Effect of Diluents on The Synergistic Solvent Extraction and

May 25, 2010 - Maria A. Petrova,*,† Victoria I. Lachkova,‡ Nikolay G. Vassilev,§ and Sabi G. Varbanov§. UniVersity of Chemical Technology and Me...
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Ind. Eng. Chem. Res. 2010, 49, 6189–6195

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Effect of Diluents on The Synergistic Solvent Extraction and Separation of Trivalent Lanthanoids With 4-Benzoyl-3-Phenyl-5-Isoxazolone and tert-Butylcalix[4]Arene Tetrakis(N,N-Dimethyl Acetamide) and Structural Study of Gd(III) Solid Complex By IR and NMR Maria A. Petrova,*,† Victoria I. Lachkova,‡ Nikolay G. Vassilev,§ and Sabi G. Varbanov§ UniVersity of Chemical Technology and Metallurgy, Department of General and Inorganic Chemistry, 8 Kliment Ohridski blVd. BG-1756 Sofia, Bulgaria, UniVersity of Forestry, Department of Ecology, 10 Kliment Ohridski blVd., BG-1756 Sofia, Bulgaria, Institute of Organic Chemistry with Center of Phytochemistry, Bulgarian Academy of Sciences, Block 9, Acad. G. BoncheV street, BG 1113, Sofia, Bulgaria

The solvent extraction of 14 lanthanoid(III) ions from a chloride medium with acidic chelating extractant 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) and tert-butylcalix[4]arene tetrakis(N,N-dimethylacetamide) (S) as a synergistic agent in C6H6 has been studied. In all cases, Ln(III) ions were extracted as Ln(PBI)3 · S species. Positive values of the synergistic coefficients show that all lanthanoids were extracted synergistically upon the addition of compound S to the chelating extractant. The values of the equilibrium constant and the separation factors were calculated. The synergistic solvent extraction of five selected lanthanoid ions (La3+, Nd3+, Eu3+, Ho3+, and Lu3+) with a mixture of HPBI and S from a perchlorate medium in other diluents such as CHCl3, C2H4Cl2, and CCl4 has been studied too. It was found that the values of the equilibrium constant increased in the order CHCl3 < C2H4Cl2 < CCl4. The effect of the diluents on the metal extraction and separation has been discussed. A solid complex of Gd(III) with HPBI and S was synthesized. The composition of the complex was determined as Gd(PBI)3 · S using elemental analysis, IR, and 1H NMR data. 1. Introduction In recent decades, the development of macrocyclic chemistry has contributed widely to the separation science.1-3 Many calixarene derivatives with different functional groups such as ketono, ester, and amide have been prepared and studied as cyclic extractants.4-7 The solvent extraction of trivalent lanthanoid and actinoid ions with various calixarenes has been investigated by Arnaud-Neu et al.,8-12 Kuznetsova et al.,13-15 Bu¨nzli et al.,16,17 Ludwig et al.,18,19 etc. It was noted that the cavity size, the position and kind of donor groups, and the ligand’s hydrophobicity have a pronounced impact on the extraction power and selectivity. The use of calixarenes in separation chemical technology has been discussed in the review of Ludwig.20 The nuclear industry is particularly interested in phosphorus-organic derivatives of calix[n]arenes for the extraction of the main radioactive contaminants from wastes before their confinement.21 Recently, Li et al.22 have established that the combination of calix[4]arene carboxyl derivative and primary amine N1923 ([C19H19∼C11H23]2CHNH2) in chloroform causes not only synergism in the extraction of rare earth metals from a chloride medium but also the enhanced extraction selectivity of mutual elements. The capability of hexaphosphinoylated p-tert-butylcalix[6]arene toward the extraction of actinides was observed by Bu¨nzli and co-workers23 and its good separation ability of UO22+ and Th(IV) over La(III), Eu(III), and Yb(III). A synergistic effect of almost 6 orders of magnitude occurred in the extraction of Eu from 0.1 M HNO3 * To whom correspondence should be addressed. E-mail: ma@ uctm.edu. † University of Chemical Technology and Metallurgy. ‡ University of Forestry. § Bulgarian Academy of Sciences.

