Dideoxygenated calix[4]arene crown-6 ethers enhanced selectivity for caesium over potassium and rubidium

Article abstract of DOI:10.1039/a905682c

Calix[4]arene crown-6 ethers derived from dideoxygenated calix[4]arene exhibit enhanced extraction selectivity for caesium over potassium; the crystal stucture of the uncomplexed calix[4]arene monobenzocrown-6 ether exists in the 1,3-alt conformation in the solid state.

Full text of DOI:10.1039/a905682c

Dideoxygenated calix[4]arene crown-6 ethers enhanced selectivity for caesium
over potassium and rubidium
Richard A. Sachleben,*a Agathe Urvoas,a Jeffrey C. Bryan,a Tamara J. Haverlock,a Benjamin P. Hayb and
Bruce A. Moyera
a Oak Ridge National Laboratory, PO Box 2008, MS-6119/4500S, Oak Ridge, TN 37831, USA. E-mail: sik@ornl.gov
b Pacific Northwest National Laboratory, PO Box 999/MS KI-83, Richland, WA 99352, USA.
Received (in Columbia, MO, USA) 2nd December 1998, Revised 6th July 1999, Accepted 6th July 1999
Calix[4]arene crown-6 ethers derived from dideoxygenated
calix[4]arene exhibit enhanced extraction selectivity for
caesium over potassium; the crystal stucture of the un-
complexed calix[4]arene monobenzocrown-6 ether exists in
the 1,3-alt conformation in the solid state.
calix[4]arene 2a,15 which exhibit Cs+/Rb+ and Cs+/K+ selectiv-
ities significantly greater than that of comparable 1,3-dialkox-
ycalix[4]arene crown-6-ethers 4a–c.
Competitive extractions of alkali metal nitrates by the six
calix[4]arenes 3a–3c and 4a–c were performed essentially as
described previously,14 with the organic phase consisting of
0.025 M of calixarene crown ether in 1,2-dichloroethane and the
aqueous phase consisting of a mixture of alkali metal nitrates:
1.00 M LiNO3, 1.00 M NaNO3, 0.100 M KNO3, 0.0020 M
RbNO3 and 0.0011 M CsNO3. Metal ion concentrations were
determined for both phases, from which distribution ratios DM
= [M+]organic/[M+]aqueous were calculated directly. Caesium
was analyzed by g spectrometry, while lithium, sodium,
potassium and rubidium were analyzed, subsequent to stripping
of the organic phase with 1.0 mM HNO3, by inductively
coupled plasma atomic emission spectroscopy. From the data
given in Table 1, it can be seen that in each case where the
calixarene is changed from dioctyloxycalix[4]arene 4a–c to the
dihydrocalix[4]arene 3a–c the extraction strength for caesium,
expressed in terms of DCs, decreases by roughly an order of
magnitude. However, an even larger decrease is observed for
the extraction of the other alkali metals, to the point for K+ and
Rb+ where no extraction was measured. This is despite the fact
that the aqueous phase used in these experiments contained
significantly higher concentrations of Li+, Na+, K+ and Rb+ than
Cs+. As a consequence, selectivity for extraction of caesium (as
expressed by SCs/M = DCs/DM) increases. Since only upper
