formation in RTIL (5) is more complicated than that one in
molecular solvents [30, 31]. In general, both ions forming
the ionic liquid (its cation Z? and anion X-) may react
with cesium complex constituents. Z? is competing with
cesium for the ligand, while X- solvates cesium, resisting
complex formation:
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1ꢂnþp
CsXn1ꢂn þ ZmLmþ ꢀ CsLX
þ mZþ
ð5Þ
nꢂp
The NMR chemical shift data indicate that crown ether does
not substitute all RTIL anions X- in coordination sphere of
cesium in 1:1 complexes. This observation agrees well with
X-ray structural data for cesium complexes with 18-crown-
6 in solid state [32, 33] and aqueous solution [34], and with
classical molecular dynamics simulations [35]. In all these
structures cesium is located above mean oxygen plane of
the crown-ether ring since its size is larger than the cavity
size of 18C6 (170 and 130 pm respectively [24]).
Thus the exposed part of Cs? may strongly interact with
RTIL anions making coordination number equal to 8 or 9
as it is observed crystallographically for molecular
solvents.
On the other hand, a strong influence of the ionic liquid
cation, Z?, on log K1, DH and DS values of complex for-
mation is observed when one compares RTIL I and II,
indicating Z?-crown ether interactions of various intensity.
Such an interaction has also analogues among molecular
solvents. For example, for AN, nitromethane (NM) and
even for chloroform 18-crown-6 ꢀ 2AN, 18-crown-
6 ꢀ 2NM and 18-crown-6 ꢀ 2CH2Cl2 solvates have been
isolated and their structures have been determined by X-
Ray crystallography [36–39]. Hence, both cation and anion
of RTIL have an impact on the resultant stability constant
of chelated compound.
The stability constants for the formation of 18-crown-6
complexes in RTIL are all larger than in water. The reaction
thermodynamic quantities, however, differ much among the
solvents. The complexation is sufficiently more exothermic
in RTIL I to III than in water. On the other hand, the
reaction entropies are more negative than in water.
In conclusion, the complexation of 18-crown-6 with
cesium ion mainly reflects the different solvation of 18-
crown-6 and also the different degree of solvent structure.
In general, RTILs provide good promises of complex sta-
bility contribution in cesium extraction processes.
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