ANALYST. MAY 1992, VOL. 117 853 trans-Cyclohexano Crown Ethers as Ion Sensors in Cation-selective Electrodes R. D. Tsingarelli, L. K. Shpigun,” V. V. Samoshin, 0. A. Zelyonkina, M. E. Zapolsky, N. S. Zefirov and Yu. A. Zolotov N.S. Kurnakov Institute of General and Inorganic Chemistry, USSR Academy of Sciences, Leninsky Prospect 3 I , Moscow I I 7907, Russia Several trans-cyclohexano crown ethers have been synthesized and studied as sensor materials for different metal cations in PVC-matrix membranes of ion-selective electrodes. The potentiometric selectivity of the membranes has been found t o correlate with the chemical structure peculiarities of the incorporated compound and can be changed by variation of the substituent attached t o the cyclohexane fragment. Some of the compounds show superior selectivity for potassium over many other metal cations.Keywords: Ion-selective electrode; macroc yclic crown ether; sensor material Ion-selective electrodes (ISEs) with liquid membranes con- taining electrically neutral lipophilic macrocyclic or non-cyclic ion carriers are widely used for the determination of different metal ions in various samples.’-3 The significance of macro- cyclic compounds as cation-sensor materials in ISEs has been recognized, and increasing interest has been focused on the molecular design of these structures. This has resulted in the development of ISEs based on synthesized ionophores.4 The function mechanism of such sensors is based on a cation- transfer reaction at the membrane interface (or at the aqueous solution-organic phase boundary) by means of reversible complexation of the cation by the neutral carrier introduced into the membrane.5 In fact, the potentiometric selectivity of these compounds for various equally charged metal cations is related to the ratio of their corresponding complex stability constants,6 and has been found to correlate with their extraction constants.7 Although a number of compounds have been synthesized, the molecular design of a sensor with given analytically relevant ion selectivity still remains empirical.”.” Hence, some specifically complexing macro-heterocyclic com- pounds, e .g . , some crown ethers,5-10 either do not act as ionophores in membranes or do not reveal an electrode activity 1 1 -12 (q Hd ’;- OH The present paper describes a comparative study on the potentiometric selectivity of a series of solvent polymeric membranes, containing a number of trans-cyclohexano crown ethers (trans-CCEs) and their 4-alkyl derivatives as the sensor material, in ISEs.The results are discussed in terms of the chemical structure peculiarities and of the possibility of complex formation between these compounds and alkali or alkaline earth metal cations. Experimental Reagents All the crown compounds investigated were obtained from appropriate cyclohexano glycols and oligoethylene glycol ditosylates by well-known cyclization procedures13 (Fig. 1). Cyclohexano glycols in turn were prepared by the acid- catalysed reaction of the corresponding epoxides with oligo- ethylene glycols.14-15 The wide variety of potential reactants makes this approach to the synthesis of trans-CCEs flexible and highly promising. .e *A f3 0 OH I OH ),) H2S04,CHC13 R AK TsO 0 OTs NaH, 1 ,4-dioxane R w R w I: n = 2, R = H VI: n = 2, R = CH3 XI: n = 2, R = (CH3I3C II: n = 3, R = H 111: n = 4, R = H V: n = 6, R = H VII: n = 3, R = CH3 VIII: n = 4, R = CH3 X: n = 6, R = CH3 XII: = 3, R = (CH3)3C XIII: n = 4, R = (CH3)3C XV: n = 6, R = (CH3)3C IV: n = 5, R = H IX: n = 5, R = CH3 XIV: n = 5, R = (CH3I3C Fig. 1 Synthesis of the crown compounds investigated * To whom correspondence should be addressed.854 ANALYST, M 4 Y 1992, VOL. 117 Table 1 Potentiometric selectivity coefficients of various membranes containing trans-cyclohexano crown ethers -logkr:.Mn+ (n = 5 ; P=O.95) trans-CCE 15C5 (I) 18C6 (11) 2 1 C7 (111) 24C8 (IV) 27C9 (V) 15C5 (VI) 18C6 (VII) 21C7 (VIII) 24C8 (IX) 27C9 (X) 15C5 (XI) 18C6 (XII) 21C7 (XIII) 24C8 (XIV) 27C9 (XV) H-CCE- CH3- CCE- C(CHs)j-CCE- Li+ Na+ 2.18 0.50 1.80 0.73 2.37 0.94 2.36 0.86 1.68 0.69 1.20 0.00 - 2.30 1.20 2.07 1.01 2.19 0.31 1.00 0.29 1.80 0.05 2.40 1.30 1.43 0.95 2.51 1.06 2.03 1.10 0.17 0.45 2.23 2.18 2.86 2.05 0.03 0.15 2.57 2.86 2.68 2.05 0.26 0.31 2.08 3.14 2.76 2.24 0.18 0.50 2.50 2.68 3.08 1.55 0.26 0.77 1.80 1.77 2.09 0.99 -0.13 0.70 1.90 2.20 1.50 1.00 0.10 0.60 2.80 2.84 2.80 2.04 0.00 0.15 2.17 2.68 2.41 1.56 0.18 0.36 2.26 2.82 2.58 1.69 0.19 0.43 0.88 1.72 1.22 0.76 0.25 0.68 1.43 1.34 1.36 0.75 0.45 1.00 2.61 3.15 2.88 2.35 0.00 0.20 2.29 1.80 2.44 1.96 0.08 0.30 2.67 3.20 3.20 2.29 0.05 0.21 2.41 2.53 2.09 1.24 0 -1 -2 + r 8: a cn -I 0 -1 -2 -3 Li + Na+ K+ Rb+ Cs+ (0.068) (0.098) (0.133) (0.149) (0.165) Mg2+ Ca2+ Sr2+ Ba2+ (0.074) (0.104) (0.120)(0.138) Fig.2 Dependence of the potentiometric selectivity coefficients (log k) on the cationic radius (Y) for (a) alkali metal cations and ( 6 ) alkaline earth metal cations. The values in parentheses on the abscissa are the cationic radii of the metal ions in nanometres. 