Molecular baskets in supercritical CO2

Article abstract of DOI:10.1021/ac960850q

Calixarenes are synthetic macrocyclic compounds, described as "molecular baskets" as they possess high ionophoric selectivity and form inclusion complexes with many important ionic guests. In our initial work, hexameric and tetrameric tert-butylcalixarenes, unfunctionalized at the lower rim, are shown to be separable on a diol column using supercritical fluid chromatography with methanol/chloroform-modified CO₂ as mobile phase. The variation in capacity factors for these calixarenes was studied as a function of modifier composition. However, the solubility of these molecular baskets in unmodified supercritical CO₂ is enhanced by fluorination at the upper rim. For example, when p-allylcalix[4]arene is derivatized by a thiol-ene addition reaction with heptadecafluorodecanethiol, CF₃(CF₂)₇(CH₂)₂SH, a solubility of >0.12 mol L^-1 in supercritical CO₂ is measured for the p-heptadecafluorodecylthio-n-propylcalix[4]arene at 60 °C and 200 atm. However, subsequent lower rim functionalization to form the tetrahydroxamate derivative, while reducing the solubility, allows supercritical fluid extraction of Fe(III) by the fluorinated calix[4]arene ligands to be studied as a function of temperature and pressure and monitored using UV/visible and atomic absorption spectrometry. In particular, the visible absorption spectra obtained for the extracted Fe(III)-calix[4]arene tetrahydroxamate complex, collected in dimethyl sulfoxide, are indicative of octahedral Fe(III) complexation in a manner similar to that displayed by water-soluble siderophores. Studies on the efficiency and selectivity of Fe(III) extraction are also reported.

Full text of DOI:10.1021/ac960850q

Anal. Chem. 1997, 69, 2207-2212
Mo le c u la r Ba s k e t s in S u p e rc rit ic a l CO2
J e re m y D. Gle nnon,*,†,‡ Sha ron Hutc hins on, Ste phe n J . Ha rris , Andre w Wa lke r,
†
†
†
§
†
M. Anthony Mc Ke rve y, a nd Conor C. Mc Sw e e ne y
Department of Chemistry, University College Cork, Cork, Ireland, and School of Chemistry, Queen’s University,
Belfast, N. Ireland
Calixarenes are synthetic macrocyclic compounds, de-
scribed as “molecular baskets” as they possess high
ionophoric selectivity and form inclusion complexes with
many important ionic guests. In our initial work, hex-
americ and tetrameric ter t-butylcalixarenes, unfunction-
alized at the lower rim, are shown to be separable on a
diol column using supercritical fluid chromatography with
2
methanol/ chloroform-modified CO as mobile phase. The
variation in capacity factors for these calixarenes was
studied as a function of modifier composition. However,
the solubility of these molecular baskets in unmodified
supercritical CO is enhanced by fluorination at the upper
2
rim. For example, when p-allylcalix[4]arene is derivatized
by a thiol-ene addition reaction with heptadecafluorode-
canethiol, CF
3
(CF
2
)
7
(CH
2
)
2
SH, a solubility of >0 .1 2 mol
-
1
L
in supercritical CO
2
is measured for the p-heptade-
cafluorodecylthio-n -propylcalix[4 ]arene at 6 0 °C and 2 0 0
atm. However, subsequent lower rim functionalization to
form the tetrahydroxamate derivative, while reducing the
solubility, allows supercritical fluid extraction of Fe(III)
by the fluorinated calix[4 ]arene ligands to be studied as
a function of temperature and pressure and monitored
using UV/ visible and atomic absorption spectrometry. In
particular, the visible absorption spectra obtained for the
extracted Fe(III)-calix[4 ]arene tetrahydroxamate com-
plex, collected in dimethyl sulfoxide, are indicative of
octahedral Fe(III) complexation in a manner similar to
that displayed by water-soluble siderophores. Studies on
the efficiency and selectivity of Fe(III) extraction are also
reported.
Fig u re 1 . Chemical structures of the macrocyclic calixarenes:
p-tert-butylcalix[4]arene (C1); p-tert-butylcalix[6]arene (C2); p-hepta-
decafluorodecylthio-n-propylcalix[4]arene (C3); p-tert-butylcalix[4]-
t
arene tetraethyl acetate (C4 ); p-heptadecafluorodecylthio-n-propylcalix-
[4]arene tetraethyl acetate (C4); p-tert-butylcalix[4]arene tetrahydroxamic
t
acid (C5 ); p-heptadecafluorodecylthio-n-propylcalix[4]arenetetra-
hydroxamic acid (C5).
