Short communication
2+
Stability Constants of Ca2+, Cu2+, Zn2+, UO2 , Mn2+, Co2+ and Ni2+ Complexes of Tetraethyl p-tert-Butylcalix[4]arene Tetraacetate in Nitrobenzene Saturated with Water
Emanuel Makrlik,1'* Petr Vanura2 and Pavel Selucky2
1 Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen 2 Institute of Chemical Technology, Prague, Technickä 5, 166 28 Prague 6, Czech Republic 3 Nuclear Research Institute, 250 68 Rež, Czech Republic * Corresponding author: E-mail: makrlik@centrum/sz Received: 13-01-2009
Abstract
From extraction experiments and y-activity measurements, the exchange extraction constants corresponding to the general equilibrium M2+ (aq) + SrL2+ (nb) ^ ML2+ (nb) + Sr2+ (aq) taking place in the two-phase water-nitrobenzene system (M2+ = Ca2+, Cu2+, Zn2+, UO2+, Mn2+, Co2+, Ni2+; L = tetraethyl p-tert-butylcalix[4] arene tetraacetate; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Further, the stability constants of the ML2+ complexes in water saturated nitrobenzene were calculated; they were found to increase in the cation order Mn2+ < Co2+ < Cu2+ < Zn2+ < UO2+, Ni2+ < Ca2+.
Keywords: Divalent metal cations, calix[4] arene compound, strontium dicarbollylcobaltate, extraction and stability constants, water - nitrobenzene system
1. Introduction
Calix[n]arenes are a well-known family of ma-crocyclic molecules with many potential applications in various branches of chemistry. Because of their simple one-pot preparation, easy derivatization and unique com-plexation abilities, calix[n]arenes are widely used as the building blocks for the constructions of more sophisticated molecular systems. Their unique three-dimensional pre-organization make them very attractive as the receptors for the complexation of cations, anions, and even neutral molecules. Calix[n]arenes find applications as selective binders and carriers, as analytical sensors, as catalysts and model structures for biomimetic studies.12 In the field of host-guest chemistry, many studies have focused on the binding ability of calixarene derivatives with carbonyl groups at their lower rims toward metal ions, predominantly alkali and alkaline-earth, but also transition and heavy metal cations,3-12 and even toward HO+.13-21
Dicarbollylcobaltate anion and some of its halogen derivatives are very useful reagents for the extraction of alkali metal cations (especially Cs+), and also-in the presence of polyoxyethylene compounds (e.g., crown ethers, PEG 400, Slovafol 909) - for the extraction of Sr2+ and Ba2+ from aqueous solutions into an organic polar phase, both under laboratory conditions for purely theoretical or analytical purposes,22,23 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive wa-ste.24,25
Recently, the solvent extraction of Ba2+, Pb2+ and Cd2+ into nitrobenzene by using synergistic mixture of strontium dicarbollylcobaltate and tetraethyl p-tert-butyl-calix[4]arene tetraacetate (see Scheme 1) has been investi-gated.26 In the present work, the stability constants of Ca2+, Cu2+, Zn2+, UO22+, Mn2+, Co2+ and Ni2+ complexes with the mentioned calix[4]arene ligand were determined in the organic phase of the water-nitrobenzene extraction system.
Scheme 1. Structural formula of tetraethyl p-tert-butylcalix[4]are-ne tetraacetate (abbrev. L).
2. Experimental
Tetraethyl p-tert-butylcalix[4]arene tetraacetate was purchased from Fluka, Buchs, Switzerland. Cesium dicar-bollylcobaltate, CsDCC, was supplied by Katchem, Rež, Czech Republic. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A nitrobenzene solution of hydrogen dicarbollylcobaltate (HDCC)22 was prepared from CsDCC by the method described elsewhere.27 The equilibration of the nitrobenzene solution of HDCC with stoichiometric Sr(OH)2, which was dissolved in an aqueous solution of Sr(NO3)2 (0.2 mol dm-3), yielded the corresponding Sr(DCC)2 solution in nitrobenzene. The radionuclide 85Sr2+ (DuPont, Belgium) was of standard radiochemical purity.
