Specific Cooperative Effect of a Macrocyclic Receptor for Metal Ion

Oct 10, 2012 - Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan...
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Specific Cooperative Effect of a Macrocyclic Receptor for Metal Ion Transfer into an Ionic Liquid Hiroyuki Okamura,†,‡ Atsushi Ikeda-Ohno,§,∥ Takumi Saito,⊥ Noboru Aoyagi,‡ Hirochika Naganawa,‡ Naoki Hirayama,# Shigeo Umetani,∇ Hisanori Imura,† and Kojiro Shimojo*,‡ †

Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan Division of Chemistry for Nuclear Engineering, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan § Reaction Dynamics Research Division, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ∥ School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ⊥ Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan # Department of Chemistry, Faculty of Science, Toho University, Funabashi 274-8510, Japan ∇ Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ‡

S Supporting Information *

ABSTRACT: An intramolecular cooperative extraction system for the removal of strontium cations (Sr2+) from water by use of a novel macrocyclic receptor (H2βDA18C6) composed of diaza-18-crown-6 and two β-diketone fragments in ionic liquid (IL) is reported, together with X-ray spectroscopic characterization of the resulting extracted complexes in the IL and chloroform phases. The covalent attachment of two β-diketone fragments to a diazacrown ether resulted in a cooperative interaction within the receptor for Sr2+ transfer, which remarkably enhanced the efficiency of Sr2+ transfer relative to a mixed β-diketone and diazacrown system. The intramolecular cooperative effect was observed only in the IL extraction system, providing a 500-fold increase in extraction performance for Sr2+ over chloroform. Slope analysis and potentiometric titration confirmed that identical extraction mechanisms operated in both the IL and chloroform systems. Extended X-ray absorption fine structure spectroscopy revealed that the average distance between Sr2+ and O atoms in the Sr2+ complex was shorter in IL than in chloroform. Consequently, Sr2+ was held by H2βDA18C6 more rigidly in IL than in chloroform, representing an important factor dominating the magnitude of the intramolecular cooperative effect of H2βDA18C6 for Sr2+. Furthermore, competitive extraction studies with alkaline earth metal ions revealed that the magnitude of the intramolecular cooperative effect depended on the suitability between metal ion size and the cavity size of H2βDA18C6. Sr2+ was successfully recovered from IL by controlling the pH in the receiving phase, and the extraction performance of H2βDA18C6 in IL was maintained after five repeated uses.

I

organic solvent systems. ILs enable the transfer of metal ions without the requirement for neutralizing the cationic charge, because charged species are extracted by ion exchange with the cationic or anionic constituent of the ILs (cation-exchange19 or anion-exchange20 mechanism, respectively). In comparison to organic solvents, ILs show remarkably high extraction performance for metal ions when certain neutral ligands such as crown ethers,19,21−26 calixarenes,27−30 CMPO,31−34 TODGA,35,36 and TPEN37 are used, because of the change in extraction mechanism from an ion-pair extraction in organic solvent systems to a cation-exchange extraction in IL-based systems, in which metal ion transfer does not depend on the acid and salt

onic liquids (ILs) have attracted considerable interest, in many fields of chemistry and industry, as a new class of solvents.1 ILs can be designed by selecting different combinations of cations and anions, which enables the synthesis of tailor-made solvents with specific desired properties.2−4 The features of ILs offer great potential as reaction media in a variety of fields including analytical chemistry,5−8 electrochemistry,9−11 and material chemistry.12−14 Over the past decade, several water-immiscible hydrophobic ILs have been widely investigated as extracting phases for the replacement of conventional organic diluents in liquid−liquid extraction systems.15−18 Liquid−liquid extraction is one of the most effective analytical methods for separation, purification, and removal of target substances. One of the greatest advantages of applying ILs to liquid−liquid extraction lies in the fact that IL-based extraction systems often cause unusual extraction behavior, which is not observed in conventional © 2012 American Chemical Society

Received: July 24, 2012 Accepted: October 10, 2012 Published: October 10, 2012 9332

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concentrations in the aqueous phase.19 In contrast, β-diketonetype anionic ligands transfer metal ions through a typical proton-exchange extraction and/or an anion-exchange extraction in IL-based systems.20,38−43 Furthermore, we reported that the employment of β-diketone together with crown ethers in IL extraction systems resulted in a selective synergistic effect for lighter lanthanides.44,45 Beyond their application to metal ion extraction, the outstanding ability of ILs as ion exchangers has in recent years also enabled the partitioning of metal nanoparticles46,47 and proteins.48,49 We have been interested in the development of macrocyclic receptors suitable for application in IL-based extraction systems. The cyclic framework of 4,13-diaza-18-crown-6 provides an interesting platform for the complexation of metal ions and can be readily functionalized with specific ligands to enhance the cation-binding ability and the ́ selectivity.50,51 For example, Ferreirós-Martinez et al.51 reported the complexation properties of the macrocyclic decadentate receptor N,N′-bis[(6-carboxy-2-pyridyl)methyl]4,13-diaza-18-crown-6 toward Sr2+ in aqueous solution. There has, however, been little progress in the development of novel macrocyclic receptors for use in IL-based extraction systems.27−30 In a preliminary communication,52 we synthesized a macrocyclic receptor (H2βDA18C6, Figure 1) composed of

