Anal. Chem. 1999, 71, 1021-1026
Effect of Structural Variation within Lipophilic Lariat Ether Phosphonic Acid Monoesters on the Selectivity and Efficiency of Competitive Alkali Metal Cation Extraction into Chloroform Wladyslaw Walkowiak,† Grace Ndip, and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Lipophilic lariat ether phosphonic acid monoethyl esters with systematic crown ether ring size variation from 12crown-4 to 24-crown-8 are utilized for competitive alkali metal cation extractions from aqueous solutions into chloroform. Effective alkali metal cation extraction from weakly acidic, neutral, and basic aqueous solutions is achieved. With 4, 5, and 6 oxygens in the crown ether rings, selectivities for Li+, Na+, and K+, respectively, are observed. An 18-crown-6 phosphonic acid monoester exhibits excellent extraction selectivity for K+ with K+/ Li+ and K+/Na+ > 100. The lipophilic group attachment site, as well as the crown ether ring size, is shown to influence the extraction selectivity for the lariat ether phosphonic monoesters.
Attachment of a functional sidearm to the framework of a macrocylic polyether (crown ether) produces a lariat ether.1,2 The potential of crown ethers as the next generation of specific extracting agents for metal ions was markedly enhanced by the incorporation of a pendant acidic group.3-7 Ion exchange of the proton from the acidic function with a metal ion eliminates the need to concomitantly transfer an aqueous-phase anion into the organic diluent to preserve electroneutrality. This factor is of immense importance for potential practical applications that would involve highly hydrophilic chloride, nitrate, and sulfate anions. Applications of proton-ionizable lariat ethers as metal ion-complexing agents have been reviewed.8-11 † Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclaw University of Technology, 50370 Wroclaw, Poland. (1) Gokel, G. W.; Dishong, D. M.; Diamond, C. J. J. Chem. Soc., Chem. Commun. 1980, 1053-1054. (2) Gokel, G. W. In Cation Binding by Macrocycles. Complexation of Cationic Species by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, 1990; pp 253-310. (3) Helgeson, R. C.; Timko, J. M.; Cram, D. J. J. Am. Chem. Soc. 1973, 95, 3023-3025. (4) Newcomb, M.; Cram, D. J. J. Am. Chem. Soc. 1977, 97, 1257-1259. (5) Nakamura, H.; Takagi, M.; Ueno, T. Talanta 1979, 26, 921-927. (6) Frederick, L. A.; Fyles, T. M.; Gorprasad, N. P.; Whitfield, D. M. Can. J. Chem. 1981, 59, 1724-1733. (7) Bartsch, R. A.; Heo, G. S.; Kang, S. I.; Liu, Y.; Strzelbicki, J. J. Org. Chem. 1982, 47, 457-460. (8) Bartsch, R. A. Solvent Extr. Ion Exch. 1989, 7, 829-854. (9) Brown, P. R.; Bartsch, R. A. In Inclusion Aspects of Membrane Chemistry; Atwood, J. L., Ed.; Kluwer Academic Publishers: Boston, 1991; pp 1-57.