With the continuing search for new synthetic molecular
receptors capable of host-guest relationships with ions and
neutral molecules, calixarenes have become attractive compounds
for attempting to construct in vitro systems that mimic the in vivo
catalytic activity of enzymes.3 Shinkai et al. showed that selected
ligand groups arranged on the lower rim of the calixarene cavity
yielded novel binding sites for transition metal cations. Some
chemically modified calixarenes have been reported to be effective
ionophores in ion-selective electrodes5,6 and in a CHEMFET for
the analysis of sodium and caesium. A primary amine-selective
polymeric membrane electrode based on a calix[6]arene iono-
phore has been reported.8 In this laboratory, the solid-phase
extraction of transition metal ions has been demonstrated and
applied in ion chromatography using hydroxamate dextran-coated
4
Calixarenes are synthetic macrocyclic compounds, described
as “molecular baskets” as they possess high ionophoric selectivity
and form inclusion complexes with many ions. Calixarenes are
the macrocyclic products of phenol-formaldehyde condensations
and have been shown to act as efficient extractants of alkali metals
when modified at the phenolic oxygen atoms.1,2 The basic
structure of a calixarene can be seen from Figure 1, with n ) 1
calix[4]arene (a tetramer), n ) 2 (a pentamer), and n ) 3 (a
hexamer). Positions for functionalization are available at the upper
rim, for example, R ) H or tert-butyl group and at the lower rim
or phenolic positions, R′.
7
(
3) Gutsche, C. D. Calixarenes, Monographs in Supramolecular Chemistry; Royal
Society of Chemistry: Cambridge, U.K., 1989.
(4) (a) Shinkai, S.; Shiramama, Y.; Satoh, H.; Manabe, O.; Arimura, T.; Fujimoto,
K.; Matsuda, T. J. Chem. Soc., Perkin Trans. 2, 1 9 8 9 , 1167-1171. (b)
Shinkai, S.; Kawabata, H.; Arimura, T.; Matsuda, T.; Satoh, H.; Manabe, O.
J. Chem. Soc., Perkin Trans. I, 1 9 8 9 , 1073-1074. (c) Shinkai, S.; Otsuka,
T.; Arake, K.; Matsuda, T. Bull. Chem. Soc. Jpn. 1 9 8 9 , 62, 4055-4057.
(5) Cadogan, A.; Diamond, D.; Smyth, M. R.; Svehla, G.; McKervey, M. A.;
Seward, E. M.; Harris, S. J. Analyst, 1 9 9 0 , 225, 1207.
(6) Kimura, K.; Miura, T.; Matsuo, M.; Shona, T. Anal. Chem. 1 9 9 0 , 62, 110.
(7) Brunink, J.; Haak, J. R.; Bomer, J. G.; Reinhoudt, D. N.; McKervey, M. A.;
Harris, S. J. Anal. Chim. Acta. 1 9 9 1 , 254, 75.
†
University College Cork.
This paper is dedicated to John T. Glennon, a great motivator and father.
Queen’s University.
‡
§
(
(
1) Schwing-Weill, M. J.; McKervey, M. A. In Calixarenes:A Versatile Class of
Macrocyclic Compounds; Vincens, J., Bohmer, V., Eds.; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1991.
2) Arnaud-Neu, F.; Collins, E. M.; Deasy, M.; Ferguson, G.; Harris, S. J.; Kaitner,
B.; Lough, A. J.; McKervey, M. A.; Marques, E.; Ruhl, B. L.; Schwing-Weill,
M. J.; Seward, E. M. J. Am. Chem. Soc. 1989, 111, 8681.
(8) Chan, W. H.; Shiu, K. K.; Gu, X. H. Analyst 1 9 9 3 , 118, 863-867.
S0003-2700(96)00850-5 CCC: $14.00 © 1997 American Chemical Society
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997 2207

9
10
Fe3+, as the guest in a “host-guest” cavity, using unmodified SF-
CO containing a new fluorinated calix[4]arene tetrahydroxamic
acid. The extraction of other metal ions using this ligand is also
studied. The solubilities of the fluorinated and nonfluorinated
macrocyclic ligands are studied at different pressures and tem-
peratures.