0, n = 1; A , n = 2; 77, n = 3; 0, n = 4; and X , n = 5 Membranes The plasticized polymer membranes containing the above- mentioned macrocycles were prepared by a known proce- dure.16 the membranes included (mass ratio in percentages): macrocycle, 1.0; poly(viny1 chloride) (PVC), 33.0; and o-nitrophenyl octyl ether, 66.0. Reference PVC membranes, which did not contain macrocycles, were also prepared.The thickness of the membrane thus obtained was 0.1-0.2 mm. All the membranes were conditioned before use by soaking for 24 h in 0.02 mol dm-3 potassium chloride. Electrode System and e.m.f. Measurements The membranes were studied with use of electrochemical cells of the following type: Ag-AgCllKCI(O.1 mol dm-3)( Bridge [Sample electrolyte ~Membranel~KCl(0.1 mol dm-3)AgCl-Ag The conditioned membrane discs (10 mm in diameter) were assembled in a commercial electrode body (NPO Analit Pribor, Tbilisi). A double-junction reference electrode was also used (Model 90-02: Orion Research, Cambridge, MA, USA) and the outer filling solution was 0.1 mol dm-3 ammonium nitrate. The e.m.f. measurements were carried out at 295 t 2 K, with use of a digital Radelkis (Budapest, Hungary) OP-208 pH meter.Potentiometric selectivity coefficients for different metal cations relative to potassium ions (kgo2, Mrt+) were determined by the separate-solution method with the 0.1 mol dm-3 respective metal ion s0lutions.1~ Results and Discussion The membrane potentiometric selectivity for alkali and alkaline earth metal cations, referred to K+, was studied. Table 1 lists the selectivity coefficients obtained. It is shown that the values -log /cR"+',~"+ ( n = 1,2) for each metal cation depend significantly both on the size of the ring and on the nature of the substituent R in the cyclohexane fragment. All the trans-CCEs studied reveal selectivity towards alkali metal cations, especially K+ and Rb+.Fig. 2(a) shows that the dependence of -log /cRO+~,~+ on the cationic radius for alkali metals has a maximum at 0.133 nm, irrespective of the size of the crown ether ring (when R = H). The order of selectivity is as follows: K+, Rb+ , Cs+, Na+, Li+. The values of -log kR"tf,Li+ are 1.0-2.5 orders of magnitude lower than those for other alkali metals. It is interesting to note that the above-mentioned experimental results, espe- cially the highest potentiometric selectivity towards K+, are in agreement with those for the crown ethers not containing the cyclohexane fragment. 18 On the other hand, for alkaline earth metal cations, the analogous curves have minima at 0.098-0.113 nm [Fig. 2(b)]. The order of the selectivity in this instance is different for each macrocycle, but all log kr?,M'+ values are essentially lower than those for alkali metals, except Li+.This indicates that the known hole-size concept19-20 cannot be used to predict the potentiometric selectivity of membrane electrode systems. Fig. 3 shows the dependence of kR"+',M"+ on the size of the macrocycle. These curves are non-monotonic for all metal ions, especially for the alkyl derivatives of trans-CCEs. Hence, for tert-butyl derivatives (XI-XV), there are two distinct minima at 18-crown-6 (18C6) and 24-crown-8 (24C8) ( n = 3 and 5 ) , which alternate with three maxima at 15-crown-5 (15C5), 21-crown-7 (21C7) and 27-crown-9 (27C9) (n = 2 , 4 and 6). The highest selectivity for K+ compared with alkaline earth metal cations and Li+ is observed for mem- branes containing tert-butyl-trans-cyclohexano-18-crown-6 (XII) and tert-butyl-trans-cyclohexano-24-crown-8 (XIV).The influence of other alkali metal cations is relatively low only for compound XII. The membranes containing methyl-trans- cyclohexano-15-crown-5 (XI) are sensitive to the cations Rb+, Na+ and K+. Hence, the introduction of a fairly well-branched alkyl substituent into the cyclohexane fragment results in a signifi- cant change in the cation selectivity of the membrane.ANALYST, MAY 1992. VOL. 117 855 0 - 1 - 2 + C r 55 0 -I - 1 -2 -3 a) 1 I I 1 I b) / I I I I 1 2 3 4 5 6 I I I I I d) X 1 I I 1 I 2 3 4 5 6 R n I I 1 I I 2 3 4 5 6 Fig. 3 Dependence of the potentiometric selectivity coefficients (log kP,9' Mrt+) on the number (n) of binding-site atoms in the macrocyclic ring.