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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

First Report on the Separation of Trivalent Lanthanides from Trivalent Actinides Using an Aqueous Soluble Multiple N‑Donor Ligand, 2,6-bis(1H‑tetrazol-5-yl)pyridine: Extraction, Spectroscopic, Structural, and Computational Studies Arunasis Bhattacharyya,*,†,# Trilochan Gadly,‡ Avinash S. Kanekar,† Sunil K. Ghosh,§,# Mukesh Kumar,∥,# and Prasanta K. Mohapatra*,†,# †

Radiochemistry Division, ‡Bio-Organic Division, §Food Technology Division, and ∥Radiation Biology and Health Sciences Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India # Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India S Supporting Information *

ABSTRACT: A terdentate multiple N donor ligand, 2,6-bis(1Htetrazol-5-yl)pyridine (H2BTzP), was synthesized, and its complexation with trivalent americium, neodymium, and europium was studied using single-crystal X-ray diffraction, attenuated total reflectance-fourrier transform infrared spectroscopy, time-resolved fluorescence spectroscopy, UV−vis absorption spectrophotometry. Higher complexation strength of BTzP toward trivalent actinide over lanthanides as observed from UV−vis spectrophotometric study resulted in an effective separation of Am3+ and Eu3+ in liquid−liquid extraction studies employing N,N,N′,N′-tetra-n-octyl diglycolamide in the presence of BTzP as the aqueous complexant. The selectivity of BTzP toward Am3+ over Eu3+ was further investigated by DFT computations, which indicated higher metal−ligand overlap in the Am3+ complex as indicated from the metal−nitrogen bond order and frontier molecular orbital analysis of the BTzP complexes of Am3+ and Eu3+.

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INTRODUCTION Separation of trivalent actinides and lanthanides is one of the most challenging issues encountered in the back end of the nuclear fuel cycle, as until today not a single ligand is available that possesses all the desirable properties required for process development. Over the years, a large number of S and N donor ligands have been explored for the Ln/An separation studies.1 S donor ligands, having very high selectivity for the trivalent actinides over the lanthanides, have limited stability under radiation and oxidizing conditions.2 Musikas et al. reported a higher complexation efficiency of an aqueous soluble N donor anion, namely, azide (N3−) for the Am3+ over the lanthanides.3 Choppin et al. have also investigated the lanthanide and actinide complexation with different aqueous soluble aminocarboxylates, and the actinide selectivity observed was attributed to the coordination of the N atoms of the aminocarboxylates.4 N donor heteropolycyclic ligands such as terpyridine (Figure 1a) were explored as the lipophilic actinide selective extractants for the separation of trivalent actinides and lanthanides in a synergistic combination with α-bromo-octanoic acid. A separation factor (SF = distribution ratio values of the metal ions DAm/DEu) value of 7.2 (Table S1) could be achieved using terpyridine-based extraction system.5 Increasing the © XXXX American Chemical Society

number of N atoms in the pyridine rings helped in enhancing the actinide selectivity of this class of ligands. Tri-(2,4,6-(2pyridyl))-l,3,5-triazine (TPTZ; Figure 1b), where the central pyridine ring is replaced by a 1,3,5-triazine ring, showed an SF of 10.1 for Am3+ over Eu3+ in a synergistic combination with αbromo-decanoic acid.6 Replacement of one of the two lateral pyridine rings of terpyridine by a 1,2,4-triazine ring, namely, 1,2,4-triazinyl bipyridine (TBipy; Figure 1c), also resulted in an enhancement of separation factor to ∼28.7 Bis(1,2,4-triazinyl) pyridine (BTP; Figure 1d), where both the lateral pyridine units of terpyridine were replaced with 1,2,4-triazine units, showed the SF value as high as greater than 100 for Am3+ over Eu3+.8 Another class of heteropolycyclic N donor ligands have also been exhaustively studied for the separation of trivalent actinides and lanthanides, where both the lateral pyridine rings in terpyridine were replaced with the five-membered di- or triazole rings.9 In the case of 2,6-bis(imidazolyl) pyridine (Figure 1e), where the two N atoms in the five-membered rings are present in the 1,3-positions, poor selectivity (SF < 10) was reported.10 Another lipophilic bis-diazolylpyridine derivative Received: January 18, 2018

A

DOI: 10.1021/acs.inorgchem.8b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic structures of various N donor ligands evaluated for Ln/An separation studies: (a) terpyridine; (b) TPTZ; (c) TBipy; (d) BTP; (e) 2,6-bis(imidazolyl) pyridine; (f) 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine; (g) 2,6-bis(1,2,4-triazolyl) pyridine; (h) 2,6-bis(1,2,3triazolyl) pyridine; (i) 6-(tetrazol-5-yl)-2,2′-bipyridine; (j) BTzP.

of the ligand both for Am3+ and Eu3+.13 Higher selectivity for the trivalent actinides over the lanthanides is expected if both the lateral pyridine units of terpyridine molecule can be replaced by the tetrazolyl moiety, 2,6-bis (tetrazol-5-yl) pyridine (BTzP; Figure 1j). BTzP and its derivatives were explored earlier to sensitize the luminescence of the lanthanides by making their complexes, where very high quantum yields of the metal-centered luminescence in the Eu 3+ and Tb3+ complexes of BTzP derivatives were reported.14 Smirnov et al. prepared the aryl derivative of BTzP to make it lipophilic and used it for the separation study of trivalent actinides and lanthanides in a synergistic combination with chlorinated cobalt dicarbollide.15 The drawback of this extraction system was again their poor solubility in a suitable diluent, and hence, they performed all of their studies in fluorinated solvents, namely, mnitrobenzotrifluoride and phenyl trifluoromethyl sulfone (also known as FS-13). The lipophilic 2,6-bis(1,2,4-triazin-5-yl) pyridine/bipyridine/phenanthroline (BTP, BTBP, or BTPhen) derivatives show promising selectivity for the trivalent actinides over the lanthanides,11,16 but the major drawback in their use is again their poor solubility in suitable organic solvents, namely, n-dodecane and kerosene. This limitation was overcome by attaching multiple hydrophilic substitution, namely, sulfonic acid or alcoholic −OH moieties at the appropriate positions in the BTP, BTBP, or BTPhen derivatives making them aqueous soluble. The marginal selectivity of diglycolamide (DGA)-based ligands for Eu3+ over Am3+ was enhanced further by selectively holding the Am3+ ion in the aqueous phase employing these

