Anal. Chem. 1994,66, 4332-4336
Alkali Metal, Alkaline Earth Metal, and Ammonium Ion Selectivities of Dibenzo-lG=Crown-S Compounds with Functional Side Arms in Ion-Selective Electrodes Akira Ohki,t Jian Ping Lu, Xiaowu Huang, and Richard A. Bartsch* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Potentiometric selectivities of 11 dibenzo-16-crown-5 compounds for alkali metal, alkaline earth metal, and ammonium ions have been determined in solvent polymeric membrane electrodes. The ionophores bear one or two pendent groups on the central carbon of the threecarbon bridge in the polyether ring. Side-armvariation includes (O)N(CzH5)2, and OCH~C(O)N(C~HII)~ Units. Attachment of a propyl group to the ring carbon that bears an extended, oxygen-containing side arm increases the selectivityfor Na+ relative to larger alkali metal and alkaline earth metal cations. For a given side arm, a linear relationship is obtained when the enhancement in Na+ selectivity produced by attachment of a geminal propyl group is plotted against the diameter of the interference ion. Potentiometricresponses of the dibenzo-16-crown-5 compounds are rationalized in terms of the crown ether ring size and the oxygen basicity, conformational positioning, and rigidity of the side arm. In earlier work, we prepared lariat ether' derivatives of dibenzo16-crown-5, such as syn-(R)-dibenzo-lkrown-5oxyaceticacid (1 with R = H or alkyl) in which an oxyacetic acid group (OCHz COZH)is attached to the central carbon of the threecarbon bridge in the polyether ring. Such proton-ionizable lariat ethers in their ionized forms are efficient and selective agents for the solvent extraction of alkali metal, alkaline earth metal, and lanthanide ions and their transport through liquid membranes-" We have also ' Permanent address: Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Kagoshima 890, Japan. (1) Gokel, G. W.; Dishong, D. M.; Diamond, C. J. J. Chem. Soc., Chem. Commun. 1980, 1053-1054. (2) Strzelbicki, J.; Bartsch, R. A Anal. Chem. 1981, 53, 1894-1899. (3) Bartsch, R A.; Heo, G. S.; Kang, S. I.; Liu,Y.; Strzelbicki, J.]. Oyg. Chem. 1982, 47, 457-460. (4) Charewicz, W. A; Heo, G. S.; Bartsch, R. A. Anal. Chem. 1982,54,20942097. (5) Bartsch, R A,; Liu, Y.; Kang, S. I.; Son, B.; Heo, G. S.; Hipes, P. G.; Bills, L. J. Org. Chem. 1983, 48, 4864-4869. (6) Tang, J.; Wai, C. M. Anal. Chem. 1 9 8 6 , 58, 3233-3235. (7) Charewicz, W. A.; Walkowiak, W.; Bartsch, R A Anal. Chem. 1987, 59, 494-496. (8) Tang, J.; Wai, C. M. J. Membr. Sci., 1988, 35, 339-345. (9) Bartsch, R A Solvent Extr. Ion Etch. 1989, 7, 829-854. (10) Walkowiak, W.; Charewicz, W. A.; Kang, S. I.; Yang, I. W.; Pugia, M. J.; Bartsch, R A Anal. Chem. 1990, 62, 2018-2021. 4332 Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
investigated the influence of nonionizable side arms, such as OCHzCOzH (unionized) ,12 OCHzCOzCzH5,12and OCHzC(0)NRR' with R = H or alkyl (CZ-CS) and R' = alkyl (C2-C6),12J3upon the complexation of Lit, Na+, and K+ by dibenzo-16-crown5 compounds in solvent polymeric membrane electrodes. In the present study, we have determined the potentiometric selectivities of solvent polymeric membrane electrodes containing dibenzo-lkrownd compounds 2- 12 (Figure 1) as ionophores for five alkali metal, four alkaline earth metal, and ammonium ions. Within the series of compounds 3-7,the sidearm variation on the central carbon of the three-carbon bridge in the polyether ring includes OCH3, OCHzCHzOCH3, OCHZCOZCZH~, OCHzC(0)N(C&)z, and OCHzC(O)N(CsH132 units; while in 8-12,a geminal propyl group is attached to the central carbon in addition to the oxygen-containing side arm. EXPERIMENTAL SECTION Chemicals. Poly(viny1 chloride) (PVC) with an average polymerization degree of 1100 was purchased from Wako Pure Chemical Industries (Osaka, Japan). o-Nitrophenyl octyl ether (NPOE) and potassium tetrakis(pchloropheny1)borate(KTpClPB) were obtained from Dojindo Laboratories (Kumamoto, Japan). Alkali metal and alkaline earth metal chlorides, ammonium chloride, and tetrahydrofuran WF) were reagent-grade chemicals. Deionized water was prepared by passing distilled water through three Barnstead D8922 combination cartridges in series. Dibenzo-16-crown-5compounds 3,543, and 10-12 are known compound^.