The application of ionic liquid-based system in the extraction of palladium: synthesis, characterization and computer calculation of palladium complexes

Abstract

In view of the recycling and the preconcentration of palladium (Pd) from aqueous solutions, a sulfur-bearing extractant, 1,3-diethyl-imidazole-2-thione, in an ionic liquid organic phase was evaluated for palladium extraction. The extraction conditions were examined followed by mechanism studies. The aqueous Pd(II) was extracted using a neutral extraction mechanism, which can avoid or decrease the loss of ionic liquid used in traditional methods and highlight the green credentials of the ionic liquids. The extraction system also provides a new method for the preparation of metal complexes crystals. Investigations of the extracted complexes of palladium(II) with the EEImT ligand were conducted using single crystal X-ray diffraction and computational methods. The results showed that the Pd(II)–EEImT complexes with both 1 : 1 and 1 : 2 stoichiometry can be produced during extraction, rather than simply one structure. The cis geometry of the Pd(EEImT)₂Cl₂ complex was more favorable than the trans geometry. This was further explained by computer calculations, which suggested that the cis configuration with a larger dipole was energetically more stable than the trans configuration.

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Introduction

The widespread applications of palladium (Pd) in chemical catalysis,¹ alloy membrane preparation² and the fine chemical industry³ indicate the growing demand of palladium, not only in high quality but also in great quantity. Therefore, under the overarching pressure of natural resources and the environment, the separation and purification of palladium from both natural ores and industry waste are profound. Given this, different methods, such as electrolytic deposition,⁴ solid phase extraction,⁵ and ion exchange,⁶ were developed. Considering the separation ability and complexity, the traditional solvent extraction is still the most convenient and economical method for large-scale applications.^7,8

Although solvent extraction has gained long-run development, efficient green extraction systems are still needed. Recent studies have reported the solvent extraction of palladium using ionic liquids, which are considered advantageous for their versatility and “green” credentials,⁹ as either the organic phase or the extractant. For example, A. Cieszynska et al. probed the use of trihexyl(tetradecyl)phosphonium chloride for palladium extraction from hydrochloric acid solutions.¹⁰ Katsuta and Shoichi et al.¹¹ developed a well-founded method to selectively separate palladium and platinum by trioctylammonium-based mixed ionic liquids, which are recyclable, easy to handle, safe, and environmentally friendly. Sasaki, Kotoe et al.¹² used the ionic liquid, betainium bis(trifluoromethanesulfonyl)imide, to extract Pd(II), Rh(III) and Ru(III) from aqueous HNO₃ solutions, which proceeded via coordination between betaine and metal ions and the cation exchange of the formed complex with a proton.

During the abovementioned researches, the metal ions were extracted to the organic ionic liquid phase through an ion exchange mechanism, which are shown as follows,^13–15 where the subscripts (aq), (IL) and (or) denote an aqueous phase, ionic liquid phase and organic phase, respectively.

2H_(aq)⁺ + PdCl_4(aq)²⁻ + 2R₃R′PA_(IL) ⇔ (R₃R′P)₂PdCl_4(or) + 2HA_(aq)

Mⁿ⁺_(aq) + mL_(IL) + x[C_n min][TFSA]_(IL) ⇔ [M(L)_mⁿ⁺(TFSA⁻)_x]_(IL) + x[C_n min⁺]_(aq)

PtCl_6(aq)²⁻ + 2[C₄ min] PF_6(IL)⁺ ⇔ ([C₄ min])₂·PtCl_6(or)²⁻ + 2PF_6(aq)⁻

These mechanisms will certainly lead to the loss of ionic liquids. Therefore, to facilitate extraction, the volume of the ionic liquids phase would be relatively large, with an aqueous phase volume ratio of about 1 : 2,¹⁶ which means higher cost for applications. On the other hand, because most waste aqueous sources containing Pd(II) are acidic, and the yields of the extraction with ion exchange will fluctuate with the varying concentration of aqueous hydrogen ions,¹⁷ an extra procedure is needed to adjust the pH. Nevertheless, these disadvantages can be avoided through neutral extraction.

