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Worthy extraction and uncommon selectivity of 4f-ions in ionic liquid medium: 4-acylpyrazolones and CMPO Maria Atanassova Petrova ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00038 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Sustainable Chemistry & Engineering

Worthy extraction and uncommon selectivity of 4f-ions in ionic liquid medium: 4-acylpyrazolones and CMPO

Maria A. Petrovaa,b,c

a

University of Chemical Technology and Metallurgy, Department of General and Inorganic Chemistry, 8 Kliment Okhridski blvd., 1756 Sofia, Bulgaria b University of Grenoble Alpes, Grenoble, France c CNRS, LEPMI, UMR 5279, 1130 rue de la piscine, 38000 Grenoble, France E-mail: [email protected]

Abstract The synergistic solvent extraction of three selected lanthanoid ions (Ce3+, Eu3+ and Lu3+) with 4-(4-fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one or 3-methyl-4-(4-methylbenzoyl)-1phenyl-pyrazol-5-one (HL) in combination with N,N-diisobutyl-2(octylphenylphosphoryl)acetamide (CMPO) or 5,11,17,23-tert-butyl-25,26,27,28tetrakis(dimethylphosphinoylmethoxy)calix[4]arene, (C4) in four ionic liquids of the imidazolium family [C1Cnim][Tf2N], (n = 4, 6, 8 and 10) has been investigated. The interaction between the extractants in deuterochloroform has been studied by 1H, 13C, and 31P NMR spectra and NOESY. On the basis of the experimental data, the values of the distribution ratios have been calculated. The logD values decrease as a function of n, i.e. with a trend following that of the hydrophobicity of ILs’ cations: 4>6>8>10 using the chelating ligand with the bulkier substituent (-CH3). The influence of the para-substituted 4acylpyrazolone and the synergistic agent on the extraction process have been discussed. A synergistic effect less than one order of magnitude occurs in the extraction of Ln(III) ions with mixtures of HL and CMPO or C4. The values of separation factors of Eu/Ce and Lu/Eu pairs have been evaluated. Keywords: ionic liquids, lanthanoids, chelating ligand, synergistic extraction, separation

INTRODUCTION The use of ionic liquids (ILs) as an innovative organic medium for the solvent extraction of a large variety of metals from Li+ to Pu4+ is a promising development in separation science and technology owing to environmental concerns.1-5 ILs are diluents that consist entirely of ions and are liquids at unusually low temperatures. The boiling point of water was chosen as a reference point most likely for historical reason. Due to the growing interest in sustainable, “green” chemistry ILs have immense potential to replace volatile organic diluents in many chemical engineering processes. They possess fundamentally unique combinations of properties such as being nonvolatile, nonflammable and able to dissolve both ionic and nonionic species. The employment of ILs as “environmentally friendly alternative diluents” is in accordance with some of the twelve principles of the green chemistry: 1) it is better to prevent waste, 2) designing safer chemicals, 5) safer solvents and auxiliaries and 12) 1 ACS Paragon Plus Environment


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inherently safer chemistry for accident prevention. At the same time, a subset of ILs retains important properties of certain conventional molecular diluents as immiscibility with water. In addition, the physico-chemical properties of individual IL can be tuned over wide ranges by varying the constituent cations or anions.6,7 To date, the only cation species studied have been based on pyridinium, imidazolium and quaternary ammonium moieties and bulky anions such as hexafluorophosphate (PF6˗) and bi(trifluoromethylsulfonyl)amide (Tf2N˗: (CF3SO2)2N˗) that have been assessed as valuable, because they have relative air and water stability and favorable viscosity, and density as diluents.8, 9 In most reports on solvent extraction processes incorporating ILs, only hydrophobic were considered, so the number is limited to fluorinated anions. In the case of hexafluorophosphate, the hydrolysis of the anion leading to the formation of hazardous hydrofluoric acid is major disadvantage. During liquid/liquid extraction of metals, the presence of a mineral acid, most often nitric or hydrochloric, as well as the existence of inorganic salts to fix the ionic strength lead to a significant increase of the ILs’ miscibility into the aqueous phase, which can be considered as only key drawback of this type of hydrophobic ILs involving 1-alkyl-3-methyl-imidazolium cations and bis(trifluoromethylsulfonyl) imide anion, already well recognized as unique extraction innovative media.10 A family of engineering-purpose ILs derived from Aliquat 336 (methyltrialkyl(C8-C10) ammonium chloride) has attracted attention in the extraction systems as synergistic agents added to improve the extraction yield of lanthanoids in combination with chelating compounds in molecular diluents (C6H6, CCl4, C6H12, CHCl3, C2H4Cl2) and thus, undermining the green character of the system in some extent i.e. Aliquat 336 was used as rare earths extractant more early than it was known as a novel IL.11-13 The remarkable results described by Dai and co-workers in 1999 showing the excellent extraction of Sr2+ in IL medium with dicyclohexane-18-crown-6 suggested vast opportunities for application in separation technology and opened new avenues for their employment in solvent extraction as a branch of science and engineering.14 Though many single ligand/IL systems investigated since the appearance of Dai pioneering study, there are only few articles, which report the advantage of mixed ligand systems, for f-elements and strontium ion only, generating synergistic effect.15-23 The phenomenon known as synergism in solvent extraction has been extensively studied during the last 60 years in various molecular diluents. To go a step further one may wanted to tackle the question of synergism in ILs. Undoubtedly, the two fundamental questions arise after examination of the wealth of data provided in the literature: What are the rules governing synergism in ionic liquid extraction medium? Can this phenomenon be mastered under the frame of the general model already developed for VOCs untill 1958 when it was discovered.24 In this fundamental study, the solvent extraction of three trivalent ions Ce3+, Eu3+ and Lu3+ chosen as representatives of the beginning, middle and the end of the 4f-series, was performed in four water-immiscible imidazolium type ILs, [C1Cnim+][Tf2N˗], n=4, 6, 8, 10, as diluents and two popular β-diketone type anionic ligands from 4-acylpyrazolone family as chelating extractants applied alone and in combination with CMPO compound, a bifunctional organophosphorus molecule having P=O and C=O groups (Figure 1) with the goal to determine the synergistic enhancement and selectivity, although, the data interpretation and the slope analysis results assessment are maybe insufficient at the viewpoint of extraction mechanism research in ionic diluents.