10.1021/ac980767y CCC: $18.00 Published on Web 01/22/1999
© 1999 American Chemical Society
Although a wide variety of lariat ether carboxylic acids have been reported, only few proton-ionizable lariat ethers with more acidic pendant groups have been described. These include lariat ether phosphonic acid monoalkyl esters,12,13 a lariat ether phosphinic acid,14 and lariat ether phosphoric acid monoalkyl esters.15-18 For such compounds, only very limited information is available regarding the influence of structural variation within the protonionizable lariat ether upon the metal ion complexation behavior.8,11-13,19 For the family of lipophilic lariat ether phosphonic acid monoethyl esters 1-4, the number of methylene units between
the crown ether ring and the ionizable group was systematically varied.12 For competitive solvent extraction of alkali metal cations from aqueous solutions into chloroform, proton-ionizable lariat ethers 1 and 2 were selective for Na+, as would be predicted for a 16-crown-5 crown ether ring size.20 The change to Li+ selectivity (10) Bartsch, R. A.; Ramesh, V.; Bach, R. O.; Shono, T.; Kimura, K. In Lithium Chemistry: An Experimental and Theoretical Overview; Sapse, A. M., Schleyer, P. v. R., Eds.; Wiley: New York, 1995; pp 393-476. (11) Habata, Y.; Akabori, S. Coord. Chem. Rev. 1996, 148, 97-113. (12) Koszuk, J. F.; Czech, B. P.; Walkowiak, W.; Babb, D. A.; Bartsch, R. A. J. Chem. Soc., Chem. Commun. 1984, 1504-1505. (13) Pugia, M. J.; Ndip, G.; Lee, H. K.; Yang, I.-W.; Bartsch, R. A. Anal. Chem. 1986, 58, 2723-2726. (14) Sachleben, R. A.; Burns, J. H.; Brown, G. M. Inorg. Chem. 1988, 27, 17871790. (15) Habata, Y.; Ikeda, M.; Akabori, S. Tetrahedron Lett. 1992, 33, 3157-3160. (16) Habata, Y.; Akabori, S. Tetrahedron Lett. 1992, 33, 5815-5818. (17) Habata, Y.; Ikeda, M.; Akabori, S. J. Chem. Soc., Perkin Trans. 1 1992, 2651-2655. (18) Habata, Y.; Ugajin, H.; Akabori, S. J. Org. Chem. 1994, 59, 676-678. (19) Walkowiak, W.; Brown, P. R.; Shukla, J. P.; Bartsch, R. A. J. Membr. Sci. 1987, 32, 59-68.
Analytical Chemistry, Vol. 71, No. 5, March 1, 1999 1021
observed for ligands 3 and 4 was attributed to primary coordination of the metal ion with the ionized group, since the sidearms are now too long to allow for simultaneous coordination of the metal ion with the cyclic polyether unit and the ionized phosphonic acid monoester group. We now report results for competitive solvent extraction of alkali metal cations from aqueous solutions into chloroform by the lipophilic lariat ether phosphonic acid monoethyl esters 5-14
in which the sidearm length is held constant and the crown ether ring size is systematically varied from 12-crown-4 to 24-crown-8 and for the 18-crown-6 phosphonic acid monoester 15 in which
the lipophilic group attachment site has been varied. Model compounds 16 and 17 with lipophilic and phosphonic acid monoethyl ester groups, but no crown ether units, are also examined. EXPERIMENTAL SECTION Reagents. Sources of inorganic reagents and chloroform were the same as previously described.20-23 Syntheses of lipophilic (20) Walkowiak, W.; Kang, S. I.; Stewart, L. E.; Ndip, G.; Bartsch, R. A. Anal. Chem. 1990, 62, 2022-2026. (21) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 1894-1899. (22) Charewicz, W. A.; Heo, G. S.; Bartsch, R. A. Anal. Chem. 1982, 54, 20942097. (23) Charewicz, W. A.; Bartsch, R. A. Anal. Chem. 1982, 54, 2300-2303.
1022 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999
crown ether phosphonic acid monoethyl esters 5-16 and model compound 18 have been reported.19,24 Synthesis of Ethyl 4(2)-(1-Octyl)phenylphosphonate (17). By adaptation of a reported general procedure,25 a mixture of 1-phenyloctane (114 g, 0.60 mol), PCl3 (298 g, 2.17 mol), and AlCl3 (93 g, 0.70 mol) was heated gently for 3 h. To the mixture, POCl3 (107 g, 0.70 mol) was added very slowly and the resulting mixture was heated for 1 h. The inorganic salts were filtered and washed 3 times with petroleum ether. The filtrate and washings were combined, and the upper layer was separated and evaporated in vacuo. The residue was distilled in vacuo (bp 145-160 °C/0.6 Torr) to give 68.3 g (40%) of 4(2)-(1-octyl)phenyldichlorophosphine which was used directly in the next step. (By 1H NMR spectroscopy, the product was shown to be an approximately 3:1 mixture of the para and ortho