H2βDA18C6 was used was compared with the extraction behavior when 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP), N,N′-dibenzyl-4,13-diaza-18-crown-6 (DBzDA18C6), and a mixture of HPMBP and DBzDA18C6 were used. In addition, the mechanisms of Sr2+ extraction in the [C2mim][Tf2N] and chloroform systems were determined by slope analysis and potentiometric titration. We clarified the relationships between the extraction performance of H2βDA18C6 for Sr2+ and the structures of the Sr2+ complexes by X-ray spectroscopic characterization of the Sr2+ complexes extracted into the [C2mim][Tf2N] and chloroform phases. Chemical structures of the ILs and the extractants employed in this study are shown in Figure 1.

■

EXPERIMENTAL SECTION Experimental details corresponding to the potentiometric titration, distribution of the macrocyclic receptor, X-ray structure determination, and extended X-ray absorption fine structure (EXAFS) spectroscopy are given in the Supporting Information. Reagents. The extractants HPMBP (Tokyo Chemical Industry Co., >98.0%) and DBzDA18C6 (Aldrich Chemical Co., 97%) were used without further purification. The synthesis of H2βDA18C6 was performed according to published procedures.52,55 The ILs [Cnmim][Tf2N] (n = 2, 4, and 6) were synthesized according to a published procedure49,56 with several minor modifications. The precursors [Cnmim][Br] were recrystallized from acetonitrile at least three times and subsequently treated with activated carbon. [Cnmim][Tf2N] were colorless, and no bromide content could be detected in the IL by an AgNO3 test. Chloroform (Wako Pure Chemical Industries, HPLC grade) was washed five times with ultrapure water prior to use. All other chemicals were of analytical or guaranteed reagent grade and were used as received. Ultrapure water (18.2 MΩ), produced with a Direct-Q (Millipore), was used throughout this study. Liquid−Liquid Extraction. Extracting phases were prepared by dissolving each extractant in [Cnmim][Tf2N]. For comparison with the performance of IL, a conventional organic solvent, chloroform, containing each extractant was also prepared by the same procedures. Aqueous phases were prepared by dissolving Sr(NO3)2 in a N-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (HEPES) buffer solution to generate a 0.01 mM solution. The pH of the aqueous solutions was adjusted by the addition of either HNO3 or LiOH into the HEPES buffer solution. Equal volumes of the extracting and aqueous solutions were mixed and shaken mechanically at 25 °C for 30 min to attain equilibrium (effect of shaking time on the extraction is shown in Figure S-1 in Supporting Information). Following the separation of the two phases by centrifugation, Sr2+ in the extracting phase was back-extracted into a 0.01 M HNO3 solution. The concentrations of Sr2+ in the aqueous phase and the receiving phase were determined by use of an inductively coupled plasma mass spectrometer (HewlettPackard HP 4500) to obtain the extractability (= [Sr2+]ext/ [Sr2+]ini × 100), the distribution ratio (= [Sr2+]ext/[Sr2+]aq), and the degree of back extraction (= [Sr2+]rec/[Sr2+]ext × 100). The subscripts ext, aq, rec and ini denote the extracting phase, aqueous phase, receiving phase, and initial condition, respectively. The equilibrium pH values of the aqueous phases were also measured.

Figure 1. Chemical structures and abbreviations for ionic liquids and extractants.

diaza-18-crown-6 and two 4-acyl-5-pyrazolone-type β-diketones, and we reported efficient transfer of Sr2+ by use of H2βDA18C6 in the IL extraction system. Strontium-90 (90Sr) is a radioactive fission product found in spent nuclear fuels and is a matter of serious concern because of its relatively long half-life (28.8 years) and high energy of β-decay.53 Following its incorporation into the human body, 90Sr accumulates in the bones, exerting severe adverse effects on human health.54 Thus, the removal of 90Sr from contaminated soil, seawater, and highlevel radioactive wastes represents an urgently required and challenging task. In the present study, we provide new and detailed data on the extraction performance of H2βDA18C6 for Sr2+ into 1ethyl-, 1-butyl-, and 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imides ([Cnmim][Tf2N]; n = 2, 4, and 6) and chloroform. The extraction behavior of Sr2+ when 9333