silicas and immobilized calixarene hydroxamates. Hydroxamic
acids are known to form stable chelates with a large number of
metal ions11 and are well-known for their biochelation activity, as
they constitute the strong complexing donor atoms in many
microbial siderophores.12 A silica-bonded tetrameric calix[4]arene
ester stationary phase has also been used in the chromatographic
separation of amino acid esters.13 Using a silica-bonded calix[4]-
arene tetradiethylamide, Glennon et al.14 were able to selectively
2
EXPERIMENTAL SECTION
+
retain Na ions over other alkali metal ions. Brindle et al. recently
Reagents. Tetrameric and hexameric tert-butylcalixarene
derivatives were initially chosen for study. p-tert-butyl-substituted
reported the synthesis, characterization, and chromatography of
silica-bonded calixarenes,15 while Gebauer and co-workers16
showed that calix[4]arene chemically bonded to silica gel behaves
predominantly as a reversed-phase material. There have been
few reports on the use of calixarenes in chromatography apart
from these. Thin-layer chromatography (TLC) and high-pressure
liquid chromatography (HPLC) have been used for the analysis
of reaction mixtures of calixarenes, with examples of the latter
using normal- and reversed-phase chromatography.17 Recently
calixarenes were shown to modify selectivities in capillary elec-
trophoresis.18
A considerable amount of research has been reported on the
synthetic macrocyclic compounds, the calixarenes, but nothing
has been published in the area of supercritical fluid chromatog-
raphy (SFC) or supercritical fluid extraction (SFE). Extensive
work has been done in the field of SFE and SFC using â-diketones
t
t
derivatives of calix[4]arene (C1 , C4 , C5 ) and calix[6]arene (C2 )
were synthesised according to previously published procedures2,10
(
Figure 1). The calixarene solutions for injection into the SFC
system were made up in chromatographic grade chloroform
Merck, Darmstadt, Germany) in the concentration range 10-2
(
-
0-4 M. New calix[4]arenes, fluorinated at the upper rim, were
1
synthesized as described below for SFE applications.
3 2 7
P reparation of Heptadecafluorodecanethiol, (CF (CF ) -
(
CH
2
)
2
SH). A 50.00 g (0.087 mol) aliquot of 1,1,1,2,2,3,3,4,4,-
5
,5,6,6,7,7,8,8-heptadecafluoro-10-iododecane (Aldrich 37052-5)
was added to 6.62 g (0.087 mol) of thiourea in 200 mL of ethanol
and the resultant mixture refluxed for 3 days. The resulting
isothiourea iodide was converted to its mercaptan by addition of
3
5
.23 g (0.131 mol) of sodium hydroxide and refluxed for 2 h. The
ethanol was removed, and the product was distilled as a colorless
oil under reduced pressure using a water pump; 21.74 g (52%
yield) of the product was isolated: 1H NMR (270 MHz) δ 1.67
1
9-22
and dithiocarbamates as chelating agents.
However, the
research published on macrocyclic compounds as chelating agents
in SFE and SFC is limited. Wai and co-workers23 used super-
critical fluids together with the macrocyclic, tert-butyl-substituted
dibenzobistriazolo crown ether to quantitatively extract Hg2 ions
from sand and cellulose-based filter papers. Similar to calixarenes,
crown ethers are selective ligands that form stable complexes with
metal ions based on the ionic radius-cavity size compatibility
concept, unlike the dithiocarbamates and â-diketones, which are
less selective agents and complex with a number of metals and
non-metals. Wai found that the crown ethers have very low
(
1H, t, SH), δ 2.25-2.36 (2H, m, CH
CH CH SH). Anal. Calcd: C, 25.01%; H, 1.05. Found: C, 25.40%;
H, 1.46.
2
SH), δ 2.72-2.79 (2H, m,
2
2
+
P reparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Hep-
tadecafluoro(1 0 -thiodecyl)-n -propylcalix[4 ]arene (C3 ). To
.5 g (0.002 57 mol) of p-allylcalix[4]arene,24a were added 4.93 g
0.0103 mol) of CF (CF (CH SH (prepared above) in 10 mL
of dry CHCl and 120 mg of azoisobutyronitrile (AIBN, free-radical
1
(
3
2
)
7
2 2
)
3
thiol ene addition catalyst). The mixture was refluxed for 4 h
under nitrogen and after a further addition of 120 mg of AIBN,
the mixture was stirred under reflux for a further 4 h. The pale
yellow solution was filtered and concentrated in vacuo to afford
C3 as a pale yellow solid. The reaction was monitored by the
disappearance of the IR band at 1636.4 cm-1, which is character-
2
solubilities in pure SF-CO ; however with the introduction of 5%
methanol, the solubility increased 1 order of magnitude. Other
metal ions, Cd2+, Co2+, Mn2+, Ni2+, Pb2+, Au3+, and Zn2+, were
extracted from filter paper under the same conditions as for Hg2+.
This paper reports the SFC and SFE of the hexameric and
tetrameric p-tert-butylcalixarenes and demonstrates extraction of
istic of the allyl group. Purification using flash chromatography
(
9) Ryan, N.; Glennon, J. D.; Muller, D. Anal. Chim. Acta 1 9 9 3 , 283, 344.
on silica gel with methylene chloride/ hexane (1:2) as eluent
(
10) Hutchinson, S.; Kearney, G. A.; Horne, E.; Lynch, B.; Glennon, J. D.;
McKervey, M. A.; Harris, S. J. Anal. Chim. Acta 1 9 9 4 , 291, 269-275.
