Complexation of Lanthanides with Glutaroimide-dioxime: Binding

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Complexation of Lanthanides with Glutaroimide-dioxime: Binding Strength and Coordination Modes Seraj A. Ansari,†,‡ Yanqiu Yang,†,# Zhicheng Zhang,*,† Kevin J. Gagnon,§ Simon J. Teat,§ Shunzhong Luo,*,# and Linfeng Rao† †

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India # Institute of Nuclear Physics and Chemistry, CAEP, Mianyang, 621900, China § Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: The complexation of lanthanides (Nd3+ and Eu3+) with glutaroimide-dioxime (H2L), a cyclic imide dioxime ligand that has been found to form stable complexes with actinides (UO22+ and NpO2+) and transition metal ions (Fe3+, Cu2+, etc.), was studied by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. Lanthanides form three successive complexes, M(HL)2+, M(HL)L, and M(HL)2+ (where M stands for Nd3+/Eu3+ and HL− stands for the singly deprotonated ligand). The enthalpies of complexation, determined by microcalorimetry, show that the formation of these complexes is exothermic. The stability constants of Ln3+/H2L complexes are several orders of magnitude lower than that of the corresponding Fe3+/H2L complexes but are comparable with that of UO22+/H2L complexes. A structure of Eu3+/ H2L complex, identified by single-crystal X-ray diffractometry, shows that the ligand coordinates to Eu3+ in a tridentate mode, via the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. The relocation of protons of the oxime groups (−CHN−OH) from the oxygen to the nitrogen atom, and the deprotonation of the imide group (−CH−NH−CH−) result in a conjugated system with delocalized electron density on the ligand (−O−N−C−N−C−N−O−) that forms strong complexes with the lanthanide ions.

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INTRODUCTION Glutaroimide-dioxime, a tridentate ligand containing nitrogen (of the imide group) and oxygen (of the oxime groups) donor atoms, has recently been shown to form strong complexes with UO22+ ion,1 Fe3+, and other transition metal ions,2 moderately strong complexes with NpO2+ ion,3 and weak complexes with alkaline metal ions.4 Due to its strong binding ability with many metal cations (e.g., UO22+) and its redox capability in adjusting/ controlling the oxidation states of metal ions (e.g., NpO2+), glutaroimide-dioxime has been intensively studied for its applications in separation processes such as the sequestration of uranium from seawater5−10 and actinide separations in spent nuclear fuel cycle.3 Complexation of glutaroimide-dioxime with trivalent lanthanide elements is of high interest at both applied and fundamental levels. At the applied level, because trivalent lanthanides (and trivalent actinides as well) are the key components in the separation processes of the spent nuclear fuel cycle, their complexation with glutaroimide-dioxime could determine the efficiency of the separation processes using glutaroimide-dioxime. At the fundamental level, thermodynamic and structural data for the complexation of lanthanides © XXXX American Chemical Society

with glutaroimide-dioxime are not available at present. For the actinyl cations with a linear configuration such as UO22+ or NpO2+, the tridentate glutaroimide-dioxime coordinates only via their equatorial plane and the ML2 complex is the limiting species.1,3 On the contrary, with the spherical lanthanide ions (Ln3+) with a coordination number of 8−9, the glutaroimidedioxime ligand could form up to the ML3 complex. Besides, the lanthanide and UO22+ ions have different cationic charges and radii, which could be reflected by the binding strength with glutaroimide-dioxime, provided that the binding is predominantly ionic. In this work, the stability constants and the enthalpy of complexation of glutaroimide-dioxime with Eu3+ and Nd3+ were determined by thermodynamic measurements including potentiometry, spectrophotometry, and microcalorimetry. Nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffractometry were used to reveal the structural aspects of the complex. The data for lanthanide complexes with Received: November 18, 2015

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DOI: 10.1021/acs.inorgchem.5b02653 Inorg. Chem. XXXX, XXX, XXX−XXX

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L KCl) was replaced with 1 mol/L NaCl to prevent the clogging of the electrode junction due to the low solubility of KClO4. In the potentiometric titrations, the concentration of hydrogen ions is determined from the measured electromotive force (EMF). The EMF in acidic and basic regions can be expressed by eqs 2 and (3), respectively.

glutaroimide-dioxime are discussed by comparing with previous data for UO22+ and Fe3+.