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RESULTS AND DISCUSSION Acid Dissociation Constants of the Extractants. The extractant H2βDA18C6 exhibited the following acid dissociation equilibria: H4β DA18C62 + ↔ H3β DA18C6+ + H+

(1)

H3β DA18C6+ ↔ H 2β DA18C6 + H+

(2)

−

H 2β DA18C6 ↔ Hβ DA18C6 + H

+

Hβ DA18C6− ↔ β DA18C62 − + H+

(3) (4)

where eqs 1 and 2 correspond to the acid dissociations of two β-diketone moieties, and eqs 3 and 4 represent the acid dissociations of two nitrogen atoms from the diaza-18-crown-6 moiety. The values of the acid dissociation constants (pKa) of the extractants (H2βDA18C6, DBzDA18C6, and HPMBP) used in the present study were determined by potentiometric titration. The measurements were carried out in H2O−dimethyl sulfoxide (DMSO) (1:1) at a constant ionic strength of 0.1 M LiCl because the extractants were insoluble in water. The pKa values of the extractants were calculated as follows: H2βDA18C6, pKa1, pKa2 < 2.34, pKa3 = 7.88, pKa4 = 8.65; DBzDA18C6, pKa1 = 5.85, pKa2 = 7.50; HPMBP, pKa = 3.73 (titration curves are shown in the Supporting Information, Figures S-2−S-4). Although the pKa1 and pKa2 values (βdiketone moieties) of H2βDA18C6 were estimated to be below 2.34, accurate values could unfortunately not be determined because of the strong acidity of this extractant. The pKa1 and pKa2 values of H2βDA18C6 were much lower than the pKa value of HPMBP because of local electrostatic repulsion between the protons of the β-diketone moieties and the protonated aza sites. The pKa3 and pKa4 values (diaza-18crown-6 moiety) of H2βDA18C6 were higher than the pKa1 and pKa2 values of DBzDA18C6. The shifts of pKa3 and pKa4 values in H2βDA18C6 are likely attributable to the intramolecular electrostatic interaction between the protonated aza sites and negatively charged oxygen atoms of the β-diketone moieties, implying that H2βDA18C6 exists as a dizwitterionic species over the pH range, from the weakly acidic conditions to the neutral region. Extraction Behavior of Sr2+. The extraction behavior of 2+ Sr in chloroform and [Cnmim][Tf2N] (n = 2, 4, and 6) systems with H2βDA18C6 as a function of pH in the aqueous phase is shown in Figure 2. The extraction performance of H2βDA18C6 in these four systems was compared with the extraction performance of β-diketone HPMBP, diazacrown ether DBzDA18C6, and a mixture of HPMBP and DBzDA18C6. The concentration of DBzDA18C6 was equal to that of H2βDA18C6, but the concentration of HPMBP was 2-fold greater than that of H2βDA18C6, to ensure the same number of functional β-diketone groups. In the chloroform system (Figure 2a), little partitioning of Sr2+ was observed when HPMBP or DBzDA18C6 was used. In contrast, quantitative extraction of Sr2+ was achieved when the mixture of HPMBP and DBzDA18C6 was used. This improvement in extraction efficiency was attributed to the synergistic effect, which was generated by the combination of HPMBP and DBzDA18C6. HPMBP neutralizes the positive charge of Sr2+, whereas DBzDA18C6 removes the coordinated water molecules from Sr2+, thereby forming a more hydrophobic metal complex. Sr 2+ extraction was also accomplished with H2βDA18C6 composed of diaza-18-crown-6 and β-diketone

Figure 2. Extraction behavior of Sr2+ into (a) chloroform and (b−d) [Cnmim][Tf2N] (n = 2, 4, and 6) phases as a function of pH in the aqueous phase. (Red symbols) H2βDA18C6 system; (green symbols) HPMBP and DBzDA18C6 system; (black symbols) HPMBP system; (blue symbols) DBzDA18C6 system. Aqueous phase, [Sr2+] = 0.01 mM; extracting phase, [H2βDA18C6] = 1 mM, [HPMBP] = 2 mM, and [DBzDA18C6] = 1 mM.