The extraction experiments were carried out in 10 cm3 glass test-tubes covered with polyethylene stoppers: 2 cm3 of an aqueous solution of M(NO3)2 (M2+ = Ca2+, Cu2+,
Zn2+, UO2+, Mn2+, Co2+, Ni2+) of the concentration in the range from 1 x 10-3 to 1 x 10-2 mol dm-3 and microa-mounts of 85Sr2+ were added to 2 cm3 of a nitrobenzene solution of tetraethyl p-tert-butylcalix[4]arene tetraacetate and Sr(DCC)2, whose initial concentrations varied also from 1 x 10-3 to 1 x 10-2 mol dm-3 (in all experiments, the initial concentration of tetraethyl p-tert-butylcalix[4]arene tetraacetate in nitrobenzene, CiLn,nb, was always equal to the initial concentration of Sr(DCC)2 in this medium, CiSnr,(nDbCC) ). The test-tubes filled with the solutions were shaken for 12 hours at 25 °C, using a laboratory shaker. Then the phases were separated by centrifugation. Afterwards, 1 cm3 samples were taken from each phase and their y-activities were measured using a well-type NaI(T1) scintillation detector connected to a y-analyzer NK/350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of strontium, DSr, were determined as the ratios of the measured radioactivities of 85Sr2+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
Regarding the results of previous papers,22, 28-30 the two-phase water- M(NO3)2 (M2+ = Ca2+, Cu2+, Zn2+, UO2+, Mn2+, Co2+, Ni2+)-nitrobenzene-Sr(DCC)2 extraction system can be described by the following general equilibrium
M (aq ) + Sr'" (nb) o M {nb) + Sr'" (aq) (1)
with the corresponding exchange extraction constant Kex(M2+, Sr2+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex(M2+, Sr2+) one can write22,28
=	-logK'^^. (2)
where KiM2+ and KiSr2+ are the individual extraction constants for M2+ and Sr2+, respectively, in the water-nitrobenzene system. Knowing log KCa2+ = -11.2 (inferred from References 28 and 29), log = -11.5,31 log KZn,+ log KNi,+ = -11.6,31 log KUo,+= -11.8,31 log KMn,+ = -11.1,31
Table 1. Equilibrium data for the M2^ and ML2+ cations in the two-phase water-nitrobenzene extraction system at 25 °C (M2+ = Ca2+, Sr2+, Cu2+, Zn2+, UO2+, Mn2+, Co2+, Ni2+; L = tetraethyl p-tert-butylcalix[4]arene tetraacetate; for the meaning of the constants see text).
Quantity	Ca2+	Sr2+	Cu2+	Zn2+	UO2+	Mn2+	Co2+	Ni2+
log KM2+	-11.2a	-10.7b	-11.5c	-11.6c	-11.8c	-11.1c	-11.4c	-11.6c
log Kex(M2+, Sr2^)^	-0.5	-	-0.8	-0.9	-1.1	-0.4	-0.7	-0.9
log Kex(M2+, SrL2+)e	1.3	-	-0.7	-0.3	-0.3	-0.6	-0.7	-0.1
log ^„b(ML2+)f	8.0	6.2g	6.3	6.8	7.0	6.0	6.2	7.0
a Inferred from Refs 28 and 29. b Inferred from Refs 28 and 30. c Ref. 31. d Calculated from Eq. (2) using data from Refs 28-31. e Calculated from Eq. (5). f Calculated from Eq. (6) using data from Refs 28-31 and 36. ^ Ref. 36.
log KCo2+ = -11.431 and finally, log KSr2+ = -10.7, which was inferred from References 28 and 30, the single exchange extraction constants Kex(M2+, Sr2+) were simply calculated on the basis of Eq. (2). The corresponding data are given in Table 1.
Previous results32-35 showed that the two-phase wa-ter-M(NO3)2 (M2+ = Ca2+, Cu2+, Zn2+, UO2+, Mn2+, Co2+, Ni2+)-nitrobenzene-L (L = tetraethyl p-tert-butylca-lix[4]arene tetraacetate)-Sr(DCC)2 extraction system (see Experimental), chosen for determination of stability of the complex ML2+ in nitrobenzene saturated with water, can be characterized by the main chemical equilibrium
M''{aq) + SrL'^(nb)o ML'"(nb) + Sr"(aq) (3)
with the general equilibrium extraction constant Kex(M2+, SrL2+):
(4)
It is necessary to emphasize that tetraethyl p-tert-butylcalix[4]arene tetraacetate is a considerably hydrop-hobic ligand, practically present in the nitrobenzene phase only, where it forms the very stable complexes ML2+ with the mentioned divalent cations. Taking into account the conditions of electroneutrality in the organic and aqueous phases of the system under study, the mass balances of the divalent cations studied at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratio of strontium, DSr = [SrL2+]nb/ [Sr2+]aq, combined with Eq. (4), we gain the final expression for Kex(M2+, SrL2+) in the form
(5)
where CiMn,(aNqO ) is the initial concentration of M(NO3)2 (M2+ = Ca2+, 3(:u2+, Zn2+, UO22+, Mn2+, Co2+, Ni2+) in thej aqueous phase of the system under consideration.