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EXPERIMENTAL SECTION

Chemicals. Glutaroimide-dioxime, represented as H2L in this paper, was synthesized by reacting glutaronitrile and hydroxyl amine (1:2 mol ratio) in a 1:1 ethanol/water mixture at 80−90 °C (Scheme 1). Detailed procedures for the synthesis and purification have been

Scheme 1. Synthesis of Glutaroimide-dioxime (H2L)

(2)

E = E 0 + RT /F × ln(K w /[OH−]) + γOH[OH−]

(3)

where R is the gas constant, F is the Faraday constant, T is the absolute temperature, and Kw = [H+][OH−]. The terms, γH and γOH are the electrode junction potentials for the hydrogen or hydroxide ions, which is assumed to be proportional to the concentration of hydrogen or hydroxide ions. Prior to each titration, an acid/base titration with standard HClO4 and NaOH was performed to obtain the electrode parameters of E0, γH, and γOH. These parameters allowed the calculation of hydrogen ion concentrations from the measured EMF in the subsequent titration. In a typical titration, a solution (about 50 mL) containing appropriate amounts of Nd3+/Eu3+ and ligand was titrated with a standard solution of solution of HClO4 or NaOH. Multiple titrations were conducted with solutions of different concentrations of the metal ions (CM), the ligand (CL), and total proton (CH). The potentiometric titration data were analyzed to obtain the stability constants of the metal/ligand complexes by the Hyperquad 2008 program.12 Microcalorimetry. Microcalorimetric titrations at 25 °C were performed on an isothermal titration microcalorimeter (TAM-III) that measures the heat flow between the reaction vessel, reference vessel, and a heat sink maintained at a constant temperature. Both the chemical and the electrical calibrations were performed to validate the performance of the instrument. The chemical calibration was carried out by measuring the heat of protonation of tris(hydroxymethyl)aminomethane (THAM) at 25 °C. The obtained value (−47.6 ± 0.3 kJ/mol) was in good agreement with the literature data.13 The titrand (H2L with Nd3+ or Eu3+) was contained in the 750 μL reaction vessel and stirred with a gold propeller at 80 rpm. The titrant (50 mmol/L NaOH) was added into the vessel through a Hamilton 250 μL syringe stepwise, 5 μL per addition. About 50 injections were made in each experiment. Multiple titrations using different CM, CL, and CH were performed to reduce the uncertainty of the results. The reaction heat, after correcting for the dilution heat from a separate blank titration, includes the heats generated by all reactions that occurred in the reaction vessel, including the complexation, deprotonation, and acid/ base neutralization. In conjunction with the equilibrium constants obtained by potentiometry, the enthalpy of complexation was calculated by using the HyperDeltaH program.14 NMR Spectroscopy. NMR experiments were carried out on a Bruker Avance DPX 300 spectrometer, operated at 300 MHz for the measurement of 1H signals. 1H and 13C NMR spectra were collected with two series of samples in D2O: one contained H2L at different pH, the other contained H2L and La3+ with different CL/CM ratios at different pH. Due to its diamagnetic nature, La3+ was used as a representative trivalent lanthanide. The 1H NMR spectra were recorded with a standard pulse program ZG30, averaging 32 scans for each spectrum. The obtained 1H NMR spectra were referenced to the solvent peak (HOD, 4.79 ppm). The 13C NMR spectra were collected with a decoupler on the pulse program ZGDC, each averaging over 10000 scans. Luminescence Spectroscopy. Luminescence emission spectra and lifetime of Eu3+ were recorded at 25 °C on a Horiba Jobin Yvon IBH Fluorolog-3 fluorimeter, adapted for time-resolved measurements. A sub-microsecond xenon flash lamp (Jobin Yvon, 5000XeF) was the light source, coupled with a double-grating excitation monochromator for spectral selection. A thermoelectrically cooled single photon detection module (Horiba Jobin Yvon IBH, TBX-04-D) equipped with a fast-rise-time photomultiplier tube, a wideband width preamplifier, and a picosecond constant-fraction discriminator was used as the detector. The luminescence emission spectra were