moieties, although any superiority over the mixture system of HPMBP and DBzDA18C6 was negligible. The extractability of Sr2+ plateaued at around 80%. In alkaline aqueous solution (pH > ca. 8.0), the distribution ratio of H2βDA18C6 between the chloroform and aqueous phases decreased because the hydrophobicity of the extractant was reduced by proton dissociation (Figure S-5, Supporting Information). In the [C2mim][Tf2N] system (Figure 2b), HPMBP provided very little indication of the efficiency of Sr 2+ extraction. In contrast, DBzDA18C6 was capable of quantitative transfer of Sr2+ into [C2mim][Tf2N], and its extraction performance was greatly enhanced in the IL. This positive result was attributed to the involvement of C2mim+ as a cationexchanger (details of this process will be described later under Determination of Extraction Mechanism). Similar extraction behavior was observed in the HPMBP and DBzDA18C6 mixed system relative to the use of only DBzDA18C6, implying that HPMBP barely acted as a synergist in [C2mim][Tf2N]. This was a negative result relative to the chloroform system. Surprisingly, H2βDA18C6 enabled the extraction of Sr2+ from more acidic aqueous solutions compared with the mixture system of HPMBP and DBzDA18C6. Namely, H2βDA18C6 exhibited a remarkably high extraction performance for Sr2+ in the [C2mim][Tf2N] system. The synergistic effect was not observed in the mixture system of HPMBP and DBzDA18C6 in [C2mim][Tf2N]. However, the covalent attachment of two β-diketone fragments to a diazacrown ether generated a cooperative interaction within the molecule, which significantly increased the efficiency of Sr2+ transfer in the [C2mim][Tf2N] system. We termed this enhancement of the extraction efficiency “intramolecular cooperative effect”. Intramolecular cooperative effect denotes the phenomenon that combining two different types of extractants leads to an increase of the extraction performance due to their cooperative interactions within the molecule, which is discriminated from general synergistic effect (the mixture system of two separate extractants). To assess the difference in the extraction performance of H2βDA18C6 between the [C2mim][Tf2N] and chloroform systems, an extraction test of Sr2+ using 9334

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H2 βDA18C6 was carried out under identical aqueous conditions ([Sr2+] = 0.01 mM, pH 7.0). It was confirmed that H2βDA18C6 showed about a 500-fold increase in extraction performance in [C2mim][Tf2N] relative to chloroform, when the distribution ratio of Sr2+ in the [C2mim][Tf2N] and chloroform systems was compared. In addition, we confirmed that the Sr2+ transfer with H2βDA18C6 was enhanced in [C4mim][Tf2N] and [C6mim][Tf2N] systems (Figure 2c,d) compared with in the chloroform system. These results indicated that ILs can provide an appropriate environment to enhance the performance of H2βDA18C6. The intramolecular cooperative effect of H2βDA18C6 was observed only in the IL systems and not in the chloroform system. When the magnitude of intramolecular cooperative effect was estimated by comparing the extraction performance of H2βDA18C6 with that of the HPMBP and DBzDA18C6 system, the magnitude of the intramolecular cooperative effect was higher in the [C2mim][Tf2N] system than in the [C4mim][Tf2N] and [C6mim][Tf2N] systems. Therefore, [C2mim][Tf2N] was used as an IL in the following studies. Determination of Extraction Mechanism. The extraction mechanisms of Sr2+ with the series of extractants employed in this study were investigated by slope analysis. In the DBzDA18C6 system in [C2mim][Tf2N], linear plots with a slope of 1.09 ± 0.01 were obtained for the DBzDA18C6 concentration (Figure 3a), indicating that one DBzDA18C6

combined mechanism of cation-exchange and proton-exchange reactions of Sr2+ for one C2mim+ and one proton as follows: Sr 2 + + H(DBzDA18C6)+IL + C2mim+IL ↔ Sr(DBzDA18C6)2 +IL + H+ + C2mim+

(5)

where the subscript IL denotes the [C2mim][Tf2N] phase. In the HPMBP and DBzDA18C6 mixed system in chloroform and [C2mim][Tf2N], slope analysis was conducted (Figure 4). In chloroform, the slopes obtained for DBzDA18C6

Figure 4. Slope analysis of Sr2+ extraction by use of HPMBP and DBzDA18C6 mixture in (open symbols) chloroform system and (solid symbols) [C2mim][Tf2N] system. (a) Dependency on DBzDA18C6 concentration: [Sr2+] = 0.01 mM; [HPMBP] = 2 mM; pH 7.7 (chloroform) or pH 7.6 ([C2mim][Tf2N]). (b) Dependency on HPMBP concentration: [Sr2+] = 0.01 mM; [DBzDA18C6] = 1 mM; pH 7.7 (chloroform) or pH 7.6 ([C2mim][Tf2N]). (c) Dependency on pH of aqueous phase: [Sr2+] = 0.01 mM; [HPMBP] = 2 mM; [DBzDA18C6] = 1 mM.

concentration, HPMBP concentration, and aqueous-phase pH were 0.81 ± 0.02, 1.74 ± 0.04, and 1.77 ± 0.07, respectively. These results indicated that one DBzDA18C6 molecule and two HPMBP molecules coordinated to Sr2+ and that the Sr2+ transfer was accompanied by the release of two protons to form the neutral adduct complex Sr(PMBP)2(DBzDA18C6). The synergistic extraction equilibrium equation can be represented as follows:

Figure 3. Slope analysis of Sr2+ extraction by use of DBzDA18C6 in the [C2mim][Tf2N] system. (a) Dependency on DBzDA18C6 concentration: [Sr2+] = 0.01 mM, pH 7.9. (b) Dependency on pH of aqueous phase: [Sr2+] = 0.01 mM, [DBzDA18C6] = 1 mM.

