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Article

Manipulating Electronic Effect of Nitrogen Donor-Based Ligands for Efficient Complexation and Separation of Palladium from Highly Acidic Solution

by
Yuyang Gan
†,
Yimin Cai
†,
Song Huang
,
Xiaowei Li
*,
Wen Feng
and
Lihua Yuan
*
College of Chemistry, Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2025, 30(7), 1533; https://doi.org/10.3390/molecules30071533
Submission received: 14 February 2025 / Revised: 25 March 2025 / Accepted: 28 March 2025 / Published: 30 March 2025
(This article belongs to the Special Issue 30th Anniversary of Molecules—Recent Advances in Inorganic Chemistry)
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Abstract

:
Nitrogen donor-based ligands are highly promising extractants for palladium separation from high-level liquid waste (HLLW). However, the electronic effect of these ligands, a critical factor influencing their complexation ability with Pd(II), remains largely unexplored. Herein, three picolinamide-based ligands were designed and synthesized, each featuring substituents with distinct electronic effects at the para-position of the pyridine (electro-donating methoxyl group for L-I, hydrogen for L-II, and electro-withdrawing ester group for L-III). The concurrent processes of Pd(II) coordination and ligand protonation enable the manipulation of pyridine nitrogen electronegativity, resulting in a tunable Pd(II) extraction performance. Notably, L-I exhibits the highest extraction efficiency at low acidities (≤1 M HNO3) but the lowest extraction at high acidities (≥3 M HNO3), whereas L-III shows the poorest efficiency at low acidities but the best performance at high acidities. The Job plot analysis and ESI-HRMS results reveal a 1:1 and 2:1 (L/Pd) stoichiometry in the Pd(II) complexation process. The single crystal X-ray analysis of Pd(NO3)2(L-II)2 complex confirms a four-coordinated Pd(II) center, with two pyridine nitrogen atoms and two monodentate nitrate oxygens forming a quadrangular geometry. Density functional theory (DFT) calculations further indicate that the formation of 2:1 (L/Pd) complexes is energetically favored, and the stronger basicity of the nitrogen atoms correlates with a higher Pd(II) binding affinity and increased susceptibility to protonation.

Graphical Abstract

1. Introduction

Nuclear power, as a high-density and green energy supply with negligible CO2 emissions, has long been considered as a promising alternative for traditional fuel energy with the ever-increasing worldwide concern about global warming [1,2,3]. However, the generation of large amounts of nuclear wastes with high radio- and chemo-toxicity, which causes severe threats to biological and environmental systems, has become one of the most pivotal factors stymieing the further development of nuclear industries [4,5,6]. The proper treatment of the harmful fission products (FPs) is an essential yet challenging task for the subsequent disposal of nuclear wastes [7,8,9]. Among these FPs, palladium exists in large quantities up to 1 kg/t in fast reactors at a burn up of 33 GWd/t [10]. These palladium ions can seriously impede the solidification of high-level liquid waste (HLLW), the raffinate of the plutonium and uranium reduction extraction (PUREX) process aiming at removing Pu and U from nuclear wastes, due to the poor solubility of Pd species in the glass matrix [11]. On the other hand, palladium is an important precious metal with widespread applications in the fields of catalysis [12], medical science [13], corrosion resistance alloys [14], electronics [15], etc. However, the extremely low abundance of Pd in the Earth’s crust (less than 3 ng/g) is far from enough to meet the growing industrial demands, leading to an explosive growth in its price in recent years [16]. It is worth noting that only 107Pd presents a very weak β-emission that is acceptable for most industrial uses, and this isotope only accounts for ca. 17% of the total Pd resources in HLLW [17]. Therefore, the recovery of Pd from HLLW is not only necessary for nuclear waste treatment, but also desirable for commercial interests.
The solvent extraction technique is currently the best developed method for metal ion separation from HLLW, which possesses attractive merits including easy operation, low energy consumption, and so on [18,19]. This method relies heavily on the use of ligands with high efficiency and selectivity. Different kinds of ligands have been designed for Pd(II) extraction from HLLW in the past years [20] (Figure 1A), mainly focusing on oxygen-based ligands (e.g., CMPO and DMDBTDMA), sulfur-based ligands (e.g., TIPS), and nitrogen-based ligands (e.g., Tz2 [21] and PyTri [22]). However, most oxygen-based ligands inevitably face drawbacks regarding selectivity due to the coextraction towards lanthanides and other fission products, such as Sr(II) and Cs(I) [23,24]. Although sulfur-based ligands exhibit a higher selectivity for Pd(II), they can hardly be employed under highly acidic conditions because of severe protonation [25]. In comparison, nitrogen-based ligands demonstrated a higher potential for Pd(II) separation from simulated HLLW [21,22]. Since Pd(II) belongs to the soft acids, nitrogen donor atoms as soft bases generally exhibit an excellent selectivity towards Pd(II) [26]. In addition, the chemical structures of these nitrogen-based ligands meet the CHON principle, and thus they are more environmentally friendly as no secondary pollution is generated after incineration [27]. It was found that the Pd(II) extraction efficiency of nitrogen-based ligands is largely dependent on their structural characteristics. For example, altering the alkyl side chains can affect the hydrophobicity and steric hindrance of ligands [22]. Diverse types of n-heterocycles exhibit different efficiencies for Pd(II) separation [21,28]. Changing the number and direction of nitrogen donors in ligands also results in distinct Pd(II) separation abilities. Disclosing the relationship between ligand structures and Pd(II) complexation and extraction efficiency is an important fundamental task for ligand/material design and the development of related separation processes. However, the influence of the electronic effect of ligands, one of the key factors determining the Pd(II) coordination strength of donor atoms, on palladium complexation and extraction still remains an unexplored territory.
In such a background, this work reports on the design and synthesis of three new pyridine-based ligands with different substituents that show various electronic effects on the nitrogen coordination sites (Figure 1B). L-I and L-III are decorated with an electron-donating methoxyl group and an electron-withdrawing ester group, respectively, at the para-position of the pyridine nitrogen atom, while L-II is not substituted. Their Pd(II) extraction ability and complexation mechanisms were investigated comparatively by solvent extraction experiments, spectroscopic methods, single crystal X-ray diffraction analysis, and DFT calculations.