MATERIALS AND METHODS Reagents 2 ACS Paragon Plus Environment

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4-(4-Fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one (HPMFBP) and 3-methyl-4-(4methylbenzoyl)-1-phenyl-pyrazol-5-one (HPMMBP) were obtained according to a literature procedure already described in detail.25 The commercial product N,N-diisobutyl-2(octylphenylphosphoryl)acetamide (CMPO) with a purity higher than 98.6% (ChemosGmbH) was used as received. The lower rim substituted calix[4]arene, C4, was synthesized according to the method already described, Fig. 1.26

Figure 1. Structural formulas of 4-aroyl-3-methyl-1-phenyl-pyrazol-5-ones, CMPO and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene. The diluens 1-butyl-, 1-hexyl-, 1-octyl-, and 1-decyl-3-methyl-imidazoliumbis(trifluoromethanesulfonyl)imide, (purity, 99.5%) were purchased from Solvionic (Toulouse, France) and dried in vacuum (0.2 mbar) at a raised temperature of 60±1oC during 3 h prior to use. Stock solutions of the lanthanoid ions were prepared from their oxides and CeCl3·7H2O (Fluka, puriss) by dissolving in concentrated hydrochloric acid and diluting with distilled water to the required volume. Arsenazo III (Fluka, Switzarland) was of analytical grade purity as were the other reagents used. A 1002 mg/L europium ICP standard was obtained from Fluka (Switzerland). All samples were prepared with ultra-pure water (Millipore). Solvent extraction of lanthanoids All experiments were performed at room temperature (22±2)oC. Extraction experiments were carried out by mixing aqueous and organic phases in 1:1 v/v ratio (0.8 cm3) for 2 hours (1500 rpm), which was more than sufficient for attaining equilibrium. The top layer contained the aqueous and the bottom layer included the organic IL phase. After the separation of the phases by 2 min centrifugation (5000 rpm), the lanthanoid ion concentration in the aqueous phase was determined spectrophotometrically using Arsenazo III (S-20 Spectrophotometer Boeco).27 ICP-OES (Varian 72 ES) was used for Eu3+ measurements only including the calixarene ligand. The concentration of the metal ion in the organic phase was obtained by material balance. Note that [C1Cnim+][Tf2N˗] alone do not extract Ln(III) ions under the experimental conditions. Extractant solutions were prepared by precisely weighted samples. The acidity of the aqueous phase at equilibrium was measured by a pH meter (pH 211 HANNA, USA) with an accuracy of 0.01 pH unit. The ionic strength was maintained at 0.1 M (Na, H) Cl. The initial concentration of the lanthanoid ions was 2.5 × 10-4 mol/dm3 in all experiments. The distribution ratio (D) was calculated as (total metal concentration in the extraction phase) / (total metal concentration in the aqueous phase). The synergistic enhancement upon addition of a second extractant can be assessed using synergistic coefficients calculated as SC = log (D1,2 / D1 + D2) where D1,2, D1, and D2 denote the distribution coefficient of a metal ion using mixture of extractants (D1,2) and the same extractants separately (D1 and D2). The metal separation can be assessed using separation factors (S.F.) determined as S. F. = D(Z+n) /D(Z). 3 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION Solvent extraction of Ln(III) ions with two 4-aroyl-3-methyl-1-phenyl-pyrazol-5-ones in IL media, [C1Cnim+][Tf2N˗] Beside classical β-diketones, a novel family of enolizable ligands named 4-aryl-5-pyrazolones has been handled for a long time (near 60 years) to extract almost all metal ions in the periodic table due to their high extraction ability and great separation power.28-30 The knowledge concerns the coordination behavior in IL media is totally unknown and continue to be very scarce as only handful of papers are present in the literature dealing with the complexation of acylpyrazolones with f-ions acceptors.21,22,31 Much more developed is instead the chemistry including β-diketones, in particular thenoyltrifluoroacetone (HTTA) molecule.32-35 The fundamental metal:proton/ligand stoichiometry for the extraction of Ln3+ ions into [C1Cnim+][Tf2N˗] have been studied by measuring the equilibrium partitioning of fions between aqueous and IL phases as functions of the aqueous acidity and the ligand concentrations. A detailed NMR study on the possible interactions between a series of ILs [C1Cnim+][Tf2N˗] and some widely applied for f-ions extractants (acidic or neutral) shows that no interactions between them occurred in chloroform solutions independently on the length of the imidazolium alkyl chain or on the structure and acidity of the ligand.36 Since IL phase has large amounts of exchangeable ionic species,10 Ln3+ ion can be extracted not only as neutral complex incorporating anions of the chelating ligand (LnL3 and LnL3·HL) but also as cationic [LnL2]+ or anionic [LnL4]˗ species. It is well documented in extraction systems applying molecular diluents.11-13,37 So, the following extraction equilibria can be considered: i) neutral species21,22 Ln3+(aq) + 3HL(o) ↔ LnL3(o) + 3H+(aq) (1) 3+ + Ln (aq) + 4HL(o) ↔ LnL3·HL(o) + 3H (aq) (2) ii) anionic species 8, 11-13 Ln3+(aq) + 4HL(o) + [C1Cnim+][Tf2N˗](o) ↔ [C1Cnim]+[LnL4]˗(o) + 4H+(aq) + [Tf2N˗](aq) (3) In order to determine the stoichiometric values, relationship between logarithmic distribution ratio of each metal ion and equilibrated pH at fixed HL concentrations (Figures 2, 3, S1 and S2, see Supporting Information) as well as those between logD versus ligands concentrations (Figures S3 and S4) was studied. The distribution ratios of Ln(III) ions increase with an increase in the concentration of HPMMBP (Figures 2 and 3) or HPMFBP (Figures S1 and S2) in the oil phase. It is seen from the figures that the conventional slope analysis was not feasible because most of the [H+] and extractant dependencies are strongly nonlinear ascribed to the mutual solubilities of the H2O, H+, [C1Cnim+] and [Tf2N˗] entities in the lower or upper phases.10 Although slope analysis could not clearly identify a single, unique, metal-extractant species in the IL phase, the slopes are approximately 1÷2.3 (HPMMBP) and 1÷2 (HPMFBP), suggesting a maximum HPMFBP:Ce3+ ratio of 3 (the slope is ~2.6, Figure S4). So, the existence of multiple extraction equilibria that vary in importance as the aqueous solution conditions change for light/middle/heavy 4f-ion could be evoke as well as for a certain ion. While not quantitative, the slopes demonstrate that anionic 1:4 (Ln3+:HL) complexes are not extracted into [C1Cnim+][Tf2N˗] under the applied experimental conditions. The dependencies of Ln(III) ion distribution ratios on pH of the aqueous phase have shown slopes in the range 0.97÷2.3 (more higher for Ce3+, 2÷2.3) for all cases utilizing HPMMBP as a chelating agent, and in the frame of 0.9÷2 (as well more higher slope values for Ce3+, 2.3÷3.5), when HPMFBP was implemented. This means that 1 or 2 molecules (maximum 3 for Ce3+ cases) of the acidic ligand are involved in the extraction process and the 4 ACS Paragon Plus Environment