11) Agrawal, Y. K.; Patel, S. A. Anal. Chem. 1 9 8 0 , 52, 327.
yielded C3 as a white solid (4.25 g, 66% yield): 1H NMR (270
MHz) δ 1.79-1.87 (8H, m, SCH
SCH CH CH ), 2.45-2.54, (16H, m, SCH
CF ), 2.68-2.74 (8H, m, SCH CH (CF
Ar, H ), 4.22 (4H, d, ArCH AR,H
s, OH). Anal. Calcd: C, 38.35%; H, 2.41. Found: C, 38.54%; H,
.66.
P reparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Hep-
2 2 2
CH CH ), δ 2.25-2.38 (8H, m,
(
(
12) Glennon, J. D.; Woulfe, M. R.; Senior, A. T.; NiChoileain, N. Anal. Chem.
2
2
2
2
CH
2
CH
2
and SCH
2
CH
2
-
1
9 8 9 , 61, 1474-1478.
(
2
)
7
2
2
2 7
) ), 3.44 (4H, d, ArCH
2
-
(
(
(
(
13) Glennon, J. D.; Horne, E.; Kearney, G. A.; Harris, S. J.; McKervey, M. A.
b
2
a
), 6.84, (8H, s, ArH), 10‚18 (4H,
Anal. Proc. 1 9 9 4 , 31 (Jan), 33-35.
14) Glennon, J. D.; Horne, E.; Hall, K.; Cocker, D.; Kuhn, A.; Harris, S. J.;
McKervey, M. A. J. Chromatog., A 1 9 9 6 , 731, 47-55.
15) Brindle, R.; Albert, K.; Harris, S.; Tr o¨ ltzsch, C.; Horne, E.; Glennon, J. D. J.
Chromatog., A 1 9 9 6 , 731, 41-46.
2
16) Friebe, S.; Gebauer, S.; Krauss, G. J.; Goemar, G.; Kruger, J. J. Chromatogr.
Sci., 1 9 9 5 , 33, 281.
17) Berger, T. A. J. Chromatog. 1 9 8 9 , 478, 311-324.
18) Shohat, D.; Grushka, E. Anal. Chem. 1 9 9 4 , 66, 747-750.
19) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1 9 9 4 ,
tadecafluoro(1 0 -thiodecyl)-n -propylcalix[4 ]arene Tetraethyl
Ester (C4 ). A mixture of 0.8 g (0.000 862 mols) of p-allylcalix-
(
(
(
[
4]arene tetraethyl ester,24b 1.65 g (0.00345 mols) of CF
3 2 7
(CF ) -
(CH SH, and 80 mg of AIBN in 10 mL of dry benzene were
2 2
)
2
8, 1190-1193.
20) Laintz, K. E.; Wai, C. M.; Yonker; C. R.; Smith, R. D. Anal. Chem. 1 9 9 2 ,
4, 2875-2878.
21) Jahn, K. R.; Wenclawiak, B. W. Fresenius Z. Anal. Chem. 1 9 8 8 , 330, 243-
45.
refluxed under an inert atmosphere for 4 h. After this time, a
further portion of AIBN (80 mg) was added to the solution, and
(
(
6
2
(24) (a) Gutsche, C. D.; Levine, J. A. J. Am. Chem. Soc. 1 9 8 2 , 104, 2652. (b)
Harris, S. J.; Woods, J. G.; Rooney, J. M. U.S. Patent 4642362, February 10,
1987.
(
(
22) Laintz, K. E.; Yu, J.-J.; Wai, C. M. Anal. Chem. 1 9 9 2 , 64, 311-315.
23) Wang, S.; Elshani, S.; Wai, C. M. Anal. Chem. 1 9 9 5 , 67, 919-923.
2208 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997

the reaction contents were refluxed for an additional 4 h. The
yellow solution was filtered under gravity and concentrated in
vacuo to afford a pale yellow waxy solid. Purification via methanol
trituration at room temperature yielded C4 as a white solid (1.75
g, 71% yield): Anal. Calcd: C, 40.46%; H, 2.97. Found: C, 40.80;
H, 3.12.
P reparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Hep-
ta d e c a flu o r o (1 0 -th io d e c yl)-n -p r o p ylc a lix[4 ]a r e n e -
tetrahydroxamic Acid (C5 ). To a cooled solution of 1.0 g
restrictor. Samples were loaded into an open-ended glass tube
(3.0 × 0.5 cm i.d.), packed with glass wool at both ends and
mounted inside a stainless steel cartridge (5.5 × 0.76 cm i.d.,
volume 2.5 mL). Extracted samples were collected in a liquid
trap containing chloroform or dimethyl sulfoxide (DMSO). A
variable heated restrictor, set at a temperature 5 °C below the
oven temperature, was used in these studies. Pressure was varied
in the range 200-400 atm, and the temperature of the extractor
was set between 60 and 120 °C. SFE extracts were analyzed using
SFC, spectrophotometry, AAS, and NMR, where appropriate.
(
(
0.00 108 mol) of p-allylcalix[4]arene tetraethyl ester and 1.2 g
0.0172 mol) of hydroxylamine hydrochloride (BDH, Poole,
2
Unmodified SF-CO was used for all the SFE extractions.
Dorset, U.K.) in a mixture of 40 mL of methanol and 20 mL of
THF was added a solution of 1.21 g (0.0216 mol) of KOH in a
mixture of 12 mL of methanol and 6 mL of THF dropwise at -5
Solubility Measurements. Calixarene samples (50-500 mg)
were loaded into a glass tube (5.4 × 0.5 cm i.d.), plugged with
glass wool at both ends of the tube, and weighed. The glass tube
was inserted into the stainless steel extraction vessel, tightened
into the heating block, and statically extracted for 30 min at
different temperatures and pressures. The remaining cell volume
was determined to be 1.8 mL. The extraction cell was then vented
into a collection vessel containing 3 mL of chloroform or DMSO.
The glass tube was removed from the extraction cell and weighed.
The loss in weight corresponded to the amount extracted by 1.8
°
C. After stirring for 5 h at -5 °C and then 4 days at room
temperature, all volatiles were removed in vacuo. Dilute acetic
acid was added to the residue and the p-allylcalix[4]arene-
tetrahydroxamic acid precipitate was isolated by filtration as an
off-white solid (0.70 g, 74% yield), which was fluorinated without
further purification. A solution of 0.7 g (0.000 799 mol) of
p-allylcalix[4]arene tetrahydroxamic acid in 10 mL of dry chloro-
form containing 1.53 g (0.0032 mol) of CF
3
(CF
2
)
7
(CH
2
)
2
SH and
mL of CO
2
.
1
05 mg of AIBN was refluxed for 4 h under nitrogen. After this
Supercritical Fluid Extraction of Fe(III). For the metal
extraction experiments, 50 µL of Fe(III) nitrate solution in
methanol [0.05 mg of Fe(III)] was spiked onto filter paper (2 × 1
cm). The filter paper was allowed to dry and was then spiked
with 40 µL of deionized water. The wet filter paper along with a
set quantity of the fluorinated calixarene (C3 or C5 , in the range
20-40 mg) was loaded into a glass tube (3.0 × 0.5 cm i.d.),
plugged with glass wool at both ends and also between the filter
paper and the calixarene powder. The glass tube was again
mounted inside the stainless steel extraction vessel and statically
time, a further portion of 105 mg of AIBN was added and the
solution refluxed for an additional 4 h. The yellow solution was
filtered and concentrated in vacuo to give C5 as a yellow solid,
which was purified by successive precipitation from methylene
chloride/ hexane (1:1) and then chloroform to isolate a white solid
(
1.22 g, 55% yield): IR ν 3258 (m, NHOH); 1669 cm-1, (s, CdO).
Anal. Calcd C, 37.78; H, 2.59; N, 2.00. Found: C, 38.98; H, 2.99;
N ) 2.51%.
Instrumentation. Supercritical Fluid Chromatography. All the
chromatography was performed using an SF3 supercritical fluid
chromatographic system (Gilson Medical Electronics, supplied
by Anachem, Luton, UK). The SFC system was microprocessor
controlled, allowing independent programming of mobile phase,
pressure, composition, and flow rate, and consisted of a cooling
extracted using unmodified SF-CO
2
for 20 min, initially at 300 atm.
Extractions were carried out at temperatures between 60 and 120
°C. The extraction cell was vented into a collection vessel
containing 8 mL of chloroform (or 8 mL of DMSO using C5 ) for
15 min (dynamic extraction). Similarly, a pressure variation study
was carried out between 200 and 400 atm, while the temperature
was kept constant at 60 °C. UV/ visible spectra of the extracted
samples were recorded in the collecting solvent, using a 1 cm
cuvette and a Shimadzu UV 260 spectrophotometer. AAS analysis
of extracted samples collected in chloroform was carried out by
solvent evaporation to dryness followed by acid digestion using
concentrated nitric acid at temperatures just below boiling. After
5-6 h digestion, the calixarene material disappeared and the clear
solutions were cooled to room temperature. The digested solu-
tions were transferred to volumetric flasks and diluted to 10 mL
for aspiration into a Perkin-Elmer 2380 atomic absorption spec-
trometer.