2. Results and Discussion

2.1. Solvent Extraction

The solvent extraction behaviors of the ligands toward Pd(II) were investigated starting with extraction kinetics. Given the high solubility of both the extractants and Pd(II)-loaded complexes in 3-nitrobenzotrifluoride, this solvent was selected as the diluent in all extraction experiments to prevent precipitation, emulsification, and third-phase formation. These experiments were carried out by shaking the mixture of the organic phase (solutions of ligands in 3-nitrobenzotrifluoride) and aqueous phase (Pd(NO3)2 in HNO3) with the ratio of 1:1 (v/v) and detecting the residual Pd(II) concentrations of the aqueous phase at different contact times. As shown in Figure 2A, at 3 M HNO3, L-I, L-II, and L-III showed very fast extraction kinetics toward Pd(II) with extraction percentages of over 96% within 1 min. Our previous work suggested that the hydrophobicity of ligands controlled by different alkyl side chains could produce significant differences in Pd(II) extraction kinetics, with the ligand bearing the shortest alkyl side chains showing much faster kinetics [22]. Therefore, one of the determinants responsible for the ultrafast extraction observed here could be the presence of short alkyl side chains in these ligands, which leads to lower hydrophobicity and faster metal ion transfer at the interface during extraction. At equilibrium, all of the three ligands exhibit a quantitative extraction for Pd(II). Based on these results, the extraction time was fixed at 2 h to ensure the equilibria of all the subsequent extraction experiments.
The extraction results at different HNO3 concentrations ranging from 0.01 M to 6 M are presented in Figure 2B and Table S4. Despite the presence of similar coordination sites, the three ligands were found to exhibit distinct extraction behaviors toward Pd(II) ions. At 0.01 M HNO3, L-I bearing an electron-donating methoxyl group demonstrates the highest Pd(II) extraction (94%) among the three ligands, while L-III with an electron-withdrawing ester group extracts only 71% of the Pd(II). The results suggest that the higher basicity of the nitrogen donor atom produces a positive effect on the Pd(II) extraction due to the stronger binding affinity of donor atoms towards Pd(II). As the aqueous phase is subjected to the change in HNO3 concentrations from 0.01 M to 3 M HNO3, the three ligands experience an increased extraction for Pd(II) to almost quantitative extraction. This phenomenon can be rationalized by the fact that the increased acidity leads to higher concentrations of NO3− anions in the aqueous phase, which facilitates Pd(II) extraction by being co-extracted into the organic phase to keep the charge balance of both aqueous and organic phases [29,30]. When the HNO3 concentration is further elevated to 6 M, the Pd(II) extractions by L-I and L-II decrease dramatically to 54% and 67%, respectively, indicating that the serious protonation of ligands at such a high acidity is the dominating factor to effect the extraction performance. Interestingly, although L-III shows a lower extraction efficiency at low acidities (0.01–0.2 M HNO3), it exhibits a remarkably higher extraction towards Pd(II) at the acidities ranging from 4 to 6 M than other two ligands. The results above demonstrate a unique but undiscovered phenomenon for Pd(II) extraction systems. At low acidities, the protonation of ligands shows a relatively low impact on Pd(II) extraction. Therefore, the ligands decorated with electron-donating groups exhibit a better extraction ability. However, the pattern is reversed at high acidities, in which cases the protonation of ligands plays the leading role for affecting the extraction, and the donor atoms with higher basicity result in much severer protonation, thus yielding low extraction. This conclusion will be discussed in detail and further verified by DFT calculations (vide post). Moreover, we studied the extraction performance and structural changes in three ligands after radiation. After irradiation at 200 kGy, the 1H NMR analysis revealed that the structures of the three ligands remained almost unchanged. Additionally, extraction experiments demonstrated that all the three ligands only exhibited a very slight reduction in palladium extraction efficiency (Figures S4–S6 and S10), indicating that these ligands have a good radiation resistance.
In order to further evaluate the efficiencies of the extraction systems and to probe the stoichiometry of ligands and Pd(II) during extraction, the loading capacities were measured by increasing the concentrations of Pd(II) in the aqueous phase gradually while keeping the concentration of the ligands in organic phases unchanged. As presented in Figure 2C, for the three ligands, when increasing the concentration of Pd(II) from 0.42 mM to 1.33 mM, the [L]/[Pd]org. values decrease dramatically from ca. 4.9 to 1.1–1.5, reflecting the increased extraction amounts of Pd(II) ions from the aqueous phase to the organic phase. At the Pd(II) concentrations higher than 2.03 mM, the [L]/[Pd]org. values level off and no significant reduction was further observed, even at 4.12 mM Pd(II). The lowest [L]/[Pd]org. values for L-I, L-II, and L-III are 1.3, 1.4, and 1.3, respectively, suggesting that each Pd(II) ion could bind to one or two ligands during extraction. These two kinds of stoichiometry, i.e., 1:1 and 2:1 (L/Pd), were confirmed by the Job plot method and ESI-HRMS, respectively. As shown in Figure 2D–F, with the change in the [L]/([L] + [Pd]) ratio when fixing the total concentration of the ligand and Pd(II) at 2 mM, the amount of extracted Pd(II) into the organic phase changes and exhibits the highest values at [L]/([L] + [Pd]) = 0.5 for the three ligands, indicating that the main stoichiometry under this condition is 1:1 (L/Pd). The ESI-HRMS analysis was performed using L-II as the representative ligand (Figure 3). The peaks located at m/z 293.0750, 408.0157, and 648.1449 are assigned to [2 L-II • Pd]2+, [L-II • Pd • NO3]+, and [2 L-II • Pd • NO3]+, respectively, confirming the coexistence of the 2:1 and 1:1 (L/Pd) complexes during extraction.
The selectivity of ligands toward Pd(II) was evaluated using a simulated HLLW solution containing 17 competing ions, such as Cs(I), Sr(II), and Ru(III). The composition and concentrations of this solution were similar with those found in actual HLLW (Table S1). Extraction results indicate that the presence of competing ions affects the extraction of Pd(II) to different degrees for the three ligands (Figure 4). L-I and L-II can recover 90.2% and 94.7% of Pd(II), respectively, with all other competing ions being negligibly extracted (E% < 5%). These results demonstrate an excellent selectivity of L-I and L-II toward Pd(II). L-III also exhibits a good selectivity for Pd(II) over most of the coexisting ions except for a slightly higher extraction for Cs(I) (10.6%) and Ru(II) (10.8%), leading to a relatively lower Pd(II) extraction of 88.6%. As shown in Table S3, the three ligands exhibit markedly higher distribution coefficients for palladium over other metals, confirming their high selectivity for Pd(II). To further investigate the selectivity of these ligands for Pd(II), additional extraction experiments were conducted using Th(IV) and U(VI) as representative radioactive actinides (Figure S11). The results demonstrate that all three ligands exhibited a markedly lower extraction efficiency for these actinides compared to Pd(II), confirming their high selectivity for Pd(II) over actinide species. The separation factors (SFPd/M) calculated from the extraction results are all higher than 50, further proving the excellent extraction selectivity of these nitrogen-based ligands for Pd(II). Using L-II as the representative ligand, we further performed extraction experiments from simulated HLLW in n-octanol, which results in a slightly declined extraction for Pd(II) (83%) and an acceptable selectivity (Figure S12), suggesting that the further replacement of 3-nitrobenzotrifluoride by n-octanol is feasible in regard to its lower price.