In our work, the sulfur-bearing extractant, 1,3-diethyl imidazole-2-thione (EEImT), was first used as the extractant to extract aqueous palladium to the ionic liquid organic phase, i.e., 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIm]NTF₂). The extracted complexes were formed through a neutral coordination mechanism rather than an ion exchange mechanism, which avoids the loss of ionic liquid and saves the volume of organic phase. The extraction system also provides a new method for the quick preparation of the regularly-shaped metal complexes crystals under certain conditions. Two novel PdCl₂–thione crystals were obtained. X-ray crystallography and computer calculations were conducted to elucidate the construction and configuration of the Pd(II)–EEImT complexes. The extraction was barely dependent on the hydrochloric acid concentration under certain conditions. The entire system was verified to be effective and stable.

Experimental section

Reagent and materials

1,3-diethyl imidazole acetate salt and 1-ethyl-3-methyl imidazolium bis((trifluoromethyl) sulfonyl) imide were purchased from Lanzhou Greenchem ILS, LICP, CAS, China. PdCl₂ was purchased from Guangfu Institute of Fine Chemicals (Tianjin, China). All the reagents (sulfur, sodium bicarbonate, methanol, acetonitrile, methylene dichloride, acetone) were of AR grade and used without additional purification. Distilled water was used to prepare the aqueous solutions in all experiments.

Synthesis of 1,3-dimethyl-imidazole-2-thione

The synthesis of thione was performed according to a previously reported procedure¹⁸ to give 1,3-diethyl imidazole-2-thione in 63% yield. 1H NMR (300 MHz, DMSO): δ = 7.378 (s, 1H), 4.051 (t, 4H), 1.247 (trip, 6H). 1,3-dimethyl-imidazolium acetate (2.0 g) was stirred with sulfur (0.1 g) in an round-bottom flask at 50 °C. After several minutes, the mixture turned yellow to dark brown. Subsequently, 10 mL acetonitrile was added after 24 h. The mixture was leached with a filter to eradicate sulfur. Then, acetonitrile was distilled under vacuum. The product was dissolved in 15 mL methylene dichloride. Deionized water and a sodium bicarbonate solution (5%) were used to wash the oil to remove the 1,3-diethyl imidazole acetate salt and acetic acid. Then, methylene dichloride was distilled under vacuum. The product was recrystallized at room temperature to give almost colorless crystals. This procedure was shown as the following scheme (Scheme 1):

Scheme 1 Synthesis of 1,3-dimethyl-imidazole-2-thione.

Extraction procedure

Extraction was conducted using 0.01–0.05 mM of EEImT in 0.5 mL [EMIm]NTF₂ as the organic phase. The feed solutions contained 0.001 mM of palladium(II) chloride in 0.1 mM 5 M HCl. The two phases with a volume ratio of the aqueous phase to organic phase (R_A/O) of 10, were shaken mechanically in an orbital shaker for 0.5–20 min and separated with a centrifuge at 2000 rpm for 5 min. Atomic absorption spectrophotometry (3150, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China) was used to determine Pd(II) concentrations in the aqueous solutions before and after extraction. High performance liquid chromatography (LC2000, TianMei, Shanghai, China) was used to analyze the consumed extractant after extraction. Each experiment was carried out 3 times and the standard deviations did not exceed 5%. The extraction percentage (E (%)) and distribution coefficient (D) of Pd(II) were calculated from the following equations:

Crystal growth and analysis

Single crystals suitable for the X-ray diffraction study were grown by slow solvent evaporation. Ethanol water solutions (10 mL) containing 0.0282 mM PdCl₂ and one or two equivalents of EEImT were stirred and filtered with a funnel and then volatilized in a serum bottle at 298 K for two weeks. Reddish brown needle-like crystals were obtained.