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rest of the charge of Ln(III) could be compensated by [Tf2N˗] anions present in the organic phase.8,20,31 Therefore, the plausible mechanism for the extraction of Am(III) by benzoylpyrazolone proposed by Rama and co-researchers can be modified and rewritten for 4f-ions under the present study:31 Ln3+(aq) + 2HL(o) + x[C1Cnim+][Tf2N˗](o) ↔ Ln(L)2-x·(HL)x·(Tf2N˗)x(o) + (2-x)H+(aq) + x[C1Cnim+](aq) (4) It can be seen from the obtained results (Figures 3 and S2) that the logD values decrease as a function of n, i.e. with a trend following that of the hydrophobicity of ILs’ cations and with the decrease of IL components concentration in the aqueous phase i.e. [C1C4im+][Tf2N−]>[C1C6im+][Tf2N−]>[C1C8im+][Tf2N−]>[C1C10im+][Tf2N−] for Ln3+/HPMMBP/ILs systems. However, when HPMFBP is used, the following order n=10>n=4>n=8>n=6 could be distinguished. A detailed q-NMR study performed by Atanassova et al.10 on the IL cation and anion solubilities in the upper phase (pHeq range 0.58) shows that for a given aqueous phase composition (DCl, DNO3, DClO4 in the absence of NaClO4 and metal), the solubility presents a decreasing equal trend as n increases, in accordance with the hydrophobicity of the cation: [C1C2im+]=[Tf2N−]= 40 mM, [C1C4im+]=[Tf2N−]= 17 mM, [C1C6im+]=[Tf2N−]= 6 mM. For chain length n equal to 10 and 8, the values are below the detection limit i.e. negligible transferred amount of entities [C1Cnim+] and [Tf2N−].10 On an environmental approach, it is effortless to accomplish that the increase of the alkyl chain length of imidazolium cations will reduce the unenviable IL’s component solubility in weak acidic and ionic aqueous media (µ≤0.1) without NaClO4. In addition, Dietz and co-researchers38 reported that the cation exchange mechanism operates for Sr2+ extraction with dicyclohexano-18-crown-6 into [C1C5im+][Tf2N−] and [C1C6im+][Tf2N−]. Mixed extraction is observed for [C1C8im+][Tf2N−], while a neutral complex is extracted in the case of [C1C10im+][Tf2N−].38 For the extraction of trivalent lanthanoids in molecular diluent CHCl3, the observed dependence is in accordance with the decreasing pKa values of the substituted pyrazolones. The obtained equilibrium constants (logKL) with 4-fluorophenyl terminal group are larger than those of 4-methyl.25 The 4-acylpyrazolone (HPMFBP) having a lower pKa value due to an electron withdrawing substituent (–F) tends to be a stronger acid (3.52) in comparison with the case when electron donating (–CH3) groups (4.02) are introduced in the benzoyl moieties. The substituted pyrazolones (HPMFBP or HPMMBP) cause significant quantitative but not qualitative changes in the solvent extraction of lanthanoids in CHCl3 solution because in both cases LnL3·HL adducts were established. It was found, in the present study, that HPMMBP is a little bit more efficient than HPMFBP or demonstrate equal coherence (Ce3+, 5x10-3 M HL). The La(III) ion extraction behavior in the IL medium is influenced neither qualitatively nor quantitatively by the nature of the 4-acylpyrazolone substituents: logKL= -2.33 for 3-methyl1-phenyl-4-(4-trifluoromethylbenzoyl)-pyrazol-5-one (-CF3) or logKL= -2.16 for 3-methyl-1phenyl-4-(4-phenylbenzoyl)-pyrazol-5-one (-C6H5),22 while in molecular diluent, CHCl3, the corresponding equilibrium constants are -3.24 and -6.21, respectively. As derived from these results, when this family of ligands (4-acylpyrazolones) are dissolved in [C1Cnim+][Tf2N˗] they extract 4f-ions more or less similarly, i.e. with comparable efficiency in contrast with the large distinction perceived in molecular diluents. Fluorinated substituents have often been introduced into extractants in order to increase the acidity by the electron-withdrawing effect of the fluorinated group. Improved understanding of solvation phenomena in both aqueous and organic oil media, development of complexing agents that do not create environmental hazards or waste disposal complications, when their utility is accomplish (CHON principle) can be suggested for future research emphasis.