^^^^^ The new lariat ethers, sym-(1,4-dioxapentyl)dibenzo-lkrown-5 (4) and sp-(l,4dioxapentyl)(propyl)dibenzo-lkrown-5 (9),were prepared as follows. After removal of the protecting mineral oil from 1.02 g of potassium hydride (35%dispersion in mineral oil, 9.0 mmol) by washing with pentane under nitrogen, 3.0 mmol of sym-(hydroxy)dibenz0-16-crown-5~~ or sym-(hydroxy) (propyl)dibenzo-lkr0wn-5~~ in 100 mL of dry THF was added. The mixture was stirred for 30 min at room temperarture and 0.56 mL (6.0 "01) of 1-bromo-2-methoxyethane was added dropwise. (11) Wai, C. M.; Du, H. S. Anal. Chem. 1990, 62, 2412-2414. (12) Ohki, A Lu,J. P.; Bartsch, R A Anal. Chem. 1 9 9 4 , 66, 651-654. (13) Ohki, A; Maeda, S.; Lu,J. P.; Bartsch, R A Anal. Chem. 1994,66,17431746. (14) Heo, G. S.; Bartsch, R A; Schlobohm, L. L.; Lee, J. G . ] 0%.Chem. 1981, 46, 3574-3575. (15) Hayashita, T.: Goo, M.-J.; Lee, J.-C.; Kim, J. S.; Kryzkawski, J.; Bartsch, R. A Anal. Chem. 1 9 9 0 , 62, 2283-2287. 0003-2700/94/0366-4332$04.50/0 0 1994 American Chemical Society
0
II
R
OCH2COH
n
1
A' -
When R = H
When A = C3H7
3
-OCH3
8
4
-0CHzCH20CH3
9
5
-0CHzC02CzH5
10
6
-OCHZC(O)N(QH5)Z
11
7
-OCH2C(O"CSH11)2
12
Figure 1. Structures of dibenzo-16-crownd and related lariat ether derivatives.
The mixture was refluxed for 4 h, and water was carefully added to destroy the unreacted potassium hydride. The THF was evaporated in vacuo, and dichloromethane (150 mL) and water (100 mL) were added to the residue. The mixture was shaken, and the dichloromethane layer was separated, washed with water (2 x 50 mL), dried over magnesium sulfate, and evaporated in vacuo to give the crude product. For 4, the crude product was recrystallized from hexanes (50 mL) to give a 77%yield of white crystals: mp 62-63 "C; IR (deposit from CDCl3 solution on a NaCl plate), 1256 (C-0) cm-I; 'H NMR (CDCld, 6 3.40-3.55 (s, 3 H); 3.62-4.40 (m, 17 H); 6.83-7.05 (m, 8 H). Anal. Calcd for CzzHzsOi: C, 65.35; H, 6.93. Found: C, 65.56 H, 6.96. For 9, the crude product was chromatographed on alumina with dichloromethane as eluent to give a colorless oil in 83%yield: IR (neat), 1257 (C-0) cm-l; lH NMR (CDC13), 6 0.94-1.01 (t, 3 H), 1.401.55 (m, 2 H), 1.88-2.00 (m, 2 H), 3.40 (s, 3 H),3.55-3.62 (t, 2 H), 3.90-4.35 (m, 14 H), 6.81-6.96 (m, 8 H). Anal. Calcd for C25Ha07: C, 67.24; H, 7.67. Found: C, 67.46; H, 7.77. Preparation of PVC Membranes. PVC (50 mg), NPOE (100 mg), the lariat ether (5.0 mg), and KTpClPB (1.0 mg) were dissolved in 1.5 mL of THF. An aliquot of the THF solution was poured onto a porous poly(tebatluoroethy1ene)W E ) 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 W C tube with the coated F'TFE membrane was fixed on a Denki JSagaku 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 membrane electrode were carried out at 24-25 "C with a voltage meter (Fisher Scientific Accumet 50 pH meter), a doublejunction AgAgCl reference electrode @KK No. 4083), and a magnetic stirrer to agitate the sample solution. The electrode cell was Ag-AgCl/ 0.10 M NaCl/WC membrane/sample solution/O.lO M NH4N03/ 3.0 M KCl/Ag-AgCl. Single ion activities were obtained as described previously.12 The selectivity coefficients (KN~,M~O~) were determined by the fixed interference method.16 The constant