2
jacket on the CO pump head, a pressurized cell, an absorbance
detector, a pressure regulation valve and a pressure relief valve
in the column oven. The detection of peaks was carried out using
a Dynamax UV-1 UV/ visible variable-wavelength detector (Rainin
Instrument Co. Inc., supplied by Anachem) with the wavelength
set at 280 nm. Injections were made with a Rheodyne (Cotati,
CA, supplied by Anachem) Model 7125 injector with a 10 µL loop.
A 24.5 × 0.46 cm i.d. LiChrospher 100 DIOL (5 µm) column
(
2
Merck) was used with CO modified with methanol/ chloroform
as the mobile phase. The chromatographic data were recorded
on a Phillips Model PM 8241 single-pen recorder. Pressure, flow
rate, and percent modifier conditions for the analysis were
microprocessor controlled. Temperature was manually set using
the oven temperature controls. Methanol/ chloroform was added
as modifier using a Gilson 5SC 306 pump. The carbon dioxide
cylinder with dip tube was obtained from Irish Oxygen (Cork,
Ireland).
Supercritical Fluid Extraction. All extractions were performed
using an Isco SFX supercritical fluid extraction system (Isco Inc.,
supplied by Jones Chromatography, Hengoed, Mid Glamorgan,
UK). The SFE system was controlled by a 260D Series pump
controller, featuring pressure automatic refill and continuous flow.
It consisted of a syringe pump, heated extractor block, and
The SFE of Fe(III) in the presence of Cu(II), Mn(II), and Pb-
(II) was also studied using C5 . Aliquots (40 µL) of each metal
nitrate solution in 0.5 M HNO
3
(0.04 mg of metal) were first spiked
onto filter paper (2 × 1 cm), with 40 µL of deionized water as
described above. The wet filter paper along with 40 mg of C5
was loaded into a glass tube (3.0 × 0.5 cm i.d.), as previously
described, and statically extracted for 20 min at 350 atm and 60
°C (optimum conditions). The extraction cell was vented into a
collecting vessel containing 8 mL of DMSO for 15 min. The
collected sample was analyzed directly using atomic absorption
analysis, following addition of acid.
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997 2209

RESULTS AND DISCUSSION
The selectivity of the host calixarenes for ionic and neutral
guest complexes is largely determined by the size of the cavity
and the nature of the functionalization. Thus, the tetrameric p-tert-
+
butylcalix[4]arenetetraacetic acid tetraesters exhibit Na selectiv-
ity, while the hexameric equivalent is Cs+ selective as shown by
picrate extraction 2 and ion-selective electrode measurements.
Apart from the sulfonated calixarenes and some others, calixare-
nes are in the main water insoluble, preferring nonaqueous
solvents such as chloroform. To date, no literature has been found
demonstrating the transport of these calixarene macrocycles in
supercritical fluids. The chemical structures of the macrocyclic
calixarenes are shown in Figure 1. The work reported here
initially focused on the chromatographic behavior of selected
2
calixarene derivatives on a diol column using SF-CO , with a view
to demonstrating the retention, stability, and identity of these
molecular baskets in methanol/ chloroform-modified supercritical
CO
Calixarenes exhibit characteristic ultraviolet spectra, displaying
a pair of absorption maxima near 280 and 288 nm, when recorded
in CHCl or dioxane. The ratio of intensities at these two
2
.
3
wavelengths is a function of the ring size and ranges from 1.3 for
tetramers to 0.78 for the octamers.25 The wavelength of detection
was thus chosen at 280 nm.
Calixarenes unfunctionalized at the lower rim are considered
to be stable macrocyclic compounds; for example, p-tert-butylcalix-
[
6]arene (C2 ) is known to bind an equimolar amount of chloro-
form in a host-guest complex that is stable to heating at 257 °C
for 6 days under high vacuum. On the other hand, p-tert-
butylcalix[4]arenetetraacetic acid tetraester is known to undergo
a monohydrolysis reaction to yield the triester monoacid, a
reaction inhibited by Na+ ion.26 Chromatographic studies were
carried out with the calixarenes C1 and C2 and a number of p-tert-
butylcalixarenes functionalized at the lower rim such as the
tetraester, using as mobile phase methanol/ chloroform-modified
Fig u re 2 . SFC of calixarenes C1 and C2 using (a) 10% chloroform
tr) 3.39 min (C1), tr ) 7.33 min (C2), (b) 10% chloroform/methanol
(70:30) tr ) 1.95 min (C1), tr ) 2.59 min (C2), and (c) 10% methanol
as modifier tr ) 1.62 min (C1), tr ) 2.06 min (C2); 10% modifier, 3
2
CO at 55 °C. Characteristic calixarene ultraviolet and proton
mL min-1, 3.6 kpsi, 55 °C, and 280 nm).