with mixtures of dicarbollide and derivatives of calix[4]arene, but practically zero extraction has been obtained with sulphonated calixarenes.24 The authors have also reported that the synergistic effect is strongly diluent-dependent and that the highest distribution ratios were obtained when the diluent was chlorobenzene. As a part of a systematic study of the synergistic solvent extraction of the lanthanoids, the present work was undertaken to investigate the effect of diluents (C6H6, CCl4, CHCl3, C2H4Cl2) on the extraction of trivalent ions of the 4f series (with the exception of radioactive Pm) with a mixture of a chelating extractant 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) and tert-butylcalix[4]arene tetrakis(N,N-dimethylacetamide) (S) as well as to elucidate the nature of the complexes extracted into the organic phase and to determine the possibilities for separation of the lanthanoid metals. The stoichiometry and the structure of synthesized solid complexes of Gd(III) with these extractants were determined by IR, 1H NMR, and elemental analysis. 2. Experimental Section 2.1. Reagents. The commercial product 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) with a purity higher than 97% (Fluka, Switzerland) and tert-butylcalix[4]arene tetrakis(N,Ndimethylacetamide) (Fluka, purum g97%) were used as supplied. Stock solutions of the lanthanoid ions were prepared from their oxides (Fluka, puriss) by dissolving them in concentrated hydrochloric or perchloric acid and dilution with distilled water to the required volume. The diluents were benzene and 1,2-dichloroethane (p.a., Merck, Germany), carbon tetrachloride, and chloroform (Fluka, p.a.). Arsenazo III (Fluka, Switzerland) was of analytical purity, as were the other reagents used.