described elsewhere.4 The obtained ligand was characterized by 1H NMR and potentiometric titration. All the other chemicals were Analytical Reagent grade and were used without further purification. The stock solutions of Nd(ClO4)3 and Eu(ClO4)3 were prepared by dissolving Nd 2 O 3 (s) and Eu 2 O 3 (s) in perchloric acid. The concentrations of Nd3+ or Eu3+ and the free acid in the stock solution were determined by EDTA titration (using methyl thymol blue as the indicator in a hexamethylenetriamine buffered solution of pH 7−8) and Gran titration, respectively.11 Deionized Milli Q water (18.2 MΩ· cm) was used for the preparation of solutions. All the experiments were conducted at 25 °C and at the ionic strength of 1 mol/L (NaClO4). For NMR experiments, the solutions of glutaroimide-dioxime were prepared in D2O (99.8%-d, Cambridge Isotopic Laboratory) and the pH was adjusted with NaOD (40% NaOD in D2O, Cambridge Isotopic Laboratory) and DClO4 solutions (68% DClO4 in D2O, Cambridge Isotopic Laboratory). A stock solution of La3+ in D2O was prepared by dissolving La2O3(s) with DClO4 and then diluting with D2O. The concentrations of La3+ and D+ in the stock solution were determined by EDTA and Gran titrations, respectively. Spectrophotometry. UV−vis absorption spectra of the Nd3+ solutions were collected in the wavelength region 555−610 nm (0.1 nm interval) on a double beam Varian Cary-5G spectrophotometer using 10 mm path length quartz cells. The temperature of the sample and reference cell holders was maintained at 25 °C by water circulation from a thermostated water bath. Solutions of the ligand and Nd3+ with different CL/CNd ratios were placed in the sample cell and titrated with NaOH. A spectrum was taken after adding an appropriate aliquot of NaOH into the cell and mixing thoroughly for about 2 min. The mixing time was found to be sufficient to complete the complexation reaction. Usually 15−20 spectra were recorded in each set of titration. The stability constants of the metal/H2L complexes were calculated by nonlinear least-squares regression analysis using the HyperSpec program,12 based on equilibrium reaction 1. Ln 3 + + i H+ + j L2 − = Ln(HiLj)(2j − i − 3) −

E = E 0 + RT /F × ln[H+] + γH[H+]

(1)

βij = [Ln(HiLj)(2j − i − 3) − ]/([Ln 3 +][H+]i [L2 −]j ) where L2− represents the doubly deprotonated form of glutaroimidedioxime (H2L→ 2H+ + L2−) and Ln3+ represents trivalent lanthanide cation. Potentiometry. Potentiometric titrations were performed using an autotitration unit consisting of a double jacket glass titration cell and a Metrohm dosimat (907 Titrando) connected with a pH electrode (Orion model 8102). The temperature of the titration cell was maintained at (25 ± 0.1) °C by circulating water from a constant temperature water bath. An inert atmosphere was maintained in the titration cell by passing Ar gas to prevent the sorption of CO2 in the solution during titration. The original electrode filling solution (3 mol/ B

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Inorganic Chemistry obtained in the wavelength region of 550−725 nm (0.5 nm/step, 5 nm bandwidth) by excitation at 395 nm (5 nm bandwidth). The luminescence lifetime data were obtained at an excitation of 395 ± 5 nm, and the decay curve was followed at an emission wavelength of 612 ± 5 nm. Signals were acquired using an IBH Data Station Hub, and the data were analyzed using the commercially available DAS-6 decay analysis software package from Horiba Jobin Yvon IBH. Single Crystal X-ray Diffractometry. Lightly yellow single crystals of the Eu3+/H2L complex were obtained from a 1 mol/L NaClO4 solution (2.5 mL) containing 20 μmol of Eu(ClO4)3, 55 μmol of H2L, and 15 μmol of NaOH. Crystallographic data were collected on beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Lab. Samples were mounted on MiTeGen kapton loops and placed on the goniometer head of a Bruker D8 diffractometer equipped with a PHOTON100 CMOS detector operating in shutterless mode. The samples were in a 100(2) K nitrogen cold stream provided by an Oxford Cryostream 700 Plus low temperature apparatus. Diffraction data were collected using synchrotron radiation monochromated by using silicon(111) to a wavelength of 0.7749(1) Å. An approximate full-sphere of data was collected using a combination of phi and omega scans with scan speeds of 1 s per 4 degrees for the phi scans, and 1 and 3 s per degree for the omega scans, at 2θ = 0 and −45, respectively. The structure was solved by intrinsic phasing (SHELXT) and refined by full-matrix least-squares on F2 (SHELXL2014).15 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon atoms were geometrically calculated and refined as riding atoms, all hydrogen atoms on O and N atoms were found in the Fourier difference map, and their distances were fixed and refined as riding atoms. Additional crystallographic information is summarized in Table 1.

