2018
Anal. Chem. 1990, 62, 2018-2021
Effect of Structural Variations within Lipophilic Dibenzocrown Ether Carboxylic Acids on the Selectivity and Efficiency of Competitive Alkali-Metal Cation Solvent Extraction into Chloroform Wladyslaw Walkowiak,] Witold A. Charewicz,' Sang Ihn Kang, 11-Woo Yang,2 Michael J. Pugia, and Richard A. Bartsch* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Competltive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by a series of lipophilic dlbenzocrown ethers wlth pendant carboxylic acld groups has been Investigated. Both the extraction selectivity and efflciency are Influenced by variations of the crown ether ring size and the attachment site of the ilpophillc group. Preorganlzation of the binding site by proper positioning of the ilpophlilc group enhances extraction seiectlvity. Very high Na' extraction selectivity (Na+/K+ = 32, Na+/LI+ = 66 and no detectable extraction of Rb' or Cs') was obtained with sym-decyldbenzo-16-crown-5-oxyacetic acld.
INTRODUCTION In his recent review of crown ethers as solvent extraction agents for metal ions, McDowell points out that when a proton-ionizable group is attached to the cyclic polyether framework, the molecule is both a cation exchanger and a coordinator ( I ) . This arrangement has the potential for providing an extraction system with greater selectivity and efficiency than one in which an organophilic acid is simply mixed with a crown ether. In earlier work (2,3),we examined competitive alkali-metal cation extraction from aqueous solutions into chloroform by dibenzocrown ether carboxylic acids 1-3. I t was found that these proton-ionizable ionophores were of insufficient lipophilicity to remain completely in the organic phase during extraction of alkali-metal cations from alkaline aqueous phases. To avoid such complications in extraction behavior, a lipophilic group was attached either to each benzene ring or to the carboxylic acid containing sidearm of the dibenzo16-crown-5 compound 2 to produce the lipophilic dibenzo16-crown-5-carboxylic acids 4 and 5 , respectively (3-5). Compounds 4 and 5 were found to be sufficiently lipophilic to remain completely in the chloroform phases even when the contacting aqueous solutions of alkali-metal cations were highly alkaline (3,5). Although the overall alkali-metal cation extraction behavior was similar for structural isomers 4 and 5, the former gave enhanced Na+/K+ selectivity and excluded Li+ from the chloroform phase (5). Hence the lipophilic group attachment site was found to exert some influence upon extraction behavior. The efficiencies and selectivities of competitive alkali-metal cation extraction into toluene (6) and of alkali-metal cation transport across chloroform and toluene liquid membranes Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 50-370 Wroclaw, Poland.
*Present address: Department of Chemistry, Korea Military
Academy. Seoul 130-09, Korea.
0 C H2C OZH
I O I1
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-Y - CHzCHzCHz-CyH20CHzCk$CH21Ct$0CHz)2CH2-
and liquid surfactant membranes by 4 and 5 were subsequently compared (5-8). We now report results for solvent extraction of alkali-metal cations from aqueous solutions into chloroform by lipophilic dibenzocrown ether carboxylic acids 6-12. By comparison of these results with those reported earlier for 4 and 5, the influence of the crown ether ring size and the lipophilic group attachment site upon extraction selectivity and efficiency may be assessed.