It is worth mentioning that at high complexes concentrations (0.282 mol L⁻¹), the organic phase become rather viscous. The addition of 2 mL ethanol will easily wash out the Pd–EEImT complexes, which also can form regularly-shaped needle like crystals. This procedure was not reported previously, and it may represent a novel method to prepare crystals of metal complexes.

Single crystal X-ray diffraction was conducted using an area detecting system (Bruker-Nonius SMART APEX II CCD) and graphite monochromated Mo-Kα radiation (λ = 0.71000 Å). A hemisphere of data was measured using a strategy of omega scans of 0.5° per frame. Empirical absorption corrections were applied. The structures were solved by a combination of direct methods and difference Fourier syntheses. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were calculated in the ideal positions riding on the parent carbon atoms. The unit cell parameters were obtained by a full-matrix least-squares refinement. Data collection, integration, and absorption corrections were performed using the APEX222 software suite (Bruker).¹⁹ The computing structure solution and refinement was carried out using the SHELXL-97 software package (Bruker).²⁰

Crystal data and details of the structure determination are summarized in Table 1. CCDC 994393 and 994500 contain the ESI† for the structures described in this paper.

Table 1 Crystal data and details of the structure determination

	Pd2(EEImT)2Cl4	Pd(EEImT)2Cl2
CCDC number	994393	994500
Formula	C10H24Cl4N4Pd2S2	C10H24Cl2N4PdS2
Formula weight	667.09 g mol^−1	489.79 g mol^−1
Crystal system	Tetragonal	Monoclinic
Space group	P4̄21c	C2/c
Temperature (K)	298(2)	293(2)
a(Å)	10.954(3)	18.078(19)
b(Å)	10.954(3)	9.268(1)
cÅ)	19.840(5)	13.131(14)
α(°)	90	90
β(°)	90	113.059(2)°
γ(°)	90	90
Cell volume (Å^3)	2380.6(8)	2024.26(40)
Z	4	4
Total/unique reflections	10 996	4756
Rint	0.1138	0.0291
R1[I > 2σ(I)]	0.0484	0.0309
wR2 [I > 2σ(I)]	0.0931	0.0759

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X-ray powder diffraction was carried out using a D8 Advance X-ray diffractometer with a graphite monochromator and Cu-K radiation (λ = 0.15418 nm).

Calculation methods

All calculations were performed using Gaussian-09.²¹ Optimization for the geometrical structures in this investigation was first conducted by the Hartree–Fock method using the STO-3G basis set.²² Symmetric and internal coordinate constraints were applied during optimization. Harmonic frequency calculations were carried out at the same level to determine the sum of the electronic and thermal free energies. None of the imaginary frequencies were observed in all cases. The single point energies of the optimized structures were determined using the density functional hybrid model Becke3LYP at the 6-31+G(d,p) basis set for hydrogen, carbon, nitrogen, sulfur, chlorine atoms, and palladium treated by the Hay-Wadt effective core potential.²³ All energies reported in this work, unless otherwise noted, are the free energies at 298 K and 1 atm.

Results and discussion

Extraction of Pd(II) with EEImT/[EMIm]NTF₂

The influence of the mixing time, hydrochloric acid and EEImT concentrations on the extraction of palladium(II) with EEImT/[EMIm]NTF₂ were examined to study the optimal conditions.

Establishment of the optimum time to reach equilibrium was conducted by mixing 5 mL aqueous phase (0.1 mM PdCl₂ in 0.1 mM HCl) and 0.5 mL organic phase (2 mM EEImT in [EMIm]NTF₂ and [EMIm]NTF₂ alone) from 0.5 to 30 min. The ionic liquid, [EMIm]NTF₂, itself can extract only 8% of Pd(II) after 15 min of contact, whereas the addition of EEImT results in 98% extraction of Pd(II) within 3 min of contact. This indicates a fast and stable equilibrium for Pd(II) extraction with EEImT/[EMIm]NTF₂. In addition, through the entire extraction process, there was no observable change in the volumes of the two phases, and no emulsion was observed between the interphase, which further represents the easy separation of the two phases.

