Anal. Chem. 1997, 69, 2207-2212
Molecular Baskets in Supercritical CO2 Jeremy D. Glennon,*,†,‡ Sharon Hutchinson,† Stephen J. Harris,† Andrew Walker,† M. Anthony McKervey,§ and Conor C. McSweeney†
Department of Chemistry, University College Cork, Cork, Ireland, and School of Chemistry, Queen’s University, Belfast, N. Ireland
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. 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′. †
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. ‡ §
S0003-2700(96)00850-5 CCC: $14.00
© 1997 American Chemical Society
Figure 1. Chemical structures of the macrocyclic calixarenes: p-tert-butylcalix[4]arene (C1); p-tert-butylcalix[6]arene (C2); p-heptadecafluorodecylthio-n-propylcalix[4]arene (C3); p-tert-butylcalix[4]arene tetraethyl acetate (C4t); p-heptadecafluorodecylthio-n-propylcalix[4]arene tetraethyl acetate (C4); p-tert-butylcalix[4]arene tetrahydroxamic acid (C5t); p-heptadecafluorodecylthio-n-propylcalix[4]arenetetrahydroxamic 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.4 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 CHEMFET7 for the analysis of sodium and caesium. A primary amine-selective polymeric membrane electrode based on a calix[6]arene ionophore 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 (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, 1989, 1167-1171. (b) Shinkai, S.; Kawabata, H.; Arimura, T.; Matsuda, T.; Satoh, H.; Manabe, O. J. Chem. Soc., Perkin Trans. I, 1989, 1073-1074. (c) Shinkai, S.; Otsuka, T.; Arake, K.; Matsuda, T. Bull. Chem. Soc. Jpn. 1989, 62, 4055-4057. (5) Cadogan, A.; Diamond, D.; Smyth, M. R.; Svehla, G.; McKervey, M. A.; Seward, E. M.; Harris, S. J. Analyst, 1990, 225, 1207. (6) Kimura, K.; Miura, T.; Matsuo, M.; Shona, T. Anal. Chem. 1990, 62, 110. (7) Brunink, J.; Haak, J. R.; Bomer, J. G.; Reinhoudt, D. N.; McKervey, M. A.; Harris, S. J. Anal. Chim. Acta. 1991, 254, 75. (8) Chan, W. H.; Shiu, K. K.; Gu, X. H. Analyst 1993, 118, 863-867.
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997 2207
silicas9 and immobilized calixarene hydroxamates.10 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 retain Na+ ions over other alkali metal ions. Brindle et al. recently 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 electrophoresis.18 A considerable amount of research has been reported on the synthetic macrocyclic compounds, the calixarenes,3 but nothing has been published in the area of supercritical fluid chromatography (SFC) or supercritical fluid extraction (SFE). Extensive work has been done in the field of SFE and SFC using β-diketones and dithiocarbamates as chelating agents.19-22 However, the research published on macrocyclic compounds as chelating agents in SFE and SFC is limited. Wai and co-workers23 used supercritical 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 β-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 solubilities in pure SF-CO2; 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 (9) Ryan, N.; Glennon, J. D.; Muller, D. Anal. Chim. Acta 1993, 283, 344. (10) Hutchinson, S.; Kearney, G. A.; Horne, E.; Lynch, B.; Glennon, J. D.; McKervey, M. A.; Harris, S. J. Anal. Chim. Acta 1994, 291, 269-275. (11) Agrawal, Y. K.; Patel, S. A. Anal. Chem. 1980, 52, 327. (12) Glennon, J. D.; Woulfe, M. R.; Senior, A. T.; NiChoileain, N. Anal. Chem. 1989, 61, 1474-1478. (13) Glennon, J. D.; Horne, E.; Kearney, G. A.; Harris, S. J.; McKervey, M. A. Anal. Proc. 1994, 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 1996, 731, 47-55. (15) Brindle, R.; Albert, K.; Harris, S.; Tro ¨ltzsch, C.; Horne, E.; Glennon, J. D. J. Chromatog., A 1996, 731, 41-46. (16) Friebe, S.; Gebauer, S.; Krauss, G. J.; Goemar, G.; Kruger, J. J. Chromatogr. Sci., 1995, 33, 281. (17) Berger, T. A. J. Chromatog. 1989, 478, 311-324. (18) Shohat, D.; Grushka, E. Anal. Chem. 1994, 66, 747-750. (19) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1994, 28, 1190-1193. (20) Laintz, K. E.; Wai, C. M.; Yonker; C. R.; Smith, R. D. Anal. Chem. 1992, 64, 2875-2878. (21) Jahn, K. R.; Wenclawiak, B. W. Fresenius Z. Anal. Chem. 1988, 330, 243245. (22) Laintz, K. E.; Yu, J.-J.; Wai, C. M. Anal. Chem. 1992, 64, 311-315. (23) Wang, S.; Elshani, S.; Wai, C. M. Anal. Chem. 1995, 67, 919-923.
