Anal. Chem. 1999, 71, 672-677
Ex t ra c t io n S e le c t ivit ie s o f Cro w n Et h e rs fo r Alk a li
Me t a l Ca t io n s : Diffe re n c e s b e t w e e n
S in g le -S p e c ie s a n d Co m p e t it ive S o lve n t
Ex t ra c t io n s
†
†
†
†
Ga lina G. Ta la nova , Na za r S. A. Elka rim , Robe rt E. Ha ne s , J r., Hong-Sik Hw a ng,
‡
,†
Robin D. Roge rs , a nd Ric ha rd A. Ba rts c h*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, and Department of
Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336
different metal binding abilities, such as cyclohexano-CEs.3
Therefore, the capability of a CE for separating metal ion species
is most often assessed from metal ion distribution ratios deter-
mined in either single-species or competitive alkali metal salt
extractions.
It has often been assumed that selectivities based upon
extrapolations made from single-species extraction data will closely
resemble the competitive behavior. However, dramatic selectivity
changes between single-species and competitive processes have
been reported for closely related metal ion transport across liquid
membranes by macrocyclic ionophores.4 To the best of our
knowledge, comparable studies of selectivities in single-species
and competitive solvent extractions of alkali metal cations by CEs
have not appeared in the literature.
We now provide a systematic comparison of the separating
abilities of a wide variety of 18- to 24-membered benzo- and
cyclohexano-group-containing CEs 1 -1 2 (Chart 1) in single-
species and competitive extractions of alkali metal salts, mostly
picrates, from aqueous solutions into chloroform. In view of the
unexpected results for NaPic extractions with dibenzo-24-crown-8
Separation factor values for pairs of alkali metal cations
determined in competitive solvent extractions of alkali
metal picrates from aqueous solutions into chloroform by
a variety of benzo- and cyclohexano-group-containing
crown ethers vary significantly from extrapolations based
upon the results of single-species extraction experiments.
For almost all of the crown ether-alkali metal cation
combinations examined, the separation factor values are
greater for competitive solvent extraction. In view of the
unexpected results for sodium picrate extraction by
dibenzo-2 4 -crown-8 , the solid-state structure of the iso-
lated complex was determined.
Due to their unique capabilities for the selective complexation
of alkali metal cations, crown ethers (CEs) have been widely
1
utilized for the separation of these metal ion species. For potential
practical applications, the behavior of these macrocyclic iono-
phores in solvent extraction from aqueous solutions containing
mixtures of different alkali metal cations is important. Extraction
selectivities of CEs for alkali metal cations depend on the
compositions and stabilities of the complexes which they form
with the metal salts in the two-phase extraction systems. In the
simplest cases, when CEs bind only one metal cation per ligand
molecule, their separation efficiencies may be determined by
comparing the extraction constants values for different alkali metal
cations. However, the composition of the alkali metal complexes
formed by macrocyclic ligands may vary with the cation identity,
especially when the size of the metal ion is incompatible with that
(
3 ), the solid-state structure of the isolated complex was deter-
mined by X-ray diffraction.
EXPERIMENTAL SECTION
Reagents. Dibenzo-CEs 1 -3 and dicyclohexano-CEs (mixed
isomers) 5 and 6 are commercially available compounds. The
known5 unsym-dibenzo-18-crown-6 (4 ) and the polybenzo-CEs
7
-1 0 were obtained in improved yields as will be described
6
elsewhere. sym-Tricyclohexano-18-crown-6 (1 1 ) and sym-tricy-
clohexano-21-crown-7 (1 2 ), both as mixtures of isomers, were
synthesized by the procedure provided below. By a similar
procedure with the reaction carried out for 20 h at 65 °C, dibenzo-
2
of the CE cavity. This makes comparison of the corresponding
extraction constants impossible. Also, the determination of alkali
metal cation extraction constants for CEs may be difficult,
particularly when the ligands are mixtures of isomers that possess
2
1-crown-7 (2 ) was hydrogenated to give the corresponding
dicyclohexano-21-crown-7 (6 ) in 97% yield, which is appreciably
†
higher than that obtained by the reported procedure with RuO
Texas Tech University.
The University of Alabama.
2
‡
catalyst.5
(
1) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L., Chem. Rev., 1 9 9 5 ,
5, 2529-2586; Macrocyclic Compounds in Analytical Chemistry, Zolotov, Yu.
9
(3) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1 9 9 1 ,
91, 1721-2085.
(4) Christensen, J. L.; Lamb, J. D.; Brown, P. R.; Oscarson, J. L.; Izatt, R. M.
Sep. Sci. Technol. 1 9 8 1, 16, 1193-215; Strzelbicki, J.; Bartsch, R. A. J. Membr.
Sci. 1 9 8 2 , 10, 35.
(5) Pedersen, C. J. J. Am. Chem. Soc. 1 9 6 7 , 89, 7017-36.
(6) Hanes, R. E., Jr.; Lee, J. C.; Ivy, S. N.; Rodgers, R. D.; Bartsch, R. A.,
unpublished work.
A. Ed.; Wiley: New York, 1997; Moyer, B., in Comprehensive Supramolecular
Chemistry, volume ed. Gokel, G. W.; Pergamon: New York, 1996; vol.1, pp
3
77-416; de Jong, F.; Visser, H. C., in Comprehensive Supramolecular
Chemistry, volume ed. Reinhoudt, D. N.; Pergamon: New York, 1996; vol.
0, pp 13-52.
1
(
2) Hiraoka, M. Crown Compounds. Their Characteristics and Applications;
Elsevier: Amsterdam, Oxford, New York, 1982; Chapter 3.
672 Analytical Chemistry, Vol. 71, No. 3, February 1, 1999
10.1021/ac981068o CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/22/1998