Anal. Chem. 1995, 67, 2405-2408
Sodium Ion-Selective Electrodes Based on Dibenzo-I6-crown-5 Compounds with Pendent Ester Groups Akira Ohki,t Jian Ping Lu, Johnny L. Hallman, Xiaowu Huang, and Richard A. Bartsch*
Department of Chemistry and Biochemistry, Texas Tech UniveEity, Lubbock, Texas 79409-1061
Selectivities of 21 dibenzo-16-crown-5compounds with pendent ester groups for alkali metal, allraline earth metal, and ammonium ions have been determined in sokent polymeric membrane electrodes. The ionophores include alkyl sym-R-dibenzo-16-crown-5-o~acetates with R = hydrogen, linear alkyl, branched alkyl, fluoroalkyl, alkenyl, alkynyl, and phenyl groups. For lariat ethers with a hydrogen, linear alkyl group, or branched alkyl group attached to the ring carbon that bears the ester-containing side arm, the Na+/K+ selectivity increases as the bulkiness of the geminal substituent is enhanced; whereas the Na+/Li+selectivity exhibits a weaker inverse relationship to the size of R. On the other hand, the Na+/K+and Na+/ Li+ selectivities are insensitive to variations in the size and shape of the ester alkyl groups. For eight of the lariat ether ester ionophores, very high Na+/LP selectivities of 103.8-103.9are observed. In previous we have investigated the influence of attaching one or two pendent groups to the central carbon of the threearbon bridge in dibenzo-lkrown-5 upon the potentiometric response of the lariat ethers4 in solvent polymeric membranes. Side arm variation included alkyl, OCH3, OCH$02H, OCHr C02C2H5,and OCH~C(O)NRZ(with R = H or alkyl) units. Attachment of a geminal-propyl group to the polyether ring carbon that bears an oxygencontaining side arm markedly increases the selectivity for Na+ relative to larger cations. It was proposed that the alkyl group preorganizes the binding site by orienting the functional side arm over the polyether cavity in a conformation that requires minimal structural reorganization for Na+ complexation.' In the present study, we have examined the influence of structural variation in alkyl sym-R-dibenzo-lkrow-~xyacetates 1-21 (Figure 1) upon the selectivityfor Na+ in solvent polymeric membrane electrodes. In the series of ethyl sym-R-dibenzo-18 crown-5oxyacetates 1-13, the geminal R substituent is varied to include hydrogen, h e a r alkyl, branched alkyl, fluoroalkyl, alkenyl, alkynyl, and phenyl groups. For the series of alkyl symdibenzo-lkrown-5oxyacetates 1 and 14-17 and alkyl sym+ Permanent address: Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Kagoshima 890, Japan. (1) Ohki, A; Lu. J. P.; Bartsch. R A. Anal. Chem. 1994, 66, 651-654. (2) Ohki, A.; Maeda, S.; Lu,J. P.; Bartsch, R A Anal. Chem. 1994, 66, 17431746. (3) Ohki, A.; Lu,J. P.; Huang, X.; Bartsch, R. A. Anal. Chem. 1994, 66, 43324336. (4) Gokel, G. W.; Dishong, D. M.; Diamond, C. J.J. Chem. Soc., Chem. Commun. 1980. 1053-1054.
0003-2700/95/0367-2405$9.00/0 0 1995 American Chemical Society
0 II
xHZco a0 0D /
Co4 *
/
13 14 15 16 17 18 19 20 21
Figure 1. Structures of lariat ether esters.