10.1021/ie100328v  2010 American Chemical Society Published on Web 05/25/2010

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2.2. Apparatus. An S-20 Spectrophotometer Boeco (Germany) was used for measuring absorbances, and a pH 211 HANNA digital pH meter was used for pH measurements in the solvent extraction studies. The melting points (uncorrected) of the solid complexes were measured on a Boetzius microheating plate, PHMK05 (Germany). Infrared spectra (400-4000 cm-1) were recorded on a FTIR Bruker IFS113v IR spectrometer in KBr pellets. The 1H NMR spectra were recorded on Bruker Avance II+ 600 MHz high performance digital FT-NMR spectrometers in CD3OD. The 1H NMR chemical shifts are given relative to TMS. 2.3. Solvent Extraction Procedure. Equal volumes (10 cm3) each of the aqueous and organic phases were shaken mechanically for 45 min at room temperature, which was sufficient to reach equilibrium. After phase separation, the metal concentration in the aqueous phase was determined photometrically (654 nm) using Arsenazo(III).25 The acidity of the aqueous phase was measured by a pH meter with an accuracy of 0.01 pH unit. The ionic strength was maintained at 0.1 M with NaCl/HCl or NaClO4/HClO4. The initial concentration of the metals was 2.5 × 10-4 mol/dm3 in all experiments. 2.4. Preparation of Metal Complexes. 2.4.1. Complex of Gd(III) with 4-Benzoyl-3-phenyl-5-isoxazolone (HPBI). A solution of 55.8 mg (0.15 mmol) of GdCl3 · 6H2O in 1.0 cm3 of absolute ethanol was added at room temperature to a solution of 119.4 mg (0.45 mmol) of 4-benzoyl-3-phenyl-5-isoxazolone in 1.0 cm3 of absolute ethanol and stirred for 2 h. The formed white solid product was separated by centrifugation and washed three times with 1.5 cm3 of absolute ethanol. It was dried in a high vacuum over P2O5. The crystal product melts at 212-213 °C. Yield: 142 mg (98%). Analysis for C48H30N3O9Gd (M 950.036 g/mol), Calcd, %: C, 60.69; H, 3.18; N, 4.42. Found, %: C, 60.34; H, 3.48; N, 4.11. IR spectrum (KBr): ν(CdO) 1643 (vs) cm-1; ν(Ar) 1581, 1498-1483 (s) cm-1; ν (for monosubstituted Ar in benzoyl) 786, 697 cm-1. The 1H NMR (600 MHz, CD3OD, 20 °C) spectrum consists of a very broad signal at 7.2 ppm for all aromatic protons. 2.4.2. Complex of Gd(III) with 4-Benzoyl-3-phenyl-5isoxazolone (HPBI) and tert-Butylcalix[4]Arene Tetrakis(N,N-dimethylacetamide) (S) (1:3:1). A colorless solution of 37.2 mg (0.1 mmol) of GdCl3 · 6H2O in 1.0 cm3 of absolute ethanol was added at room temperature to a solution containing 98.3 mg (0.1 mmol) of S and 79.6 mg (0.3 mmol) of 4-benzoyl3-phenyl-5-isoxazolone in 1.0 cm3 of absolute ethanol. The reaction mixture was stirred at room temperature for 3 h, and a white solid product was separated via centrifugation, washed three times with 1.5 cm3 of ethanol, and dried in a high vacuum over P2O5. The crystal product melts at 286-287 °C. Yield: 172 mg (88%). Analysis for C108H114N7O17Gd · H2O (M 1957.40 g/mol), Calcd, %: C, 66.27; H, 5.97; N, 5.01. Found, %: C, 66.36; H, 5.68; N, 5.18%. IR-data (KBr): ν (CdO) 1651 (vs) cm-1; ν (Ar), 1601(s), 1500-1485(s) cm-1; ν (Ar-C-H), 1196 (m) cm-1; ν (for 1,2,3,5-tetra substituted Ar), 1439 (m), 909 (m) cm-1; ν (C-H in t-Bu), 3433-2962 (m) cm-1; ν (for monosubstituted Ar in benzoyl) 760, 697 cm-1. 1H NMR (CD3OD, 62 °C): δ 1.22 (s, 9H, tert-Bu), 2.95 (s, 3H, CH3), 3.06(s, 3H, CH3), 3.43 (d, J ) 11.2 Hz, 1H, 1/2 CH2), 4.62 (d, J ) 11.2 Hz, 1H, 1/2 CH2), 4.68 (s, 2H, OCH2), 7.2 (bs, 10H, Ph-PBI), 7.29 (s, 2H, Ar).

3. Results and Discussion 3.1. Solvent Extraction of Ln3+ with 4-Benzoyl-3-phenyl5-isoxazolone (HPBI). The solvent extraction of Ln3+ with a solution of HPBI in C6H626 and in CHCl327 was studied previously. The metal extraction can be represented by the equation 3+ + Ln(aq) + 3HPBI(o) T Ln(PBI)3(o) + 3H(aq)

(1)

where Ln3+ denotes lanthanoids, and “aq” and “o” denote aqueous and organic phases, respectively. The equilibrium constant KI can be expressed as KI )

3 [Ln(PBI)3](o)[H+](aq) 3 [Ln3+](aq)[HPBI](o)

) DI

3 [H+](aq) 3 [HPBI](o)

(2)

where DI is the distribution ratio. The extraction behavior of the lanthanoid ions using HPBI in CCl4 and 1,2-dichloroethane was studied as a function of pH and [HPBI]. The plots of log DI (DI is the distribution ratio for the extraction with HPBI alone) vs pH and log[HPBI] were linear with slopes equal to 3 (these plots are not shown in the figures). So, the extraction of Ln(III) ions can be described by eq 1. The relationship between the distribution ratio DI and the equilibrium constant KI can be expressed as log KI ) log DI - 3pH - 3log[HPBI]

(3)