EXPERIMENTAL SECTION Reagents. Sources of inorganic reagents were the same as those reported previously ( 2 , 3 , 6 ,7). Demineralized water was prepared by passing distilled water through three Barnstead D8922 combination cartridges in series. Reagent grade chloroform was treated by shaking 4 times with demineralized water to remove the stabilizing ethanol and saturate the chloroform with water. Syntheses of lipophilic dibenzocrown ether carboxylic acids 6-9 have been reported (9). Synthesis of Lipophilic Dibenzocrown Ether Carboxylic Acids 10-12. Under nitrogen 0.80 g (20 mmol) of NaH (60%
0003-2700/90/0362-2018$02.50/0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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dispersion in mineral oil) was washed with dry pentane to remove Table I. Diameters of Crown Ether Cavitiesa and the mineral oil and was suspended in 100 mL of dry THF. To Alkali-Metal Cations* in Angstroms the stirred mixture, 10.0 mmol of the appropriate crown ether alcohol 13-15 (IO)in 25 mL of dry THF was added during 15 min. alkaliThe mixture was stirred for 1 h and a solution of 2.30 g (15mmol) metal crown ether cavity diameter cation diameter of methyl bromoacetate in 25 mL of dry THF was added dropwise. The mixture was stirred for 3 days at room temperature and cooled 14-crown-4 1.2-1.5 Li+ 1.20 in an ice bath, and water (25mL) was added slowly. The solution 16-crown-5 2.0-2.4 Na+ 1.90 was acidified to pH = 1 with 6 N HCl and stirred for 2 h at room 19-crown-6 3.0-3.5 K+ 2.66 temperature. The THF was evaporated in vacuo and the acidic 22-crown-7 4.7-5.0 Rb+ 2.96 aqueous mixture was extracted with CH2C12. The combined cs+ 3.38 CHICll extracts were washed with water (two 100-mL portions), dried over MgSO,, and evaporated in vacuo to give a crude product Estimated from CPK space-filling models. *Reference 11. which was chromatographed on deactivated silica gel with EtOAc as eluent to give the lipophilic dibenzocrown ether methyl ester. With this procedure, identical results for alkali-metal cation The methyl ester was refluxed with 0.75g of NaOH in 25 mL solvent extraction into chloroform by crown ether carboxylic acid of water under nitrogen for 4 h. The solution was cooled to 0 OC 9 were obtained by three different co-workers during a two-year and acidified to pH = 1 with 6 N HC1. Extraction with CH2C12 period. (three 50-mL portions), washing with water (50 mL), drying over R E S U L T S A N D DISCUSSION MgS04,evaporation in vacuo, and chromatography of the residue on deactivated silica gel with EtOAc-EtOH (101)as eluent gave Synthesis of Lipophilic Dibenzocrown Carboxylic the lipophilic dibenzocrown ether carboxylic acid. Acids 10-12. Three new lipophilic dibenzocrown ether carMethyl sym-decy1dibenzo-14-crown-4-oxyacetate (16) was obboxylic acids 10-12 were prepared by two-step reactions from 1110 (0) tained in 65% yield as an oil. IR (neat): 1762 (-), the corresponding lipophilic crown ether alcohols 13-15 as cm-'. 'H NMR (CDC13): 6 0.80-1.80(m, 21 H), 2.20 (m, 2 H), shown in eq 1. 3.50-4.80 (m, 13 H), 6.90(s, 8 H). MS: 518.6 (M'). Hydrolysis acid (10)in 60% yield gave sym-decyldibenzo-14-crown-4-oxyacetic as a white solid with mp 117-119 OC. IR (KBr): 3600-2650 (COOH), 1733 (C=O); 1255,1110( C - 0 ) cm-'. 'H NMR (CDC13): 0.80-1.30(m, 21 H), 2.20(m, 2 H), 3.60-4.40(m, 10 H), 5.60 (br s, 1 H), 6.95 (s, 8 H). Anal. Calcd for C&&0,'0.5H20: C, 68.80; H, 8.27. Found: C, 68.97;H, 7.93. 16 I3 -cnf~$n~Methyl syrn-decyldibenzo-16-crown-5-oxyacetate (17)was ob17 I L - cn,cn,ocn,cntained in 79% yield as a white solid with mp 52-53 OC. IR (deposit I0 15 - cn,icn,ocn,l,cn, on NaCl plate): 1760 (C=O), 1250,1110(C-0) cm-'. 'H NMR (CDC13): 6 0.80-2.00 (m, 21 H), 3.65 (s, 3 H), 3.70-4.95 (m, 14 Solvent E x t r a c t i o n of Alkali-Metal Cations from H), 6.93 (s, 8 H). MS: 558.6 (M+). Hydrolysis gave symAqueous Solution into Chloroform by Lipophilic Didecyldibenzo-16-crown-5-oxyacetic acid (11) in 61% yield as a benzocrown Ether carboxylic Acids. In a previous inwhite solid with mp 102-102.5 "C. IR (KBr): 3630-2280 (COOH), vestigation of alkali-metal cation extraction into chloroform 1776,1740(C=O), 1120 (C-0) cm-'. 'H NMR (CDC13): 6 0.80 by crown carboxylic acid 2, it was found that selectivity orders (t, 3 H), 1.20-1.45 (m, 16 H), 1.85-1.90 (m, 2 H), 3.55-5.00 (m, and efficiencies for competitive extractions in multi-ion sys14 H), 6.70-6.95 (m, 8 H). Anal. Calcd for C31H4408:C, 68.36; tems were quite different from expectations based upon the H, 8.14. Found: C, 68.07;H, 8.15. results of single ion extractions (2). Therefore, competitive Methyl sym-decyldibenzo-19-crown-6-oxyacetate (18) was obtained in 82% yield as a colorless oil. IR (neat): 1753,1747 extractions were utilized in this investigation. (C=O), 1255,1113(C-0) cm-'. 