Sr 2 + + 2HPMBPorg + DBzDA18C6org ↔ Sr(PMBP)2 (DBzDA18C6)org + 2H+

2+

molecule was required to extract Sr , in that a 1:1 complex, Sr(DBzDA18C6)2+, was formed. The slope of the logarithmic distribution ratio versus pH plots was 1.13 ± 0.01 (Figure 3b). Considering the result of the slope analysis and the acid dissociation constants of DBzDA18C6, it was envisaged that the protonated extractant H(DBzDA18C6)+ would participate in Sr2+ extraction under aqueous conditions from pH 7 to 9 and that one proton from H(DBzDA18C6)+ would be released to transfer the divalent cation Sr2+. It is worthy of note that the number of protons released is not equivalent to the valence of Sr2+. This fact implied that the charge balance during the partitioning process was maintained by cation exchange with C2mim+. To elucidate the involvement of C2mim+ in Sr2+ extraction using DBzDA18C6, the dependence of distribution ratio of Sr2+ on C2mim+ concentration in the aqueous phase was investigated (Figure S-6, Supporting Information). The distribution ratio of Sr2+ was reduced with increasing C2mim+ concentration. The slope of the logarithmic distribution ratio versus logarithmic C2mim+ concentration in the aqueous phase plots was −1.09 ± 0.02. These data indicated that the transfer of Sr2+ with DBzDA18C6 into [C2mim][Tf2N] proceeded via a

(6)

where the subscript org denotes the chloroform phase. In contrast, in [C2mim][Tf2N], although the slopes obtained for the DBzDA18C6 concentration and the aqueous-phase pH were 1.09 ± 0.01 and 1.11 ± 0.01, respectively, the extraction dependency of Sr2+ on the HPMBP concentration in the mixture system was negligible (except for the high HPMBP concentration region). That is, HPMBP barely behaved as a synergist in [C2mim][Tf2N] because partitioning of Sr2+ into [C2mim][Tf2N] was possible without neutralizing the cationic charge of Sr2+ by HPMBP. In addition, the slope of the logarithmic distribution ratio versus logarithmic C 2mim+ concentration in the aqueous phase plots was −0.56 ± 0.03 (Figure S-6, Supporting Information). These results suggested that the extraction mechanism of Sr2+ in the mixture system of HPMBP and DBzDA18C6 was almost identical to the combined mechanism of proton- and cation-exchange reactions in the system with only DBzDA18C6, as shown in eq 5. However, when the minor dependence on HPMBP concentration and the slight difference in extraction behavior with DBzDA18C6 are considered, it is obvious that HPMBP 9335

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participates in the extraction under high HPMBP concentration conditions (cf. eq 6), albeit to a small extent, which led to the increased value of the slope for C2mim+ concentration relative to the DBzDA18C6 system. In the Sr2+ transfer with H2βDA18C6 in chloroform and [C2mim][Tf2N] systems, slope analysis was performed as a function of logarithmic H2βDA18C6 concentration (Figure 5a)

Figure 6. (a, b) Crystal structure of [Sr(βDA18C6)(DMSO)] from different perspectives; (c) illustration of Sr2+ primary coordination sphere encapsulated by the diaza-18-crown-6 moiety and two oxygens of the β-diketone fragments. Green = Sr2+; red = oxygen; blue = nitrogen; yellow = sulfur; black = carbon. Hydrogens are not shown for clarity.

Figure 5. Slope analysis of Sr2+ extraction by use of H2βDA18C6 in (open symbols) chloroform system and (solid symbols) [C2mim][Tf2N] system. (a) Dependency on H2βDA18C6 concentration: [Sr2+] = 0.01 mM; pH 7.0 (chloroform) or pH 6.2 ([C2mim][Tf2N]). (b) Dependency on pH of aqueous phase: [Sr2+] = 0.01 mM; [H2βDA18C6] = 1 mM.