2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine (Figure 1f) containing two N atoms at the adjacent positions in the lateral five-membered rings appeared promising with a separation factor for Am3+ over Eu3+ in the range of 100. However, the requirement of lipophilic counteranion, namely, α-bromo-octanoic acid, could not be avoided here.9a The presence of two adjacent N atoms in the two lateral fivemembered heterocyclic rings is probably the prerequisite to achieve high selectivity for the trivalent actinides over the lanthanides. Two different bis-triazolylpyridine varying the position of the N atoms in the triazole ring, namely, 2,6bis(1,2,4-triazolyl) pyridine9b (Figure 1g) and 2,6-bis(1,2,3triazolyl) pyridine11 (Figure 1h) derivatives were evaluated for the separation of trivalent actinides and lanthanides, and higher selectivity was noticed in the case of the 1,2,3-triazolyl derivative, where all the three N atoms in the triazole ring were present adjacent to each other. Efforts were also made to substitute one of the two lateral pyridine rings of the terpyridine with the tetrazole ring, namely, 6-(tetrazol-5-yl)2,2′-bipyridine12 (Figure 1i), where the stability constant value for Cm3+ was reported to be higher than that of Eu3+ by 2 orders of magnitude from the luminescence studies. The drawback of this ligand, however, was its poor solubility in the solvents to be used for Ln/An separations, and therefore, no two-phase liquid−liquid extraction data are reported with this ligand. To enhance the lipophilicity, they further modified the ligand by substituting one of the protons in the pyridine rings by a tert-butyl group, but that resulted into poor extractability B


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Inorganic Chemistry

Figure 2. Absorption spectra of Am3+ (a) and Nd3+ (c) upon successive addition of BTzP and molar absorptivity of the Am3+ (b) and Nd3+ (d) complexes of BTzP as calculated by fitting the absorption spectrophotometric titration data in methanol/water (9:1) mixture at pH 4 containing 0.1 M NaNO3. vacuum, which gave the desired white solid 0.95 g (58%). 1H NMR (500 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): δ ppm 8.34 (3H, br s) (Figure S1 in the Supporting Information). Reagents and Chemicals. N,N,N′,N′-Tetra-n-octyl diglycolamide (TODGA) was procured from Thermax Ltd. and used after checking for purities by 1H NMR, high-resolution mass spectrometry (HR-MS), and high-performance liquid chromatography (HPLC). n-Dodecane and isodecanol (SD Fine chemicals Pvt. Ltd.) were used as procured. Suprapur nitric acid (Merck) was used for preparing the dilute acid solutions, which was done with Milli-Q (Millipore) water. All the other reagents were of analytical reagent (AR) grade. Laboratory stock solutions of 241Am tracer were used, while 152,154Eu was procured from Board of Radiation and Isotope Technology (BRIT), Mumbai. All the radiotracers were used after checking their radiochemical purities by α/γ ray spectrometry. Single-Crystal X-ray Diffraction Study. For the structural analysis the single crystals of Nd 3 + complex of BTzP (C21H41N27Nd2O16) were grown in methanol/water medium devoid of any other salt or foreign reagents (i.e., sulphanilic acid or NaNO3 used in solvent extraction and complexation studies) to allow the growing the pure crystals of the complex of diffraction quality. A suitable crystal was selected and mounted on a SuperNova, Single source at offset, Titan diffractometer. The crystal was kept at 293(2) K during data collection. The structure was solved with the olex2 structure solution program using Charge Flipping18 and refined with the ShelXL19 refinement package using least squares minimization. FTIR Study. Infrared spectra of BTzP and its Nd3+ complexes were recorded in the solid state in argon atmosphere using the Jasco FTIR

aqueous soluble BTP, BTBP, or BTPhen derivatives resulting in SF values higher than 100.11,17 However, the aqueous soluble BTzP has, to our knowledge, never been used for the separation of trivalent lanthanide/actinide ions making a strong case for such investigations. The present work, on the separation of Am3+ and Eu3+ employing unsubstituted BTzP molecule (Figure 1j, R = H) as the actinide selective aqueous complexant, where Eu3+ was selectively extracted using diglycolamides, is the first of its kind, and the results appear highly encouraging. The complexation behavior of Am3+ and different lanthanides with BTzP were also performed systematically using UV−vis absorption, luminescence, Fourier transformed infrared (FTIR) spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT)-based calculations.

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EXPERIMENTAL SECTION

Synthesis of 2,6-Bis(1H-tetrazol-5-yl)pyridine (BTzP). BTzP was synthesized as per the method described in the literature,14a where a solution of pyridine-2,6-dicarbonitrile (0.1 g, 0.77 mmol), sodium azide (0.376 g, 5.8 mmol), and ammonium chloride (0.310 g, 5.8 mmol) in dimethylformamide (DMF; 35 mL) was stirred under argon at 125 °C for 24 h. After it cooled, the reaction mixture was filtered to remove inorganic salts. DMF was removed under reduced pressure, and subsequently dilute hydrochloric acid (0.1 M, 5 mL) was added. The resulting suspension was stirred for 1 h and refrigerated. The precipitate was filtered and washed with cold water and dried under C


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Inorganic Chemistry

Table 1. Extraction of Am3+ and Eu3+ Using 0.05 M TODGA in Different Diluents from 0.02 M Sulphanilic Acid at pH 4 Containing 1 M NaNO3 in the Absence and Presence of 1.4 mM BTzP in the absence of BTzP diluent

a

DAm

in the presence of BTzP

DEu

SF

n-dodecane

6.9 ± 0.3

30.3 ± 0.8

4.4 ± 0.2

CHCl3 octanol xylene

0.50 ± 0.03 0.70 ± 0.04 10.1 ± 0.2

0.60 ± 0.02 5.2 ± 0.1 12.7 ± 0.3

1.2 ± 0.1 7.4 ± 0.4 1.26 ± 0.04

0.006 0.054 0.001 0.049 0.002

DAm

DEu

± ± ± ± ±

0.37 ± 0.04 3.97 ± 0.07a 0.03 ± 0.002 2.69 ± 0.1 0.050 ± 0.007

0.001 0.001a 0.0004 0.007 0.0004

SF 62 74 30 55 25

± ± ± ± ±

12 2a 12 8 6

Organic phase: 0.1 M TODGA in n-dodecane; aqueous phase: 0.9 mM BTzP at pH 9 containing 1 M NaNO3.