'H NMR (CDC13): 6 0.80-2.00 For lipophilic dibenzocrown ether carboxylic acids 6-12, (m, 21 H), 3.70(S,3 H), 3.75-4.65 (m, 18 H), 6.95 (s, 8 H). MS: there was no detectable loss of the ionophore from the chlo602.7(M+). Hydrolysis gave sym-decyldibenzo-19-crown-6-oxy- roform phases during extraction (Figures 1-3,filled squares). acetic acid (12) in 69% yield as a colorless oil. IR (neat): Thus attachment of an octyl or a decyl group to the sidearm 3600-2525 (COOH), 1782 (C=O), 1252,1120(C-0). 'H NMR (in 6-8) or polyether framework (in 9-12) of dibenzocrown (CDC13): 6 0.80-1.90 (m, 21 H), 3.50-4.70(m, 18 H), 6.00 (br s, ethers 1-3 provided sufficient lipophilicity. 1 H), 6.95(s,8 H). Anal. Calcd for C33H409: C, 67.32;H, 8.22. Effect of Ring Size Variation f o r Dibenzocrown EthFound: C, 68.98;H, 8.03. ers w i t h an Octyl G r o u p Attached to the Sidearm. Apparatus. Concentrations of alkali-metal cations in aqueous Ionophores 6,5, 7, and 8 are a series of dibenzocrown ethers phases were determined with Dionex Model 10 and Model 2000 ion chromatographs. Organic complexing agent concentrations in which the sidearm contains both the carboxylic acid in the chloroform phases were measured with Cary Model 17 and function and the lipophilic group, but have different crown Shimadzu Model 260 ultraviolet-visible spectrophotometers. A ether ring sizes of 14-crown-4, 16-crown-5, 19-crown-6, and Fisher Scientific Accumet Model 620 pH meter with a Corning 22-crown-7, respectively. Crown ether cavity diameters as No. 76050 or No. 476193 glass body combination electrode was estimated from Corey-Pauling-Koltun (CPK) space-filling used for the pH measurements. Solvent extraction samples in models are compared with diameters of the alkali-metal separatory funnels were shaken with a Burrell Model 25 wrist cations in Table I. action shaker. For competitive solvent extractions of aqueous alkali-metal Extraction Procedure. An aqueous solution of the alkalication (0.25 M in each) solutions by 0.050 M lipophilic dimetal chlorides with hydroxides for pH adjustment (5.0 mL, 0.25 M in each) and a chloroform solution (5.0 mL of the complexing benzocrown ether carboxylic acids 6-8 in chloroform, data for agent (0.050 M)) were shaken for 30 min in a 30-mL separatory the metals concentrations in the chloroform phase vs the funnel at room temperature. The 5.0-mL phases were separated equilibrium pH of the aqueous phase are shown in Figure 1. and the equilibrium pH of the aqueous phase was measured. Of Extraction selectivity orders and selectivity coefficients and the organic phase, 4.0 mL was removed and shaken with 5.0 mL maximal metals loadings for 6-8 are compared with that reof 0.1N HC1 for 30 min to strip the metal cations from the organic ported previously for 5 ( 4 ) in Table 11. (The selectivity phase into an aqueous solution for analysis by ion chromatography. coefficient is the ratio between the organic phase concentraA small sample (10-20 pL) of the stripped organic phase was tions of the best extracted cation and the indicated cations removed and diluted with CHCl, in a 10-mL volumetric flask and at pH = 10.0. The percent metals loading of the chloroform the absorption was measured at 273-274 nm to determine the phase was calculated by assuming that a 1:l metal ion to crown concentration of the complexing agent in the chloroform layer.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18. SEPTEMBER 15, 1990
2020
I
I b)
a)
Kp
1
Ic)
4 L
6
8
IO
G
8
6
1 0 4
6
8
DH
Figure 1. Molar concentrations of metals (X103) in the chloroform phase vs the equilibrium pH of the aqueous phase for competitive extraction of 0.25 M alkali-metal cations by 0.050 M (a) 6, (b) 7, and (c) 8. Mdar chloroformphase complexing agent concentrations (X102) are shown as solid squares.
Table 11. Effect of Crown Ether Ring Size Variation on Selectivity and Efficiency of Competitive Alkali-Metal Cation Extraction from Aqueous Solutions into Chloroform by Lipophilic Dibenzocrown Ether Carboxylic Acids 5-8
6
8
1
0
PH
IO
Figure 2. Molar concentrations of metals (X103) 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 9. Molar chloroform-phase complexing agent concentrations (X 10') are shown as solid squares.