2.2. UV-Vis Spectrophotometric Studies

The complexation behaviors between the ligands and Pd(II) ions in acetonitrile were explored by UV-Vis spectroscopic studies. Job plot experiments were first carried out by fixing the total concentrations of ligand and Pd(II) ions at 20 μM. The results shown in Figure 5A–C indicate the 1:1 complexation stoichiometry for the three ligands with Pd(II). In order to determine the stability constants for these ligands binding to Pd(II), UV-Vis titration experiments were performed in acetonitrile at 20 μM of ligand and varying concentrations of Pd(II) ions from 0 to 30 μM (Figure 5D–F). The stability constants were obtained employing global analysis by the website (http://supramolecular.org/), which follow the order of L-I (1.7 × 104) > L-II (4.6 × 103) > L-III (5.4 × 102) (Figure S13), suggesting again that the high electronegativity of donor atoms is beneficial for complexing metal ions under neutral conditions.

2.3. X-Ray Crystallographic Studies

In order to achieve more insights into the coordination information between the ligands and Pd(II), a yellowish single crystal of Pd(NO3)2(L-II)2 was obtained by slowly evaporating ether into solutions of 2 mM L-II and Pd(NO3)2. The detailed bond information is listed in Table S2. Pd(NO3)2(L-II)2 crystallized in the monoclinic Pca21 space group with lattice parameters a = 15.9841(5) Å, b = 12.9740(5) Å, and c = 15.1889(5) Å. In the crystal structure, the central Pd(II) ion is four-coordinated with two nitrogen atoms from pyridine groups and two monodentate nitrate anions in a nearly quadrangular geometry (Figure 6A,B). The cis configuration of the complex is likely favored due to reduced steric hindrance compared with the trans isomer. On account of the presence of nitrate anions, the Pd(NO3)2(L-II)2 complex can further stack through intermolecular hydrogen bonds between nitrate oxygen atoms and pyridine hydrogen atoms (Figure 6C).