Under the experimental conditions, the extraction of Ln(III) ions with tert-butylcalix[4]arene tetrakis(N,N-dimethylacetamide) was not detectable. 3.2. Synergistic Solvent Extraction of Ln3+ with Mixtures of HPBI- S in C6H6. The overall reaction of the generally accepted synergistic extraction of lanthanoid ions is given by eq 3 where m and n correspond to the number of PBI-’s and S’s, respectively, in the extracted adduct, Ln(PBI)m · nS. 3+ + Ln(aq) + mHPBI(o) + nS(o) T Ln(PBI)m · nS(o) + mH(aq) (4)

To determine the number of PBI-’s involved in the extracted adduct, the variation of the distribution ratio of Ln(III) with respect to the concentration of H+ at a constant concentration of HPBI and S was investigated as well as the variation of the distribution ratio of Ln(III) with respect to the concentration of HPBI at constant concentrations of H+ and S. The distribution ratio of lanthanoid ions extracted with HPBI increases by the addition of calix[4]arene (S). This is attributable to the formation of a more lipophilic adduct with S. The log DI,S vs pH and logDI,S vs [HPBI] in the extraction of Ln(III) using a mixture of HPBI-S are shown in Figures 1 and 2. The plots show straight lines with a slope close to 3 for all Ln(III)’s, which implies that three HPBI’s participated in the synergistic extraction of Ln3+. Plots of log DI,S vs [S] in the extraction of Ln(III) from a chloride medium at pH and [HPBI] constant concentrations into C6H6 are shown in Figure 3. The slopes are close to 1. These results indicate that the complex Ln(PBI)3 forms an adduct with one molecule of S. So the adducts Ln(PBI)3 · S are extracted. This is in conformity with the results obtained by Dukov et al. for lanthanoid extraction with 4-benzoyl-3methyl-1-phenyl-2-pyrazolin-5-one(HP) and calix[4]arene28 or calix[8]arene29 with PdO donor groups. However, the formation of a 1:3:2 adduct Ln(TTA)3 · 2S was reported during the extraction of Ln(III) with thenoyltrifluoroacetone (HTTA)

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Figure 1. Log DI,S vs pH for the extraction of lanthanoid elements with a HPBI-S mixture at [HPBI] ) 1.25 × 10-3 mol/dm3 and [S] ) 1.25 × 10-4 mol/dm3 in C6H6.

Figure 2. Log DI,S vs [HPBI] for the extraction of lanthanoid elements with a HPBI-S mixture at [S] ) 1.25 × 10-4 mol/dm3 in C6H6: La, pH ) 2.10; Pr, pH ) 2.05; Eu, pH ) 1.95; Tb, pH ) 1.95; Ho, pH ) 1.70; Tm, pH ) 1.60; Lu, pH ) 1.45. Ce, pH ) 2.05; Nd, pH ) 2.00; Sm, pH ) 2.05; Gd, pH ) 2.00; Dy, pH ) 1.85; Er, pH ) 1.75; Yb, pH ) 1.50.

Figure 3. Log DI,S vs [S] for the extraction of lanthanoid elements with a HPBI-S mixture at [HPBI] ) 1.25 × 10-3 mol/dm3. La, pH ) 2.15; Pr, pH ) 2.05; Eu, pH ) 1.95; Tb, pH ) 1.95; Ho, pH ) 1.90; Tm, pH ) 1.80; Lu, pH ) 1.65. Ce, pH ) 2.15; Nd, pH ) 2.15; Sm, pH ) 1.95; Gd, pH ) 2.05; Dy, pH ) 1.85; Er, pH ) 1.95; Yb, pH ) 1.55.

and p-tert-butylcalix[4]arene fitted with phosphinoyl pendant arms as a synergistic agent in CHCl3.30

Thus, the synergistic extraction equilibrium in eq 4 can be rewritten as eq 5:

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The values of log KI,26 log KI,S, and log βI,S are given in Table 1. The equilibrium constants are based on the assumption that the activity coefficients of the species do not change significantly under the experimental conditions. That is, they are concentration constants. It is seen from Table 1 that the values of the overall equilibrium constant logKI,S increase with increasing atomic number of the metals as expected from their decreasing ionic radii. To compare the extraction ability of the synergistic mixture HPBI-S with those of the chelating extractant alone, the pH50 values (values of pH where log D ) 0) are gathered in Table 1. A difference between the pH50 values was observed for the lanthanoid ions between 0.8 and 0.5 pH unit upon the addition of S. Thus, the lower pKa value (1.23) due to the electron delocalization induced by the isoxazolone moiety makes 4-acyl5-isoxazolones an interesting class of β-diketones with potential application as reagents for the extraction and separation of metal ions from strong acid media.31,32 The synergistic enhancement can be assessed using synergistic coefficients calculated as S.C. ) log (D1,2/D1 + D2), where D1,2, D1, and D2 denote the distribution ratio of a metal ion using a mixture of extractants (D1,2) and the same extractants separately (D1 and D2). The values of the synergistic coefficients are also given in Table 1 ([HPBI] ) 1.25 × 10-3 mol/dm3, [S] ) 1.25 × 10-4 mol/dm3, pH ) 2.10). These data show that the formation of the adducts Ln(PBI)3 · S causes a synergistic effect between 1.3 and 2.5 orders of magnitude. The values of the separation factors (S.F.) for the synergistic solvent extraction of the lanthanoid ions are given in Table 2. They represent the ratio of the distribution coefficients of two lanthanoids, viz., D(z+n) and Dz. When the two metals form the same types of complexes, the S.F. can be determined as S.F. ) K(z+n)/Kz. Such a binary system seems excellent for the separation of lighter lanthanoids (La-Nd) from the other nonadjacent 4f elements. It should also be noted that a considerable loss of separation selectivity is observed across the 4f series (Table 2). It is seen that the SF values for heavier pairs are a little bit higher than those obtained when HPBI was used alone (Table 1). The comparison of the data found in the present study for the separation factor between Lu and Eu (10.5) is similar to or remains almost the same as those found for the extraction of the same lanthanoids with mixtures of HTTA-calix[4]arene,30 HP-calix[4]arene,28 and HP-calix[8]arene.29 It is relevant at this stage to point out that the type of usable chelating extractants and calix[n]arenes does not have too much importance on the separation efficiency between the heavier 4f metals. But the value for the SF for the Eu-La (7.41) is 19,30 8.2,28 and 8.529 times smaller than those when the respective above-mentioned mixtures were used. It is interesting to note that HTTA, which

Table 1. Values of the Equilibrium Constants KI, KI,S, and βI,S; pH50; Synergistic Coefficients for Lanthanoids Extraction with the HPBI-S Mixture in C6H6; and Separation Factors for Lanthanoids Extraction with HPBI Alonea pH50

S.F.

Ln3+

log KI26

log KI,S

log βI,S

S.C.

HPBI

HPBI-S

HPBI

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

-1.06 -0.72 -0.56 -0.24 -0.20 0.65 1.10 1.20 1.29 1.40 1.50 1.58 1.65 1.72

5.32 5.56 5.74 5.85 6.04 6.16 6.26 6.36 6.53 6.65 6.75 6.90 7.02 7.18

6.38 6.28 6.30 6.09 5.84 5.51 5.16 5.16 5.24 5.25 5.25 5.32 5.37 5.46

2.48 2.38 2.40 2.19 1.94 1.61 1.26 1.26 1.34 1.35 1.35 1.42 1.47 1.56

3.25 3.14 3.08 2.98 2.83 2.68 2.53 2.50 2.47 2.43 2.40 2.37 2.35 2.32

2.43 2.35 2.29 2.25 2.19 2.15 2.11 2.07 2.02 1.97 1.94 1.90 1.86 1.81

2.18 1.44 2.08 2.75 2.82 2.82 1.23 1.23 1.25 1.23 1.20 1.17 1.14

a

The values of the equilibrium constants are calculated on the basis of the 36 experimental points; statistical confidence is 95%, and standard deviation is less than (0.05. 3+ + Ln(aq) + 3HPBI(o) + S(o) T Ln(PBI)3 · S(o) + 3H(aq)

(5)

The equilibrium constant can be expressed as KI,S )

