Won-Zin Oh
Korea Atomic Energy Research Institute
P. O. Box 105
Taejon 305-600, Korea

Jong Seung Kim
Department of Chemistry
Konyang University
Nonsan 320-711, Korea

Jong Kuk Kim,^†
Department of Chemical Engineering
Konyang University
Nonsan 320-711, Korea.

Kune Woo Lee and Wang Kyu Choi
Korea Atomic Energy Research Institute
P. O. Box 105
Taejon 305-600, Korea

1,3-Dipropyloxycalix[4]arene crown ethers (1 and 2) were successfully synthesized in the fixed 1,3-alternate conformation with good yields by the reaction of corresponding 1,3-dipropyloxycalix[4]arenes (3) with pentaethylene glycol ditosylate and dibenzodimesylate 4, respectively in acetonitrile in the presence of cesium carbonate as a base. Complexation of the corresponding calix[4]arene 1 and 2 toward alkali metal ions using single flux method through supported liquid membrane system was found to give a high cesium selectivity. Co-transport ion (NO₃^-) concentration of feed solution provided the best extraction result for cesium ion in 4 M HNO₃. SLM experiment using compound 2 gave an exceptional selectivity of cesium ion over sodium ion and permeation coefficient of cesium was estimated to 8.91 cm/hr.

It has been deeply considered that the burial of vitrified reprocessed high level activity liquid wastes (HLW) and medium level activity liquid wastes (MLW) are of very importance in view of our environment. Recently, it was reported that MLW containing radioactive metal ions having 20-30 years half lives such as Sr²⁺, Cs⁺, and Co²⁺ etc., can be treated by evaporation or other techniques such as chemical precipitation and ion exchange to concentrate their radioactive waste into the smallest possible volume, so to speak, volume reduction process (1).

Calix[4]arene molecule has been well documented to be a useful three dimensional molecular building block for the synthesis of molecules with specific properties (2). It is known that the calix[4]arenes are able to exist in the following four different conformations: cone, partial cone, 1,2-alternate, and 1,3-alternate (3). 1,3-Distal capping of calix[4]arene at the lower rim (OH) has been accomplished with polyether linkage such as calixcrown ether, calix-doubly-crowned, and double-calix-crown. More recently, Reinhoudt has reported that 1,3-dialkoxycalix[4]arene-crown-6 derivatives as selective ionophores for cesium ion were successfully prepared (4). Those observation was due to a complexation of cesium ion not only with the crown ether moiety but also with the two rotated aromatic nuclei (cation/p -interaction) of the 1,3-alternate conformation.

It was reported that benzo group in crown ether moiety makes the ether linkage more rigid and more lipophilic, resulting better complexation in organic medium (5). However, preparation of calix[4]arene dibenzocrown ether in which two benzo groups are introduced into the ether linkage is not as easy as that of calix[4]arene absent dibenzo group. So, in this paper, we report that facile synthesis of calix[4]arene dibenzocrown ether 2 and its complexation ability to compare with calix[4]arene crown ether 1 not containing dibenzo groups. Their uses as selective cesium extractants by SLM experiments are described.

Melting points were taken by the use of a Mel-Temp of Fisher-Johns melting point apparatus without any correction. IR spectra were obtained with a Perkin-Elmer 1600 Series FT-IR on potassium bromide pellet and on deposited KBr window in the case of soild product and oil, respectively, are recorded in reciprocal centimeters. ¹H and ¹³C NMR spectra were recorded with a 400 MHz (Bruker ARX-400) and an 100 MHz spectrometer, respectively, the chemical shifts (d) reported downfield from the internal standard, tetramethylsilane. Elemental analysis was performed by Vario EL of Elemental Analyzer in Korea Basic Science Institute in Seoul, Korea. FAB⁺ mass spectra were obtained from JEOL-JMS-HX 110A/110A High Resolution Tendem Mass Spectrometry in Korea Basic Science Institute in Taejon, Korea.