spectrophotometer model FT/IR 6300 with attached single reflectance attenuated total reflectance (ATR) model ATR PRO450-S. UV−Vis Spectrophotometric Study. To study the Am3+ and Nd3+ complexation, f→f transition peak of Am3+ and Nd3+ at 506 and 580 nm, respectively, were monitored by incremental addition of the BTzP (2.5 and 44 mM for the titration of Am3+ and Nd3+, respectively; Figure 2) to a 2 mL solution of 0.2 mM 241Am3+ and 10 mM Nd3+ in methanol/water (9:1) mixture at pH 4 and fixed ionic strength of 0.1 M NaNO3. The detailed experimental conditions and methodology used for the calculation of conditional stability constant values is provided in the Supporting Information. TRFS Study. All the luminescence studies on Eu3+ complexation with BTzP both in solution and solid phase were performed by a Horiba PTI Quantamaster (QM 400) series of steady-state and lifetime spectrofluorometer. To determine the conditional stability constants for the Eu3+ complexes of BTzP in the solution phase, the titration of 2 mL volume of 0.1 mM of Eu(NO3)3 in methanol/water (9:1) mixture at pH 4 and fixed ionic strength of 0.1 M NaNO3 was performed with incremental addition of 1 mM BTzP solution in the same medium at pH 4, where the change in the emission spectra of Eu3+ ion due to the electric dipole 5D0−7F4 transition in the wavelength range of 675−710 nm upon complexation was followed in a time-resolved fluorescence spectroscopic study (Figure S2a in the Supporting Information). In the titration study the ligand-to-metal ratio was varied from 0 to 10, until the change in the luminescence spectra became negligible with addition of further BTzP solution. Repeated attempts were made to grow the single crystals of Eu3+ complex of BTzP following the similar procedure as the Nd3+ complex; however, the crystals were not up to the grade for the single-crystal XRD studies. Time-resolved fluorescence spectroscopy (TRFS) studies were therefore performed with the solid Eu3+ complex of BTzP. For the luminescence study in the solid phase, the crystals of the Eu3+ complex of BTzP were mounted on the sample chamber using a quartz slide with the geometry adjusted to maximize the signalto-noise ratio. Distribution Measurements. Equal volumes (usually 0.5 mL) of the aqueous mixture containing 1.4 mM BTzP at pH 4 using 0.02 M sulphanilic acid buffer or pH 9 solution containing 0.9 mM BTzP in the presence of 1 M NaNO3 and the respective radiotracer, 241Am (∼8 kBq/ml) or 152,154Eu (∼6 kBq/ml)) and the organic (containing 0.05 or 0.1 M TODGA in different diluents) phases were equilibrated in a thermostated water bath at 25 ± 1 °C for 1 h to enable complete attainment of equilibrium and were subsequently centrifuged, and suitable aliquots (100 μL) from both phases were removed and assayed radiometrically using a well-type NaI(Tl) scintillation counter (Para Electronics) coupled with a multichannel analyzer (ECIL). Sulphanilic acid is used as a buffer in the extraction experiments at pH 4, as this region of pH is quite sensitive, and in the presence of sulphanilic acid only the pH could be maintained at pH 4 after equilibration. Moreover, another reason for choosing sulphanilic acid as the buffering agent was its noncomplexing nature with trivalent actinides and lanthanides as indicated from our previous study.20 The distribution ratio values of the metal ions (DM) were calculated as the ratio of counts per unit time per unit volume of the organic phase to that in the aqueous phase. The concentrations of Am and Eu used in these studies were 1 × 10−7 and 1 × 10−5 M, respectively. Each

experiment was performed in triplicate, and the accepted data were within the relative standard deviation of 5%. Computational Methodology. Geometry Optimization and Molecular Orbital Analysis. The hybrid B3LYP (Becke’s threeparameter nonlocal hybrid exchange correlation functional) functional was reported to be suitable for the f-block metal ion complexes to predict the geometry, energetic, and the nature of bonding because of the inclusion of the nonlocal Hartree−Fock contribution in the exchange functional.21 Single-crystal XRD showed the ion pair type of complex for Nd3+ complex, which was further supplemented in case of the Eu3+ complex from the luminescence study. In case of computational study, therefore, the structure of the cationic and anionic part of the Nd3+, Eu3+, and Am3+ complexes as observed for the Nd3+ complex was taken as the guess structure and allowed to relax. The minimum-energy structures of the cationic and anionic parts of the Am3+, Nd3+, and Eu3+ complexes of BTzP were, therefore, calculated using the B3LYP functional employing the split valence plus polarization (SVP) basis set22 as available in the Turbomole 7.0 suite of program.23 In the case of all the Ln3+ and Am3+ an effective core potential (ECP) core potential was used, 28 electrons in the core of lanthanides (i.e., Nd and Eu),24 whereas the number of core electrons was 60 for Am.25 In case of Am3+ and Eu3+, the high-spin septet was found to be the ground-state configuration, whereas for Nd3+ complex quartet was the ground-state configuration. In case of all the complexes, the value of ⟨S2⟩ was found to be very close to the (S + 1) ideal values, which indicated negligible spin contamination for all the complexes. Calculation of Vibrational Frequencies. Vibrational frequencies of the Nd3+ complex of BTzP were calculated at the B3LYP level of theory at a fixed geometry observed from the SCXRD study using defTZVP basis set for Nd3+ along with small core (28 electrons) ECP. All the other elements present in the complex were treated at the allelectron level using def-TZVP basis set. The vibrational spectra generated were compared with the experimentally observed spectra of the Nd3+-BTzP complex.