3 [Ln(PBI)3S](o)[H+](aq) 3 [Ln3+](aq)[HPBI](o) [S](o)

) DI,S

3 [H+](aq)

(6)

3 [HPBI](o) [S](o)

where DI,S is the synergistic distribution ratio. The overall equilibrium constant KI,S can be determined by the following equation taking logarithms on both sides of eq 6: log KI,S ) log DI,S - 3log[HPBI] - log[S] - 3pH

(7)

The formation of mixed adducts in the organic phase can be expressed by the equation: Ln(PBI)3(o) + S(o) T Ln(PBI)3 · S(o)

(8)

The equilibrium constant βI,S for the organic phase synergistic extraction is given by equation βI,S )

[Ln(PBI)3S](o) KI,S ) [Ln(PBI)3](o)[S](o) KI

(9)

The equilibrium constant βI,S for the organic phase synergistic reaction can be determined as log βI,S ) log KI,S - log KI

(10)

Table 2. Values of Separation Factors between the Lanthanoid Ions Obtained Using Mixture HPBI-S in C6H6 S.F.

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

1.7

2.6 1.51

3.3 1.94 1.94

5.2 2.93 1.94 1.52

7.4 3.98 2.63 2.0 1.32

8.7 5.0 3.3 2.57 1.65 1.25

10.9 6.3 4.16 3.23 2.0 1.32 1.23

16.2 9.1 6.0 4.67 3.0 2.3 1.8 1.54

21.4 12.3 8.1 6.3 4.0 3.0 2.45 1.9 1.32

26.9 15.5 10.2 7.94 5.12 3.80 3.02 2.45 1.65 1.23

38.0 21.8 14.45 11.22 7.24 5.37 4.26 3.38 2.34 1.77 1.4

50.1 28.8 19.0 14.7 9.5 7.0 5.6 4.4 3.0 2.3 1.8 1.3

72.4 41.6 27.5 21.3 13.8 10.5 8.3 6.6 4.5 3.3 2.6 1.8 1.4

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Figure 4. Log DI,S vs pH for the extraction of lanthanoid elements with a HPBI-S mixture at [HPBI] ) 1.25 × 10-3 mol/dm3 and [S] ) 1.25 × 10-4 mol/dm3. Table 3. Values of the Equilibrium Constants KI, KI,S, and βI,S and Synergistic Coefficients for Lanthanoids Extraction with the HPBI-S Mixture in Different Diluentsa Ln3+ La Nd Eu Ho Lu

diluent

log KI

log KI,S

log βI,S

S.C.

CHCl3 C2H4Cl2 CCl4 CHCl3 C2H4Cl2 CCl4 CHCl3 C2H4Cl2 CCl4 CHCl3 C2H4Cl2 CCl4 CHCl3 C2H4Cl2 CCl4

-1.33 -1.25 0.82 -0.5427 -0.46 1.26 0.0627 0.16 1.77 0.3627 0.82 2.23 0.7027 1.24 2.63

3.18 3.29 6.6 3.87 3.95 7.07 4.54 4.76 7.65 5.02 5.17 8.17 5.45 5.50 8.56

4.51 4.54 5.75 4.41 4.41 5.81 4.48 4.60 5.88 4.66 4.35 5.94 4.75 4.26 5.93

0.61 0.64 1.88 0.51 0.51 1.91 0.58 0.70 1.98 0.76 0.45 2.04 0.85 0.36 2.03

27

a The values of the equilibrium constants are calculated on the basis of the 36 experimental points; statistical confidence is 95%, and standard deviation is less than (0.05.