(propyl) dibenzo-lkrown-5oxyacetates 4 and 18-2 1, the geminal group is held constant and the ester alkyl group of the lariat ether ester is varied. EXPERIMENTAL SECTION
Chemicals. Poly(viny1 chloride) (PVC) with an average polymerization degree of 1100 and dibenzyl ether @BE) were purchased from Wako Pure Chemical Industries (Osaka,Japan). o-Nitrophenyl octyl ether (NPOE) and potassium tetrakis(pchlorophenyl)borate (KTpClPB) were obtained from Dojindo Laboratories (Kumamoto, Japan). Alkali metal, alkaline earth metal, ammonium chlorides, and tetrahydrofuran (THF)were reagent-grade chemicals. Deionized water was prepared by passing distilled water through three Barnstead D8922 combination cartridges in series. Lariat ether ethyl esters l5and 46 were prepared by the published methods. The lariat ether carboxylic acid precursors for lariat ether ethyl esters 2 , 3 ,and 5-7 have been r e p ~ r t e d . ~ - ~ (5) Bartsch, R A; Heo, G. S.: Kang, S. I.; Liu, Y.; Strzelbicki, J.J. Og. Chem. 1982, 47, 457-460. (6) Hayashita, T.;Goo, M.-J.; Lee. J. C . ; Kim, J. S.; Krzykawski, J.; Bartsch, R. A. Anal. Chem. 1990, 62. 2283-2287. (7) Bartsch, R. A; Liu, Y.; Kang, S. I.: Son, B.; Heo. G. S.; Hipes, P. G.; Bills, L. J. J. 0%. Chem. 1983, 48, 4864-4869.
Analytical Chemistry, Vol. 67, No. 74, July 15, 1995 2405
Table I. Selectivities of Ethyl symR-Dibenzo~l6-crown-5~oxyacetates 1-1 3 for Na+ over Alkali Metal, Alkaline Earth Metal, and Ammonium Ions
lariat ethef compd 1 2
3 4
5 6
7 8 9 10 11 12 13
-log
R
H CH3 C2H5 C3H7 C4H9 CBH17
CioHzi % )3( ( 2CH (CW3CCHz C6F13 C6H5 (CH3)zCzCH CtjHi3CEC
gab
Li+
0.00 -0.35 -0.49 -0.54 -0.57 -0.59 -0.62 -0.67
3.87c 3.74 3.73 3.73c 3.78 3.75 3.75 3.67 3.68
-0.81 -0.50' -0.60/
3.83 3.85 3.90
K' 0.45c 1.36 1.49 1.4gC 1.54 1.55 1.59 1.73 1.70 1.26 1.39 1.25 0.94
KN,,MP"'
Rb+
Cs+
Mg2+
Ca2+
Sr2+
Ba2+
NH4+
0.8gd
1.46d
3.81d
3.4od
2.63d
2.56d
1.97d
2.16d
2.61d
3.8W
3.84d
2.7W
3.18d
3.2W
2.18 2.35 2.35
2.65 2.75 2.69
3.83 3.81 3.86
3.75 3.69 3.80
2.65 2.72 2.66
3.18 3.12 3.12
3.27 3.30 3.32
1.98 1.79 1.53
2.35 2.37 1.92
3.79 3.74 3.75
3.78 3.79 3.80
2.65 2.72 2.71
3.12 3.11 2.84
2.57 2.99 2.61
Structures for the lariat ether esters are shown in Figure 1. Reference 12. Data from ref 1. Data from ref 3. e Value for CHZ-CH. 'Value for HCsC. (I
The carboxylic acid precursors for lariat ether esters 8-13 were prepared by adaptations of the reported methods. Lariat ether ethyl esters 2, 3, and 5-13 were synthesized by refluxing for 18 h the appropriate lariat ether carboxylic acid in ethanol to which a few drops of concentrated sulfuric acid catalyst had been added. The condensate was passed through a Soxhlet thimble containing anhydrous sodium sulfate to absorb the evolved water. Lariat ether esters 14-21 were prepared by first converting the appropriate lariat ether carboxylic acid into the corresponding lariat ether acid chloride by reaction with 2 equiv of oxalyl chloride in benzene at room temperature for 10 h. The resultant lariat ether acid chloride was dissolved in THF, and 4 equiv of pyridine were added. The appropriate alcohol was added, and the solution was stirred at room temperature for 3 h. Structures for lariat ether esters 2,3,and 5-21 were confirmed by 'H NMR and IR spectra and by combustion analysis. A detailed description of the syntheses will be provided elsewhere. Preparationof PVC Membranes. PVC (50 mg), NPOE (100 mg), the lariat ether ester (5.0 mg), and KTpClPB (1.0 mg) were dissolved in 1.5 mL of THF. An aliquot of the THF solution was applied to a porous polytetrafluoroethylene (PTFE) membrane attached to a PVC tube, and the solvent was allowed to evaporate for 15-20 min. Addition of the THF solution and evaporation were repeated eight or nine times. The resulting PVC tube with the coated PTFE membrane was fixed on a Denki Kagaku Keiki @KK, Musashino, Tokyo, Japan) No. 7900 electrode body. An internal filling solution of 0.10 M NaCl was added to the electrode. The electrode was conditioned by soaking in 0.10 M NaCl solution for 12 h before use. Measurements. Potentiometric measurements with a m e m brane electrode were carried out at 24-25 "C with a voltage meter (Fisher Scientific, Accumet 50 pH meter), a double junction AgAgCl reference electrode @KK No. 4083), and a magnetic stirrer to agitate the sample solution. The electrode cell was Ag-AgC1/ 0.10 M NaCl/WC membrane/sample solution/O.lO M NH4N03/3 M KCl/Ag-AgC1. Single ion activities were obtained as described earlier.' Selectivity coefficients ( K ~ , , , M were ~ ~ determined ~) by the J. L.; Whaley, L. W.: Desai, D. H.; Pugia. M. P.; Bartsch, R. A.] Membr. Srd. 1991,56,195-206. (9) Hayashita,T.; Goo, M.-J.; Kim, J. S.; Bartsch, R A Tule& 1991,38,14531457.
(8) Brown, P. R.; Hallman,
2406 Analytical Chemisfry, Vol. 67,No. 14, July 15, 1995
fixed interference method.I0 The constant background concentrations of interfering ions were 0.50 M for Li+, 5.0 x M for K+, 0.10 M for Rb+ and Cs', 1.0 M for Mg*+, and 0.50 M for other alkaline earth metal cations and NH4+. For a given solvent polymeric membrane electrode system, the selectivity was determined twice for each of two independently prepared membranes. The average value for the potentiometric selectivity was calculated from the values obtained for the four measurements. The standard deviation of the log Pot values from the average was less than 0.05. RESULTS AND DISCUSSION The alkyl sym-Rdibenzo-lkrown-50xyacetates1-2 1 (Figure 1) were incorporated into solvent polymeric membranes in which PVC was the polymer and NPOE was the membrane solvent. For the ion-selective electrodes (ISEs) prepared from these membranes, selectivities for Na+ relative to other alkali metal cations, alkaline earth metal cations, and NH4+ were determined by the fixed interference method.1° Nerstian responses (59 mV/decade) were obtained for all of the solvent polymeric membrane electrodes. ISEs prepared with lariat ether esters 1-21 were selective for Na+ over all of the other cations tested. Selectivity coefficients expressed as log values for Na+ over other alkali metal cations, alkaline earth metal cations, and NH4+ are presented in Table 1. Lariat Ether Ethyl Esters with Linear or Branched gemAlkyl Groups. Lariat ether esters 2-9 have a linear or branched gem-alkyl group and an ester-containing side arm attached to the central carbon of the three-carbon bridge in the dibenzo-16 crown-5 ring. Compared with lariat ether esters 2-7, which have values for 8 and 9,which linear gem-alkyl groups, the log have branched gem-alkyl groups (isopropyl and neopentyl, respectively), are more negative by 0.14-0.37. (It was not possible to synthesize an analogous compound with a geminal tert-butyl group." ) Thus the change from a linear to a branched gemalkyl group is found to enhance the Na+/K+ selectivity. This tendency is also observed for the Na+/Rb+ and Na+/Cs+ selectivities. On the other hand, the lariat ether ethyl esters 2-7 with ~~~
~
(10) Recommendation for Nomenclature of Ion-Selective electrodes. Pure Appl. Chem. 1976,48, 127-132. (11) Kim, J. S. Doctoral Dissertation, Texas Tech IJniversity, 1993, pp 30-31.