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NMR interaction study between 4-(4-methylbenzoyl)-3-methyl-1-phenyl-pyrazol-5-one and CMPO The probable interactions between the acidic (HPMMBP) and neutral (CMPO) extractants applied in the present study were investigated at different molar ratios in CDCl3 by NMR experiments (Figures S5-S7). The disadvantageous role of such potential interaction leading to destruction of the synergism in solvent extraction has been already noted in the literature.21,22,25 The chemical shifts of selected proton and carbon resonances, which appear in areas free of other signals and belong to closed to the coordinating centres groups, as well as CMPO phosphorus signals are listed in Table S1. As seen, no substantial shifts were detected upon mixing. The chemical shift differences were negligible even in phosphorus resonances (Figure S7), which are highly sensitive to structural and environmental changes. The latter is an indication that no interactions occur in chloroform solutions, independently on HPMMBP/CMPO proportions. The interactions were additionally studied by NOESY experiment of the 1:1 mixture. As illustrated on Figures 4 and S8-S10 too, only intramolecular interactions for both compounds were registered, while no intermolecular cross peaks were observed despite relatively high for such experiment concentration of 0.1 M. For instance, HPMMBP CH3-3 and CH3-Ar interact only with CH-2+6 and CH-3+5 of the aroyl group, respectively (Figure S8). As no interaction has been detected between the dissolved species (various ligands and ILs) in CDCl3, benzene and acetonitrile, it can be supposed that the influence of the organic medium is negligible in this particular case, too.21, 36 Synergistic solvent extraction and selectivity of Ln(III)ions with mixtures of 4-aroyl-3methyl-1-phenyl-pyrazol-5-ones and CMPO The liquid/liquid extraction of Ce3+, Eu3+ and Lu3+ in [C1C4im+][Tf2N-] were carried out in the presence of two extractant molecules acidic/neutral couple and the corresponding obtained data are presented in Figure 5 for HPMFBP-CMPO and Figures S11 and S12 for HPMMBP-CMPO combination. The values of D ratios of the mixed systems are greater than those of individual extractants,19,31 and this clearly reveals the positive synergistic extraction effects under the applied conditions. Evidently, the synergistic enhancement in IL media is realized but it is difficult to establish the nature of the exact extracting Ln3+ complex formed in this oil phase. Probably the arrangement of one unique synergistic complex is a unfavorable extraction mechanism and the organic phase synergistic reaction could not be expressed by one equation, taking into account the intensive ion exchange i.e. loss of IL cation or anion to the aqueous phase, in contrast to molecular diluents.8 Although the plots of logD vs. pH are not so linear, the slopes close to 0.6-1.4 can be derived for the two systems indicating that at minimum one chelating ligand is deprotonated and contribute two oxygen donors to the lanthanoid complex. This is even more evident from the extraction experiments logDlog[HL]IL giving straight lines with slopes between 0.72-2.04 (Figure S13). The log-log plots of D vs. [CMPO] are not presented in the figures but the established slopes are between 0.46 and 2.7. When benzoylacetone(HBA) was used as an acidic ligand in the presence of trioctylphosphine oxide (TOPO), the ternary complex Eu(BA)(TOPO)32+ has been assumed to be extracted via ion exchange by Okamura et al.20 Hirayama and co-researchers also established the mixed extracted species in [C1C4im+][Tf2N−] of the type [La(TTA)2(18C6)]+ and [La(TTA)(18C6)]2+ (18-crown-6).17 Therefore, in the presence of the second extractant, CMPO molecule, the lanthanoid synergistic extraction can be expressed by the equations: 6 ACS Paragon Plus Environment