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RESULTS AND DISCUSSION

Solvent Extraction Study. Diglycolamide-based extractants, namely, TODGA is reported to show marginal selectivity toward Eu3+ over Am3+ with separation factor values ranging between 1 and 10 depending upon the experimental conditions.26 The selectivity of BTzP can be evaluated by comparing the extraction behavior of Am3+ and Eu3+ by TODGA in absence vis-à-vis presence of BTzP in the aqueous phase. Solvent-extraction studies of Am3+ and Eu3+ were, therefore, performed using TODGA as the organic extractant in the absence and presence of BTzP, which acted as the aqueous complexing agent. Studies were further performed to check whether the selectivity can be improved by fine-tuning the nature of the organic diluent. Of the various diluents evaluated, the result was found to be most promising for n-dodecane, where the extraction of both Am3+ and Eu3+ decreased by orders of magnitude in the presence of BTzP in the aqueous phase (Table 1). This decrease was, however, much more D


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Inorganic Chemistry significant in the case of Am3+ as compared to that obtained with Eu3+ resulting in a significant enhancement in the separation factor (SF = DEu/DAm) from 4.4 to 62 when the extraction was performed from aqueous phase at a pH value of 4. The experiment was further performed at an elevated pH (pH 9) of the aqueous phase, where both the protons of the two tetrazole rings of BTzP are expected to be dissociated (pKa1 = 1.5 and pKa2 = 7.0) resulting in a stronger complexation of BTzP and enhancement in the selectivity (SF). TODGA concentration in the organic phase when increased from 0.05 to 0.1 M the SF value was found to be 74, and this appeared quite promising as far as Ln/An separation is concerned. Extraction capabilities of Am3+ and Eu3+ can be further fine-tuned by varying the TODGA and BTzP concentrations. However, this was considered beyond the scope of this work, which focused on other aspects such as determination of aqueous complexation constants and to give a logical explanation to the increasing SF values. Complexation Study of Am3+, Nd3+, and Eu3+ with BTzP in Solution Phase. Solvent-extraction studies showed the selectivity of BTzP for trivalent actinides (i.e., Am3+) over the lanthanides (i.e., Eu3+). It was, therefore, of interest to compare the complexation behavior of the trivalent actinides vis-à-vis lanthanides. UV−Vis spectrophotometric titration studies were performed to get the quantitative and comparative complexation formation constants of trivalent actinides/ lanthanides with BTzP. Both Am3+ and Nd3+ are known to show the absorption bands centered at ∼505 and ∼580 nm, respectively, due to the f→f transitions, which are affected by the ligand field. A number of literature reports are available, where the changes in these peaks of Am3+ and Nd3+ were followed to understand their complexation behavior.27 The change in these absorption peaks during the complexation with BTzP was, therefore, followed to calculate the conditional stability constants of the Am3+ and Nd3+ complexes of BTzP in methanol/water (9:1) mixture at pH 4 at a fixed ionic strength of 0.1 M NaNO3 (Figure 2). The results are given in Table 2, which clearly shows that both Am3+ and Nd3+ form ML and ML3 types of complexes with BTzP. Am3+ forms stronger complex than Nd3+ indicating the selectivity of BTzP toward the trivalent actinide ion over the trivalent lanthanide ion. Eu3+ complexation with BTzP was also studied by following the change in the emission spectra of Eu3+ ion upon complexation in the wavelength range of 675−710 nm (Figure S2a in the Supporting Information) in a TRFS study. The results, here, also indicate the formation of ML and ML3 types of complexes similar to that observed in the cases of Am3+ and Nd3+ from the UV−vis absorption spectrophotometric titrations. The luminescence spectra of the different species (Eu3+, EuL, and EuL3) present during the course of the titration of Eu3+ with BTzP obtained by fitting the titration data are shown in Figure S2b in the Supporting Information. The excitation and emission spectra of the Eu3+ complex of BTzP at a ligand-to-metal ratio of 10 in methanol/water (9:1) mixture at pH 4 and fixed ioinic strength of 0.1 M are shown in Figures S3 and S4 in the Supporting Information. The number and nature of species formed during the course of the titration of Eu3+ with BTzP was further confirmed from the lifetime measurement at different metal-to-ligand ratios during the titration (Figure S5 and Table S2 in the Supporting Information), which indicates the formation ML type of complex with the lifetime values of 370−400 μs, whereas the other species (ML3), formed at higher BTzP concentration, has a lifetime value of greater than

Table 2. Conditional Stability Constants of the Trivalent Actinides and Lanthanide Complexes of BTzP and Analogous Ligands at 298 K ligand Figure 1g (R = -C6H4− CH3)

medium acetonitrile/ water (95:5)

metal

species

log β

Cm3+

ML2

9.7 ± 0.2

31

ML3 ML ML3 ML

14.0 ± 0.3 2.8 ± 0.1 10.3 ± 0.2 3.5 ± 0.3

11

ML2 ML ML ML2 ML3 ML ML2 ML3 ML

6.8 2.4 3.2 6.6 9.7 2.3 4.8 7.3 1.7

± ± ± ± ± ± ± ± ±

0.3 0.1 0.2 0.2 0.3 0.3 0.3 0.4 0.2

ML2 ML3 ML ML2 ML3 ML2

4.0 5.7 0.9 2.1 3.7 9.3

± ± ± ± ± ±

0.2 0.3 0.4 0.5 0.3 0.2

ML3 ML ML3 ML

13.8 ± 0.3 4.0 ± 0.1 11.1 ± 0.2 5.4 ± 0.3

ML3 ML ML3 ML ML3

14.1 ± 0.2 4.7 ± 0.2 10.6 ± 0.3 5.3 ± 0.2 12.8 ± 0.1

Eu3+ Figure 1h (R = −CH2− CH2− CH2−OH)

methanol/ water (3:1)

1 mM HClO4

Am3+

Eu3+ Cm3+

Eu3+

0.44 M HNO3

Cm3+

Eu3+

Figure 1i (R = H)

ethanol/water (95.6:4.4)

Cm3+

Eu3+ BTzP (Figure 1j; R = H)

methanol/ water (9:1)