Table 1. Crystallographic Data and Structure Refinement for [Eu4(HL)6(H2O)6ClO4)6]·8.5H2O(s), Where HL− Stands for the Singly Deprotonated Glutaroimide-dioxime Ligand (H2L) Shown in Scheme 1 empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions

volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to θ = 27.706° Absorption correction Max. and min transmission refinement method data/restraints/ parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute structure parameter largest diff peak and hole

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RESULTS AND DISCUSSION Stability Constants of Glutaroimide-dioxime Complexes with Nd3+ and Eu3+. Potentiometry for Nd3+ and Eu3+. Representative potentiometric titrations for the complexation of Nd3+ (upper figures) and Eu3+ (lower figures) with H2L are shown in Figure 1. The titrations were conducted in two directions, from lower to higher pCH (note: pCH = −log[H+]) using NaOH or reversely using HClO4. The best model fitting of the titration data in the pCH region from 3 to 7.8 includes the formation of three complexes, M(HL)2+, M(HL)L, and M(HL)2+, where M stands for Nd3+ or Eu3+ and H2L the glutaroimide-dioxime ligand. The stability constants of these complexes are listed in Table 2. The data for the region of pCH above 7.8 were not used for the calculation because precipitation occurred at pCH ≥ 8. Absorption Spectrophotometry for Nd3+. Figure 2 shows representative spectrophotometric titrations for the complexation of Nd3+ with glutaroimide-dioxime. The initial solution contained H2L and Nd3+ at a molar ratio (CL/CNd) of 2.5 and pCH about 3.5. As aliquots of 10 mmol/L NaOH were added stepwise, the intensity of the absorption band of Nd3+ at 575 nm, corresponding to the hypersensitive 4I9/2 → 4G5/2, 2G7/2 transition,16 decreased with simultaneous increase in the band intensity around 585 nm. Similar to the observations in potentiometric titrations, precipitation occurred when the solution pCH was around 8. As a result, only the data in the pCH region of 3.5−7.8 were used for calculating the stability constants. Factor analysis of the spectra suggested the presence of two absorbing species in the titration system, i.e., the free Nd3+ cation and one Nd3+/H2L complex. A model including the formation of Nd(HL)2+ provides good fits to the data with the equilibrium constant of log β(Nd(HL)2+) = (19.9 ± 0.3). This value agrees very well with that obtained by potentiometry (Table 2). However, it is noted that, in the same pCH region,

C30 H77 Cl6 Eu4 N18 O50.5 2318.63 100 (2) K 0.7749 Å orthorhombic P21212 a = 20.5454(18) Å, α = 90° b = 25.326(2) Å, β = 90° c = 13.8827(12) Å, γ = 90° 7223.6(11) Å3 4 2.132 mg/m3 4.702 mm−1 4564 0.120 × 0.080 × 0.020 mm3 1.930° to 36.649° −31 ≤ h ≤ 31, −38 ≤ k ≤ 38, −21 ≤ l ≤ 21 115763 27515 [R(int) = 0.0714] 99.7% semiempirical from equivalents 0.876 and 0.550 full-matrix least-squares on F2 27515/1098/1138 1.037 R1 = 0.0382, wR2 = 0.0951 R1 = 0.0441, wR2 = 0.0983 0.480(9) 4.599 and −1.370 e·Å−3

three complexes including M(HL)2+, M(HL)L, and M(HL)2+ were identified with the potentiometric data, but only the Nd(HL)2+ complex was observed with the spectrophotometric data. The reason for not detecting the other species by spectrophotometry is probably 2-fold: (i) the absorption spectra of the complexes (M(HL)2+, M(HL)L, and M(HL)2+) are very similar, and (ii) the other species, namely M(HL)L and M(HL)2+, are minor in the pCH region of the spectrophotometric titrations. It is envisaged, therefore, that H + potentiometry could provide a more accurate description of the complexation system involving complexes that differ only in the degree of protonation. Luminescence Spectroscopy for Eu3+. A representative luminescence titration is shown in Figure 3. The luminescence titration for Eu3+ was conducted in a similar manner as that in the spectrophotometric titrations for Nd3+, with both the metal ion and the ligand in the initial solution (CL/CEu ≈ 2) titrated with NaOH. The spectra contain features originating from electronic transitions from the lowest excited state of Eu3+, 5D0, to the ground state manifold, 7F1 (590 nm), 7F2 (615 nm), and 7 F4 (695 nm).17 In many Eu3+ complexation systems, the intensity of the 5D0 → 7F2 band (an electric dipole transition, 615 nm) generally increases upon complexation due to replacement of water molecules in the coordination environC