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(1) Ln3+(aq) +HLIL +CMPOIL +2[C1Cnim+]IL ↔ Ln[(L)(CMPO)]IL2+ + H+(aq) + 2[C1Cnim+](aq), with participation at least of one chelating ligand and one synergistic agent. (5) The results show that under the experimental conditions, Ln3+ ions are extracted as cationic complexes but the stoichiometry of the extracted species in the organic IL phase depends on pH, Ln3+, HL or CMPO concentrations, solubility of ILs’ components in the aqueous phase etc.: (2) Ln3+(aq) +nHLIL + (1+y)CMPOIL +2[C1Cnim+]IL ↔ Ln(L)n(CMPO)(1+y)IL2+ + nH+(aq) + 2[C1Cnim+](aq), where n = 1 or 2; y = 0, 1 or 2. (6) The proposed complex is minimum four coordinated, but lanthanoid cations generally prefer higher coordination numbers with 8 or 9 rather common.8,37 So [Ln(L)(CMPO)(1+y)]2+ complex must contain other ligands to form stable coordinately saturated species in [C1Cnim+][Tf2N-], protonated HL viz. [Ln(L)(HL)(CMPO)(1+y)]2+. Water molecules or IL phase anions [Tf2N-] could directly coordinate the metal center. The four conditions for synergistic extraction of a metal with mixture including acidic/neutral ligands in molecular diluents were indicated before many years leading to a considerable effect22,40 i.e. formation of a metal chelate complex. The second extractant is capable of displacing any coordinated water rendering it less hydrophobic. The second compound is less coordinated than the first one and the maximum coordination number of the metal and the obtained architecture are favorable. It is evident that when ILs are employed, the extraction ability of a neutral ligand is almost equal to that of the principle acidic compound even at low concentrations. Furthermore, one may declare that the metal complexation in ionic phase with neutral ligands is more convenient and beneficial reaction. Although, more detailed research regarding the extraction mechanism is necessary, the obtained results via slope analysis seem to be consistent with the previously reported conclusions in the literature about metal complexations in ionic liquids. The europium ion was chosen for investigation of the individual effect of IL cation on the extractability. Two synergistic systems employing various-chain imidazolium cations (n=4, 6, 8, 10) and aqueous chloride solutions (µ=0.1) were studied. The obtained data are presented in Figure 6 (HPMFBP-CMPO) and Figure S12 (HPMMBP-CMPO). As was pointed out in previous section the values of distribution ratios increased in the order [C1C4im+][Tf2N−]>[C1C6im+][Tf2N−]>[C1C8im+][Tf2N−]>[C1C10im+][Tf2N−], which can be clearly seen for Eu3+/HPMMBP/CMPO/ILs systems. Similar tendency was not found for the other synergistic mixture including HPMFBP (n=4>n=10>n=8>n=6). The values of the synergistic coefficients calculated in the present study as well as some previously published data are given in Table 1. It is seen that lanthanoids are extracted synergistically using HPMFBP-CMPO and HPMMBP-CMPO mixtures, (SC>0). However, the data show that the addition of CMPO molecule to the substituted pyrazolones causes a rather small synergism, i.e. the values are below one order of magnitude. In addition, the synergistic increase produced in molecular diluents (CHCl3, C6H6) for europium ion extracted by the chelating extractant HPMFBP and five phosphorus-containing synergistic agents is between two and six orders of magnitude.25,39 Although, the synergistic compound is not the same, the lowering of SCs in IL media is clearly noticeable in conformity with previous results.21,22 Concerning the synergistic effects in [C1C4im+][Tf2N-], the two systems display different efficiency across the 4f-series. Namely diminish from Ce3+ to Lu3+ when HPMFBP is chelating ligand (trend usually observed in conventional molecular systems)40 and increase in HPMMBP-CMPO mixture as already perceived by Okamura et al.20 for the three β7 ACS Paragon Plus Environment


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diketone-TOPO combinations. In all of the investigated IL systems, the magnitude of synergistic effect has increased in the order La3+HPMMBP>HPMFBP, while the lanthanoid extraction increases in the order HPPPBP[C1C8im+][Tf2N−]>[C1C10im+][Tf2N−]. The use of acidic/neutral mixture of molecules leads to more effective solvent extraction of various metals applying VOCs and owing to few recent studies this tendency was confirmed to be true in ILs as well. The four studied 4-aroyl-3-methyl-1-phenyl-pyrazol-5-one molecules possessing distinct acidic properties have demonstrated different extraction capability in VOCs in contrast to IL medium, whereas as dissolved these compounds have almost equal efficiency independently from the pKa values.21,22,25 The subsequent revision of the already available library of extracting compounds must be suggested with respect to update the knowledge in this field. From a practical standpoint, the design of an IL-based extraction system for a target metal recuperation form aqueous solutions requires not only minimal loss of IL into the aqueous phase (minimal contribution of ion exchange to overall extraction process), but also adequate extraction efficiency and quite good selectivity over average potential delivered by molecular diluents. Acknowledgments The author is grateful to FP7-PEOPLE-Marie Curie Actions-IEF for the financial support of the project INNOVILLN (622906) and thanks Assoc. Prof. Dr. Sabi Varbanov (IOCCP, Bulgarian Academy of Sciences) providing the calixarene ligand. Dr. Atanassova express her gratitude to Assoc. Prof. Dr. Vanya Kurteva (IOCCP, BAS) for stimulating NMR 10 ACS Paragon Plus Environment