equivalent stoichiometry (1:1) of the complex was consistent with the result of the slope analysis. One DMSO molecule was incorporated into the Sr−βDA18C6 unit during the crystallization process. Sr2+ was encapsulated in the cavity of the diaza18-crown-6 ring, and coordinated to four oxygen and two nitrogen atoms from the diaza-18-crown-6 moiety. Furthermore, the encapsulated Sr2+ was further coordinated to two oxygen atoms from two β-diketone moieties. As illustrated in Figure 6c, the total coordination number (CN) in the Sr2+ primary coordination sphere was 9, consisting of four oxygens (ODA18C6) and two nitrogens (NDA18C6) from the diaza-18crown-6 moiety, two oxygens from the β-diketone moiety (OβDK), and one oxygen from DMSO (ODMSO) with average coordination distances of 2.663, 2.742, 2.519, and 2.442 Å, respectively. The subscripts DA18C6 and βDK denote diaza18-crown-6 and β-diketone, respectively. The β-diketone moiety could only coordinate to the Sr2+ center in a unidentate fashion via one of its two coordinative oxygens, likely because the carbon chain connecting the β-diketone and diaza-18crown-6 fragments was too short to give the β-diketone moiety the steric margin for bidentate coordination. Unfortunately, preparation of single crystals of Sr−βDA18C6 complex extracted into [C2mim][Tf2N] proved difficult because of the inherent properties of ILs, such as negligible volatility and immiscibility with poor solvents. Nevertheless, the crystallographic information obtained for the Sr(βDA18C6)(DMSO) complex served as a valuable reference for the structural determination of extracted complexes by EXAFS spectroscopy discussed in the next section. Structural Comparison of Extracted Complexes in Chloroform and [C2mim][Tf2N]. On the basis of crystal structure data for the Sr(βDA18C6)(DMSO) complex, the structural arrangements of the Sr−βDA18C6 complexes extracted into chloroform and [C2mim][Tf2N] were investigated by EXAFS spectroscopy.20,57−60 Experimental details are presented in Supporting Information. The k3-weighted Sr K-edge EXAFS spectra for Sr2+ extracted into the chloroform phase (shown in blue) and the [C2mim][Tf2N] phase (shown in red) are presented in Figure 7a, together with the data for a structural reference of Sr(βDA18C6)(DMSO) crystals (shown in black). Their corresponding Fourier transforms (FTs) are shown in Figure 7b. The Sr(βDA18C6)(DMSO) complex displayed three distinguishable peaks in its EXAFS-FT spectrum at R + Δ = 2.0, 2.8, and 3.9 Å (where Δ represents

and aqueous-phase pH (Figure 5b). In chloroform, the slopes obtained for H2βDA18C6 concentration and aqueous-phase pH were 1.00 ± 0.03 and 1.92 ± 0.15, respectively. Surprisingly, similar slopes were observed in [C2mim][Tf2N], which were 1.03 ± 0.01 and 1.73 ± 0.02, respectively. These results indicated that one molecule of H2βDA18C6 participated in the Sr2+ extraction and that two protons were released from H2βDA18C6 for the partitioning of Sr2+ in both systems. Furthermore, we confirmed that the distribution ratio of Sr2+ in both extraction systems was independent of the initial Sr2+ concentration. These results suggested that the extracted species Sr(βDA18C6) did not dimerize in these extraction systems. From the slope analysis, the transfer of Sr2+ with H2βDA18C6 proceeded via a proton-exchange reaction in both chloroform and [C2mim][Tf2N] systems as follows: Sr 2 + + H 2β DA18C6ext ↔ Sr(β DA18C6)ext + 2H+

(7)

In spite of the identical extraction mechanism in both chloroform and [C2mim][Tf2N] systems, H2βDA18C6 provided a remarkably high extraction performance for Sr2+ in the [C2mim][Tf2N] system relative to the chloroform system. The different solvent properties of IL and chloroform might have an influence on the performance of H 2 βDA18C6 for Sr 2+ extraction. Solid-State Structure of Sr−βDA18C6 Complex. We assumed that the enhancement of the intramolecular cooperative effect of H2βDA18C6 in ILs was attributable to the unique solvent properties of ILs. The strength of the electrostatic interaction between the metal ions and the extractants can be sensitive to the physicochemical properties of solvents, which may give rise to structural differences in the extracted complexes between the chloroform and [C2mim][Tf2N] systems. To confirm this hypothesis, the solid-state molecular structure of the extracted complex formed in chloroform was characterized by X-ray diffraction (XRD). Details of sample preparation and data analysis are presented in Supporting Information. The crystal structure obtained is illustrated in Figure 6. The molecular formula of the crystallized complex was determined to be [Sr(βDA18C6)DMSO]. The 9336