background concentrations of interfering ions were 0.10 or 0.50 or 5.0 x M for K+, 2.0 x lo2or 0.10 M M for Lit, 1.0 x for Rb+ and Cs+, 0.10 or 0.50 M for NH4+, 1.0 M for Mgz+,and 0.50 M for Ca2+,Sr2+,and Ba2+. For a given solvent polymeric membrane electrode system, the potentiometric 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 from the average value for log Pot was less than 0.05. RESULTS AND DISCUSSION Potentiometric Selectivities of Ionophores 2- 12. Ionophores 2-12 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, potentiometric selectivities for Na+ relative to other alkali metal cations, to alkaline earth metal cations and to NH4+ were determined by the fixed interference method.16 Nernstian responses (59 mV/decade) were obtained for these solvent polymeric membrane electrodes. In Table 1are recorded the selectivities (log K N ~ , M for~ the ~ ~ISEs ) based on ionophores 2-12. When dibenzo-l&crown-5 (2) was utilized as the ionophore in the solvent polymeric membrane, positive log K N ~ , M values ~o~ were observed for M = Kf and Rb+. Thus the ISE exhibits a greater potentiometric response to K+ and Rb+ than to Na+. The response to monovalent cations decreases in the order K+ > Rb+ > Na+ > Cs+ > NH4+ > Li+. From comparison of the cavity diameter of dibenzo-lkrown-5 and the ionic diameters of the alkali metal ions and NH4+,l7J8both Na+ and Id+ are expected to form inclusion or nesting complexes,lg with Na+ fitting the polyether cavity much better than Lit. On the other hand, K+, Rb+, Cs+, and NH4+ are too large to fit within the cavity and should form perching c o m p l e x e ~ which , ~ ~ are anticipated to be less stable than nesting complexes. Although selective response to Na+ is expected for the ISE containing ionophore 2, it does not exhibit such selectivity. This is ascribed to a higher hydration energy of Na+ compared with K+ and Rbt.12.20 Lariat ethers 3-7 are substituted dibenzo-lkrown-5 compounds which have one or more oxygen atoms in their side arms. These oxygen atoms are potential ligation sites for cationic species. (16) Recommendation for Nomenclature of Ion-Selective Electrodes. Pure Appl. Chem. 1976,48, 127-132. (17) Lamb, J. D.; Izatt, R M.; Christensen, J. J. In Progress in Macrocyclic Chemistry; Izatt, R M., Christensen, J. J., Eds.; Wiley: New York, 1981; Vnl. 2, pp 41-90. (18) Izatt, R M.; Bradshaw, J. S.; Nielsen, S. A; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1985,85, 271-339. (19) Cram, D. J.; Trueblood, K. N. In Hast Guest Complex Chemistry. Macrocycles; Vogtle, F., Weber, E., Eds.; Springer-Verlag: New York, 1985; pp 135-188. (20) Burgess, J. Metal Ions in Solution;Wiley: New York, 1978; p 186.
Analytical Chemistry, Vol. 66,No. 23, December 1 , 1994
4333
Table 1. Potentiometric Selectivitles of Lariat Ethers 2-12 for Na+ over Alkali Metal, Alkaline Earth Metal, and Ammonium Ions
lariat etheP cmpd
R
2 3
H H H H H H C3H7 C3H7 C3H7 C3H7 C3H7
4 5
6 7 8 9 10 11 12 a
-log
R'
Li+
H 2.48' OCH3 3.08' OCHzCHzOCH3 3.23 OCHzCOzCzHs 3.87' 3.02b OCHzC(O)N(CzH5)2 OCHZC(O)N(C~HI~)Z 3.14c OCH3 2.99' OCHzCH20CH3 3.49 OCHzCOzCzHs 3.73' 2.84' OCHzC(0)N(CzH5)2 2.91' OCHzC(O)N(CsHii)z
KNa,MPOf
K+
Rb+
Cs+
Mg2+
Ca2+
Sr2+
Ba2+
NH4+
-0.42b -0.07b 0.46 0.45b 0.7gb 0.98c 0.34' 1.34 1.49' 1.98' 2.13c
-0.06 0.29 0.81 0.88 1.42 1.48 0.35 1.96 2.16 2.62 2.77
0.48 0.88 1.49 1.46 2.05 2.18
3.36 3.53 3.67 3.81 3.76 3.88 3.73 3.81 3.80 3.83 3.85
2.49
2.22
2.88
2.50 2.62 2.63 2.31 2.41 2.53 2.62 2.70 2.66 2.71
2.62 2.59 3.08 2.56 1.40 1.56 3.09 3.63 3.18 2.76 2.68
0.94 1.48 1.93 1.97
1.08
2.53 2.61 3.07 3.18
3.20 3.40 2.90 3.19 3.25 3.78 3.84 3.55 3.63
2.11
2.13 1.68 3.03 3.20 3.02 3.12
Structures of the lariat ethers are shown in Figure 1. Data from ref 12. Data from ref 13.