In this study, from the extraction experiments and 7-activity measurements (see Experimental) by means of Eq. (5), the logarithms of the constants Kex(M2+, SrL2+) were determined as reviewed in Table 1. Moreover, with respect to References 32-35, for the extraction constants Kex(M2+, Sr2^) and Kex(M2+, SrL2+) defined above, as well as for the stability constants of the complexes ML2+ and SrL2+ in nitrobenzene saturated with water, denoted by ^nb(ML2+) and ^nb(SrL2+), respectively, one gets
+ log (M SrL^^ ) - log {M Sr^"^ )
Finally, using the constants log Kex(M2+, Sr2+) and log Kex(M2+, SrL2+) given in Table 1, log ^nb(SrL2+) = 6.2(L = tetraethyl p-tert-butylcalix[4]arene tetraacetate),36 determined from the distribution of strontium picrate in the water-nitrobenzene system containing the considered calix[4]arene ligand, and applying Eq. (6), we obtain the stability constants of the ML2+ complexes in water-saturated nitrobenzene. These data are also summarized in Table 1. The ßnb(ML2+) values from this table indicate that the stability of the ML2+ complex cation in nitrobenzene saturated with water increases in the series of Mn2+ < Sr2+, Co2+ < Cu2+ < Zn2+ < UO2+, Ni2+ < Ca2+.
In conclusion, it should be noted that the stability constants of the complex species ML2+, where M2+ = Ba2+, Cd2+, Pb2+ and L is tetraethyl p-tert-butylcalix[4]arene te-traacetate, in water-saturated nitrobenzene are log ßnb(BaL2+) = 6.6,26 log ßnb(CdL2+) = 5.826 and log ßnnb(PbL2+) = 7.7.26 Thus, the previous data26 and the data listed in Table 1 show that the stability of the complex cation ML2+ in the mentioned medium increases in the Cd2+ < Mn2+ < Sr2^, Co2+ < Cu2+ < Ba2+ < Zn2+ < UO22+, Ni2+ < Pb2+< Ca2+ order.
4. Acknowledgement
The present work was supported by the Specific Research of the Faculty of Applied Sciences, University of West Bohemia, Pilsen, Czech Republic, and by the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6046137307.
5. References
1.	V. Böhmer, Angew. Chem., Int. Ed. Engl. 1995, 34, 713-745.
2.	C. D. Gutsche, Calixarenes Revisited. The Royal Society of Chemistry, Cambridge, 1998.
3.	A. Arduini, A. Pochini, S. Reverberi, R. Ungaro, Tetrahedron 1986, 42, 2089-2100.
4.	A. Arduini, E. Ghidini, A. Pochini, R. Ungaro, G. D. An-dreetti, G. Calestani, F. Ugozzoli, J. Inclusion Phenom. Macrocytic Chem. 1988, 6, 119-134.
5.	F. Arnaud-Neu, E. M. Collins, M. Deasy, G. Ferguson, S. J. Harris, B. Kaitner, A. J. Lough, M. A. McKervey, E. Marques, B. L. Ruhl, M. J. Schwing-Weill, E. M. Seward, J. Am. Chem. Soc. 1989, 111, 8681-8691.
6.	F. Arnaud-Neu, G. Barrett, S. J. Harris, M. Owens, M. A. McKervey, M. J. Schwing-Weill, P. Schwinté, Inorg. Chem. 1993, 32, 2644-2650.
7.	K. Ohto, E. Murakami, T. Shinohara, K. Shiratsuchi, K. Ino-ue, M. Iwasaki, Anal. Chim. Acta 1997, 341, 275-283.
8.	Z. Ye, W. He, X. Shi, L. Zhu, J. Coord. Chem. 2001, 54, 105 - 116.
9.	A. F. Danil de Namor, S. Chahine, D. Kowalska, E. E. Castellano, O. E. Piro, J. Am. Chem. Soc. 2002, 124, 12824-12863.
10.	P. M. Marcos, J. R. Ascenso, M. A. P. Segurado, J. L. C. Pereira, J. Inclusion Phenom. Macrocyclic Chem. 2002, 42, 281-288.