limits are available for the extraction of Rb+ and K+ by
calix[4]arene crown ethers 3a–c, the magnitude of the selectiv-
ity increase as compared to 4a–d can only be assigned a lower
limit. Even so, the observed trends provide evidence that
improved extraction selectivity for Cs+ can be obtained by
appropriate modification of the calixarene portion of calix[4]-
arene crown-6 ethers.
Calixarenes1 have been taking on an increasingly important role
in host–guest chemistry, in large part because they can provide
a well-organized platform for the attachment of pendant
functional groups.2,3 The incorporation of a calix[4]arene (1 in
Scheme 1) moiety into crown ethers4 has been used to develop
powerful, highly selective ionophores for alkali metal
cations,5,6 the properties of which are often dependent on the
conformation adopted by the calixarene. Whereas diethoxy-
calix[4]arene crown-4 ether in the cone conformation exhibits
high sodium selectivity,7 diethoxycalix[4]arene crown-5 ether
in the 1,3-alternate conformation exhibits selectivity for K+ over
Na+ exceeding that of Valinomycin.8 Furthermore, 1,3-alt
calix[4]arene crown-6 ethers exhibit high caesium binding9,10
with selectivities for Cs+/Na+ of ca. 104–105,9–11 a property
which has led to their study for the selective removal of caesium
from nuclear fuel reprocessing wastes.12,13 However, Cs/K
selectivities are only ca. 80–600, which is of concern since the
wastes may contain as much as 1 M K+ in a typical matrix
containing 5–7 M Na+, but only 1026–1023 M Cs.14 Having
undertaken an effort to develop extractants with improved Cs/K
selectivity, we have synthesized three new calix[4]arene crown-
6-ethers (3a–c in Scheme 1)† derived from di-dehydroxylated
Table 1 Calix[4]arene crown-6 ethers used in this study, their distribution
ratios (DM) for the extraction of alkali metal nitrates from water to
1,2-dichloroethane at 25 °C, and the corresponding selectivities (S) for Cs+
ions over Li+, Na+, K+ and Rb+ ions a
DRb
DK
DNa/1025 DLi/1025
Extractant DCs
(SCs/Rb
)
(SCs/K
)
(SCs/Na
)
(SCs/Li)
b
b
3a
3b
3c
4a
4b
4c
a
0.403
—
—
2.17
1.90
( > 58)
( > 4200)
(23000)
(32000)
b
b
b
b
0.383
0.116
3.65
—
—
—
—
( > 54)
( > 4000)
( > 150000) ( > 660000)
b
b
b
b
—
—
—
—
( > 17)
0.180
(20)
0.214
(19)
( > 1200)
0.0152
(240)
0.0141
(290)
( > 47000) ( > 200000)
22.0
(17000)
3.9
2.98
(120000)
0.99
( > 450000)
0.97
4.04
(100000)
b
1.61
0.178
(9.0)
0.0125
(130)
—
( > 640000) (170000)
b
All values are the average of two determinations. Lower limits on
measurement of distribution ratios: DLi = 5.8 3 1027, DNa = 2.5 3 1026
DK = 9.6 3 1025, DRb = 0.0070.
Scheme 1 Reagents and conditions: i, (EtO)2PHO, CCl4, Et3N, toluene,
0 °C, then K, NH3 (liquid); ii, (TsOCH2CH2O-Z-O)2Y, Cs2CO3, MeCN; iii,
C8H17I, K2CO3, MeCN, then (TsOCH2CH2O-Z-O)2Y, Cs2CO3, MeCN.
,
Chem. Commun., 1999, 1751–1752
1751