Compared with 2, the log K N ~ ,values M ~ ~for~ sym-(methoxy)dibenzo-l&crown-5 (3),which has a methoxy group for the side arm, are more negative by 0.35-0.60 for M = Lit,K+, Rb', Cs+, and NH4+. This is attributed to an increase in the Na+ complexation by 3 relative to 2 due to coordination of Na+ by the oxygen atoms in the crown ether ring and in the side arm.When a second ether oxygen is introduced into the side arm to give 4, the log K N ~ values , M ~ become ~~ more negative by 0.15 when M = Li+ and by 0.45-0.61 when M = K+, Rb+, Cs+, and NH4+ compared with the values for 3. This demonstrates coordination of the complexed Na+ by the second oxygen in the side arm as well. The enhancement in Na+ selectivity for the ISE containing 4 relative to 3 is larger when the competing ion forms a perching complex (with K+,Rb+,Cs+, or NH4+) rather than a nesting complex (with
Li+).
2 r a) R' = -OCH,
Ba
D 0
t
- -t
b) R' = -0CHZCHzOCH3
Lariat ethers 5-7 have side arms of OCHZCOZC~H~, OCH2C(O)N(C2H&, and OCHzC (O)N(CSH&, respectively. Compared with 3, for which the side arm is OCH3, the log K N ~ values M ~ ~ ~ noted for 5-7 when M = K+, Rb+, Cs+, and NH4+ are more negative by 0.49-0.59, 0.63-1.17, and 0.65-1.30, respectively. This is consistent with interaction of both the ether and the carbonyl oxygens in the side arm with the polyether cavityD complexed Na+. Examination of CPK (Corey-Pauling-Kortum) space-filling models reveals that when the side arm is oriented over the crown ether cavity, the carboxyl oxygen can occupy an apical coordination site for the complexed Na+. The Na+ selectivity increases as the pendent sidearm function is changed from ester to amide, which is in agreement with the enhanced basicity of the carbonyl oxygen in the latter.21 Also the trend for slightly 1 2 3 4 higher Na+ selectivityfor the N,N-dipentyloxyacetamideside arm Ionic diameter (A) in 7 compared with the corresponding Nfl-diethyloxyacetamide Figure 2. Differencesin log KNa,MPo'between lariat ethers with and side arm in 6 may be rationalized in terms of an anticipated without a geminal propyl group [-D = (log KNa,MPotforthe lariat ether slightly higher carbonyl oxygen basicity of the former. with a geminal propyl group) - (log KNa,MPo' for the corresponding Effect of Preorganization. Lariat ethers 8-12 have a lariat ether with a geminal hydrogen atom)] for dibenzo-16-crown-5 lariat ethers with (a) OCH3 and (b) OCH2CH20CH3 side arms versus geminal propyl group attached to the central carbon of the three the diameter of the interference ion M+. carbon bridge in the polyether ring, as well as an oxygencontaining side arm. The presence of the geminal propyl group reorganizationfor metal ion complexation.12 Such preorganization is postulated to orient the functional side arm over the cyclic of the binding site enhances ~electivity.*g~~~ polyether cavity in a conformation that requires minimal structural To probe the effect of preorganization on Na+ selectivity, the difference in log K N ~ ,values M ~ ~between ~ lariat ethers with and (21) March, J. Advanced Organic Chemistry: Wiley: New York, 1985; pp 220221.