-5.0-1.0 1 -0.5 0.05 L
4
%
Figure 2. Plot of log K N ~vs a, , for ~ lariat ~ ~ ether ~ ethyl esters with (a) NPOE and (b) DBE as the membrane solvents.
0 ,
Figure 3. Plot of log K N ~ , vs L ,ua ~ for ~ ~lariat ether ethyl esters. table 2. Na+/Li+and Na+/K+Selectivities for R’ symR-DIbenzo-16~crown-5~oxyacetates
linear gem-alkyl groups are somewhat superior to lariat ether ethyl compdO R R -log KNa,LiPot -log KNa,KPot esters 8 and 9 in Na+/Li+ selectivity. 1 H CzH5 3.87b 0.45b To probe the influence of geminal substituents on the Nat 14 H 3.83 0.48 , K ~ for ~ ~lariat ether ethyl esters 1-6, selectivity, the log K N ~ values 15 H CioHzi 3.84 0.46 16 H CH(CH3)z 3.78 0.42 8,and 9 were plotted against a,, the polarizability constantl2 (Table 17 H C(CHd3 3.80 0.51 l),for each geminal group (Figure 2a). The substituent variation 1.4gb 4 C3H7 CzHj 3.73b includes hydrogen, methyl, ethyl, propyl, butyl, octyl, isopropyl, 18 C3H7 3.73 1.54 19 C3H7 CioHzi 3.73 1.48 and neopentyl groups. The polarizability constant a, offers a 20 C3H7 CH(CH3)3 3.76 1.51 quantitative measure of substituent polarizability or bulkiness.’* 1.53 21 C3H7 C(CH3)3 3.63 A h e a r relationship is evident with a slope of 1.8 for the least Structures for the lariat ethers are shown in Figure 1. Data from squares line. It should be noted that with other substituent ref 1. constants, such as the Taft substituent constants13u* and the Taft steric constant^'^ E,, linear relationships were not obtained. Since a, values may reflect either the polarizability or bulkiness appears to be dependent on the bulkiness of the gem-alkyl group. of a substituent,Qlog K ~ Jvalues ~ Kwere ~ determined ~ ~ for the same Thus, the presence of branched gem-alkyl groups, such as series of ionophores in solvent polymeric membranes for which isopropyl and neopentyl, brings about a more favorably preorgathe membrane solvent was changed from NPOE (dielectric nized structure for Na+ complexation, resulting in an increase in constant, E = 24) to DBE ( E = 4). A plot of the log K N ~ values ,K~~~ the Na+/K+ selectivity. with DBE against a, is presented in Figure 2b. The Na+/K+ vs a, (Figure 3) shows a linear relationship A plot of log selectivity is noted to decrease for each lariat ether ethyl ester with a small negative slope (-0.2). Thus, the presence of when DBE is the membrane solvent. However, the slope of the branched gem-allryl substituents (isopropyl and neopentyl) appears correlation line is 2.0, which is very nearly the same as that found to modestly diminish the Na+/Li+ selectivity compared with that with NPOE as the membrane solvent (Figure 2a). Thus, the effect for linear gem-alkyl groups. The opposing effects of gem-alkyl of structural variation within the geminal substituent is found to group branching upon the Na+/K+ and Na+/Li’ selectivities may be independent of the polarity of the membrane solvent. Since be rationalized by