2.4. Structures Simulations and Binding Energies Calculations

The different electronegativity of the pyridine nitrogen atoms of the three ligands was confirmed by DFT calculations. As shown in Figure S14, the natural charges on pyridine nitrogen atoms obtained from the optimized structures follow the order of L-III (−0.441) > L-II (−0.446) > L-I (−0.472), suggesting the successful control of the nitrogen basicity by introducing electro-donating or electro-withdrawing groups. The binding energies of Pd(NO3)2L2 complexes in acetonitrile were firstly compared according to DFT calculations. The structural optimization was carried out at the B3LYP/6-31G* level of the theory referring to the geometry of single crystals. As shown in Figure 7A–C, the binding energies of Pd(NO3)2(L-I)2, Pd(NO3)2(L-II)2, and Pd(NO3)2(L-III)2 are calculated to be −46.62 kcal/mol, −41.90 kcal/mol, and −37.28 kcal/mol, respectively, suggesting again that the increased electronegativity is beneficial for Pd(II) complexation in neutral conditions. These results can also rationalize the trend observed from solvent extraction experiments at low acidities. However, the extraction abilities of the ligands at high acidities (>3 M HNO3) follow the trend of L-III > L-II > L-I, which is the opposite of that at low acidities. This could be owing to the higher degree of protonation of the donor atoms with higher electronegativity. To prove this speculation, the protonation energies of the three ligands were calculated. As shown in Figure 7D,E, the protonation energy of L-I (−42.73 kcal/mol) is a little bit higher than that of L-II (−41.08 kcal/mol), indicating that the higher electronegativity leads to easier protonation. Compared with L-I and L-II, L-III has a higher protonation energy (−38.33 kcal/mol), which suggests that L-III is much more difficult to be protonated and this is in great agreement with its markedly higher extraction ability for Pd(II) at high acidities.
Since the formation of 1:1 (L/Pd) complexes was demonstrated in Job plot experiments, their possible configurations were also optimized referring to reported structures of picolinamide with metal ions [31]. As presented in Figure 7G–I, these ligands may also bind to Pd(II) ions according to the pyridine nitrogen and amide oxygen in a bidentate manner, with the cooperation of two monodentate nitrate anions. However, their binding energies (−15.05 kcal/mol for Pd(NO3)2(L-I), −17.41 kcal/mol for Pd(NO3)2(L-II), and −14.08 kcal/mol for Pd(NO3)2(L-III)) are profoundly higher than those of PdL2 complexes, indicating that the formation of PdL2 complexes is more energetically favored. Furthermore, the double free energy difference (ΔΔE) values between Pd(NO3)2(L)2 and protonated L are −3.89, −0.82, and 1.05 kcal/mol for ligands L-I, L-II, and L-III, respectively. These results suggest that L-I and L-II exhibit a greater propensity to form 2:1 complexes with Pd(II) compared with L-III [32].

3. Materials and Methods

3.1. General

Chemical reagents used for ligand synthesis were purchased from Energy Chemical Co., Ltd. (analytically pure, Shanghai, China) and Sigma-Aldrich (Shanghai, China) in their purest forms. Metal nitrates in analytical purity utilized in extraction experiments were provided by Aladdin Industrial Corporation (Chengdu, China). Organic solvents employed in spectroscopic experiments were purchased from Chengdu Chron Chemicals Co., Ltd., Chengdu, China.
NMR experiments were carried out with a Bruker AVANCE AV II-400 MHz spectrometer (Bruker, Karlsruhe, Germany). UV-Vis data were collected on a SHIMADZU UV-2600i (Kyoto, Japan). The concentrations of metal ions were examined by a PerkinElmer ICP optima 8000 (Waltham, MA, USA). ESI-HRMS data were collected on a Bruker Daltonics Micro TOF-Q II (SHIMADZU, Kyoto, Japan).

3.2. Synthesis of Ligands

The ligands were synthesized through condensation reactions between N-ethyl-4-methylaniline and different monoacid precursors. In a general procedure, N-ethyl-4-methylaniline (1.00 g, 7.40 mmol), monoacids (7.40 mmol), EDCI (1.70 g, 8.88 mmol), HOBt (1.09 g, 8.88 mmol), and Et3N (0.899 g, 8.88 mmol) were mixed in 50 mL of DCM. The solution was refluxed for 24 h at 45 °C under Ar atmosphere and then was washed with water (50 mL × 3). The pure products were collected by fast column chromatography (DCM/MeOH, 30:1, v/v), and the detailed characterization data and figures are presented in Supplementary Materials.

3.3. Extraction Procedure

In solvent extraction experiments, the organic phase was prepared by dissolving extractants into 3-nitrobenzotrifluoride, which was selected as the diluent in this work due to its perfect solubility towards all these ligands and their extracted species. Before extraction, the organic phase was contacted with the HNO3 solution of corresponding concentrations with the ratio of 1:1 (v/v). The aqueous phase containing 1 mM of Pd(II) was prepared by dissolving Pd(NO3)2 in nitric acid (1–6 M). In a general extraction procedure, the organic and aqueous phases were mixed (0.5 mL/0.5 mL) and shaken vigorously for a required period of time. Then, the concentrations of Pd(II) ions in aqueous phases were tested by ICP-AES or ICP-MS. The values of extraction percentages (E%), distribution ratios (D), and SFPd/M of Pd(II) were obtained according to the Equations (1), (2), and (3), respectively:
E   % = c o r g . c t o t .
D = c o r g . c a q .
S F P d / M = D P d D M
where caq., corg., and ctot. stand for the concentration of Pd(II) ions in the aqueous phase, organic phase, and the initial Pd(II) concentrations in aqueous phase before extraction, respectively.

3.4. DFT Calculations

DFT calculations were conducted employing Gaussian 09 package [33]. Becke-Lee-Young-Parr [34,35] (B3LYP)/SDD-6-31G* was used for structural optimization in the gas phase, in which the 6-311++G** basis set was used for the C, H, O, and N, and the SDD basis set was used for Pd(II). Calculations were performed without any symmetry restrictions. The minimum energies of the optimized structures were confirmed by frequency calculations. The single-point energies were calculated in acetonitrile using the SMD solvation model at the B3LYP/SDD-6-311++G** level of theory. The binding energies were calculated as the differences in total energies of products and reactants. For the energy calculations of generated Pd2+ complexes, Pd(NO3)2(H2O)2 was used as the raw reagent as it is the main species in HNO3 solution (Figure S15). For the calculation of protonation energies, H3O+ was used as the raw reagent.