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investigation and fruitful discussion. The help of Dr Bruno Vincent and Dr Maurice Coppe (NMR Centre at the Chemical Faculty of the Strasbourg University) for the NMR measurements of this work is greatly appreciated. The help of Dr. Sarah Bureau (Equipe Géochimie, ISTerre/CNRS/UJF, Grenoble) for europium ICP measurements of this work is greatly acknowledged as well. Supporting Information LogD vs. pH and log[HL] for different systems as well as NMR spectra and NOESY experiments for HPMMBP+CMPO interaction. This material is available free of charge via the Internet at… Notes The author declares no competing financial interest. REFERENCES (1) Liu, Y.; Chen, J.; Li, D. Application and perspective of ionic liquids on rare earths green separation, Sep. Sci. Technol. 2012, 47, 223-232. (2) Rao, P.; Venkatesan, K.; Rout, A.; Srinivasan, T.; Nagarajan, K. Potential applications of room temperatures ionic liquids for fussion products and actinide separation, Sep. Sci. Technol. 2012, 47, 204-222. (3) Chen, Y.; Wang, H.; Pei, Y.; Ren, J.; Wang, J. pH-Controlled selective separation of neodymium(III) and samarium(III) from transition metals with carboxyl-functionalized ionic liquids, ACS Sustainable Chem. Eng. 2015, 3, 3167-3174. (4) Katsuta, S.; Watanabe, Y.; Araki, Y.; Kudo, Y. Extraction of gold(III) from hydrochloric acid into various ionic liquids: relationship between extraction efficiency and aqueous solubility of ionic liquids, ACS Sustainable Chem. Eng. 2016, 4, 564-571. (5) Hirayama, N.; Deguchi, M.; Kawasami, H.; Honjo, T. Use of 1-alkyl-3methylimidazolium hexafluorophosphate room temperature ionic liquids as chelate extraction solvent with 4,4,4-trifluoro-1-(2-thyenyl)-1,3-butanedione, Talanta, 2005, 65, 255-260. (6) Turanov, A.; Karandashev, V.; Artyushin, O.; Sharova, E. Extraction of U(VI), Th(IV), and lanthanides(III) from nitric acid solutions with CMPO-functionalized ionic liquid in molecular diluents, Solvent Extr. Ion Exch., 2015, 33, 540-553. (7) Sun, X.; Waters, K. Synergistic effect between bifunctional ionic liquids and a molecular extractant for lanthanide separation, ACS Sustainable Chem. Eng. 2014, 2, 2758-2764. (8) Jensen, M.; Borkowski, M.; Laszak, I.; Beitz, J.; Rickert, P.; Dietz, M. Anion effects in the extraction of lanthanide 2-thenoyltrifluoroacetone complexes into an ionic liquid, Sep. Sci. Technol. 2012, 47, 233-243. (9) Pereiro, A.; Araujo, J.; Martinho, S.; Alves, F.; Nunes, S.; Matias, A.; Duarte, C.; Rebelo, L.; Marrucho, I. Fluorinated ionic liquids: properties and application, ACS Sustainable Chem. Eng. 2013, 1, 427-439. (10) Atanassova, M.; Mazan, V.; Billard, I. Modulating the solubilities of ionic liquid components in aqueous-ionic liquid biphasic systems: a Q-NMR investigation. ChemPhysChem. 2015, 16, 1703-1711. (11) Atanassova, M.; Dukov, I. Synergistic solvent extraction and separation of trivalent lanthanide metals with mixtures of 4-benzoyl-3-methyl-1-phenyl-2-pyrazoline-5-one and Aliquat 336, Sep. Purif. Technol. 2004, 40, 171-176. (12) Atanassova, M. Synergistic solvent extraction and separation of lanthanide(III) ions with 4-benzoyl-3-phenyl-5-isoxazolone and the quaternary ammonium salt, Solvent Extr. Ion Exch. 2009, 27, 159-171. 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 17