Unless specified otherwise, reagent grade reactants and solvents were obtained from chemical suppliers and used as received. Dry solvents were prepared as follows: tetrahydrofuran was freshly distilled from sodium metal ribbon or chunks; benzene and pentane were stored over sodium ribbon, respectively; dichloromethane was freshly distilled from lithium aluminum hydride. Acetonitrile was pre-dried from molecular seives (3Å) and distilled over diphosphorous pentaoxide. Organic diluents such as chloroform, toluene, dodecane, and NPOE were used as received from Aldrich chemical company.

25,27-Bis(1-propyloxy)calix[4]arene dibenzocrown-6, 1,3-alternate (2). 25, 27-Bis(1-propyloxy)calix[4]arene 2 (2.0 mmole) was dissolved in 50 mL of acetonitrile and added to an excess of Cs₂CO₃ (1.62 g, 5.0 mmole) and 1,2-bis[2-(2-mesyloxyethyloxy)phenoxy]ethane (1.03 g, 2.1 mmole) under N₂. The reaction mixture was refluxed for 24 h. Then acetonitrile was removed in vacuo and the residue extracted with 100 mL of methylene chloride and 50 mL of 10% aqueous HCl solution. The organic layer was separated and washed twice with water. After the organic layer was separated and dried over anhydrous magnesium sulfate followed by removing the solvent in vacuo to give a brownish oil. With TLC analysis, pure compound was found to show an only one spot (R_f=0.4). Filtration column chromatography with ethyl acetate : hexane = 1 : 6 as eluents provied pure 1,3-alternate calix[4]arene dibenzocrown ethers as a white solid in over 90 % yield. Mp 229-232 ° C. IR (KBr pellet, cm^-1); 3068 (Ar-H), 1501, 1451, 1254, 1196. ¹H NMR (CDCl₃) d 7.12-6.55 (m, 20 H, Ar-H), 4.37 (s, 4 H), 3.75 (s, 8 H, ArCH₂Ar), 3.65-3.32 (m, 12 H), 1.25-1.16 (m, J = 7.4 Hz, 4 H, OCH₂CH₂CH₃), 0.65 (t, J = 7.4 Hz, 6 H, OCH₂CH₂CH₃). ¹³C NMR (CDCl₃): ppm 157.5, 156.7, 152.0, 149.7, 134.8, 134.7, 130.3, 129.9, 124.7, 123.3, 123.0, 122.8, 122.3, 115.9, 72.7, 71.0, 69.0, 68.0, 38.7, 23.3, 10.7. FAB MS m/z (M⁺) calcd 806.21, found 806.11. Anal. Calcd for C₅₂H₅₄O₈: C, 77.41; H, 6.69. Found: C, 77.30; H, 6.71.

1,2-Bis[2-(2-methanesulfonyloxyethyloxy)phenoxy]ethane (4). Under nitrogen, to a solution of 4.40 g (13.2 mmole) of dibenzodiol and 4.08 mL (2.94 g, 29.0 mmole) of triethylamine in 100 mL of dry CH₂Cl₂ was added dropwise 2.14 mL (3.33 g, 29.0 mmole) of methanesulfonyl chloride during a period of 30 min at 0 ° C. Upon the complete addition, reaction mixture was stirred for 5 h at 0 ° C. Reaction temperature was slowly rasied upto room temperature and stirred for additional 10 h. 50 mL of 10 % aqueous sodium bicarbonate solution was added and CH₂Cl₂ layer was separated. The organic layer was washed with water (2´ 20 mL) and brine (2´ 20 mL) followed by drying over anhydrous magnesium sulfate. Removal of CH₂Cl₂ in vacuo provided a colorless oil which was recrystallized from 100 mL of diethyl ether to give 5.82 g (90 %) of desired product. Mp 158-160 ° C. IR (KBr pellet, cm^-1) 1597, 1516, 1350 (SO₂), 1181 (SO₂). ¹H NMR (CDCl₃) d 7.14-6.90 (m, 8 H, Ar-H), 4.50 (s, 4 H, -CH₂CH₂O-), 4.32 (s, 4 H, -CH₂CH₂O-), 4.25 (t, 4 H), 3.09 (s, 6 H, CH₃SO₂O-); FAB MS m/z (M⁺) calcd 490.55, found 490.23. Anal. Calcd for C₂₂H₂₆O₁₀S₂: C, 48.97; H, 5.34. Found: C, 48.90; H, 5.21.