Table 111. Effect of Lipophilic Group Attachment Site on Selectivity and Efficiency of Competitive Alkali-Metal Cation Extraction from Aqueous Solutions into Chloroform by Lipophilic Dibenzo-16-crown-5-oxyacetic Acids 4, 5, and 9
maximal
metal compound ring size 6
14C4
5'
16C5
7
19C6
8
22C7
selectivity order and selectivity coefficients"fb Na+ > Li+ > K+ > Rb+ > Cs+ 2.5 5 19 60 Na+ > K+ > Li+ > Rb+ > Cs+ 5 17 36 83 K+ > Na+ > Rb+ > Cs+ > Li+ 28 4 7 11 K+ > Rb+ > Li+ > Cs+ > Na+ 1.5 1.9 2.8 3.0
loading,
compound
selectivity order and selectivity coefficientsn
maximal metals loadings, 70
70
4b
100
68 100 100
Ratio of chloroform phase concentrations of best extracted metal ion and indicated metal ion. *At pH = 10.0. 'Data from ref 4.
carboxylate extraction complex is formed.) The lipophilic dibenzocrown ether carboxylic acids 5 and 7 exhibit extraction selectivities for Na+ and K+, respectively, as would be predicted from the relationship between the crown ether ring and metal ion diameters (Table I). On the other hand, ionophore 6, which has a dibenzo-14-crown-4 unit, is selective for Na+ rather than Li+ which would be predicted for the ring size. The quantitative metals loading calculated for 6 based upon a 1:l metal ion to crown carboxylate extraction complex differs markedly from the 1:2 metal to ligand stoichiometries which have been reported for extraction of Na+, K+, and Rb+ as single ion species into chloroform by an analogue of 6 in which the lipophilic group was butyl instead of octyl (12). Only for Li+ was a 1:l extraction complex observed in this earlier report (12). If the polyether ring were planar, the lipophilic dibenzo22-crown-7 carboxylic acid 8 would have a very large crown ether cavity. The very poor selectivity observed for 8 compared with 5-7 suggests that the polyether ring is not planar but provides three-dimensional "wrap-around" complexation of alkali-metal cations. Effect of Varying the Lipophilic Group Attachment Site for Dibenzo-16-crown-5-carboxylic Acids. Lipophilic dibenzocrown ether carboxylic acids 4,5,and 9 are structural isomers which differ only in the attachment site(s) of the
5c
9
Na+ > K+ > Li+ > Rb+ > Cs+ 10 17 30 5 Na+ > K+ > Li+ > Rb+ > Cs+
5 17 36 83 Na+ > K+ > Rb+ > Cs+ > Li+ 27 51 67 90
100
68 93
"At pH = 10.0. bData from ref 6. cData from ref 4. lipophilic group(s). Data for the competitive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by 9 are presented in Figure 2. The selectivity order, selectivity coefficients, and maximal metals loading for 9 are compared with reported data for 4 (6) and 5 ( 4 ) in Table 111. Although all three lipophilic dibenzo-16-crown-5-oxyacetic acids 4,5,and 9 are selective for Na+, the selectivity coefficients for 9 are much larger than those for 4 or 5. Thus the Na+ selectivity of the lipophilic dibenzo-16-crown-5-carboxylic acid is found to be much higher when the lipophilic group is attached to the same polyether ring carbon as the sidearm (in 9) than when lipophilicity is incorporated into the benzo groups (in 4) or the sidearm (in 5). Examination of CPK space-filling models reveals that when the octyl group in 9 points away from the polar polyether ring, the carboxylic acid group on the sidearm is positioned directly over the crown ether cavity. Hence, the much higher Na+ selectivity of 9 is ascribed to preorganization of the binding site (13). Effect of Ring Size Variation for Lipophilic Dibenzocrown Ether carboxylic Acids with Preorganized Binding Sites. For the series of lipophilic d i b e m r o w n ether carboxylic acids 10-12, the decyl groups are attached to the central carbon of the three-carbon bridge of the polyether ring which should orient the oxyacetic acid sidearm over the crown ether cavity. Hence with 10-12 the influence of ring size variation may be assessed for lipophilic dibenzocrown ether carboxylic acids with preorganized binding sites. Results for the competitive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by proton-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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20
V 0
10
4
6
8
1
0
4
6
8
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1
4
0
6
8
1
0
Figure 3. Molar concentrations of metals (X103) 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 (a) 10, (b) 11, and (c) 12. Molar chloroformphase complexing agent concentrations (X lo2)are shown as solid squares.