Am3+

Nd3+ Eu3+

ref

30

12

present work

3 ms. Wartenberg et al.28 reported a lifetime value of 2.37 ms for the ML3 type of Eu3+ complex of BTzP in aqueous medium. The higher lifetime value observed in the present work for ML3 type of complex could be due to the presence of methanol as the major solvent (90%). The conditional stability constant value of ML type of species is comparable for Am3+ and Eu3+, whereas that of ML3 type of complex is higher for Am3+ (log β3 = 14.1) than that for Eu3+ (log β3 = 12.8) with a selectivity ratio (β3(Am)/β3(Eu)) of 20. The observed selectivity in the solvent extraction studies in the presence of BTzP in the aqueous phase is a combined effect of the selectivity of TODGA alone for Eu3+ over Am3+ and that of BTzP for Am3+ over Eu3+. The selectivity of DGA derivative itself is very difficult to understand. Singlephase complexation study with tetramethyl diglycolamides (TMDGA) derivative in aqueous medium indicates the formation of stronger complexes for Am3+ than that for Eu3+,29 whereas the solvent-extraction studies indicate the selectivity of DGA derivatives for Eu3+ over Am3+.26 Several reports are available on the complexation of Am3+, Cm3+, and Eu3+ with different 2,6-bis(1,2,3-triazolyl)pyridine (BTTP) derivatives in different solvent medium. In all the cases, however, higher stability constant values are reported for the E


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Inorganic Chemistry

Figure 3. Experimental FTIR spectra of H2BTzP and Nd3+ complex of BTzP in the solid state.

Single-Crystal XRD Study. The structure of the Nd3+ complex of BTzP was elucidated by single-crystal XRD technique, and all the crystallographic parameters are listed in Table 3. All the bond distances in the Nd3+ complex of BTzP are listed in Table S3. The structure of the Nd3+ complex determined by analyzing the crystallographic data (Figures 4 and S6) indicates that the complexed species constitute a cationic part, Nd(BTzP)+ (a BTzP anion having doubly negative charge coordinates to the central Nd3+ ion, and other six coordination sites of Nd3+ are saturated by the water

trivalent actinides over the trivalent lanthanides. In the case of substituted BTTP ligands having alcoholic OH group, Am3+ formed ML2 type of species, whereas Eu3+ appeared not to form analogous species indicating the high selectivity of this ligand for trivalent actinides over the lanthanides, and this selectivity was reflected in the solvent-extraction results.11 Whereas in another literature, the formation of all three ML, ML2, and ML3 types of complexes was reported for both Cm3+ and Eu3+ with the same ligand in perchlorate as well as nitrate media. Formation of ML3 type of species helps in enhancing the selectivity in the two-phase extraction process. Higher separation factor achieved in perchloric acid medium as compared to that in nitric acid medium was due to the presence of 1:3 complex in higher extent in the perchloric acid medium.30 In the present work in spite of small difference in conditional stability constant values for the 1:3 complex of Am3+ and Eu3+ with BTzP (Δlog β(Am−Eu) = 1.3) as compared to that of Cm3+ and Eu3+ reported with BTTP derivative (Figure 1h (R = −CH2−CH2−CH2−OH)) (Δlog β(Cm‑Eu) = 2.0), an SF value greater than 60 could be achieved due to the formation of 1:3 complex in higher extent because of the higher log β values in case of BTzP (Table 2). This clearly supports the proposition by Wagner et al. that the formation of 1:3 complex with the aqueous soluble ligands in higher proportion helps in enhancement in selectivity in two-phase extraction process.30 6-(Tetrazol-5-yl)-2,2′-bipyridine (Figure 1i), where one tetrazole ring is present, formed ML2 and ML3 types of complex with Cm3+, whereas with Eu3+ ML and ML3 types of complex were observed12 which is similar to our results with BTzP. Complexation Studies of Nd3+ and Eu3+ with BTzP in Solid Phase. ATR-FTIR Study. Nd3+ complex of BTzP was synthesized in methanol/water (9:1) mixture. IR spectra of the protonated form of BTzP (H2BTzP) and its Nd3+ complex were recorded to further understand the complexation behavior of BTzP (Figure 3). In the H2BTzP molecule a sharp peak was observed at 1454 cm−1, which shifted to the lower wavenumber (1436 and 1421 cm−1) upon complexation with Nd3+. The change in ATR-FTIR spectra of H2BTzP on complexation with Nd3+ was interpreted with the help of DFT calculations, and this is discussed in the Computational Study section.

Table 3. Crystal Data and Structure Refinement Parameters for the Nd3+ Complex of BTzP empirical formula formula weight temperature, K crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg volume, Å3 Z ρcalc, g/cm3 μ, mm−1 F(000) crystal size, mm3 radiation 2Θ range for data collection, deg index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] F

C21H27N27Nd2O9 1090.17 293 triclinic P1̅ 12.6011(6) 12.9914(6) 16.1489(8) 98.184(4) 109.186(4) 97.149(4) 2429.6(2) 2 1.490 16.703 1068.0 0.854 × 0.1938 × 0.1459 Cu Kα (λ = 1.541 84) 5.908 to 140.11 −8 ≤ h ≤ 15, −14 ≤ k ≤ 15, −19 ≤ l ≤ 17 15 709 9071 [Rint = 0.1107, Rsigma = 0.1134] 9071/0/542 1.126 R1 = 0.1123, wR2 = 0.2922 R1 = 0.1337, wR2 = 0.3282 DOI: 10.1021/acs.inorgchem.8b00142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Structure of Nd3+ complex of BTzP determined by solving the SCXRD data.

molecules), and an anionic part, Nd(BTzP)2−, where two BTzP anions are coordinating to the Nd3+ ion (the remaining three coordination sites of Nd3+ are satisfied by three water molecules). The structural parameters of the Nd3+ complex are listed in Table 4, and this shows that the distances between

water (9:1) mixture similar to the procedure followed to synthesize the Nd3+ complex of BTzP. Even after repeated attempts, diffraction-quality single crystals of the Eu3+ complex of BTzP could not be isolated, and therefore single-crystal XRD study could not be performed with the Eu3+ complex. TRFS study was, however, performed with the solid Eu3+ complex of BTzP. The excitation spectra at a fixed emission wavelength of 616 nm (Figure 5) show that the most intense peak is present