consideration of the types of metal ion-lariat the polarity of the membrane solvent would be expected to ether ester complexes which are formed. As discussed above, influence the substituent polarizability, it appears that the u, values Na+ should form nesting complexes with dibenzo-lkrown-5 reflect the bulkiness of the substituent in this case rather than its compounds, while K+ should produce perching complexes. On polarizability. the other hand, both Na+ and Li+ should form nesting complexes. Based upon the relationship between the size of the alkali metal Therefore, factors that enhance complexation of Na+, such as cations and the crown ether cavity for dibenzo-16-crown-5 compreorganization of binding site, may also promote Li+ binding. It pounds,’ it is anticipated that Na+ will be accommodated within appears that the enhancement is greater for the harder alkali metal the polyether cavity and form an inclusion or “nesting”c0mp1ex.l~ cation, which diminishes the Na+/Li+ selectivity. Thus, a more On the other hand, K+ is too large to fit within the cavity and will preorganized structure results in a reduction in the Na+/Li+ produce a “perching” complex15 in which the metal ion sits on selectivity. top of the polyether ring oxygens. Preorganization of the binding The effect of varying the structure of the ester alkyl group in site by the presence of a gem-alkyl group favors the nesting alkyl sym-dibenzo-lfkrown-5ovacetates 1and 14- 17 and symcomplex and enhances the Na+/K+ se1ectivity.l Furthermore, the (propyl)dibenzo-l&crown-5oxyacetates 4 and 18-21 upon the degree of preorganization for the ester-containing side arm Na+/K+ and Na+/Li+ selectivities of solvent polymeric membrane (12) Hehre, W. J.; Pau, C. F.; Headly, A. D.; Taft, R W.J Am. Chem. Soc. 1986, electrodes into which these ionophores have been incorporated 108, 1711-1712. is recorded in Table 2. The structural variation includes ethyl, (13) Dean, J. A. Lange’s Handbook of Chemist?, 13th ed.; McGraw-Hill: New York, 1985; pp 5135-3-138. hexyl, decyl, isopropyl, and tert-butyl groups. It is readily evident (14) Taft, R. W. In Steric Effects in Organic Chemistv; Newmann, M. S., Ed.; that this structural modification produces no signifkant change Wiley: New York, 1956 p 598. in either the Na+/K+ and Na+/Li+ selectivities. If, as p r o p o ~ e d , ~ (15) Cram, D. J.; Trueblood, K. N. Host Guest Complex Chemistry. Macrocycles; Vogtle, F., Weber, E., Eds.; Springer-Verlag: New York, 1985; pp 135-188. the carbonyl oxygen of the ester-containing side arm coordinates Analytical Chemistry, Vol. 67,No. 14, July 15, 1995
2407
with the polyether ring-bound metal ion, the alkyl group of the ester function will be directed away from the crown ether cavity. Insensitivity of the Na'/K" and Na+/Li+ selectivities to large changes in the size and shape of the alkyl group portion of the ester function supports such conformational orientation.