4. Conclusions

Three new nitrogen-based ligands were designed and synthesized for the efficient separation of palladium from HLLW. The electronegativity of nitrogen donor atoms was manipulated by introducing substituents with different electronic effects. The results from solvent extraction experiments indicate that anchoring an electron-donating group to the pyridyl group benefits the extraction of Pd(II) at low acidities but decreases its performance under highly acidic conditions, while reducing the electronegativity of the nitrogen donor by adding an electron-withdrawing group at the para position results in a poorer extraction at low acidities but much better performance at high acidities. These unique results implicate that the coordination strength between the donor atoms and metal ions dominates the extraction efficiency at low acidity, while the protonation of ligands becomes the main factor affecting the extraction performance at high acidity. Both 1:1 and 2:1 (L/Pd) complexes can be formed for each of the ligands as indicated by the Job plot method and ESI-HRMS. The single crystal structure of the Pd(NO3)2(L-II)2 complex reveals a four-coordination mode of Pd(II) with two ligands cooperatively coordinated with two monodentate nitrate anions in a quadrilateral fashion. DFT calculations demonstrate the preferential formation of 2:1 (L/Pd) complexes, and the stronger basicity of nitrogen atoms leads to a higher binding affinity but easier protonation, which are congruent with the results from solvent extraction experiments. This work provides several highly efficient ligands for palladium separation from HLLW, and more importantly, demonstrates the significant influence of the electronic effect on Pd(II) complexation and extraction.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30071533/s1: Figure S1: 1H NMR spectrum of L-I, Figure S2: 1H NMR spectrum of L-II, Figure S3: 1H NMR spectrum of L-III, Figure S4: 1H NMR spectrum of L-I after irradiation, Figure S5: 1H NMR spectrum of L-II after irradiation, Figure S6: 1H NMR spectrum of L-III after irradiation, Figure S7: 13C NMR spectrum of L-I, Figure S8: 13C NMR spectrum of L-II, Figure S9: 13C NMR spectrum of L-III, Figure S10: Pd(II) extraction percentage before and after irradiation, Figure S11: Extraction of Pd2+, Th4+, and UO22+, Figure S12: Extraction of different metal ions in simulated HLLW with L-II, Figure S13: Determination of the stability constants for (A) L-I; (B) L-II; (C) L-III, Figure S14: Structural optimization and natural charges on pyridine nitrogen atoms, Figure S15: Optimized geometries of (A) H2O, (B) H3O+, (C) NO3−, and (D) Pd(NO3)2(H2O)2 for binding energy calculations, Table S1: Concentrations of the metal elements in simulated HLLW (3 M HNO3), Table S2: Crystal data and structure refinements for Pd(NO3)2(L-II)2, Table S3: The distribution ratio to different ions with three ligands in simulated HLLW, and Table S4: The distribution ratio of Pd(II) with three ligands at different HNO3 acidities.

Author Contributions

Conceptualization, Y.C., X.L. and L.Y.; methodology, Y.G., Y.C. and S.H.; software, Y.G. and Y.C.; validation, Y.G. and Y.C.; formal analysis, Y.G., Y.C. and S.H.; investigation, Y.G. and Y.C.; resources, W.F., X.L. and L.Y.; data curation, Y.G. and Y.C.; writing—original draft preparation, Y.C.; writing—review and editing, X.L. and L.Y.; visualization, Y.G. and Y.C.; supervision, Y.C., W.F., X.L. and L.Y.; project administration, X.L. and L.Y.; funding acquisition, Y.C., W.F., X.L. and L.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22206138, 22176134), the China Postdoctoral Science Foundation (2022M712229), the Fundamental Research Funds for the Central Universities (2022SCU12123), the Sichuan University Postdoctoral Interdisciplinary Innovation Fund, the Foundation of Key Laboratory of Radiation Physics and Technology of the Ministry of Education (2021SCURPT04), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM2024037).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors are grateful to Pengchi Deng and Chen Yuan at the Analytical and Testing Center of Sichuan University, Xie Han from the Shiyanjia Lab (www.Shiyanjia.com), and Pall Thordarson from the University of New South Wales for providing an open-source website (http://app.supramolecular.org/bindfit/, accessed on 13 May 2024).