Table 4. Structural Parameters in the Nd3+ Complex of BTzP Determined from SCXRD Study distance/angle/

cationic part

anionic part

torsional angle

Lig 1

Lig 2

Lig 3

Nd−Nt Nd−Np Nt−Np Nt−Nt Nt−Nd−Np Nd−Nt−Np−Nt Ndcat−Ndan

2.600, 2.643 2.759 2.777, 2.737 4.608 62.33, 60.84 4.47 9.181

2.573, 2.613 2.668 2.699, 2.712 4.557 61.96, 61.80 9.12

2.598, 2.616 2.692 2.723, 2.742 4.606 62.05, 62.20 3.28

the Nd3+ ion and the coordinating N atoms of the central pyridine ring (Np) are always longer than those of the lateral triazine rings (Nt) irrespective of whether it is in the cationic or anionic part of the complex. The Nd−Np distances are shorter in the anionic part of the complex as compared to the cationic part. There are three binegative BTzP ligands coordinating to two Nd3+ ions in the overall complex. Of these three BTzP ligands, the one (Lig 1) that is present in the cationic part asymmetrically binds to the Nd3+ with Nd−Nt distances of 2.600 and 2.643 Å. Similar asymmetric binding is also observed in case of one of the two ligands (Lig 2) in anionic part with the Nd−Nt distances of 2.573 and 2.613 Å. The other BTzP ligand (Lig 3) in the anionic part, however, binds more symmetrically with the Nd−Nt distances of 2.598 and 2.616 Å. The Nd−Np distance suggests that the Lig 2 in the anionic part approached closer to the Nd3+ ion as compared to other two ligands (Lig 1 and Lig 3) in the complex. The closer approach of Lig 2 resulted in more shifting from the coplanarity in the three coordinating N atoms and the Nd3+ ion as indicated by the higher torsional angle of Nd−Nt−Np−Nt (see Figure S7 in the Supporting Information). The two Nd3+ ions of the cationic and anionic parts are present at a distance of 9.185 Å. Time-Resolved Fluorescence Studies of Eu3+ Complex of BTzP. Eu3+ complex of BTzP was synthesized in methanol/

Figure 5. Excitation and emission spectra of Eu3+ complex of BTzP in the solid state.

at 320 nm due to ligand-to-metal charge transfer; another small charge transfer peak was also noticed at a wavelength of 267 nm. The Eu3+-centered f−f transition peaks were found to be very weak as compared to the ligand-to-metal charge transfer bands indicating high degree of ligand sensitization in the luminescence spectra of the Eu3+ complex of BTzP. The emission spectra (Figure 5) at fixed excitation wavelength of 320 nm indicates the presence of a small peak at 577 nm due to the 5D0−7F0 transition suggesting poor symmetry (C2, C4, or C4v symmetry) around the Eu3+ ion in its solid complex.32 The luminescence decay of the Eu3+ ion in its solid complex with BTzP at an emission wavelength of 616 nm was followed at two excitation wavelengths of 267 and 320 nm (Figure 6). The decay profile of the Eu3+ luminescence indicated the presence of two lifetime values of 479 ± 2 and 1841 ± 3 μs. We G


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Inorganic Chemistry

note here that in the anionic part, in the equilibrium geometry, one of the three water molecules, bonded to the metal ion, is dissociated and connected to other two metal-coordinated water and BTzP moieties through hydrogen bonding, and the metal ion became eight coordinated. Metal−nitrogen bond distances and bond orders in the cationic and anionic parts of all the three complexes are listed in Table 5. The results showed that in the cases of all the complexes the M−Nt bonds are significantly shorter in the cationic part as compared to that in the anionic part, which is in contrast with the observed results from the SCXRD study. This could be due to the consideration of the isolated complexes in the DFT calculations instead of the periodic array of the cationic and anionic units in the solid Nd3+ complex of BTzP. However, from the point of view of the understanding the bonding and interactions of the actinide and lanthanide ions with the BTzP units, computations of the isolated complexes will be helpful. In the cationic part, the M−Nt bond distances are shorter in the Eu3+ complex, which is due to the smaller ionic radius of Eu3+ as compared to the other two metal ions studied in the present work. If one compares the Am3+ and Nd3+ complexes, the M−Nt bond distances are shorter in the case of former in spite of comparable ionic radii of Am3+ and Nd3+ for a fixed coordination number.33 This indicates stronger metal−nitrogen interaction in the cationic part of the Am3+ complex, and this is also supported by the Mayer’s bond order, which shows higher bond order in the Am−Nt bonds as compared to the Nd−Nt or Eu−Nt bonds. Similar observation was also noticed in case of M−Np bonds in the cationic part. In the anionic part, however, no such increase in the M−N bond strength in the Am3+ complex was noticed. The metal−nitrogen bond orders were found to be higher anonymously in the cationic part as compared to that in the anionic part. This could be due to the sharing of the metal-based orbitals with two doubly negative BTzP molecules in the anioic part, whereas in the cationic part the metal orbitals are shared with only one BTzP molecule other than the neutral water molecules. Molecular Orbital Analysis. Frontier molecular orbitals (FMOs) generated by the DFT calculation were analyzed to further understand the nature of bonding in the Am3+ and Eu3+ complexes of BTzP. The orbitals that have contributions from both the metal- and the ligand (BTzP)-based orbitals have influence in the metal−ligand bonding. The lowest unoccupied molecular orbitals (LUMO) of both the metal ions have contributions from the metal ion and the ligand molecules (BTzP) in the cationic as well as anionic parts (Figure 8). In the LUMO of both the cationic and anionic parts of the BTzP complexes of Am3+ and Eu3+, σ-bonding interaction is noticed between the metal f-orbitals and sp2 hybridized orbitals of the tetrazolyl N atoms. The strength of this σ-bonding interaction is stronger in the Am3+ complex as compared to that in the Eu3+ complex as observed from the higher overlap between the metal and ligand orbitals in case of the former. The occupied molecular orbitals (MOs) are mainly dominated by the ligandbased orbitals in the cationic part. In the Am3+ complex, πinteraction between the f-orbitals of Am3+ and p-orbitals of the tetrazolyl N atoms was noticed (singly occupied molecular orbital (SOMO)-13), which further strengthened the metal− ligand interactions in the Am3+-BTzP complex. In the cationic part of the Eu3+ complex, however, the occupied MOs that have the metal f-orbitals deeply burried (SOMO-30) do not show any bonding interactions between the metal and the ligand