The effect of the EEImT concentration on the palladium percentage extraction (E) was investigated. The results are shown in Fig. 1. The E_Pd attains maximum (about 98%) when the molar ratio of EEImT : Pd is 4.

Fig. 1 The effect of EEImT concentration (C_EEImT) on the Pd(II) extraction rate (E_Pd). The aqueous and organic phase were in the volume ratio of 10. Aqueous Pd(II) concentration was 0.1 mM.

The influence of the hydrochloric acid concentration was then studied under the optimized conditions. The addition of HCl is clearly against neutral extraction of Pd(II), because it can lower the E_Pd from 97.8% to 88.1%, as illustrated in Fig. 2.

Fig. 2 Influence of the hydrochloric acid concentration on the Pd(II) extraction rate. Aqueous phase: 5 mL, 0.1 mM PdCl₂, organic phase: 4 mM and 5 mM EEImT in 0.5 mL [EMIm]NTF₂.

This was previously explained that Pd(II) exists in the form of PdCl₄²⁻ at higher levels of Cl⁻, thus the coordination of Pd(II) and EEImT was intervened.²⁴ It is noteworthy that the addition of 25% extra extractant will significantly shrink this decline, as the red line in Fig. 2 shows. According to many previous reports, the extraction rate will be fluctuated about ±10% with varying HCl concentration.^25–27 Nevertheless, in the case of the neutral extraction with EEImT/[EMIm]NTF₂, the extraction rate becomes less dependent on the H⁺ concentration at appropriate extractant concentration, with only −2% fluctuation in the tested range. This high hydrogen ion tolerance is more preferable for practical applications, because many palladium-recyclable effluent usually contain a variety of acids.

Back-extraction was carried out by mixing the loaded organic phase with four times the volume of stripping reagent solutions, among which the 0.5 M thiourea/1.0 M HCl and 4.0 M NH₄SCN/1.5 M NH₄OH were tested to be the most efficient. Both stripping reagents led to color fading of the loaded ionic liquid phase from reddish brown to white transparent, and above 96% Pd(II) was stripped to the aqueous phase through ICP-AES determination.

Because very high extractability was shown by prior experiments, the traditional slope method and Job's method become unsatisfactory for determining the stoichiometry of Pd–EEImT complex. In addition, HPLC analysis for the concentration of EEImT before and after extraction indicated that the consumed EEImT was less than two times but more than one time that of the extracted Pd(II), which means that there may be more than one structure existing in the extracted complexes (not shown). Therefore, X-ray single crystal diffraction combined with X-ray powder diffraction were alternatively used to determine the extracted complex.

X-ray crystallographic analysis

Because traditional methods were unsuitable in this case to elucidate the extracted complexes, single X-ray diffraction was applied alternatively. Although regularly-shaped needle-like crystals can also be obtained quickly by extraction with a high level complexes concentration, as mentioned above, there may be ionic liquid remaining on the surface of the crystal that may interfere with the X-ray single crystal diffraction. In addition, the single X-ray results may not comprehensively represent all the molecular structure. Therefore, the washed crystal was ground and analyzed by X-ray powder diffraction after being dried for 24 h at temperatures less than 80 °C for comparison with the structure-known pattern. Nevertheless, this phenomenon may provide a new clue to synthesize the crystals of metal complexes.

According to previous cases related to neutral metal extraction,^28,29 the most probable stoichiometry (1 : 1 and 1 : 2) of Pd : EEImT in the extracted complexes was proposed, and X-ray single crystal diffraction was conducted to verify the speculation.