The support is made of a porous polymer membrane of polypropylene, Celgard 2400 (Hoechst Celanese Co.) of which average pore size is 0.04m m ´ 0.12m m. Porosity was 41%. The average thickness of membrane is 25.4m m. All other chemicals used are of A.R or G.R grade unless specified otherwise.

The single stage SLM measurements were carried out with a simple two compartments permeation cell which consists of a feed solution (250 mL) separated by stripping solution (250 mL) by a liquid membrane having an effective membrane area (7.065cm²). The pores of the porous polymer support were filled with 2-nitrophenyloctyl ether (NPOE) containing extractant (calix[4]arene crown ether) under vacuum. The feed and stripping solutions were mechanically stirred at about 300 rpm at 25° C to minimize the thickness of aqueous solutions at the interface of membrane and aqueous solutions. Membrane permeabilities were determined with varying the concentration of feed phase, extractant, co-transport anion (NO₃^-) and rpm by periodical monitoring of the alkali metal concentration in the feed and stripping solution as a function of time.

The steady state permeability process of SLM is described by the following equations.

J = C P = - (dC/dt)· (V/Q)	(1)
ln (C/C0) = -QPt / V	(2)
	(3)

where J = membrane flux, C = feed metal concentration at time (t), C₀ = feed metal concentration at zero time, V = volume of feed concentration, Q = membrane area, k₁ and k_-1 = pseudo first order rate constants of the interfacial chemical reactions occurring between metal species and membrane extractant, ( = aqueous diffusion film thickness, d_o = membrane thickness, D_a = aqueous diffusion coefficient of metal species, and D_o = membrane diffusion coefficient of the complex. By considering that k₁/k_-1 is equal to K_ex, and that when fast interfacial reactions occur, Equation 3 simplifies to

(4)

When the feed solution contains metal ions at relatively high concentrations, and extraction reaction is completely shifted to the right, the steady state permeability process of SLM is described by the following equations.

(5)

Where [CCE] = total carrier concentration.
For sufficiently large values of C

(6)

Equation 6 can be integrated to

(7)

To study an influence of lipophilicity of the organic extractant when the calix[4]arene dibenzocrown ether complexes with specific metal, a series of diametrically O-alkylated calix[4]arene 3 was prepared in the cone form. It has been reported that the reaction of calix[4]arene with alkylating agents in acetonitrile in the presence of K₂CO₃ is a well established synthetic method to obtain 1,3-dialkoxycalix[4]arene in the cone form. Cyclization reaction of 1,3-dipropyloxy calix[4]arene with pentaethylene glycol ditosylate in the presence of Cs₂CO₃ was performed as shown in Scheme 1.

It is known that the cyclization reaction to give the 1,3-alternate conformers is favored by the cesium template effect. Since it was reported that benzo group in crown ether moiety makes the ether linkage more rigid and more lipophilic resulting better complexation in organic medium, we have tried a number of synthetic experiments to attach the benzo group on ether linkage. Reaction of bisphenol with bromoacetic acid in the presence of sodium hydride as a base provided dibenzodicarboxylic acid with over 90 % yield. Reduction with lithium aluminum hydride in THF gave dibenzodiol in good yield. For ditosylation of diol, reaction of diol and p-toluenesulfonyl chloride in the presence of aqueous NaOH or pyridine as a base has been widely used. However, because of a fish and disgusting smell, handling and treatment of p-toluenesulfonyl chloride or pyridine in laboratory is not recommendable. Also, the yield of tosylation by the use of known method is around 50-70%. In addition, in the present study, when we applied the previously reported method to dibenzoditosylate with 1,3-dipropyloxy calix[4]arene (3), we obtained the desired product as an almost same yield (65-70 %). Because of above reasons, we tried the dimesylation of dibenzo diol instead of ditosylation using methanesulfonyl chloride in the presence of triethylamine and obtained the dibenzo dimesylate with an over 90 % yield without any handling difficulties. Cyclization of 3 with dibenzodimesylate 4 gave 1,3-dipropyloxy calix[4]arene dibenzocrown ether (2) in the 1,3-alternate conformation as shown in Scheme 1. No other conformational isomers were observed. Compounds 1 and 2 are blocked in the 1,3-alternate conformation as inferred from the ¹H NMR spectra (CDCl₃) which shows a singlet peak of d 3.71 for the bridging methylene hydrogens of the calix[4]arene moiety. In addition, ¹³C NMR spectra of compound 1 and 2 reveal one signal at around 38 ppm, indicating the characteristic of 1,3-alternate conformation.