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Analytical Chemistry

Article

the extracted Sr−βDA18C6 complex, the geometry of the first coordination shell of the Sr2+ species extracted into chloroform can be assumed to be similar to that in the crystal structure, in that the first peak at R + Δ = 2.0 Å was assigned to the coordination of the oxygens and nitrogens from H2βDA18C6 and that there would be additional water coordination perpendicular to the diaza-18-crown-6 ring plane instead of DMSO coordination.57 The average Sr−O distance in the chloroform sample was 2.55 Å. In contrast, the Sr−βDA18C6 complex extracted into [C2mim][Tf2N] phase (data shown in red in Figure 7) displayed the first peak at a shorter distance of R + Δ = 1.9 Å, resulting in a much shorter average Sr−O distance of 2.51 Å. These results clearly indicated that Sr2+ was held by H2βDA18C6 more rigidly in [C2mim][Tf2N] than in chloroform, which would likely be an important factor dominating the magnitude of the intramolecular cooperative effect of H2βDA18C6 for Sr2+. The observed shortening of Sr− O distance in the [C2mim][Tf2N] system is possibly attributed to the strong dehydration effect caused by solvation of the extracted complex with [C2mim][Tf2N].43 Extraction Behavior of Other Alkaline Earth Metals. To obtain further insight into the intramolecular cooperative effect of H2βDA18C6, the competitive extraction of alkaline earth metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) by use of H2βDA18C6 in the [C2mim][Tf2N] system was investigated, together with extractions that used HPMBP, DBzDA18C6, and the mixture of HPMBP and DBzDA18C6 (Figure S-9, Supporting Information). The extraction behavior of Ba2+ was very similar to that of Sr2+, indicating that H2βDA18C6 also generated the intramolecular cooperative effect for Ba2+, likely because the cavity of H2βDA18C6 fits the size of Ba2+ as well as Sr2+. For the sake of comparison, the ionic radii for Ba2+, Sr2+, K+, and Na+ at CN = 9 are 1.47, 1.31, 1.55, and 1.24 Å, respectively.61 In contrast, very different extraction behavior was observed for Mg2+ and Ca2+ transfer relative to Sr2+ transfer. Mg2+ and Ca2+ were extracted by use of only HPMBP because the higher charge densities of Mg2+ and Ca2+ facilitated an electrostatic interaction with HPMBP. In addition, the sizes of Mg2+ and Ca2+ were unfavorable for the cavity of DBzDA18C6, with the ionic radii for Mg2+ at CN = 8 and Ca2+ at CN = 9 being 0.89 and 1.18 Å, respectively,61 which made the extraction of Mg2+ and Ca2+ by use of DBzDA18C6 impossible. The differences in the extraction performance between the H2βDA18C6 system and HPMBP + DBzDA18C6 system were very low for Mg2+ and Ca2+, and the intramolecular cooperative effect of H2βDA18C6 was not well observed for Mg2+ and Ca2+. The magnitude of the intramolecular cooperative effect of H2βDA18C6 depended on the size of the alkaline earth metal ion. The suitability between the size of metal ion and the cavity size of H2βDA18C6 represented another important factor in the intramolecular cooperative effect of H2βDA18C6.

Figure 7. (a) k3-weighted Sr K-edge EXAFS spectra for Sr(βDA18C6)(DMSO) crystal (black) and Sr−βDA18C6 complexes extracted into chloroform phase (blue) and [C2min][Tf2N] phase (red); (b) their corresponding Fourier transforms. Solid lines, experimental data; dotted lines, theoretical fit; peak A on FT, Sr−O and Sr−N; peak B on FT, Sr−C. Phase shifts (Δ) are not corrected on FTs.

the phase shifts), reflecting the arrangement of the Sr2+ primary coordination sphere illustrated in Figure 6c. Thus, the first peak at R + Δ = 2.0 Å corresponded to the mixture of seven oxygens comprising four ODA18C6, two OβDK, and one ODMSO (Sr−O) and two nitrogens of NDA18C6 (Sr−N). The second and third peaks at R + Δ = 2.8 and 3.9 Å were attributed to 16 carbons (Sr−C) from the H2βDA18C6 framework and one sulfur from DMSO, respectively. The obtained interatomic distances (R) listed in Table 1 were also in agreement with those obtained from the crystal data. A liquid sample of Sr−βDA18C6 complex extracted into the chloroform phase (data shown in blue in Figure 7) also exhibited the largest EXAFS-FT peak at R + Δ = 2.0 Å, whereas the other peaks at a longer R range were smaller than those for the solid Sr(βDA18C6)(DMSO) sample. It is obvious from the extraction experiments that the Sr2+ species extracted into chloroform and [C2min][Tf2N] are both forming the 1:1 complex Sr2+−βDA18C6, whose coordination arrangement is highly likely to be identical to that observed in the crystal structure (Figure 6c). It should be noted that the 16 carbons of the H2βDA18C6 framework have no direct interaction with the Sr2+ center, being more flexible than the coordinating O and N sites. The flexibility of the carbons of H2βDA18C6 framework is probably more enhanced in solution than in the solid state (i.e., crystal structure), because of the faster thermal vibration in solution. In consequence, the R value for Sr−C had a wider distribution in solution, making the corresponding FT peak almost indiscernible despite its large CN of 16. This is probably the reason why the liquid samples showed much smaller EXAFS-FT peaks above R + Δ = 2.6 Å. For this reason only the first peak at R + Δ = 2.0 Å (i.e., Sr−O and Sr−N) was taken into account for the EXAFS curve fitting of the liquid samples. Given the fact that the Sr(βDA18C6)(DMSO) crystals were obtained from a chloroform solution of