(13) Dukov, I.; Atanassova, M. Effect of diluents on the synergistic solvent extraction of some lanthanides with thenoyltrifluoroacetone and quaternary ammonium salt, Hydrometallurgy, 2003, 68, 89-96. (14) Dai, S.; Ju, Y. H.; Barnes, C. E. Solvent extraction of strontium nitrate by a crown ether using room temperature ionic liquids. J. Chem. Soc., Dalton Transactions, 1999, 1201-1202. (15) Rout, A.; Venkatesan, A.; Srinivasan, V.; Rao, V. R. Extraction of americium(III) from nitric acid medium by CMPO-TBP extractants in ionic liquid diluent. Radiochim. Acta 2009, 97, 719-727. (16) Shimojo, K.; Okamura, H.; Hirayama, N.; Umetani, S.; Imura, H.; Naganawa, H. Cooperative intramolecular interaction of diazacrown ether bearing β-diketone fragments on an ionic liquid extraction system. Dalton Trans., 2009, 4850-4852. (17) Hirayama, N. ; Okamura, H.; Kindani, K.; Imura, H. Ionic liquid synergistic cationexchange system for the selective extraction of lanthanum(III) using 2thenoyltrifluoroacetone. Anal. Sci., 2008, 24, 697-701. (18) Stepinski, D.; Jensen, M.; Dzielawa, J.; Dietz M. Synergistic effects in the facilitated transfer of metal ions into room-temperature ionic liquids. Green Chem., 2005, 17, 151-158. (19) Kumar, P.; Vincent, T.; Khanna, A. Extraction of UO22+ into ionic liquid using TTA and TBP as extractants, Sep. Sci. Technol., 2015, 50, 2668-2675. (20) Okamura, H.; Takagi, H. ; Isomura, T.; Morita, K.; Nagatani, H.; Imura, H. Higly selective synergism for the extraction of lanthanoid(III) ions with β-diketones and trioctylphosphine oxide in an ionic liquid, Anal. Sci. 2014, 30, 323-325. (21) Atanassova, M.; Kurteva, V.; Lubenov, L.; Billard, I. Comparing extraction, synergism and separation of lanthanoids using acidic and neutral compounds in chloroform and one ionic liquid: is the letter always “better”? RSC Adv. 2014, 4, 38820-38829. (22) Atanassova, M.; Kurteva, V.; Lubenov, L.; Varbanov, S.; Billard, I. Are fancy acidic or neutral ligands realy needed for synergis in ionic liquids? A comparative study of lanthanoid extraction in CHCl3 and an ionic liquid. New J. Chem., 2015, 39, 7932-7941. (23) Atanassova, M.; Kurteva, V.; Lubenov, L.; Billard, I. Solvent extraction and separation of light lanthanoids with mixtures of two chelating extractants: benzene vs. ionic liquids. Sep. Sci. Technol. 2016, 52, 290-299. (24) Blake, C.A.; Baes, C.F.; Coleman, C.F. Proc. of the second Int. Conf. on the Peaceful uses of atomic energy, Vienna, 28, 1959, p.289. (25) Atanassova, M.; Kurteva, V.; Lubenov, L.; Varbanov, S.; Dukov, I. Behavior of mixed systems based on para-substituted 4-aroyl-5-pyrazolones in the presence of phosphorus containing calix[4]arene towards lanthanoids: synergistic solvent extraction and separation. Sep. Purif. Technol. 2012, 95, 58-63. (26) Saulinier, L.; Varbanov, S.; Scopelliti, R.; Elhabiri, M.; Bünzli, J. C. G. Lanthanide complexes with a p-tert-butylcalix[4]arene fitted with phosphinoyl pendant arms, J. Chem. Soc., Dalton Trans. 22, 1999, 3919-3925. (27) Savvin, S. B. Arsenazo III. Atomizdat, Moskva, 1966, 177. (28) Marchetti, F.; Pettinari, R.; Pettinari, C. Recent advances in acylpyrazolone metal complexes and their potential applications, Coord. Chem. Rev., 2015, 303, 1-31. (29) Binnemans, K. Rare Earth beta-diketones. Chapter 225. Handbook on the physics and chemistry of rare earths. vol. 35. (Eds. K A Gschneider, J-C G Bünzli and V. K. Pecharsky). Elsevier, 2005. (30) Zolotov, Y.; Kuzmin, N. Metal extraction with acylpyrazolones, Nauka, Moskva 1977 (31) Rama, R.; Rout, A.; Venkatesan, K.; Antony, M.; Vasudeva Rao, P. Extraction behavior of americium(III) in benzoylpyrazolone dissolved in pyrrolidinium based ionic liquid, Sep. Sci. Technol. 2015, 50, 2164-2169. 12 ACS Paragon Plus Environment

Page 13 of 17

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(32) Rout, A.; Venkatesan, K.; Srinivasan, T.; Vasudeva Rao, P. Ionic liquid extractants in molecular diluents: extraction behavior of plutonium(IV) in 1,3-diketone ionic liquids. Solvent Extr. Ion Exch. 2011, 29, 602-618. (33) Kadini, K.; Imura, H. Solvent effect of ionic liquids on the distribution constant of 2thenoyltrifluoroacetone and its nickel(III) and copper(II) chelates and the evaluation of the solvent properties based on the regular solution theory. Talanta 2010, 83, 299-304. (34) Bell, J.; Luo, H.; Dai, S. Superbase-derived protic ionic liquid extractants for metal ion separation. Sep. Purif. Technol. 2014, 130, 147-150. (35) Jensen, M.; Neuefeind, J.; Beitz, J.; Skanthamukar, S.; Soderholm, L. Mechanism of metal ion transfer into room-temperature ionic liquids: the role of anion exchange, J. Am. Chem. Soc. 2003, 125, 15466-15473 (36) Kurteva, V.; Atanassova, M.; Billard, I. NMR study on the possible interactions between imidazolim based ionic liquids and extractants widely applied in solvent extraction and separation of f-ions, J. Solution Chem. 2015, 44, 2416-2430. (37) Atanassova, M.; Kurteva, V.; Billard, I. Coordination chemistry of europium (III) ion towards acylpyrazolone ligands, Anal. Sci. 2015, 31, 917-922. (38) Dietz, M.; Dzielawa, J.; Laszak, I.; Young, B.; Jensen, M. Influence of solvent structural variations on the mechanism of facilitated ion transfer into room-temperature ionic liquids, Green Chem. 2003, 5, 682-685. (39) Petrova, M.; Kurteva, V.; Lubenov, L. Synergistic effect in the solvent extraction and separation of lanthanoids by 4-(4-fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one in the presence of monofunctional neural organophosphorus extractants, Ind. Eng. Chem. Res., 2011, 50, 12170-12176 (40) Atanassova, M.; Kurteva, V. Synergism as a phenomenon in solvent extraction of 4felements with calixarenes, RSC Adv., 2016, 6, 11303-11324. (41) Sun, X.; Luo, H.; Dai, S. Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle, Chem. Rev. 2012, 112, 2100-2128. (42) Bhattacharyya, A.; Ansari, S. A.; Gadly, T.; Ghosh, S. K.; Mohapatra, M.; Mohapatra, P. K. A remarkable enhancement in Am/Eu selectivity by an ionic liquid based solvent containing bis-1,2,4-triazinylpyridine derivatives: DFT validation of experimental results. Dalton Transactions, 2015, 44, 6193-6201. (43) Shkrob, I.; Marin, T.; Jensen, M. Ionic liquid based separations of trivalent lanthanide and actinide, Ind. Eng. Chem. Res. 2014, 53, 3641-4653. (44) Atanassova, M.; Billard, I. Determination of pKaIL values of three chelating extractants in ILs: consequences for the extraction of 4f elements. J. Solution Chem. 2015, 44, 606-620. (45) Rizvi, M.; Ali, A.; Iqbal, M. Synergistic extraction of Ce(III), Tb(III) and Lu(III) with a mixture of hexafluoroacetylacetone and triphenylphosphine oxide in benzene, Radiochim. Acta, 2007, 95, 733-737. (46) Nash, K. Aqueous complexes in separations of f-elements: options and strategies for future development. Sep. Sci. Technol., 1999, 34, 911-929. (47) Nash, K.; Choppin, G. Separations chemistry for actinide elements: recent developments and historical perspective, Sep. Sci. Technol. 1997, 32, 255-274. (48) Moyer, Bruce A. Ion exchange and solvent extraction: a series of advances, vol. 19, 2010, CRC Press, Taylor&Francis group. (49) Mohapatra, P.; Sengupta, A.; Iqbal, M.; Huskens, J.; Verboom, W. Diglycolamidefunctionalized calix[4]arenes showing unusual complexation of actinide ions in room temperature ionic-liquids: role of ligand structure, radiolytic stability, emission spectroscopy, and thermodynamic studies, Inorg. Chem. 2013, 52, 2533-2541.