NMR spectra were recorded for collected fractions of the eluted
peaks from the SFC system. The results helped confirm the
identity of the chromatographic peaks and also that the calixarenes
studied remained intact at the temperatures and pressures used.
Ta b le 1 . S o lu b ilit y o f S e le c t e d Flu o rin a t e d a n d
No n flu o rin a t e d Ca lix a re n e s in S u p e rc rit ic a l CO2 a t 6 0
°
C a n d 2 0 0 a t m
Using methanol modified SF-CO
elution at or close to the t ) were observed for the calixarenes
C1 and C2 (10% modifier, 3 mL min-1, 3.6 kpsi, 55 °C, and 280
nm). With a t ) 1.12 min, C1 eluted at 1.62 min and C2 at 2.07
2
, reduced retention times (i.e.,
ligand
solubility (mmol/ L)
ligand
solubility (mmol/ L)
0
C1
C2
C3
0.62
0.50
>120
0.18
C4
C5
>94
0.10
1.64
t
C5
0
t
C4
min. Substitution of chloroform for methanol provided baseline
resolution of the p-tert-butyl tetrameric calixarene (C1 ) and the
hexameric calixarene (C2 ). Figure 2 shows the differences
obtained in the separation for C1 and C2 between 10% methanol,
Separate SFE studies at 60 °C and 200 atm on these calixarenes
indicated low solubility in unmodified SF-CO . When the collected
2
1
0% of a 70:30 chloroform/ methanol mixture and finally 10%
chloroform-modified SF-CO , at a temperature of 55 °C, a pressure
of 3.7 kpsi, and a flow rate of 3 mL min-1. Retention times of
.39 and 7.33 min for C1 and C2 , respectively, were observed
chloroform extracts were analyzed by the SFC method described
above, peaks for C1 and C2 were obtained and solubilities in the
order of 10-4 M determined (Table 1). These measured solubili-
ties agreed with values calculated by weight loss in the extraction
tube. In order to enhance the solubility of these molecular baskets
2
3
using chloroform as the modifier in comparison to 1.62 and 2.07
min using methanol as the modifier. Capacity factors, when
plotted against increasing pressure, were seen to decrease as
expected at fixed temperature and modifier percentage.
2
in SF-CO , new synthetic methodologies were developed to
fluorinate the calixarenes in the para or upper rim position.
The solubilities of the new fluorinated calixarene C3 -C5
derivatives were determined using the method described and are
given in Table 1 in millimolar units, all determined at 60 °C and
(
(
25) Czerwenka, G.; Scheubeck, L. Z. Anal. Chem. 1 9 7 5 , 276, 34.
26) Blasius, E.; Jansen, K. P.; Adrian, W.; Klautke, G.; Dorschneider, R.; Maurer,
G. P.; Nguyen, V. B.; Nguyen, T.; Scholten, G.; Strockenen, J. Z. Anal. Chem.
2
00 atm for comparison purposes. The fluorination of selected
1
9 7 7 , 88, 392.
linear ligands has been shown to enhance their solubility in SF-
2210 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997

(
a )
Fig u re 3 . UV spectra of C3 in chloroform: a standard calixarene
C3 (a) and after SFE (b-f): (300 atm, 20 min static, 10 min dynamic)
at varying temperatures (b) 70, (c) 80, (d) 90, (e) 100, and (f) 120
°
C.
(
b)
CO
metal ions and are soluble in SF-CO
2
.19,22 In particular, ligands that show good selectivity toward
would be advantageous for
2
selective SFE applications. The replacement of the para tert-butyl
groups with a fluorinated side chain greatly improves the solubility
of the calixarenes; this is clearly seen when the solubilities of C1
and C3 are compared. When 0.5 g of C3 was placed in the
extraction cell, it was observed to be completely extracted into
the collecting vessel at 60 °C and 200 atm. Further lower rim
functionalization can significantly affect the solubility of the
molecular baskets; for the tetraester C4 , which similarly disap-
peared from the extraction cell, the enhancement of solubility is
t
maintained, as can be seen when compared with C4 and C3 . On
the other hand, the calix[4]arene tetrahydroxamate shows a
significantly lower solubility than C3 but the benefit of flourination
t
remains relative to C5 ; this effect can be compensated for by
operating at higher pressures, and studies are in place to
investigate further substituent effects.
The UV spectra of C3 recorded in the region between 240
and 350 nm during the SFE of Fe(III) are shown in Figure 3.