isomers.) To 7.13 g (24.5 mmol) of 4(2)-(1-octyl)phenyldichlorophosphine, 60 mL of EtOH was added dropwise with cooling and stirring. The solution was placed under vacuum for 3 h with introduction of nitrogen by a capillary tube to remove the liberated HCl, and the solution was evaporated. The residue was dissolved in Et2O, and the solution was washed with water and evaporated in vacuo. The residue was distilled with a Kugelrohr apparatus at 0.7 Torr and an oven temperature of 210 °C to give 4.88 g (62%) of diethyl 4(2)-(1-octyl)phenylphosphonate. Anal. Calcd for C18H31O3P: C, 66.23; H, 9.57. Found: C, 65.93; H, 9.321. To a solution of 0.48 g (12 mmol) of NaOH in 1 mL of water and 20 mL of EtOH, a solution of 3.26 g (10 mmol) of diethyl 4(2)-(1-octyl)phenylphosphonate in 10 mL of EtOH was added. The reaction solution was stirred and refluxed overnight and evaporated in vacuo. The residue was dissolved in 10 mL of H2O, and 10 mL of 6 N HCl was added. The mixture was extracted with CHCl3 (3 × 25 mL). The combined extracts were dried over MgSO4 and evaporated in vacuo to give 2.26 g (76%) of ethyl 4(2)(1-octyl)phenylphosphonate as a colorless oil. Anal. Calcd for C16H27O3 P: C, 64.41; H, 9.12. Found: C, 64.10: H, 8.85. Apparatus and Procedure. The apparatus and procedure were as previously described.20 RESULTS AND DISCUSSION Solvent Extraction of Alkali Metal Cations from Aqueous Solutions into Chloroform by Lipophilic Lariat Ether Phosphonic Acid Monoethyl Esters. In earlier studies of alkali metal cation solvent extraction by lariat ether carboxylic acids, it was demonstrated that the efficiencies and selectivity orders for competitive extractions are often quite different from expectations based upon the results of single ion extractions.21 Therefore, competitive alkali metal cation extractions were utilized in this investigation. For the lipophilic lariat ether phosphonic acid monoesters 5-15 and model compound 16, there was no detectable loss of extractant from the organic phase during extraction. For model compound 17, precipitation of alkali metal phosphonate monoesters at the aqueous-organic interface was observed at pH >6. Effect of Ring Size Variation for Lariat Ether Phosphonic Acid Monoesters. The lipophilic lariat ether phosphonic acid monoesters 5, 9, 11, 13, and 14 provide a series of ionophores (24) Czech, A.; Desai, D. H.; Koszuk, J.; Czech, A.; Babb, D. A.; Robison, T. W.; Bartsch, R. A. J. Heterocycl. Chem. 1992, 29, 867-875. (25) Buchner, B.; Lockhart, B., Jr. J. Am. Chem. Soc. 1951, 73, 755-756.
Figure 1. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lariat ether phosphonic acid monoethyl esters: (a) 5, (b) 9, and (c) 11.
in which the crown ether rings are 12-crown-4, 15-crown-5, 18crown-7, 21-crown-7, and 24-crown-8, respectively. Data from the competitive solvent extraction of alkali metal cations from aqueous solution into chloroform by these proton-ionizable lariat ethers are shown in Figures 1 and 2. Although analogous lariat ether carboxylic acids only extract alkali metal cations into chloroform from basic aqueous solutions,20 the lariat ether phosphonic acid monoesters are noted to be effective extractants from weakly acidic and neutral aqueous solutions as well. In Table 1, the selectivity orders, selectivity coefficients at pH 9, and maximal metal loadings for this series of extractants are presented. (The selectivity coefficient is defined as the ratio between the organic-phase concentration of the best extracted alkali metal cation and the indicated second alkali metal cation species.) The percent metal loading of the chloroform phase was calculated based on the assumption that 1:1 metal ion to lariat ether phosphonate monoester extraction complexes are formed. For the lipophilic lariat ether phosphonic acid monoesters 5, 9, 11, 13, and 14, the maximum metal loadings are all in the range of 96-100%. Lariat ether phosphonic acid monoesters 5, 9, and 11 with crown ether ring sizes of 12-crown-4, 15-crown-5, and 18-crown-6 exhibit extraction selectivities for Li+, Na+, and K+, respectively, as would be predicted from the polyether cavity diameters.20 The selectivity orders for lariat ether phosphonic acid monoesters 5, 9, and 11 are very similar to those reported for analogous lariat