Table IV. Effect of Crown Ether Ring Size on Selectivity and Efficiency of Competitive Alkali-Metal Cation Extraction from Aqueous Solutions into Chloroform by Lipophilic Dibenzocrown Ether Carboxylic Acids 10-12
compound ring size
maximal metals loadings,
selectivity order and selectivity coefficientso
%
10
14C4
Lit > Nat > Kt > Rbt > Cst
11
16C5
12
19C6
Na+ > K+ > Lit > Rb+, Cst 32 66 NDb Kt > Nat > Rbt > Li+ > Cst
3
4
7
14
12
21
100
13
82 48
67
OAt pH = 10.0. bNot detected.
ionizable ionophores 10-12 are shown in Figure 3. Selectivity orders, selectivity coefficients, and maximal metals loadings are recorded in Table IV. For lipophilic dibenzocrown ether carboxylic acids 10, 11, and 12, the crown ether ring sizes are 14-crown-4,16-crown-5, and 19-crown-9, respectively. Each of the three extractants exhibits good-to-excellent extraction selectivity for the alkali-metal cation which should fit best within the crown ether cavity (Tables I and IV). The Li+ selectivity obtained with the lipophilic dibenzo14-crown-4-carboxylic acid 10 contrasts sharply with the Na+ selectivity noted for the lipophilic dibenzo-14-crown-4carboxylic acid 6. Thus preorganization of the binding site by proper positioning of the lipophilic group is demonstrated not only to enhance the alkali-metal cation selectivity as noted earlier for lipophilic dibenzo-16-crown-5-carboxylic acid 9 (vs
2021
4 and 5) but also to change the predominant alkali-metal cation extracted to that which should best fit the polyether cavity (in 10 vs 6). Excellent Na+ extraction selectivity is observed for the lipophilic dibenzo-16-crown-5-carboxylic acid 11. Competitive extraction coefficients are Na+/K+ = 32 and Na+/Li+ = 66 with no detectable extraction of Rbt or Cs+ into the chloroform phase. The lipophilic dibenzo-19-crown-6-carboxylic acid 12 is K+ selectivity. Although the K+/Na+ selectivity Coefficients for 12 and closely related 7 are the same (compare Tables I1 and IV), selectivity coefficients for K+ vs each of the remaining alkali-metal cations are significantly higher for 12. The maximal metals loadings for 10, 11, and 12 are 100%, 82%, and 48%, respectively. The reason for the decrease in extraction efficiency as the ring size increases is not apparent.
CONCLUSIONS For competitive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by lipophilic dibenzocrown ether oxyacetic acids, the selectivity and efficiency are strongly influenced by the crown ether ring size and the attachment site of the lipophilic groups. Attachment of the lipophilic group to the polyether ring carbon atom that bears the oxyacetic acid group orients the sidearm over the crown ether cavity and produces lipophilic dibenzocrown ether carboxylic acids 10, 11, and 12 which exhibit good-to-excellent extraction selectivity for Li+, Na+, and K+, respectively. LITERATURE CITED (1) McDowell, W. J. Sep. Sci. Technol. 1988, 23, 1251-1268. (2) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53. 1894-1899. (3) Strzelbicki, J.; Heo, G. S.;Bartsch, R. A. Sep. Scl. Technol. 1982, 17, 635-643. (4) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 2251-2253. (5) Bartsch, R. A.; Heo. G. S.;Kang, S. I.; Liu, Y.; Strzelblckl, J. J . Org. Chem. 1982, 47, 457-460. (6) Charewicz, W. A.; Heo, G. S.; Bartsch, R. A. Anal. Chem. 1982, 5 4 , 2094-2097. (7) Charewicz, W. A.; Bartsch, R. A. Anal. Chem. 1982, 54, 2300-2303. (8) Bartsch, R. A.; Charewicz, W. A.; Kang, S. 1. J. Membrene Sci. 1984, 17, 94-107. (9) Bartsch, R. A.; Liu, Y.; Kang, S. I.; Son. B.; Heo, G. S.;Hipes, P. 0.; Bills, L. J. J . Org. Chem. 1983, 48, 4664-4869. (10) Pugla, M. J.; Knudsen, 8. E.; Cason, C. V.; Bartsch, R. A. J . Org. Chem. 1987, 52, 541-547. (11) Water. A Comprehensive Treatise; Franks, F.. Ed.; Plenum: New Yak, 1973; VOl 2, pp 54-118. (12) Uhlemann, E.; Geyer, H.; Gloe, K.; Muhl, P. Anel. Chlm. Acta 1988, 185, 279-285. (13) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1988, 25, 1039-1057.
RECEIVED for review March 20,1990. Accepted May 31,1990. The research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the U S . Department of Energy (Contract DE-AS05-80ER10604and Grant DE-FG05-88ER13832). Dr. 11-Woo Yang received a postdoctoral fellowship from the Korea Science and Engineering Foundation.