11.	P. M. Marcos, S. Félix, J. R. Ascenso, M. A. P. Segurado, J. L. C. Pereira, P. Khazaeli-Parsa, V. Hubscher-Bruder, F. Ar-naud-Neu, New J. Chem. 2004, 28, 748-755.
12.	E. Makrlik, J. Budka, P. Vanura, Acta Chim. Slov. 2009, 56, 278-281.
13.	E. Makrlik, P. Vanura, Monatsh. Chem. 2006, 137, 11851188.
14.	J. Dybal, E. Makrlik, P. Vanura, Monatsh. Chem. 2007, 138, 541-543.
15.	J. Kr iz, J. Dybal, E. Makrlik, J. Budka, P. Vanura, Monatsh. Chem. 2007, 138, 735-740.
16.	J. Dybal, E. Makrlik, P. Vanura, P. Selucky, Monatsh. Chem.
2007,	138, 1239-1242.
17.	J. Dybal, E. Makrlik, P. Vanura, J. Budka, Monatsh^. Chem.
2008,	139, 1175-1178.
18.	J. Dybal, E. Makrlik, J. Budka, P. Vanura, Monatsh^. Chem. 2008, 139, 1353-1355.
19.	J. Kr iz, J. Dybal, E. Makrlik, P. Vanura, J. Lang, Supramol. Chem. 2007, 19, 419-424.
20.	J. Kriz, J. Dybal, E. Makrlik, P. Vanura, Supramol. Chem. 2008, 20, 387-395.
21.	J. Kf iz, J. Dybal, E. Makrlik, J. Budka, P. Vanura, Supramol. Chem. 2008, 20, 487-494.
22.	E. Makrlik, P. Vanura, Talanta 1985, 32, 423-429.
23.	Z. Valentova, E. Makrlik, Acta Chim. Slov. 2007, 54, 175178.
24.	V. N. Romanovskiy, I.V. Smirnov, V. A. Babain, T. A. Todd, R. S. Herbst, J. D. Law, K. N. Brewer, Solvent Extr. Ion Exch. 2001, 19, 1-21.
25.	J. D. Law, R. S. Herbst, T. A. Todd, V. N. Romanovskiy, V. A. Babain, V. M. Esimantovskiy, I. V. Smirnov, B. N. Zaitsev, Solvent Extr. Ion Exch. 2001, 19, 23-36.
26.	E. Makrlik, P. Vanura, P. Selucky, Acta Chim. Slov. 2008, 55, 430-433.
27.	E. Makrlik, Collect. Czech. Chem. Commun. 1992, 57, 289295.
28.	J. Rais, Collect. Czech. Chem. Commun. 1971, 36, 32533262.
29.	P. Vanura, Czech. J. Phys. 1999, 49 Suppl. S1, 761-767.
30.	P. Vanura, E. Makrlik, J. Rais, M. Kyrs, Collect. Czech. Chem. Commun^. 1982, 47, 1444-1464.
31.	E. Makrlik, P. Vanura, unpublished results.
32.	M. Dankova, E. Makrlik, P. Vanura, J. Radioanal. Nucl. Chem. 1997, 221, 251-253.
33.	E. Makrlik, J. Halova, M. Kyrs, Collect. Czech. Chem. Commun^. 1984, 49, 39-44.
34.	E. Makrlik, P. Vanura, J. Radioanal. Nucl. Chem. 1996, 214, 339-346.
35.	E. Makrlik, P. Vanura, Monatsh. Chem. 2006, 137,157-161.
36.	P. Selucky, E. Makrlik, P. Vanura, unpublished results.
Povzetek
Raziskovali smo ekstrakcijska ravnotežja M2+ (aq) + SrL2+ (nb) ^ ML2+ (nb) + Sr^^ (aq) v dvofaznem sistemu voda-ni-trobenzen (M2+ = Ca2+, Cu2+, Zn2+, UO2+, Mn2+, Co2+, Ni2+; L = tetraetil p-tert-butilcalix[4]aren tetraacetat; aq = vodna faza, nb = faza nitrobenzena). Določili smo konstante stabilnosti ML2+ kompleksov v vodni fazi, nasičeni z nitrobenze-nom. Ugotovili smo, da te konstante naraščajo v smeri Mn2+ < Co2+ < Cu2+ < Zn2+ < UO2+, Ni2+ < Ca2+.