Figure 6. Decay profile of the Eu3+ complex of BTzP in solid phase.

also observed a lifetime value of ∼400 μs for the ML type of species in the solution phase (methanol/water (9:1) mixture). Higher lifetime of 479 μs in the solid phase can, therefore, be attributed to the ML type of species. Our solution-phase studies in methanol/water (9:1) medium already indicated that the lifetime of the ML3 type of species is greater than 3000 μs. In the solid phase, the lifetime value of 1841 μs can, therefore, not be due to the ML3 type of species. This lifetime, however, could be due to the presence of the ML2 type of species in the solid phase. In the solid Eu3+ complex of BTzP, therefore, both the ML and ML2 types of species coexist indicating a similar kind of structure as observed for the Nd3+ complex of BTzP from the SCXRD and ATR-FTIR studies having a cationic ML and anionic ML2 units in an ion pair type of complex.

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COMPUTATIONAL STUDY Geometry Optimization and Structural Parameters. The geometries of the Am3+, Nd3+, and Eu3+ complexes of BTzP were optimized at the def-SV(P)/B3LYP level of theory (Figure 7). The structure of Nd3+ complex of BTzP determined

Figure 7. DFT-optimized geometry of the (a) cationic part and (b) anionic part separately for the Am3+, Nd3+, and Eu3+ complexes of BTzP at the SVP/B3LYP level (pale yellow: Am/Nd/Eu; bright yellow: C; magenta: N; red: O; blue: H).

from SCXRD study was considered as the starting geometry of all the complexes. The cationic and anionic part of the ion pair type complex was, however, allowed to relax separately in the geometry optimization steps, as the SCXRD results showed that two Nd3+ ions present in the cationic and the anionic parts are well-separated with a distance of 9.185 Å. It is interesting to H


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Inorganic Chemistry

Table 5. Bond Lengths and Mayer’s Bond Orders for Different M−N Bonds in the Geometry-Optimized Cationic and Anionic Part of the Complexes anionic part [ML2(H2O)3]−

cationic part [ML(H2O)6]+ M−Ntz

M−Npy

M−Ntz

M3+

bond length

Mayer’s bond order

bond length


Am3+

2.444, 2.443

0.465, 0.472

2.687

0.323

Nd3+

2.465, 2.461

0.444, 0.447

2.722

0.311

Eu3+

2.424, 2.427

0.445, 0.454

2.682

0.318

bond length 2.610, 2.533, 2.511, 2.500 2.610, 2.535, 2.521, 2.505 2.582, 2.491, 2.469, 2.468

Mayer’s bond order 0.416, 0.419, 0.380, 0.354 0.416, 0.419, 0.366, 0.352 0.415, 0.424, 0.365, 0.341

M−Npy bond length


2.698, 2.651

0.304, 0.280

2.702, 2.660

0.304, 0.277

2.645, 2.620

0.304, 0.282

Figure 8. Some of the molecular orbitals of Am3+ and Eu3+ complexes of BTzP showing the metal−ligand overlap.

whereas the tetrazole ring became more electron-rich after complexation. This supports our observation of decreasing bond order in C−N bonds in pyridine and increasing bond order in C−N and N−N bonds in the tetrazole moieties. This strengthening in the C−N bonds is more pronounced in the anionic part because of the higher electron density in the tetrazole rings (Table S5). To further understand the FTIR spectra of the Nd3+ complex of BTzP, the vibrational frequencies were generated theoretically based on DFT calculations using def-SV(P) basis sets and hybrid B3LYP functional. The results showed moderate correlation between the experimental and theoretically generated spectra (Figure 9). On the one hand, the minor differences observed could be attributed to the consideration of the isolated complex without any intermolecular interactions in the DFT calculations. The experimental FTIR spectra, on the other hand, were recorded with the single crystals of solid Nd3+ complex, where the complexes are arranged in the periodic system with significant intermolecular interactions.

orbitals. This results in the selectivity of this ligand toward Am3+ over Eu3+ as observed from the solvent-extraction studies. Calculations of Vibrational Frequencies and Interpretation of ATR-FTIR Results. This shift toward the lower wavenumber observed in the ATR-FTIR spectra (Figure 3) upon complexation with Nd3+ ion is probably due to the weakening in the C−N bonds of the central pyridine moiety of BTzP as indicated by the decrease in Mayer’s bond order (MBO) for the C2−N1 and C6−N1 bonds (Figure S8 and Table S4 in the Supporting Information). This is due to the significant withdrawal of electron density from the pyridine ring to donate the tripositive metal ion (Nd3+) both in the cationic and anionic parts of the complex (Table S5). In case of tetrazole rings, however, not much change in the bond order is observed for the C−N and N−N bonds except the increase in bond order upon complexation with Nd3+ ion in the C12−N13, C7− N8, N8−N9, and N13−N14 bonds. After complexation, the bonds between Nd3+ and N8/N13 are not as covalent as shown in the case of H2BTzP. This suggested the concentration of significant electron density toward the N aoms to delocalize over the tetrazole rings and resulted in an increase in the bond orders of the C−N and N−N bonds in the tetrazole moieties. Similar strengthening in the C−N and N−N bonds is also reported from the IR studies in the cases tetrazole derivatives on complexation with transition-metal ions, which was attributed to the enhancement in electron density in the ring after complexation.34 Cumulative electronic charges on the central pyridine and two lateral tetrazole rings were calculated using natural population analysis (NPA), which showed that upon complexation with Nd3+, a significant electron density has been transferred from the central pyridine ring to the metal ion,

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CONCLUSIONS Though one would like to use BTzP derivatives for the selective extraction of actinides, the alkyl/aryl substituted BTzP derivatives possess limited solubility in organic diluents rendering the selective extraction studies impractical. However, the unsubstituted BTzP, which is aqueous soluble, was explored for the selective complexation of trivalent actinide ions and hence, a reverse extraction scheme where the trivalent lanthanides, namely, Nd3+ and Eu3+, are preferentially extracted leading to a TALSPEAK-type separation scheme. The solventI


Inorganic Chemistry

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (A.B.) *E-mail: [email protected]. (P.K.M.) ORCID

Arunasis Bhattacharyya: 0000-0003-2527-9274 Sunil K. Ghosh: 0000-0003-2508-6181 Prasanta K. Mohapatra: 0000-0002-0577-1811 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors (A.B., A.S.K., and P.K.M.) thank Dr. P. K. Pujari, Associate Director, Radiochemistry and Isotope Group, for his keen interest in the present work. We also would like to thank the supercomputing facility of Computer Division, BARC, for the computational studies.