Pd₂(EEImT)₂Cl₄ (Fig. 3.) adopts a bridging geometry with two Pd atoms connected to two sulfur atoms to form a parallelogram in the space group P4̄21c. The bond length of Pd–S is 2.311 Å and 2.290 Å, which is comparable to those observed in the related bridging palladium complexes bearing sulfur and chlorine ligands (2.280 Å and 2.271 Å,³⁰ 2.278 Å and 2.252 Å,³¹ 2.284 Å and 2.267 Å).³² The angle between the two Pd-centered square planar structure (139.901°) is dramatically smaller than that in 165.441°,³⁰ 180.000°,³¹ 180.000°,³² and 180.000°,³³ except when the two sulfur atoms are connected together, 120.115°.³⁴

Fig. 3 Molecular structure of Pd₂(EEImT)₂Cl₄, ORTEP representation on the 50% probability level for all non-carbon atoms. Hydrogen atoms have been deleted for clarity.

Each palladium center takes the usual square planar geometry with slight distortion, because the two chlorine atoms are located at different distances from it (Pd1–Cll is 2.319 Å, whereas Pd1–Cl2 is 2.300 Å). The two Pd–S bond lengths also are slightly different (Pd1–S1 is 2.311 Å, whereas Pd1–S2 is 2.290 Å). The bonding of S with Pd induces distortion in the structural features of the diethyl-imidazole substituent.³⁵ This may be explained by the strong intermolecular hydrogen bond formed between the Cl atoms with the C3 H atom in the adjacent imidazole rings, which provides unsymmetrical tension to the Cl atoms on the Pd atoms in the opposite site.

The Pd(EEImT)₂Cl₂ (Fig. 4) crystallizes in the space group C2/c with a four substituent of center Pd atoms constituting a more symmetrical structure than that of Pd₂(EEImT)₂Cl₄. The Pd–S bond distance of 2.324 Å in Pd(EEImT)₂Cl₂ is close to those reported elsewhere,^36–39 which is slightly longer than that of Pd₂(EEImT)₂Cl₄ (2.311 Å and 2.290 Å). This indicates that the Pd–S bond is considerably stronger in Pd₂(EEImT)₂Cl₄. However, the higher degree of chelation of Pd(EEImT)₂Cl₂ (2 of chelation) can be more favorable for extraction than Pd₂(EEImT)₂Cl₄ (1 of chelation).

Fig. 4 Molecular structure of cis-Pd(EEImT)₂Cl₂, ORTEP representation on the 50% probability level for all non-carbon atoms. Hydrogen atoms have been deleted for clarity.

On the other hand, the XRD results (Fig. 5) of the extracted complexes revealed the production of both the 1 : 1 and 1 : 2 stoichiometry of the Pd(II)–EEImT complexed during extraction, not simply one structure. This is rather different from previous metal extraction studies, which claimed that the neutral extractant containing S or N atoms would extract palladium (or other metal ions) at a certain integral ratio (2 : 1 or 1 : 1) such as sulfoxide and even thione.^28,40,41 We speculated that at relatively high Pd(II) : EEImT levels, about 1 : 10 to 1 : 1, the extracted complexes are composed of both 1 : 1 and 1 : 2 stoichiometry, whereas more and more 1 : 2 stoichiometry would appear with increasing extractant concentration, for its higher molecular stability.

Fig. 5 Experimental powder X-ray diffraction pattern of (C) complexes washed from loaded ionic liquid and simulated powder patterns for (B) Pd₂(EEImT)₂Cl₄ and (A) cis-Pd(EEImT)₂Cl₂.

There is possibility of two structures for the configuration of Pd(EEImT)₂Cl₂, which involve the ligation of palladium with sulfur and chlorine atoms on diagonally opposite sides (trans, Fig. 6) and a symmetrical (cis) configuration.

Fig. 6 Optimized geometry of the trans-Pd(EEImT)₂Cl₂.

The trans configurations of palladium are in the majority as reported elsewhere^42–44 and are more energetically stable in view of the smaller steric hindrance.⁴⁵ Nevertheless, the cis configuration was obtained according to the results of X-ray crystallography. However, the powder pattern does not completely match with the XRD result of the sediment washed out of the [EMIm]NTF₂ with high levels of extracted complexes, which indicates that the trans configuration may also exist in the extracted complexes.