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Adamantiades, A.; Kessides, I. Nuclear Power for Sustainable Development: Current Status and Future Prospects. Energy Policy 2009, 37, 5149. [Google Scholar] [CrossRef]
  2. Naimoğlu, M. The Impact of Nuclear Energy Use, Energy Prices and Energy Imports on CO2 Emissions: Evidence from Energy Importer Emerging Economies Which Use Nuclear Energy. J. Clean. Prod. 2022, 373, 133937. [Google Scholar] [CrossRef]
  3. Ma, H.; Shen, M.; Tong, Y.; Wang, X. Radioactive Wastewater Treatment Technologies: A Review. Molecules 2023, 28, 1935. [Google Scholar] [CrossRef] [PubMed]
  4. Leoncini, A.; Huskens, J.; Verboom, W. Ligands for F-Element Extraction Used in the Nuclear Fuel Cycle. Chem. Soc. Rev. 2017, 46, 7229. [Google Scholar] [CrossRef]
  5. Zhang, H.; Li, A.; Li, K.; Wang, Z.; Xu, X.; Wang, Y.; Sheridan, M.V.; Hu, H.S.; Xu, C.; Alekseev, E.V.; et al. Ultrafiltration Separation of Am(VI)-Polyoxometalate from Lanthanides. Nature 2023, 616, 482. [Google Scholar] [CrossRef]
  6. Wang, Z.; Huang, L.; Dong, X.; Wu, T.; Qing, Q.; Chen, J.; Lu, Y.; Xu, C. Ion Sieving in Graphene Oxide Membrane Enables Efficient Actinides/Lanthanides Separation. Nat. Commun. 2023, 14, 261. [Google Scholar] [CrossRef]
  7. Yang, X.; Wu, W.; Xie, Y.; Hao, M.; Liu, X.; Chen, Z.; Yang, H.; Waterhouse, G.I.N.; Ma, S.; Wang, X. Modulating Anion Nanotraps via Halogenation for High-Efficiency 99TcO4−/ReO4− Removal under Wide-Ranging pH Conditions. Environ. Sci. Technol. 2023, 57, 10870. [Google Scholar] [CrossRef]
  8. Tian, P.; Ai, Z.; Hu, H.; Wang, M.; Li, Y.; Gao, X.; Qian, J.; Su, X.; Xiao, S.; Xu, H.; et al. Synthesis of Electron-Rich Porous Organic Polymers via Schiff-Base Chemistry for Efficient Iodine Capture. Molecules 2022, 27, 5161. [Google Scholar] [CrossRef]
  9. Bao, L.; Cai, Y.; Liu, Z.; Li, B.; Bian, Q.; Hu, B.; Wang, X. High Sorption and Selective Extraction of Actinides from Aqueous Solutions. Molecules 2021, 26, 7101. [Google Scholar] [CrossRef]
  10. Wang, Y.; Wu, Y.; Li, J.; Li, Q.; Yang, P.; Conradson, S.D.; Cai, Y.; Feng, W.; Yuan, L. Ultra-selective and efficient static/dynamic palladium capture from highly acidic solution with robust macrocycle-based polymers. Adv. Funct. Mater. 2023, 2304051. [Google Scholar] [CrossRef]
  11. Fujii, T.; Egusa, S.; Uehara, A.; Kirishima, A.; Yamagishi, I.; Morita, Y.; Yamana, H. Electronic absorption spectra of palladium(II) in concentrated nitric acid solutions. J. Radioanal. Nucl. Chem. 2011, 290, 475. [Google Scholar] [CrossRef]
  12. Wang, J.; Dong, G. Palladium/Norbornene Cooperative Catalysis. Chem Rev. 2019, 119, 7478. [Google Scholar] [CrossRef] [PubMed]
  13. Uehling, M.; King, R.; Krska, S.; Cernak, T.; Buchwald, S. Pharmaceutical Diversification via Palladium Oxidative Addition Complexes. Science 2019, 363, 405. [Google Scholar] [CrossRef] [PubMed]
  14. Barson, S.; Skeldon, P.; Thompson, G.; Piekoszewski, J.; Chemielewski, A.; Werner, Z.; Grötzschel, R.; Wieser, E. Corrosion Protection of Titanium by Pulsed Plasma Deposition of Palladium. Corros. Sci. 2000, 42, 1213. [Google Scholar] [CrossRef]
  15. Wang, Y.; Pang, J.; Cheng, Q.; Han, L.; Li, Y.; Meng, X.; Ibarlucea, B.; Zhao, H.; Yang, F.; Liu, H.; et al. Applications of 2D-Layered Palladium Diselenide and Its Van Der Waals Heterostructures in Electronics and Optoelectronics. Nano-Micro Lett. 2021, 13, 143. [Google Scholar] [CrossRef]
  16. Bai, Y.; Chen, L.; He, L.; Li, B.; Chen, L.; Wu, F.; Chen, L.; Zhang, M.; Liu, Z.; Chai, Z.; et al. Precise Recognition of Palladium through Interlaminar Chelation in a Covalent Organic Framework. Chem 2022, 8, 1442. [Google Scholar] [CrossRef]
  17. Parajuli, D.; Hirota, K.; Seko, N. Effective separation of palladium from simulated high level radioactive waste. J. Radioanal. Nucl. Chem. 2011, 288, 53. [Google Scholar] [CrossRef]
  18. Wang, Y.; Yang, Y.; Wu, Y.; Li, J.; Hu, B.; Cai, Y.; Yuan, L.; Feng, W. Selective Complexation and Separation of Uranium(VI) from Thorium(IV) with New Tetradentate N,O-Hybrid Diamide Ligands: Synthesis, Extraction, Spectroscopy, and Crystallographic Studies. Inorg. Chem. 2023, 62, 4922. [Google Scholar] [CrossRef]
  19. Bhattacharyya, A.; Egberink, R.J.M.; Mohapatra, P.K.; Verma, P.K.; Kanekar, A.S.; Yadav, A.K.; Jha, S.N.; Bhatacharyya, D.; Huskens, J.; Verboom, W. Remarkable Enhancement in Extraction of Trivalent F-Block Elements Using a Macrocyclic Ligand with Four Diglycolamide Arms: Synthesis, Extraction, and Spectroscopic and Density Functional Theory Studies. Inorg. Chem. 2019, 58, 14885. [Google Scholar] [CrossRef]