4334 Analytical Chemisfry, Vol. 66, No. 23, December 1, 1994
(22) Cram, D. J. Angew. Chem., 1st. Ed. Engl. 1986,25,1039-1057.
a) R' = -OCH2C02C2H5
Table 2. Slopes and Correlation Coefficients for the Plots in Figures 2b and 3a-c
R'
slope
con coeff
OCHzC (O)N(C5Hii)z OCHzCOzCzHs OCHzC (0)N (CzH5)z OCHzCHzOCH3
0.69 0.68
0.91
0.64 0.46
0.87
0.89 0.79
D
1
1
b) R' = -OCH2C(O)N(C2H&
D
Li
D
-1 1
2 3 Ionic diameter (A)
Figure 3. Differences in log K N ~ , M between ~"' lariat ethers with and for the lariat ether without a geminal propyl group [-D = (log KN~,M~"' with a geminal propyl group) - (log KN,M'"'for the corresponding lariat ether with a geminal hydrogen atom)] for dibenzo-16-crown-5 lariat ethers with (a) OCH2C02CzH5, (b) OCHZC(O)N(CZH~)~, and (c) OCH2C(0)N(C5H11)2 side arms versus the diameter of the interference ion M+.
without the geminal propyl group [-D= (log K N & M for~the ~ ~lariat ether with a geminal propyl group) - (log KN,,M~O~ for the corresponding lariat ether with a geminal hydrogen atom)] is plotted against the ionic diameter of the interference cations (M+ and M2+). Such plots are shown for dibenzo-lkrown-5 lariat
ethers with OCH3and OCH2CH20CH3side arms in Figure 2 and for correspondingcompounds with OCH~COZCZH~, OCHzC(0)N(C2H5)2, and OCHZC(O)N(C~H~~)~ side arms in Figure 3. When the oxygen-containing side arm is OCH3, only slight variation in the magniture of D is noted as the diameter of the interference ion is increased (Figure 2a). Thus this side arm is judged to be too short to undergo better positioning over the binding site when the geminal propyl group is introduced. Linear relationships between the D values and the ionic diameters are evident for lariat ethers with OCHZCHZOCH~, OCHz COZCZH~, OCHZC(O)N(CZH&,and OCHZC(O)N(C~HII)Z side arms (see Figures 2b and 3a-c, respectively). As the size of the interference cations is enhanced, the difference in Na+ selectivity between lariat ethers with and without the geminal propyl group becomes larger. The correlations include data points for monovalent alkali metal and ammonium ions, as well as divalent alkaline earth metal ions. Thus the D value is found to depend on the size of interference ion, but not its charge. It appears that a preorganized ionophore structure disfavors formation of the perching complexes1gthat are anticipated for metal cations larger than Na+ by a steric effect. As the size of the interference ion is enhanced, conformational preorganization of the side arm becomes increasingly detrimental to the stability of a perching complex for the interference ion. Slopes and correlation coefficients for the plots shown in Figures 2b and 3a-c are presented in Table 2. The slopes are appreciably greater when the side arm of the lariat ether terminates with an amide or ester group compared with a terminal ether function, even though an ether oxygen is more basic than a carbonyl oxygen.21 For the ester and amide groups, the carbonyl carbon is sp2hybridized, which requires coplanarity of the carbon atom a to the carbonyl group, the carbonyl carbon, the carbonyl oxygen, and the second oxygen atom of the ester (or the nitrogen atom of the amide). Thus the presence of a carbonyl group introduces considerable rigidity into the side arm. For the current series of lariat ether amides and esters, this helps position the carbonyl oxygen over the crown ether cavity and preorganize the binding site. On the other hand, the OCH2CH20CH3 side arm is more flexible, which provides a lower level of conformational preorganization. It is proposed that a greater preorganization of the binding sites in the lariat ether amides and esters makes them more sensitive to the size of the interference ion in a perching complex. This increases the slope of the correlation lines shown in Figures 2 and 3 for the lariat ethers with amide and estercontaining side arms. In conclusion, we have probed the influence of sidearm variation in dibenzo-lkrown-5 lariat ethers upon their potentiometric responses to alkali metal, alkaline earth metal, and ammonium ions by incorporation of these ionophores into ISE systems. Attachment of a propyl group to the ring carbon that bears an extended oxygen-containing side arm increases the Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
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selectivity for Na+ relative to larger alkali metal and alkaline earth metal ions and to ammonium ions. When the enhancement in Na+ selectivity arising from attachment of a geminal propyl group is plotted against the size of the interference ions, a linear relationship is obtained. The potentiometric responses of dibenzclkrown-5 lariat ethers can be rationalized in terms of the crown ether ring size and the oxygen basicity, conformationalpositioning, and rigidity of the side arm.
4336 Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
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-FG03094ER14416). Received for review July 1 1 , 1994. Accepted August 31,
1993.m @
Abstract published in Advance ACS Abstracts, October 1, 1994