ether carboxylic acid compounds.20 The selectivity coefficients for Li+ over the other alkali metal cations are nearly the same for the 12-crown-4 extractant 5 and its lariat ether carboxylic acid analogue. Compared with its lariat ether carboxylic acid analogue, the selectivity coefficients for the 15-crown-5 phosphonic acid monoester 9 are very similar except for some enhancement of the Na+/Li+ selectivity for the latter. On the other hand, the K+/ Na+ and K+/Li+ selectivities for the 18-crown-6 lariat ether phosphonic acid monoester 11 are considerably higher than those reported for the analogous lariat ether carboxylic acid. The K+/ Na+ and K+/Li+ selectivities for 11 exceed 100, whereas for the analogous lariat ether carboxylic acid, the K+/Na+ and K+/Li+ selectivities were 13 and 64, respectively.20 Thus, lariat ether phosphonic acid monoethyl ester 11 exhibits excellent selectivity for the extraction of K+ from an aqueous solution containing Li+, Na+, and K+. Although its lariat ether carboxylic acid analogue was Cs+selective,20 the 21-crown-7 phosphonic acid monoester 13 shows only poor extraction selectivity for Rb+ and Cs+ over the other alkali metal cations. The 24-crown-8 phosphonic acid monoester 14 exhibits essentially no differentiation and extracts Na+, K+, Rb+, and Cs+ with almost equal propensity. The carboxylic acid analogue of 14 showed a very modest selectivity for extraction of Cs+.20 Thus, the large-ring lariat ether phosphonic acid monoesters 13 and 14 are judged to possess a very low potential for use in selective alkali metal cation extractions. Analytical Chemistry, Vol. 71, No. 5, March 1, 1999
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Figure 2. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lariat ether phosphonic acid monoethyl esters: (a) 13, (b) 14, and (c) 15. Table 1. Selectivity and Efficiency of Competitive Alkali Metal Cation Extraction from Aqueous Solutions into Chloroform by Crown Phosphonic Acid Monoesters 5-15 and Model Compound 17 compd
ring size
selectivity order and selectivity coeffa at pH 9.0
max metal loading, %
5
12-crown-4
96
6
13-crown-4
81
7
14-crown-4
97
8
14-crown-4
98
9
15-crown-5
97
10
16-crown-5
87
11
18-crown-6
99
12b
19-crown-6
85
13
21-crown-7
100
14
24-crown-8
97
15
18-crown-6
100
16
cyclohexane
93
a Defined as the ratio between the organic-phase concentration of the best extracted alkali metal cation and the indicated second alkali metal cation species. b At pH 11.0.
1024 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999
With the objective of improving the modest extraction selectivity for Li+ observed with the 12-crown-4 phosphonic acid monoester 5, lariat ether phosphonic acid monoesters with 13-crown-4 (i.e., 6) and 14-crown-4 rings (i.e., 7 and 8) were examined. Data from competitive solvent extractions of alkali metal cations are shown in Figure 3 and are summarized in Table 1. The maximal metal loadings are 81% for 6 and 97-98% for 7 and 8. Compared with the 12-crown-4 phosphonic acid monoester 5, insertion of a methylene group into one of the two-carbon bridges to give the 13-crown-4 analogue 6 produces only a modest increase in the Li+ extraction selectivity. For the 14-crown-4 phosphonic acid monoesters 7 and 8, a substantial increase in the Li+ extraction selectivity is noted with considerably higher Li+ selectivity for 7 than for 8. Although 7 and 8 both have the same crown ether ring size, the attachment site for the sidearm is a two-carbon bridge of the 14-crown-4 ring of 7 and the central carbon of a three-carbon bridge in 8. Although very good Li+ selectivity is observed with for 14-crown-4 phosphonic acid monoester 7, higher Li+ selectivities were reported for the lariat ether carboxylic acid analogues of 7 and 8, which extracted only Li+ and Na+ with Li+/Na+ selectivities of 17 and 20, respectively.20 For lariat ether phosphonic acid monoesters 10 and 12, the crown ether ring sizes are 16-crown-5 and 19-crown-6, respectively, with attachment via the central carbon of the three-carbon bridge in the crown ether unit. Data obtained from the competitive solvent extractions of alkali metal cations by 10 and 12 are presented in Figure 4 and summarized in Table 1. For these two extractants, the maximal metal loading was 85-87%. Compared with the 15-
Figure 3. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lariat ether phosphonic monoethyl esters: (a) 6, (b) 7, and (c) 8.