  20. Ruhela, R.; Singh, A.K.; Tomar, B.S.; Hubli, R.C. Separation of Palladium from High Level Liquid Waste—A Review. RSC Adv. 2014, 4, 24344. [Google Scholar] [CrossRef]
  21. Li, S.; Chu, N.; Li, X.; Dong, F.; Shen, Y. Recovery of Palladium from Acidic Nitrate Media with Triazole Type Extractants in Ionic Liquid. Hydrometallurgy 2019, 189, 105148. [Google Scholar] [CrossRef]
  22. Cai, Y.; Wang, M.; Yuan, L.; Feng, W. Complexation and Separation of Palladium with Tridentate 2,6-Bis-Triazolyl-Pyridine ligands: Synthesis, Solvent extraction, Spectroscopy, Crystallography, and DFT Calculations. Inorg. Chem. 2023, 62, 9168. [Google Scholar] [CrossRef] [PubMed]
  23. Courson, O.; Lebrun, M.; Malmbeck, R.; Pagliosa, G.; Römer, K.; Sätmark, B.; Glatz, J. Partitioning of Minor Actinides from HLLW Using the DIAMEX Process. Part 1—Demonstration of Extraction Performances and Hydraulic Behaviour of the Solvent in a Continuous Process. Radiochim. Acta 2000, 88, 857. [Google Scholar] [CrossRef]
  24. Zhu, Z.; Sasaki, Y.; Suzuki, H.; Suzuki, S.; Kimura, T. Cumulative Study on Solvent Extraction of Elements by N,N,N’,N’-Tetraoctyl-3-Oxapentanediamide (TODGA) from Nitric Acid into N-Dodecane. Anal. Chim. Acta 2004, 527, 163. [Google Scholar] [CrossRef]
  25. Rizvi, G.H.; Mathur, J.N.; Murali, M.S.; Iyer, R.H. Recovery of Fission Product Palladium from Acidic High Level Waste Solutions. Sep. Sci. Technol. 1996, 31, 1805. [Google Scholar] [CrossRef]
  26. Hudson, M.; Harwood, L.; Laventine, D.; Lewis, F. Use of Soft Heterocyclic N-Donor Ligands to Separate Actinides and Lanthanides. Inorg. Chem. 2013, 52, 3414. [Google Scholar] [CrossRef]
  27. Veliscek-Carolan, J. Separation of Actinides from Spent Nuclear Fuel: A Review. J. Hazard. Mater. 2016, 318, 266. [Google Scholar] [CrossRef]
  28. Xu, L.; Ding, W.; Zhang, A.; Liu, Z. Effect of Ligand Initial Conformation and Counteranion on Complexation Behaviors of R-BTBP toward Pd(II) Contained in Highly Active Liquid Waste. Chin. Chem. Lett. 2022, 33, 3565. [Google Scholar] [CrossRef]
  29. Bhattacharyya, A.; Leoncini, A.; Egberink, R.J.M.; Mohapatra, P.K.; Verma, P.K.; Kanekar, A.S.; Yadav, A.K.; Jha, S.N.; Bhattacharyya, D.; Huskens, J.; et al. First Report on the Complexation of Actinides and Lanthanides Using 2,2′,2″-(((1,4,7-Triazonane-1,4,7-Triyl)tris(2-Oxoethane-2,1-Diyl)) tris(oxy)) tris( N, N-Dioctylacetamide): Synthesis, Extraction, Luminescence, EXAFS, and DFT Studies. Inorg. Chem. 2018, 57, 12987. [Google Scholar] [CrossRef]
  30. Ansari, S.; Mohapatra, P.K.; Leoncini, A.; Ali, S.M.; Huskens, J.; Verboom, W. Highly Efficient N-Pivot Tripodal Diglycolamide Ligands for Trivalent F-Cations: Synthesis, Extraction, Spectroscopy, and Density Functional Theory Studies. Inorg. Chem. 2019, 58, 8633. [Google Scholar] [CrossRef]
  31. Sun, M.; Xu, L.; Yang, X.; Wang, S.; Lei, L.; Xiao, C. Complexation Behaviors of a Tridentate Phenanthroline Carboxamide Ligand with Trivalent F-Block Elements in Different Anion Systems: A Thermodynamic and Crystallographic Perspective. Inorg. Chem. 2022, 61, 2824. [Google Scholar] [CrossRef] [PubMed]
  32. Fleck, M.; Wieder, M.; Boresch, S. Dummy Atoms in Alchemical Free Energy Calculations. J. Chem. Theory Comput. 2021, 17, 4403. [Google Scholar] [CrossRef] [PubMed]
  33. Frisch, M.; Trucks, G.; Schlegel, H.; Scuseria, G.; Robb, M.; Cheeseman, J.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, USA, 2009. [Google Scholar]
  34. Becke, A.D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098. [Google Scholar] [CrossRef]
  35. Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785. [Google Scholar] [CrossRef]
Molecules 30 01533 g001
Figure 1. (A) Chemical structures of some reported oxygen, sulfur, and nitrogen donor-based ligands; (B) synthesis of three nitrogen-based ligands reported in this work.
Figure 1. (A) Chemical structures of some reported oxygen, sulfur, and nitrogen donor-based ligands; (B) synthesis of three nitrogen-based ligands reported in this work.
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Molecules 30 01533 g002
Figure 2. Extraction of Pd(II) by L-I, L-II, and L-III. (A) Extraction percentage as function of contact time ([HNO3] = 3 M, [L] = 2 Mm, [Pd(II)] = 1 mM, 25 °C); (B) extraction percentage of Pd(II) as function of HNO3 concentration ([L] = 2 mM, [Pd(II)] = 1 mM, 25 °C); and (C) loading capacities of Pd(II) ([HNO3] = 3 M, [L] = 2 mM, 25 °C). (D–F) Job plot analysis ([HNO3] = 3 M, [Pd(II)] + [L] = 2 mM, 25 °C). Without being specifically stated, organic phase is 3-nitrobenzotrifluoride containing corresponding ligands and aqueous phase is HNO3 solution with Pd(NO)2.