Environmental Management, U.S. Department of Energy,
under contract DE-AC05-960R22464 with Oak Ridge National
Laboratory, managed by Lockheed Martin Energy Research
Corporation.
Notes and references
† 1H NMR and 13C NMR data were consistent with the assigned structures
for all new compounds.
‡ Crystal data for 5b: C42H42O6, 642.8 g mol21, triclinic, a = 10.634(3),
b = 17.432(4), c = 19.578(4) Å, a = 70.959(17), b = 89.846(17), g =
¯
77.52(2)°, V = 3340.6(13) Å3, T = 173(2) K, P1, Z = 4, m = 0.084 mm21
,
refinement of F2, final wR2 = 0.117, on all 8182 independent reflections.
CCDC 182/1353. See http://www.rsc.org/suppdata/cc/1999/1751 for crys-
tallographic data in .cif format.
Fig. 1 ORTEP drawing (50% probability elipsoids) of 5b. For clarity only
one molecule from the asymmetric unit is displayed, hydrogen atoms are
omitted, and only oxygen atoms are labeled. The minor disorder component
(O11A) is represented with a dashed boundary ellipsoid.
1 C. David Gutsche, Calixarenes, Monographs in Supramolecular
Chemistry, ed. J. F. Stoddard, The Royal Society of Chemistry,
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Since previous studies have shown that the 1,3-alternate
conformation is preferred for high caesium binding and
selectivity9 and that calixarenes bearing substituents smaller
than ethoxy are conformationally mobile,16 we considered the
conformational preferences of these new compounds. The
crystal structure of 3b (Fig. 1)‡ reveals that this compound
crystallizes with two molecules in the crystallographic asym-
metric unit, with the calix[4]arene portions of both molecules
clearly in the 1,3-alt conformation. The calix[4]arene crown-
6-ethers 3a–c exhibit room temperature 1H NMR (CDCl)3
where the Ar-CH2-Ar resonances of the calixarene appear as a
pair of doublets at d ~ 4.2 and ~ 3.7 with a coupling constant of
~ 15.5 Hz, while the intraannular proton on the deoxygenated
aromatic ring is found upfield at d ~ 6.0. Grynszpan and Biali17
interpreted the upfield shift of the corresponding proton in 2b in
terms of shielding by the two phenolic rings in the 1,3-alt
conformation, whereas the positions and coupling constants of
the calixarene methylenes are more consistent with the report by
Ting et al.15 where the ‘unsubstituted phenyl rings can rotate
freely’. The calixarene methylene carbon of 3b was observed at
d 35.3, similar to the values reported to be observed for the
1,3-alternate conformers in conformationally immobile calix-
[4]arenes.18 We performed VT-NMR on 3b in CDCl3 and
observed a decrease in the separation between the pair of
doublets assigned to the calixarene methylene resonances, from
216 to 187 Hz (Dd = 29 Hz), when the temperature was
decreased from 340–220 K, consistent with rapid conforma-
tional interconversion which shows upon lowering the tem-
perature. Conversely, in toluene-d8, the separation between this
pair of doublets decreased, from 340 to 298 Hz (Dd = 42 Hz),
when the temperature was increased from 170 to 370 K,
suggesting a slow conformational interconversion which speeds
up with increasing temperature. However, coalescence was not
achieved even when a solution of 3b was heated to 500 K in
nitrobenzene-d5. In (CD2Cl)2, Dd ≈ 0 over the temperature
range 240–340 K. Consequently, the solution conformation
cannot be assigned to the calixarene portion of 3b based on
these results.
11 V. Lamare, J.-F. Dozol, F. Ugozzoli, A. Casnati and R. Ungaro, Eur. J.
Org. Chem., 1998, 8, 1559; J. F. Dozol, N. Simon, V. Lamare, H.
Rouquette, S. Eymard, B. Tournois, D. DeMarc and R.-M. Macias, Sep.
Sci. Technol., in the press.
12 J.-F. Dozol, Z. Asfari, C. Hill and J. Vicens, FR 2698362 (Chem. Abstr.,
1994, 121, 189685); J.-F. Dozol, H. Rouquette, R. Ungaro and A.
Casnati, WO 9424138 (Chem. Abstr., 1995, 122, 239730).
13 B. A. Moyer, P. V. Bonnesen, R. A. Sachleben and D. J. Presley, WO
9912878 (Chem. Abstr., 1999, 130, 214967).
14 T. J. Haverlock, P. V. Bonnesen, R. A. Sachleben and B. A. Moyer,
Radiochim. Acta, 1997, 76, 103.
15 Compound 2a was prepared in the manner previously reported for 2b:
Y. Ting, W. Verboom, L. C. Groenen, J.-D. van Loon and D. N.
Reinhoudt, J. Chem. Soc., Chem. Commun., 1990, 1432; F. Grynszpan,
Z. Goren and S. E. Biali, J. Org. Chem., 1991, 56, 532.
16 C. D. Gutsche, D. Dhawan, J. A. Levine, K. H. No and L. J. Bauer,
Tetrahedron, 1983, 39, 409.
17 F. Grynszpan and S. Biali, Tetrahedron Lett., 1991, 32, 5155.
18 C. Jaime, J. de Mendoza, P. Prados, P. M. Nieto and C. Sanchez, J. Org.
Chem., 1991, 56, 3372. These authors report that the 13C chemical shifts
are relatively insensitive to the substituents at the 1 and 4 positions of the
aromatic rings.
The results reported here have significant ramifications, not
only in the effort to develop more highly selective extractants,
but also in understanding the conformational and electronic
factors which contribute to the remarkable ionophoric proper-
ties of calixarenes and calix[4]arene crown ethers. Efforts in this
area are the subject of ongoing investigations.
This research was sponsored by the Environmental Manage-
ment Science Program, Offices of Energy Research and
Communication 9/05682C
1752
Chem. Commun., 1999, 1751–1752