Figure 4. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lariat ether phosphonic acid monoethyl esters: (a) 10 and (b) 12.
crown-5 phosphonic acid monoester 9, expansion of the ring size to 16-crown-5 in 10 decreases the Na+ extraction selectivity. Similarly, in comparison with the 18-crown-6 phosphonic acid monoester 11, the K+ selectivity is diminished for the 19-crown-6 phosphonic acid monoester 12. Effect of Lipophilic Group Attachment Site Variation for Lariat Ether Phosphonic Acid Monoesters. The 18-crown-6 phosphonic acid monoesters 11 and 15 are structural isomers that differ only in the lipophilic group attachment site on the benzene ring of the ionophore. Data from competitive solvent extractions of alkali metal cations by 11 and 15 are shown in Figures 1c and 2c, respectively, and summarized in Table 1. Although the selectivity order for both extractants is K+ > Rb+ > Cs+ > Na+ g Li+ and both give 99-100% maximal metal
Figure 5. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lipophilic phosphonic acid monoethyl ester 17. Molar chloroform-phase concentrations of 17 (×102) are shown as solid squares.
loading, the selectivity coefficients for lariat ether phosphonic acid monoester 11 are decidedly higher. Thus, location of the lipophilic group remote from the metal ion complexation site in this type of proton-ionizable lariat ether appears to provide enhanced selectivity in competitive extraction of alkali metal cations. Lipophilic Phosphonic Acid Monoethyl Esters without Crown Ether Units. Model compound 17 is simply a lipophilic benzenephosphonic acid monoethyl ester. Data for competitive solvent extractions of alkali metal cations by 17 are shown in Analytical Chemistry, Vol. 71, No. 5, March 1, 1999
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Model compound 16 has the same structure as lariat ether phosphonic acid monoesters 5-14 with the exception that the crown ether ring has been replaced with a cyclohexane ring. Data from the competitive solvent extractions of alkali metal cations by model compound 16 are presented in Figure 6 and summarized in Table 1. For 16, the maximal metal loading is 93% with an extraction selectivity order of Li+ > Na+ > K+, Rb+, Cs+. The Li+/Na+ selectivity is 3.9 and the selectivity for Li+ vs K+, Rb+, and Cs+ is 5.4. The much higher Li+ selectivity for model compound 16 than 17 is proposed to arise from simultaneous interaction of Li+ with the ionized phosphonate group and the ethereal oxygen in 16. CONCLUSIONS Lipophilic lariat ether phosphonic acid monoethyl esters 5-14 effectively extract alkali metal cations from weakly acidic, neutral, and basic aqueous solutions into chloroform. The extraction selectivity order is controlled by the size and number of oxygen atoms in the crown ether ring. Within this series of ionophores, individual members with very good selectivity for Li+ (i.e., 7) and K+ (i.e., 11) are identified. Lariat ether phosphonic acid monoesters with large crown ether rings of 21-crown-7 and 24-crown-8 exhibit very poor extraction selectivity. For the structural isomeric 18-crown-6 phosphonic acid monoesters 11 and 15, higher K+ selectivity is obtained for 11 in which the lipophilic group is remote from the metal ion-binding site. Figure 6. Molar concentrations of metals (×103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extractions of 0.25 M alkali metal cations by 0.050 M lipophilic phosphonic acid monoethyl ester 16.
Figure 5. For an aqueous-phase pH Na+, K+, Rb+, Cs+ with very modest Li+ selectivity. This indicates a slight, innate Li+ selectivity for the proton-ionizable group itself. At pH 6 and above, a precipitate formed at the aqueous-organic interface which caused the extractant concentration in the organic diluent (9 in Figure 5) to plummet.
1026 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999
ACKNOWLEDGMENT This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG03-94ER14416). We thank Dr. Jacek Koszuk for the preparation of model compound 17.
Received for review July 13, 1998. Accepted November 30, 1998. AC980767Y