Figure 2. Extraction of Pd(II) by L-I, L-II, and L-III. (A) Extraction percentage as function of contact time ([HNO3] = 3 M, [L] = 2 Mm, [Pd(II)] = 1 mM, 25 °C); (B) extraction percentage of Pd(II) as function of HNO3 concentration ([L] = 2 mM, [Pd(II)] = 1 mM, 25 °C); and (C) loading capacities of Pd(II) ([HNO3] = 3 M, [L] = 2 mM, 25 °C). (D–F) Job plot analysis ([HNO3] = 3 M, [Pd(II)] + [L] = 2 mM, 25 °C). Without being specifically stated, organic phase is 3-nitrobenzotrifluoride containing corresponding ligands and aqueous phase is HNO3 solution with Pd(NO)2.
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Molecules 30 01533 g003
Figure 3. ESI-HRMS spectrum of L-II-loaded organic phase(3-nitrobenzotrifluoride) after extraction of Pd(II) ([L-II] = 2 mM, [Pd] = 1 mM, and [HNO3] = 3 M).
Figure 3. ESI-HRMS spectrum of L-II-loaded organic phase(3-nitrobenzotrifluoride) after extraction of Pd(II) ([L-II] = 2 mM, [Pd] = 1 mM, and [HNO3] = 3 M).
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Molecules 30 01533 g004
Figure 4. Extraction percentages and SFPd/M values of different ions in simulated HLLW. Organic phase was 2 mM ligand in 3-nitrobenzotrifluoride, and aqueous phase was 100 mg/L metal in 3 M HNO3. Forms of all ions used are as follows: Pd2+, Ag+, Al3+, Cd2+, Ce4+, Cr3+, Cs+, Cu2+, Fe3+, La3+, Mn2+, Na+, Ni2+, Ru3+, SeO42-, Sr2+, Y3+, and ZrO2+.
Figure 4. Extraction percentages and SFPd/M values of different ions in simulated HLLW. Organic phase was 2 mM ligand in 3-nitrobenzotrifluoride, and aqueous phase was 100 mg/L metal in 3 M HNO3. Forms of all ions used are as follows: Pd2+, Ag+, Al3+, Cd2+, Ce4+, Cr3+, Cs+, Cu2+, Fe3+, La3+, Mn2+, Na+, Ni2+, Ru3+, SeO42-, Sr2+, Y3+, and ZrO2+.
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Molecules 30 01533 g005
Figure 5. Job plot analysis of stoichiometric ratios between ligands and Pd(II) in acetonitrile at 298 K ([L] + [Pd2+] = 20 μM): (A) L-I, (B) L-II, and (C) L-III. Stacked UV-Vis titration spectra: (D) L-I, (E) L-II, and (F) L-III.
Figure 5. Job plot analysis of stoichiometric ratios between ligands and Pd(II) in acetonitrile at 298 K ([L] + [Pd2+] = 20 μM): (A) L-I, (B) L-II, and (C) L-III. Stacked UV-Vis titration spectra: (D) L-I, (E) L-II, and (F) L-III.
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Molecules 30 01533 g006
Figure 6. (A,B) X-ray crystal structure of Pd(NO3)2(L-II)2 from different views. (C) Packing structure of Pd(NO3)2(L-II)2. Gray, white, blue, red, and yellow represent C, H, N, O, and Pd atoms.
Figure 6. (A,B) X-ray crystal structure of Pd(NO3)2(L-II)2 from different views. (C) Packing structure of Pd(NO3)2(L-II)2. Gray, white, blue, red, and yellow represent C, H, N, O, and Pd atoms.
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Molecules 30 01533 g007
Figure 7. DFT optimized structures and binding energies of (A) Pd(NO3)2(L-I)2, (B) Pd(NO3)2(L-II)2, and (C) Pd(NO3)2(L-III)2. Optimized protonated structures of ligands and protonation energies of (D) L-I, (E) L-II, and (F) L-III. Optimized structures and binding energies of (G) Pd(NO3)2(L-I), (H) Pd(NO3)2(L-II), and (I) Pd(NO3)2(L-III).
Figure 7. DFT optimized structures and binding energies of (A) Pd(NO3)2(L-I)2, (B) Pd(NO3)2(L-II)2, and (C) Pd(NO3)2(L-III)2. Optimized protonated structures of ligands and protonation energies of (D) L-I, (E) L-II, and (F) L-III. Optimized structures and binding energies of (G) Pd(NO3)2(L-I), (H) Pd(NO3)2(L-II), and (I) Pd(NO3)2(L-III).
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Gan, Y.; Cai, Y.; Huang, S.; Li, X.; Feng, W.; Yuan, L. Manipulating Electronic Effect of Nitrogen Donor-Based Ligands for Efficient Complexation and Separation of Palladium from Highly Acidic Solution. Molecules 2025, 30, 1533. https://doi.org/10.3390/molecules30071533

AMA Style

Gan Y, Cai Y, Huang S, Li X, Feng W, Yuan L. Manipulating Electronic Effect of Nitrogen Donor-Based Ligands for Efficient Complexation and Separation of Palladium from Highly Acidic Solution. Molecules. 2025; 30(7):1533. https://doi.org/10.3390/molecules30071533

Chicago/Turabian Style

Gan, Yuyang, Yimin Cai, Song Huang, Xiaowei Li, Wen Feng, and Lihua Yuan. 2025. "Manipulating Electronic Effect of Nitrogen Donor-Based Ligands for Efficient Complexation and Separation of Palladium from Highly Acidic Solution" Molecules 30, no. 7: 1533. https://doi.org/10.3390/molecules30071533

APA Style

Gan, Y., Cai, Y., Huang, S., Li, X., Feng, W., & Yuan, L. (2025). Manipulating Electronic Effect of Nitrogen Donor-Based Ligands for Efficient Complexation and Separation of Palladium from Highly Acidic Solution. Molecules, 30(7), 1533. https://doi.org/10.3390/molecules30071533

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