Spectrum a is a reference spectrum of the calix[4]arene in
chloroform at a concentration corresponding to complete extrac-
tion of C3 from the extraction cartridge. Spectra labelled b-f
were recorded for chloroform solutions collected following SFE
Fig u re 4 . SFE of Fe(III) using the fluorinated calixarene C5: (a)
visible absorption spectra of the resulting Fe(III) complex in DMSO
after SFE (60 °C, 20 min static, 15 min dynamic) at varying pressures;
(b) plot of percent extraction as a function of pressure for the
calixarene hydroxamate-Fe(III) complex.
(
300 atm) at temperatures 70, 80, 90, 100, and 120 °C, respectively.
While the characteristic spectrum of a calix[4]arene is evident, it
is clear that increasing the temperature decreases the extractibility
unexpected, with results available from this and other laboratories
on the selective complexation of Fe(III) by polymeric calix[4]-
arene27 and by calix[6]arene-p-sulfonic acid,28 where a water-
soluble lilac complex is formed through complexation at the
phenolic oxygens. However, for transition metal ion extraction,
of C3 in supercritical CO
of the collected samples placed in the extraction vessel with Fe-
III)-loaded filter paper, indicated a weak band between 425 and
25 nm consistent with Fe(III) calix[4]arene complexation.
2
. Visible absorption spectral analysis
(
5
Atomic absorption analysis following digestion of the extracts
verified that only trace extraction of Fe(III) occurs. The com-
plexation of Fe(III) by this fluorinated calix[4]arene C3 is not
(
27) Deligoz, H.; Tavasli, M.; Yilmaz, M. J. Polym. Sci. Part A: Polym. Chem.
1 9 9 4 , 32, 2961-2964.
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997 2211

a macrocycle that combines the metal-sequestering ability of
siderophores with the ion-recognition properties of calixarenes
would be a novel and powerful SFE reagent. Lower rim func-
tionalization of C3 was carried out to produce the tetrahydroxamic
acid derivative, C5 . SFE of Fe(III) using this ligand was studied
at 60 °C and at pressures between 200 and 400 atm. The visible
spectra recorded for the extracts in the collecting DMSO solutions
are shown in Figure 4a. The characteristic visible absorbance
for Fe(III)-hydroxamate binding can be seen to increase with
increasing pressure up to 350 atm before dropping at 400 atm.
The visual observation of the intensity of the red color of the
collected solutions followed a similar pattern. Figure 4b shows
the corresponding percentage Fe(III) extraction values (as cal-
culated from Figure 4a) vs pressure at 60 °C indicating extraction
of Fe(III) up to 86% at 350 atm. Increasing temperature at 300
atm resulted in little variation of extraction efficiency. The
selectivity of extraction was also examined by spiking Fe(III) in
the presence of other metal ions. Solid phase immobilized p-tert-
butylcalix[4]arene tetrahydroxamate10 has been shown previously
to selectively uptake Fe(III) and Pb(II) from acidic solution in
the presence of other transition metal ions such as Mn(II). For
the spiked metal mixture, greater than 90% Pb(II) was extracted
by SFE alongside Fe(III), while the percentages of extraction of
Mn(II) and Cu(II) were measured at below 10%. Further studies
on the optimization of the selectivity of metal ion extraction by
While macrocyclic calixarenes have also been shown to extract
metal ions in liquid/ liquid4,27 and solid phase extraction studies,10
the work reported here is the first example of selective extraction
of transition metals using new functionalized molecular baskets
in SFE.
CONCLUSIONS
Macrocyclic calixarene compounds possessing basket-shaped
cavities can be designed to be soluble in supercritical fluid CO
The extraction of a host-guest complex of a calix[4]arene in
supercritical CO was observed with a heptadecafluorodecane
2
.
2
derivatized calix[4]arene hydroxamate iron complex. More selec-
tive extractions of targeted metal ions using designed molecular
baskets are inevitable.
ACKNOWLEDGMENT
This work was funded by a Basic Research Grant SC/ 95/ 224
from Forbairt, Glasnevin, Dublin, Ireland. We thank Carsten
Schmidt and Caroline Williams for their involvement in this work.
The help and advice given by Professor W. B. Jennings, Professor
of Organic Chemistry at UCC and Professor Keith Bartle at Leeds
University is gratefully acknowledged.
Received for review August 20, 1996. Accepted February
X
11, 1997.
2
molecular baskets in SF-CO are in progress.
AC960850Q
X
(
28) Scharff, J. P.; Mahjoubi, M.; Perrin, R. New. J. Chem. 1 9 9 3 , 17, 793.
Abstract published in Advance ACS Abstracts, March 15, 1997.
2212 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997