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(50) Sukhbaatar, T.; Dourdain, S.; Turgis, R.; Rey, J.; Arrachart, G.; Pellet-Rostaing, S. Ionic liquid as diluent in solvent extraction: First evidence of supramolecular aggregation of a couple of extractant molecules, Chem. Comm. 2015, 51, 159600-15963.

For Table of Contents Use Only

TOC Graphic

Worthy extraction and uncommon selectivity of 4f-ions in ionic liquid medium: 4-acylpyrazolones and CMPO Maria Atanassova The unique role of IL medium on Ln3+ complexation with acidic/neutral ligands is noticed with a reversal trend observed in molecular diluents.

Ce3+, Eu3+, Lu3+


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Ce(III)

Eu(III)

1

0.5

0.5

0.3

Lu(III) 1

2

3

4

logD

logD

0

logD

0.7 0.1

-0.5

-0.1

-1

-0.3

1.5

2

2.5

0.4 0.1

-0.2 1.1

1.3

1.5

1.7

1.9

-0.5 3x10-3 5x10-3 8x10-3

-1.5

5x10-3 1x10-2 8x10-3 3x10-3

-0.5

-1.1

pH

pH

3x10-3 5x10-3 8x10-3 1x10-2

-0.8

-0.7

-2

pH

Figure 2. LogD vs. pH for the extraction of lanthanoid(III) ions with HPMMBP alone dissolved in [C1C4im+][Tf2N-]. n=6

n=8

0.2

0.4

0.2

0

0.2

0

2.3

n=10

2.3

2.8

2.5

2.7

2.9

3.1

-0.2

-0.2

0 1.6

2.1

2.6

logD

1.8

logD

logD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4

-0.2

-0.4

-0.6 -0.4

-0.6

-0.8 -0.6

-0.8

3x10-3 5x10-3 8x10-3

-1

pH

5x10-3 8x10-3 3x10-3

-1

5x10-3 8x10-3

-1.2

-0.8

pH

pH

Figure 3. LogD vs. pH for the extraction of Eu(III) ions with HPMMBP alone dissolved in [C1Cnim+][Tf2N-], n=6, 8, 10.

ACS Paragon Plus Environment


Figure 4. 1H-1H NOESY spectrum of HPMMBP:CMPO 1:1 mixture.

Eu(III)

Ce(III) 0.8

0.3

0.7

0.25

0.6

Lu(III) 1.2

1

logD

logD

logD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2

0.8

0.5

0.15

0.4 0.1

0.6

0.05

0.4

0.3 0.2 5x10-3+1x10-3 0.1

8x10-3+1x10-3 5x10-3+7x10-4

0

1.7

1.9

2.6

pH

2.3

0.2 8x10-3+1x10-3

-0.05

0 2.1

2.1

-0.1

pH

5x10-3+1x10-3 8x10-3+1x10-3 5x10-3+7x10-4

5x10-3+7x10-4 0 1.2

1.4

1.6

1.8

2

pH

Figure 5. LogD vs. pH for the extraction of Ln(III) ions with HPMFBP-CMPO mixtures in [C1C4im+][Tf2N-].


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n=6

n=8

0.3

n=10

0.4

0.4 0.3

0.25

0.2

logD

0.2

0.2

0

0.1

logD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.15

1.5

1.7

1.9

2.1

2.3

-0.2

0

2

2.2

2.4

2.6

-0.1

0.1

-0.4 -0.2

0.05

5x10-3+1x10-3 8x10-3+1x10-3 5x10-3+7x10-4

0

5x10-3+1x10-3 8x10-3+1x10-3 5x10-3+7x10-4

-0.3

2.5

pH

5x10-3+1x10-3 8x10-3+1x10-3 5x10-3+7x10-4

-0.8

-0.4

2

-0.6

3

pH

pH

Figure 6. LogD vs. pH for the extraction of Eu(III) ions with HPMFBP-CMPO mixtures in [C1Cnim+][Tf2N-], n=6, 8, 10.


Worthy Extraction and Uncommon Selectivity of 4f ... - ACS Publications

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