2208
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
Fe3+, as the guest in a “host-guest” cavity, using unmodified SFCO2 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 temperatures. EXPERIMENTAL SECTION Reagents. Tetrameric and hexameric tert-butylcalixarene derivatives were initially chosen for study. p-tert-butyl-substituted derivatives of calix[4]arene (C1, C4t, C5t) 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 10-210-4 M. New calix[4]arenes, fluorinated at the upper rim, were synthesized as described below for SFE applications. Preparation of Heptadecafluorodecanethiol, (CF3(CF2)7(CH2)2SH). 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 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 (1H, t, SH), δ 2.25-2.36 (2H, m, CH2SH), δ 2.72-2.79 (2H, m, CH2CH2SH). Anal. Calcd: C, 25.01%; H, 1.05. Found: C, 25.40%; H, 1.46. Preparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Heptadecafluoro(10-thiodecyl)-n-propylcalix[4]arene (C3). To 1.5 g (0.002 57 mol) of p-allylcalix[4]arene,24a were added 4.93 g (0.0103 mol) of CF3(CF2)7(CH2)2SH (prepared above) in 10 mL of dry CHCl3 and 120 mg of azoisobutyronitrile (AIBN, free-radical 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 characteristic of the allyl group. Purification using flash chromatography on silica gel with methylene chloride/hexane (1:2) as eluent yielded C3 as a white solid (4.25 g, 66% yield): 1H NMR (270 MHz) δ 1.79-1.87 (8H, m, SCH2CH2CH2), δ 2.25-2.38 (8H, m, SCH2CH2CH2), 2.45-2.54, (16H, m, SCH2CH2CH2 and SCH2CH2(CF2)7), 2.68-2.74 (8H, m, SCH2CH2(CF2)7), 3.44 (4H, d, ArCH2Ar, Hb), 4.22 (4H, d, ArCH2AR,Ha), 6.84, (8H, s, ArH), 10‚18 (4H, s, OH). Anal. Calcd: C, 38.35%; H, 2.41. Found: C, 38.54%; H, 2.66. Preparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Heptadecafluoro(10-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 CF3(CF2)7(CH2)2SH, and 80 mg of AIBN in 10 mL of dry benzene were refluxed under an inert atmosphere for 4 h. After this time, a further portion of AIBN (80 mg) was added to the solution, and (24) (a) Gutsche, C. D.; Levine, J. A. J. Am. Chem. Soc. 1982, 104, 2652. (b) Harris, S. J.; Woods, J. G.; Rooney, J. M. U.S. Patent 4642362, February 10, 1987.