Calculation results

Quantum chemical calculations were performed to obtain the thermodynamic parameters for the optimized geometries of the two possible structures. The selected bond lengths (Å) and angles (deg) for Pd–EEImT complexes are presented in Table 2. The trans form was not given directly by crystal growth; therefore, the optimized structure parameters were used for comparison. The DFT (B3LYP) structure of the cis-Pd(EEImT)₂Cl₂ is in good agreement with the X-ray crystallographic data. The bond distances and angles are generally within 0.0854 Å and 2.938°. According to the parameters of the optimized trans-Pd(EEImT)₂Cl₂, the Pd–S bond length is 2.378 Å, and the angle of θ (S–Pd–Cl) is 93.065°. Further energy calculations revealed that the structure involves the ligation of similar donor atoms in a cis configuration with a lower molecular energy of −8285.910 a.u., and is more energetically feasible than that with similar donor atoms in a trans configuration (with molecular energy about −8281.184 a.u.). In addition, the cis configuration has a larger dipole moment, which will lead to a higher solubility in the polar solvent phase. This may account for the crystal growth result.

Table 2 Selected bond lengths (Å) and angles (deg) for Pd–EEImT complexes

Pd2(EEImT)2Cl4
Pd–Cl1	2.3185(35)	Cl1–Pd–Cl2	93.90(14)
Pd–Cl2	2.3002(37)	S–Pd–S	81.71(13)
Pd–S1	2.2898(34)	Pd–S–Pd	90.56(11)
Pd–S2	2.3117(34)	Cl1–Pd–S	92.08(14)
C–S	1.7588(135)	N–C–N	107.2(12)
		C–S–Pd	109.5(5)
Cis-Pd(EEImT)2Cl2
Pd–Cl	2.3503(10)	Cl–Pd–Cl	90.53(5)
Pd–S	2.3240(11)	Cl–Pd–S	92.52(3)
C–S	1.7434(45)	S–Pd–S	84.56(5)
C–S–Pd	103.701(137)	N–C–N	106.6(3)
Optimized cis-Pd(EEImT)2Cl2
Pd–Cl	2.3052	Cl–Pd–Cl	93.47
Pd–S	2.4094	Cl–Pd–S	91.59
C–S	1.7207	S–Pd–S	83.36
C–S–Pd	102.7	N–C–N	108.5
Optimized trans-Pd(EEImT)2Cl2
Pd–Cl	2.3179	Cl–Pd–S	93.25
Pd–S	2.3780	C–S–Pd	105.0
C–S	1.7217	N–C–N	105.2

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Conclusions

The extraction of Pd(II) with EEImT in the ionic liquid system was studied with a detail mechanism clarification. During the extraction experiments for the Pd(II) with EEImT, the extraction rate of Pd(II) reached 97.8% with an aqueous–organic phase volume ratio of 10 and an EEImT : Pd(II) molar ratio of 4. At the appropriate extractant concentrations, the extraction rate became barely dependent on the hydrochloric acid concentration, which shows that the EEImT is an efficient extractant for palladium. Furthermore, the ionic liquid-based extraction system also provides a quick and convenient method to prepare metal complexes crystals; however, detailed conditions are still waiting for the further exploration. The single X-ray diffraction results illustrated the Pd(II)–EEImT complex with 1 : 2 and 1 : 1 stoichiometry in detail. Under the experimental conditions, both structures existed in the extracted complexes dissolved in the ionic liquid. In addition, the percentage of the former will increase with increasing amount of extractant. The computer calculations further illustrated that the cis configuration of Pd(EEImT)₂Cl₂ possesses a lower molecular energy than the trans configuration. The larger dipole moment also promotes its stability in a polar solvent, which may explain why the crystal growth results in the cis form.

Acknowledgements

Thanks to Professor Maoxia He and Di Sun for helping with computer calculations and X-ray crystallographic analysis. This work was supported by the National Natural Science Foundation of China (Grant 21276142 and 21476129).

Notes and references

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Footnote

† CCDC 994393 and 994500. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra07563c

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