is a much poor extractant for lanthanoids than HP and HPBI (the values of the equilibrium constant are approximately 4 and 8 orders of magnitude lower26), exhibits a better separation for the lighter 4f metals. The same tendency was usually established in the solvent extraction process; the separation factors decrease until the lanthanoids extractions increase. 3.3. Effect of Diluents on the Synergistic Solvent Extraction of Ln3+ with 4-Benzoyl-3-phenyl-5-isoxazolone and tert-Butylcalix[4]arene Tetrakis(N,N-dimethylacetamide). Typical experimental distribution curves log DI,S vs pH and log DI,S vs [HPBI] using CHCl3, CCl4, and C2H4Cl2 as diluents in the synergistic solvent extraction of Ln(III) from 0.1 M perchlorate medium are shown in Figure 4 and Figure S1 (see in the Supporting Information). They are straight lines of slopes very close to 3, indicating that three protons are exchanged per Ln(III). Figure S2 (Supporting Information) shows the influence of the concentration of the synergistic agent [S] upon the distribution ratio. The lines have a slope almost equal to 1, indicating that Ln(III)’s are extracted as Ln(PBI)3 · S, in agreement with the equilibrium (eq 5). For each extraction system using different diluents, the values of log KI,S have been calculated for every experimental point according to eq 7. The averaged values are reported in Table 3. The values in Table 3 show that the change of diluent has produced a marked effect

on the equilibrium constants. For a given metal, the values of log KI,S and log βI,S increase in the order CHCl3 < C2H4Cl2 < C6H6 < CCl4. The same order was observed for Ln(III) extraction with several other β-diketones. It can be concluded that, as a whole, the increase of the diluent solvating ability hinders the extraction process.33 The weak extraction of Ln(III) with chloroform is mainly due to its interaction with the chelating extractant (HPBI).34,35 The synergistic effect depends strongly on the choice of the diluent.34 The obtained results for S.C. in Table 3 also confirmed this statement. Arichi et al. reported that only diluents containing a polar group or atom (chlorine, fluorine, or nitro-group) yield fairly high distribution ratios.34 Aromatic compounds exhibit more efficient extraction than methane or ethane derivatives.24 No correlation of the maximum distribution ratio values with the values of the dielectric constant of the diluents has been found by Kyrsˇ et al.24 (in the present study also). Distribution ratios of Eu(III) were almost the same using chloroform (34.3; 4.9) and 1,2-dichloroethane (34.7; 10) as diluents when a mixture of calix[4]arene and dicarbollide was applied. The effect of the diluents on the metal extraction with HPBI-S is shown in Figures 5 and S3 (Supporting Information). The comparison of log KI,S for Eu (various diluents) with those of the other lanthanoids (Figure S3) as well as of log KI,S for C6H6 (various metals) with those of the other diluents (Figure 5) shows that log-log plots describing the behavior of the metals and the diluents are almost parallel to one another. These regularities appear to be quite helpful to estimate equilibrium constants and to select suitable diluents. Since activity coefficients of solutes are very sensitive to the properties of the diluent, the effects of the diluent have been quantitatively explained.33,35 The separation factors of the pairs Nd/La, Eu/Nd, Ho/Eu, and Lu/Ho are given in Table 4. In most cases, the lanthanoid separation was not influenced by the change of the diluents, but a considerable increase of the S.F. for the pair Eu/Nd was found when C2H4Cl2 was used. The values of the S.F. for the Nd/La pair are considerably smaller and are not changed significantly upon the addition of the synergist. It is seen that the separation factors for lighter pairs decrease when CCl4 is used as a diluent, while the lanthanoid extraction is much more efficient with this diluent. The S.C. for the system including CCl4 is 2 orders of magnitude higher in comparison with the S.C. obtained when CHCl3 and C2H4Cl2 are used (Table 3).

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Figure 5. Comparison of log K for CHCl3, C6H6, CCl4, and C2H4Cl2 to log K for C6H6 (1, La; 2, Nd; 3, Eu; 4, Ho; 5, Lu). Table 4. Values of the Separation Factors of the Metal for Lanthanoid Extraction with HPBI and a Mixture of HPBI-S S.F. Nd/La