Table 1. Summary of EXAFS Structural Parameters for Sr−βDA18C6 Complexes scattering shell Sr−O sample Sr(βDA18C6)(DMSO) crystal Sr−βDA18C6 in chloroform Sr−βDA18C6 in [C2min][Tf2N] a

form

CNa

solid liquid liquid

c

7 7.1 7.1

Sr−N R/Åb

CNa d

2.59 (2.590 ) 2.55 2.51

c

2 2.2 2.1

Sr−C R/Åb

CNa d

2.77 (2.742 ) 2.69 2.71

c

16

R/Åb 3.48 (3.496d)

Coordination number; error CN ≤ ±20%. bInteratomic distance; error R ≤ ±0.01 Å. cFixed values. dValues found in the single-crystal structure. 9337

dx.doi.org/10.1021/ac302015h | Anal. Chem. 2012, 84, 9332−9339

Analytical Chemistry Back Extraction and Recycling Test. The recovery of extracted metals is important for separation and purification. There has, however, been little progress in the development of IL-based extraction systems, striking a balance between improvements in extraction and recovery by back extraction.24,28,37 We investigated the stripping test of Sr2+ from [C2mim][Tf2N] by controlling the pH of the receiving phase (Figure S-10, Supporting Information). The degree of back extraction of Sr2+ was enhanced as the acidity of the receiving phase was increased. As a result, quantitative recovery was achieved under low pH conditions where each extractant loses its ability to coordinate to Sr2+. In addition, a recycling test of H2βDA18C6 in the [C2mim][Tf2N] phase was conducted, following the same procedure, with a 0.01 mM HNO3 solution being used as the receiving phase, and the IL phase was then recycled. Following five cycles of forward and back extraction, the H2βDA18C6−[C2mim][Tf2N] extraction system maintained its high extraction ability for Sr2+. The leakage of H2βDA18C6 from the [C2mim][Tf2N] phase into the receiving phase was negligible, even following contact with acidic solutions (Figure S-5, Supporting Information). The IL solution with H2βDA18C6 was found to be reproducible as the extracting phase.

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REFERENCES

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CONCLUSIONS In the present study, we have investigated the facilitated transfer of Sr2+ by intramolecular cooperative effect with diaza18-crown-6 incorporating β-diketone fragments in IL. H2βDA18C6 in IL provided remarkably high extraction performance for Sr2+ compared with that in chloroform because of intramolecular cooperative interaction. The superior extraction in the IL system was believed to be caused by shortening of the average Sr−O distance in the Sr−βDA18C6 complex in the IL. Furthermore, the magnitude of the intramolecular cooperative effect of H2βDA18C6 depended on the suitability between the size of metal ion and the cavity size of H2βDA18C6. The present findings suggest that crown ethers incorporating ligand fragments are capable of forming a stable complex with metal ions in IL. In the future, it is envisaged that the attachment of appropriate ligand fragments to an azacrown ether suitable for the size of metal ions will enable the selective extraction of specific metal ions. ASSOCIATED CONTENT

S Supporting Information *

Additional text, 10 figures, and two tables showing effect of shaking time on Sr2+ extraction, titration curves, distribution behavior of H2βDA18C6, effect of C2mim+ concentration in the aqueous phase, single-crystal X-ray structure determination, EXAFS spectroscopy and structural parameters, competitive extraction behavior of alkaline earth metal ions, and stripping test (PDF file); structural information (CIF file). This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We thank the Center for Instrumental Analysis at Ibaraki University for NMR measurements. Technical support received from Y. Okamoto, S. Suzuki, and T. Yaita for EXAFS measurements and from T. Kobayashi for single-crystal XRD measurements is gratefully acknowledged. This research was financially supported by the Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (to H.O.) and a Grant-in-Aid for Young Scientists (B) (22760585 to K.S.). The EXAFS measurements at Photon Factory were performed under the approval of the Photon Factory Program Advisory Committee (Proposals 2009G537 and 2009G549).

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. 9338

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Analytical Chemistry

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