For SLMs to be industrially applicable, its stability has to be improved by the proper combination of the highly selective extractant, its high solubility in hydrophobic diluent, and high boiling point organic medium. For these reasons, 2-nitrophenyloctyl ether as a solvent is used to be applied. The experimental data of ln (C/C₀) vs time is described in Figure 1 for the two different metals (1.0 mM of initial metal concentration for Na and Cs ion, respectively). The data points fall on a straight line of slope -(Q/V)P which allows one to calculate the permeability coefficient of the SLM. This result confirms the validity of equation 2 to describe the concentration vs time data. In this experimental condition, permeation coefficient of cesium was estimated to 0.406 cm/hr. The Calix[4]arene crown ether 1 allows selective removal of cesium over sodium ion. The selectivity of cesium over sodium ion was found to increase with permeation time.

ln (C/C₀) vs time data in the case of using calix[4]arene 2 is shown in Figure 2. In vicinity of C = 0.4 mM, the data point shows two different slope. When C < 0.4 mM the data points fall on the straight line of slope -(Q/V)P confirming the validity of equation 2.

In this experimental condition, permeation coefficient of cesium was estimated to 8.91 cm/hr. But in the region of C > 0.4 mM, membrane permeability is entirely controlled by membrane diffusion. (C₀ - C) vs time data of Figure 3 shows the validity of equation 7. The Calix[4]arene crown ether 2 allows more selective removal of cesium over sodium ion than calix[4]arene crown ether 1. SLM permeation experiments according to operating variables are being continued.

Calix[4]arene crown ether and its structural analog having dibenzo group on crown ether linkage were successfully synthesized by the reaction of dipropyloxy calix[4]arene with pentaethylene glycol ditosylate and dibenzodimesylate, respectively. It is noteworthy that the organic extractant 1 and 2 showed a potential cesium selective extractability from permeation through SLM. Especially, compared with ligand 1, ligand 2 containing dibenzo unit on ether linkage showed better selectivity and efficiency for cesium over other alkali metal ions in SLM experiments. Permeation coefficient of cesium using ligand 2 was estimated to 8.91 cm/hr. Using the various kinds of Calix[4]arene crown ether synthesized, the SLM will be continued for the improvement of Cs permeability coefficient and selectivity.

1. L. CECILLE "Radioactive Waste Management and Disposal" Ed.; Elsevier Science Publisher: London, New York (1991).
2. C. D. GUTSCH, "Calixarenes" Royal Society of Chemistry: Cambridge (1989). (b) C. D. GUTSCH, "Synthesis of Macrocycles: Design of Selective Complexing Agents" R. M. IZATT, J. J. CHRISTENSEN, Eds.; p 93. Wiley: New York (1987).
3. R. UNGARO, A. POCHINI, "Frontiers in Supramolecular Organic Chemistry and Photochemistry" H.-J. SCHNEIDER, Eds.; pp 57-81. VCH: Weinheim, Germany, (1991).
4. CASNATI, A. POCHINI, R. UNGARO, F. UGOZOLLI, F. ARNAUD, S. FANNI, M. -J. SCHWING, R. J. M. EGBERINK, F. DE JONG; D. N. REINHOUDT, "Synthesis, Complexation, and Membrane Transport Studies of 1,3-Alternate Calix[4]arene-crown-6 Conformers: A New Class of Cesium Selective Ionophore" J. Am. Chem. Soc., 1995, 117, 2767.
5. Z. ASFARI, C. BRESSOT, J. VICENS, C. HILL, J-F. DOZOL, H. ROUQUETTE, S. EYMARD, V. LAMARE, B. TOUMOIS, "Doubly Crowned Calix[4]arenes in the 1,3-Alternate Conformation as Cesium-Selective Extractants in SLM" Anal. Chem. 1995, 67, 3133.

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