Eu/Nd

Ho/Eu

Lu/Ho

diluent HPBI HPBI-S HPBI HPBI-S HPBI HPBI-S HPBI HPBI-S CHCl3 6.16 C2H4Cl2 6.16 2.75 CCl4

4.89 4.57 2.95

3.98 4.16 3.23

4.67 6.45 3.80

1.99 4.57 2.88

3.02 2.57 3.31

2.18 2.63 2.51

2.69 2.14 2.45

Thus, the choice of diluent is very important not only for synergistic enhancement but also for the effective separation of metals. 3.4. IR and 1H NMR Spectra of the Complexes. The elemental analysis and IR and NMR studies of the isolated solid complexes give a possibility to make conclusions about the composition and the structure of the complexes. The characteristic bands of the IR spectrum of 4-benzoyl-3-phenyl-5isoxazolone are as follows: a band at 1289 cm-1 typical for a CdC-O group and a broad band around 3056 cm-1 for an intramolecular hydrogen bond. Such a band is not observed in the complex of 4-benzoyl-3-phenyl-5-isoxazolone with Gd(III), as the OH- group is deprotonated in the complex formation. The position of the bands typical for an Ar group are at the same place in the IR spectra of the 4-benzoyl-3-phenyl-5isoxazolone and its complex with Gd(III). The interaction of the GdCl3, HPBI, and calix[4]arene S at a molar ratio of 1:3:1 results in the formation of a solid complex with a melting point of 286-287 °C and a coordination number of 10. The expected 1H NMR resonances of HPBI are significantly broadened and overlapped upon coordination, while the signals of S are relatively less influenced. The chemical shifts and coupling constants of S were extracted from the 1H NMR spectrum of Gd(PBI)3S at 62 °C, while the broad and overlapped signals of PBI- in the spectra of Gd(PBI)3S and Gd(PBI)3 are much less informative. There is no change in the symmetry of ligand S upon coordination, and all resonances of S remain in the same form as for the free S ligand, which indicates uniform interaction of all CdO bonds with the Gd3+ ion within the 1H NMR time scale. The suggested structure of the complex is presented in Figure 6 on the basis of elemental analysis, IR, and 1H NMR data. 4. Conclusion Fourteen lanthanoid(III) ions have been extracted with a chelating extractant 4-benzoyl-3-phenyl-5-isoxazolone (HPBI)

Figure 6. Structure of the solid complex Gd(PBI)3S suggested on the basis of IR and NMR data.

and tert-butylcalix[4]arene tetrakis(N,N-dimethylacetamide) (S) as Ln(PBI)3 · S species. A moderate synergistic effect, up to 2 orders of magnitude, was established for lighter lanthanoids. The diluents’ effect on the synergistic solvent extraction of La, Nd, Eu, Ho, and Lu with the HPBI-S mixture was investigated. The values of the equilibrium constant KI,S and log βI,S increased in the order CHCl3 < C2H4Cl2 < C6H6 < CCl4. The formation of ternary complexes of Gd(III), HPBI, and the used calix[4]arene was studied at a molar ratio of the reagents of 1:3:1. The stoichiometry (Gd(PBI)3S) and the structure of the solid complex were suggested using elemental analysis, IR, and 1H NMR data. Acknowledgment Financial support by the National Research Fund of Bulgaria for the purchase of a Bruker Avance II+ 600 NMR spectrometer in the framework of the Program “Promotion of the Research Potential through Unique Scientific Equipment” (Project UNA17/2005) is gratefully acknowledged. Supporting Information Available: Plots of log DI,S vs [HPBI] and [S] and a comparison of log K’s of La, Nd, Eu, Ho, and Lu to the log K of Eu (Figures S1-S3). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Gutsche, C. D. In Calixarenes. Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, U. K., 1998. (2) Lumetta, G. J.; Rogers, R. D.; Gopalan, A. Calixarenes for Separations. ACS Symp. Ser.; American Chemical Society: Washington, D.C., 2000; Vol. 757, chapter 12. (3) Asfari, Z.; Bo¨hmer, V.; Harrowfield, J.; Vicens, J. Calixarenes 2001; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001.

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ReceiVed for reView February 12, 2010 ReVised manuscript receiVed April 27, 2010 Accepted May 11, 2010 IE100328V