(a) and (b): R = H; (c) and ( d ) : R = CH,; (e) and 0: R = C(CH3)3. A,'Li+; B, Na+; C, Rb+; D, Cs+; E, Mg*+; F, Ca2+; G, W+; and H, Ba2+ A B n = 0-4; R = H, CH3, C(CH3)3 Fig. 4 Two chair conformations of thc six-membered ring in trans-CCEs R = CH3, C(CH3)3 R = C(CH& Fig. 5 derivatives of the trans-CCEs Two interconverting forms of the complexes for tert-butyl Some of the teatures of trans-CCE membrane activity can be explained in terms of the conformational properties of these compounds. A six-membered ring in a trans-CCE can adopt two chair conformations, which differ in substituent orientation (Fig. 4): C-0 bonds are axial in conformation A and equatorial in conformation B. Nuclear magnetic reso- nance (NMR) studies of the equilibrium (A G= B) have shown that it is strongly shifted to the conformation B (>90%) when R = H.1"*1-23 When R = CH3, conformation A is predomi- nant to nearly the same extent, and when R = C(CH,)?, conformation A becomes virtually the sole conformation.This regularity is in accord with conformational properties of R-substituted cyclohexanes; i. e . , the destabilization of the axial position of the alkyl group (B) increases with an increase in its volume.'4 An interesting conformational feature of trans-CCEs is the alteration of the proportion of conformers with the increasing size of the macrocycle: the population of conformers (A) decreases on passing from trans-cyclohexano-15-crown-5 (I) to trans-cyclohexano-18-crown-6 (11), and increases again on passing from I1 to trans-cyclohexano-21-crown-7 (111).13-21-23 Apparently, this is a related peculiarity of macrocycles. Owing to the ring trans fusion, the chair-chair interconver- sion of the six-membered ring is accompanied by a marked change in the conformation of the macrocycle. The macro- cycle in conformer A has on oval form resulting from trans diaxial orientation of the bridge fragment 0-C-C-0. It is856 ANALYST, MAY 1992, VOL. 117 known that 18-crown-6 has an analogous oval form (with symmetry C) in the crystalline state or in a non-polar solvent.22-27 In the.complexes of 18-crown-6, with most metal cations this macrocycle adopts a ring-shaped conformation (with symmetry D), and all the fragments 0-C-C-0 have a gauche conformation (dihedral angle about 60").13.28 For trans-CCE, such a conformation of the macrocycle is attain- able only in conformer B. Indeed, it has been shown that the crown ether VII in complexes with inorganic salts adopts conformation B14,15 in spite of destabilization by the axial methyl group. The conformational energy of the tert-butyl group (23 kJ mol-1)24 is comparable to the energy difference between the chair and twist conformations of the cyclohexane ring (22 kJ mol-1).23 Therefore, one can expect two intercon- verting forms of complexes for the tert-butyl derivatives (Fig. 5 ) . The axial substituent R destabilizes the complexes of compounds VI-XV compared with complexes of unsubsti- tuted compounds I-V. As the ionophore properties of neutral carriers are determined mainly by the stability of their complexes,5J9 so it is believed that the change in complex- formation ability, with the introduction of a substituent R, is the result of conformational perturbation. The destabilization of complexes makes the substituted trans-CCE more sensitive to conformational features of the macrocycle, e.g., to the alteration of conformer population with an increase in the size of the macrocycle (see above). This results in an alteration of the selectivity coefficients, which increase with an increase in the size of the substituent R.Conclusion The potentiometric selectivity of some synthesized trans- CCEs as sensor materials in PVC-matrix membrane elec- trodes has been studied. The selectivity for potassium by some compounds is close to that of electrodes based on valinomycin.It was observed that the properties of trans-CCEs as ion carriers can be modified via conformational change caused by variation of substituents attached to the cyclohexane frag- ment. This offers the possibility of purely conformational control of crown ether complexation. References 1 Oggeneuss, P., Morf. W. E., Oesch, U., Amman, D., Prctsch, E.. and Simon, W., Anal. Chim. Acta, 1986, 180, 155. 2 Koryta, J . . Anal. Chim. Acta, 1986, 183. 1. 3 Koryta. J . , Anal. Chim. Acta. 1988, 206. 1. 4 Amman, D., Morf, W. E . , Anker, P., Meier, P. C., Prctsch, E., and Simon, W.. Ion Sel. 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