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. Preparation of p-1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Heptadecafluoro(10-thiodecyl)-n-propylcalix[4]arenetetrahydroxamic Acid (C5). To a cooled solution of 1.0 g (0.00 108 mol) of p-allylcalix[4]arene tetraethyl ester and 1.2 g (0.0172 mol) of hydroxylamine hydrochloride (BDH, Poole, 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 °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]arenetetrahydroxamic 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 chloroform containing 1.53 g (0.0032 mol) of CF3(CF2)7(CH2)2SH and 105 mg of AIBN was refluxed for 4 h under nitrogen. After this 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 jacket on the CO2 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 (Merck) was used with CO2 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
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. Unmodified SF-CO2 was used for all the SFE extractions. 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 mL of CO2. 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 extracted using unmodified SF-CO2 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 solutions were transferred to volumetric flasks and diluted to 10 mL for aspiration into a Perkin-Elmer 2380 atomic absorption spectrometer. 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 HNO3 (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-tertbutylcalix[4]arenetetraacetic acid tetraesters exhibit Na+ selectivity, 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, calixarenes 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 calixarene derivatives on a diol column using SF-CO2, with a view to demonstrating the retention, stability, and identity of these molecular baskets in methanol/chloroform-modified supercritical CO2. Calixarenes exhibit characteristic ultraviolet spectra, displaying a pair of absorption maxima near 280 and 288 nm, when recorded in CHCl3 or dioxane. The ratio of intensities at these two 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 chloroform in a host-guest complex that is stable to heating at 257 °C for 6 days under high vacuum. On the other hand, p-tertbutylcalix[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-tertbutylcalixarenes functionalized at the lower rim such as the tetraester, using as mobile phase methanol/chloroform-modified CO2 at 55 °C. Characteristic calixarene ultraviolet and proton 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. Using methanol modified SF-CO2, reduced retention times (i.e., elution at or close to the t0) were observed for the calixarenes C1 and C2 (10% modifier, 3 mL min-1, 3.6 kpsi, 55 °C, and 280 nm). With a t0 ) 1.12 min, C1 eluted at 1.62 min and C2 at 2.07 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, 10% of a 70:30 chloroform/methanol mixture and finally 10% chloroform-modified SF-CO2, at a temperature of 55 °C, a pressure of 3.7 kpsi, and a flow rate of 3 mL min-1. Retention times of 3.39 and 7.33 min for C1 and C2, respectively, were observed 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. (25) Czerwenka, G.; Scheubeck, L. Z. Anal. Chem. 1975, 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. 1977, 88, 392.
2210 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
Figure 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 mL min-1, 3.6 kpsi, 55 °C, and 280 nm). Table 1. Solubility of Selected Fluorinated and Nonfluorinated Calixarenes in Supercritical CO2 at 60 °C and 200 atm ligand
solubility (mmol/L)
ligand
solubility (mmol/L)
C1 C2 C3 C4t
0.62 0.50 >120 0.18
C4 C5t C5
>94 0.10 1.64
Separate SFE studies at 60 °C and 200 atm on these calixarenes indicated low solubility in unmodified SF-CO2. When the collected 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 solubilities agreed with values calculated by weight loss in the extraction tube. In order to enhance the solubility of these molecular baskets in SF-CO2, 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 200 atm for comparison purposes. The fluorination of selected linear ligands has been shown to enhance their solubility in SF-
(a)
Figure 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) CO2.19,22
In particular, ligands that show good selectivity toward metal ions and are soluble in SF-CO2 would be advantageous for 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 disappeared from the extraction cell, the enhancement of solubility is maintained, as can be seen when compared with C4t and C3. On the other hand, the calix[4]arene tetrahydroxamate shows a significantly lower solubility than C3 but the benefit of flourination remains relative to C5t; 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 extraction of C3 from the extraction cartridge. Spectra labelled b-f were recorded for chloroform solutions collected following SFE (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 of C3 in supercritical CO2. Visible absorption spectral analysis of the collected samples placed in the extraction vessel with Fe(III)-loaded filter paper, indicated a weak band between 425 and 525 nm consistent with Fe(III) calix[4]arene complexation. Atomic absorption analysis following digestion of the extracts verified that only trace extraction of Fe(III) occurs. The complexation of Fe(III) by this fluorinated calix[4]arene C3 is not
Figure 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.
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 watersoluble lilac complex is formed through complexation at the phenolic oxygens. However, for transition metal ion extraction, (27) Deligoz, H.; Tavasli, M.; Yilmaz, M. J. Polym. Sci. Part A: Polym. Chem. 1994, 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 functionalization 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 calculated 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-tertbutylcalix[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 molecular baskets in SF-CO2 are in progress. (28) Scharff, J. P.; Mahjoubi, M.; Perrin, R. New. J. Chem. 1993, 17, 793.
2212
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
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 CO2. The extraction of a host-guest complex of a calix[4]arene in supercritical CO2 was observed with a heptadecafluorodecane derivatized calix[4]arene hydroxamate iron complex. More selective 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 11, 1997.X AC960850Q X
Abstract published in Advance ACS Abstracts, March 15, 1997.