Lariat Ether Esters with Unsaturated Geminal Groups. Lariat ether ethyl esters 11-13 have unsaturated geminal substituents (2-methyl-1-propenyl, phenyl, and 1-octynyl, respectively) attached to the central carbon of the three-carbon bridge in the dibenzo-lfkrown-5 ring. For the lariat ether ethyl esters 1-13, the selectivities for Na- over the larger alkali metal cations K+, Rb', and Cs' decrease as the geminal substituent is varied: branched alkyl (8,9),linear alkyl (3-7) > phenyl (12) > alkenyl (11) > alkynyl (13) > hydrogen (1). Comparison of the a, values and Naf/K' selectivities given in Table 1 for lariat ether esters 11- 13,which have unsaturated geminal substituents, with the correlation shown in Figure 2a for lariat ether esters 1-9, which have saturated alkyl groups, reveals that points for 1113 would diverge from the line. Thus, when the hybridization of the first atom of the geminal hydrocarbon group is changed from sp3to sp2 or sp, the Na+/K+ selectivity no longer correlates with a,, the substituent polarizability constant. A possible rationalization of the lower Na+/K+ selectivities observed for ionophores with unsaturated geminal substituents involves the electron-withdrawing nature of such groups. Electron withdrawal by the geminal substituent could lower the electron density on the ether oxygen of the side arm and thereby diminish the overall binding ability of the ionophore. To test this hypothesis, lariat ether ester 10, which has a gem-peduorohexyl group, was investigated. The perfluorohexyl group is saturated, but strongly electron withdrawing. As can be seen from the data presented in Table 1, the Na'/K+ selectivity for 10 is considerably lower ~~
~
~
~~~~~~~~
(16) Yamamoto. H.; Shinkai, S. Chem. Lett. 1994, 1115-1118. (17) Diamond, D.; Svehla, G.; Seward, E. M.: McKervey, M. A.Anal. Chim.Acta 1988,204, 223-231. (18) Kimura. K.; Matsuo, M.; Shono, T. Chem. Lett. 1988,615-616. (19) Kimura, K; Miura. T.; Matsuo, M.; Shono, T. Anal. Chem. 1990,62,15101513. (20) Suzuki, K.; Hayashi, IC;Tohda, K.; Watanabe, K.; Ouchi, M.; Hakushi, T.; Inoue, Y. Anal. Lett. 1991,24, 1085-1091. (21) Shono, T.; Okahara. M.; Ikeda, I.; Kimura, K; Tamura, H. J. Electroanal. Chem. 1982, 132, 99-105. (22) Tamura, H.; Kimura, K.; Shono, T. Anal. Chem. 1982, 54, 1224-1227.
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Analytical Chemistry, Vol. 67, No. 14, July 15, 1995
than those for lariat ether esters with linear alkyl geminal groups and is similar to that for ionophore 12, which has a gem-phenyl group. This supports the contention that the diminished Na+/ K* selectivities found for the lariat ether esters 11-13, which have unsaturated geminal groups, arise from the electronwithdrawing properties of these substituents.
Comparison with Other Sodium-Selective Electrodes. Very recently calix[4larene-based, sodium-selective electrodes with remarkably high log K N ~ ,values K ~ ~of~5.0-5.3 have been reported by Yamamoto and Shinkai.lG The ionophores in these ISEs are based upon a combination of calixarene and crown ether structures and give marked Na+/K+ selectivity enhancements over those obtained earlier by other workers with calii[4]arene estersl7-l9and amide^,'^,.'^ lipophilic l&crown5 derivatives,?Oand bis(l2-crown-4) When incorporated into polymeric membranes, the dibenzo-l&rown-5 lariat ether esters employed in the present study are found to have lower Na+/K" selectivities than the ionophores mentioned above as well as dibenzo-16 crown5 lariat ether amides.' On the other hand, polymeric membrane electrodes prepared with lariat ether esters 1 and 11- 17 exhibit very high Na+/Li+ selectivities (log K N ~ = , L-3.8 ~ ~to~ -3.9), ~ which surpass those reported recently for other ionophores including (type of ionophore, log KN~,L~PO~): a calii[4larene ether, -3.816; three calii[4]arene esters -2.917, -3.018, and -3.616; a bis(12-~rown-4),~~ -3.0; lipophilic l6crown-5 derivatives,?O -1.8 to - 2.5; and a dibenzo16crown-5 lariat ether amide,' -2.9. ACKNOWLEDGMENT This research was supported by the Division of Chemical Sciences of the Office of Basic Energy Sciences of the US. Department of Energy (Grant DEFG03-94ER14416). We express our appreciation to Dr. A. D. Headley of the Department of Chemistry and Biochemistry at Texas Tech University for helpful discussions concerning the polarizability constants. Received for review February 20, 1995. Accepted April 25, 1995.B AC950187Q Abstract published in Advance ACS Abstracts, June 1, 1995