Figure 9. Experimental and theoretically generated IR spectra Nd3+ complex of BTzP.

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(1) (a) Kolarik, Z. Complexation and Separation of Lanthanides(III) and Actinides(III) by Heterocyclic N-Donors in Solutions. Chem. Rev. 2008, 108, 4208−4252 and references cited therein.. (b) Dam, H. H.; Reinhoudt, D. N.; Verboom, W. Multicoordinate ligands for actinide/ lanthanide separations. Chem. Soc. Rev. 2007, 36, 367−377. (c) Lewis, F. W.; Hudson, M. J.; Harwood, L. M. Development of Highly Selective Ligands for Separations of Actinides from Lanthanides in the Nuclear Fuel Cycle. Synlett 2011, 2011, 2609−2632. (d) Hudson, M. J.; Harwood, L. M.; Laventine, D. M.; Lewis, F. W. Use of Soft Heterocyclic N-Donor Ligands To Separate Actinides and Lanthanides. Inorg. Chem. 2013, 52, 3414−3428. (e) Panak, P. J.; Geist, A. Complexation and Extraction of Trivalent Actinides and Lanthanides by Triazinylpyridine N-Donor Ligands. Chem. Rev. 2013, 113, 1199− 1236. (f) Leoncini, A.; Huskens, J.; Verboom, W. Ligands for f-element extraction used in the nuclear fuel cycle. Chem. Soc. Rev. 2017, 46, 7229−7273. (2) (a) Modolo, G.; Odoj, R. Influence of the purity and irradiation stability of cyanex 301 on the separation of trivalent actinides from lanthanides by solvent extraction. J. Radioanal. Nucl. Chem. 1998, 228 (1−2), 83−88. (b) Sole, K. C.; Hiskey, J. B.; Ferguson, T. L. An assessment of the long-term stabilities of Cyanex 302 and Cyanex 301 in sulfuric and nitric-acids. Solvent Extr. Ion Exch. 1993, 11 (5), 783− 796. (c) Marc, P.; Custelcean, R.; Groenewold, G. S.; Klaehn, J. R.; Peterman, D. R.; Delmau, L. H. Degradation of CYANEX 301 in Contact with Nitric Acid Media. Ind. Eng. Chem. Res. 2012, 51, 13238−13244. (3) Musikas, C.; Cuillerdier, C.; Livet, J.; Forchioni, A.; Chachaty, C. Azide interaction with 4f and 5f ions in aqueous solutions. 1. Trivalent ions. Inorg. Chem. 1983, 22, 2513−2518. (4) Chopping, G. R.; Liu, Q.; Sullivan, J. C. Calorimetric studies of curium complexation. Inorg. Chem. 1985, 24, 3968−3969. (5) Drew, M. G. B.; Iveson, P. B.; Hudson, M. J.; Liljenzin, J. O.; Spjuth, L.; Cordier, P.-Y.; Enarsson, Å.; Hill, C.; Madic, C. Separation of americium(III) from europium(III) with tridentate heterocyclic nitrogen ligands and crystallographic studies of complexes formed by 2,2′:6′,2″-terpyridine with the lanthanides. J. Chem. Soc., Dalton Trans. 2000, 821−830. (6) Cordier, P. Y.; Hill, C.; Baron, P.; Madic, C.; Hudson, M. J.; Liljenzin, J. O. Am(III)/Eu(III) separation at low pH using synergistic mixtures composed of carboxylic acid and neutral nitrogen polydentate ligands. J. Alloys Compd. 1998, 271-273, 738−741. (7) Hudson, M. J.; Drew, M. G. B.; Foreman, M. R. S. J.; Hill, C.; Huet, N.; Madic, C.; Youngs, T. G. A. The coordination chemistry of 1,2,4-triazinyl bipyridines with lanthanide(III) elements − implications for the partitioning of americium(III),. Dalton Trans 2003, 1675− 1685.

extraction results were highly promising and need to be evaluated with actual feeds. SCXRD results indicated that the Nd3+ complex of BTzP is ion pair type in nature, where the cationic part consists of a nine-coordinated Nd3+ ion separated by one doubly negative BTzP2− unit along with six water molecules, whereas in the anionic part Nd3+ ion is surrounded by two doubly negative BTzP2− units along with three water molecules. Experimentally observed infrared spectra of Nd3+ complex of BTzP was wellcorroborated with its theoretically generated vibration spectra. Eu3+ was observed to form similar type of complex with BTzP from the luminescence study. Both the single-phase complexation study on Am 3+, Nd3+, and Eu 3+ using UV−vis spectrophotometric or fluorescence titrations and two-phase liquid−liquid extraction studies involving Am3+ and Eu3+ suggested significant selectivity of BTzP toward trivalent actinides over lanthanides. This actinide selectivity of BTzP was nicely explained with the help of DFT-based electronic structure calculations.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00142. Separation factors for Am3+ and Eu3+ reported for various N donor ligands shown in Figure 1, 1H NMR spectra of BTzP, TRFS spectra Eu3+ complexes of BTzP in solution phase. All the bond distances in Nd3+ complex of BTzP from SCXRD study. Details of the determination of the stability constants for the Am 3+ , Nd 3+ , and Eu 3+ complexes of BTzP from UV−vis and fluorescence titration studies, Mayer’s bond order and natural charges in BTzP and its Nd3+ complex (PDF) Accession Codes

CCDC 1817100 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. J


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