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Figure 1. Potentiometric titrations for the complexation of Nd3+ or Eu3+ with glutaroimide-dioxime (t = 25 °C, I = 1 mol/L NaClO4). (O) Experimental data (left y-axis); (---) fit (left y-axis); () speciation of Nd3+ or Eu3+ (right y-axis). Experimental conditions (total mmoles of M, L, and H): (a) 0.082/0.212/0.413 (total V = 58.7 mL), titrant: 0.10 mol/L NaOH; (b) 0.070/0.168/0.167 (total V = 47.8 mL), titrant: 0.10 mol/L HClO4; (c) 0.065/0.223/0.452 (total V = 49.73 mL), titrant: 0.0506 mol/L NaOH; (d) 0.063/0.136/0.227 (total V = 49.48 mL); titrant: 0.0506 mol/L NaOH.

Table 2. Thermodynamic Data for the Complexation of Glutaroimide-dioxime (H2L) with Nd3+ and Eu3+ (t = 25 °C, I = 1 mol/ L NaClO4), in Comparison with UO22+ and Fe3+a reaction H+ + L2− → HL− 2H+ + L2− → H2L(aq) 3H+ + L2− → H3L+ Fe3+ + H+ + L2− → Fe(HL)2+ Fe3+ + H+ + 2L2− → Fe(HL)L(aq) Fe3+ + 2H+ + 2L2− → Fe(HL)2+ Nd3+ + H+ + L2− → Nd(HL)2+

I, mol/L

method

0.5/NaCl

pot, cal

0.5/NaCl

pot, cal

1/NaClO4

pot, cal sp pot, cal

Nd3+ + H+ + 2L2− → Nd(HL)L(aq) Nd3+ + 2H+ + 2L2− → Nd(HL)2+ Eu3+ + H+ + L2− → Eu(HL)2+ Eu3+ + H+ + 2L2− → Eu(HL)L(aq) Eu3+ + 2H+ + 2L2− → Eu(HL)2+ UO22+ + H+ + L2− → UO2(HL)+ UO22+ + H+ + 2L2− → UO2(HL)L− UO22+ + 2H+ + 2L2− → UO2(HL)2(aq) a

pot, cal lumi pot, cal 0.5/NaCl

log β

ΔH (kJ/mol)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

−36.1 ± 0.5 −69.7 ± 0.9 −77.0 ± 6.0 −63 ± 2 −91 ± 3 −113 ± 3 −74.1 ± 2.8

110 202 218 283 536 573 124

12.1 22.8 24.9 25.7 43.9 49.7 19.5 19.9 29.8 37.6 19.8 20.4 30.3 38.5 22.7 36.8 43.0

pot, cal

0.2 0.3 0.4 1.1 1.1 1.1 0.2 0.3 0.7 0.3 0.3 0.2 0.3 0.3 1.3 2.1 1.1

ΔS (J/mol/K)

ref

± ± ± ± ± ± ±

2 3 14 28 31 31 5

1 1 1 2 2 2 pw

−114.6 ± 3.1 −143.1 ± 3.7 −79.2 ± 2.2

186 ± 4 239 ± 10 113 ± 3

pw pw pw

−118.6 ± 4.8 −147.3 ± 3.5 −71.0 ± 6.0 −118 ± 6 −154 ± 25

182 242 197 309 307

± ± ± ± ±

pw pw 1 1 1

7 6 14 14 59

Methods: pot, potentiometry; sp, spectrophotometry; lumi, luminescence; cal, calorimetry; pw, present work.

ment of Eu3+ by the ligand, while the 5D0 → 7F1 band (a magnetic dipole transition, 590 nm) remains unchanged. However, as shown in Figure 3, all bands show a proportional decrease in the overall intensity and the shape of the

luminescence spectrum of Eu/glutaroimide-dioxime is almost invariant. One probable interpretation of the luminescence spectra in Figure 3 is that the luminescence of the Eu(III) complexes with glutaroimide-dioxime is significantly quenched D

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Figure 2. (a) Spectrophotometric titration of Nd3+ with glutaroimide-dioxime (t = 25 °C, I = 1 mol/L NaClO4). CMo/CLo/CHo in mmol/L: 8.25/ 19.72/39.44 (2.5 mL), titrant: 10 mmol/L NaOH. (b) Deconvoluted spectra of Nd3+ and Nd(HL)2+ species.

Figure 3. (a) Luminescence emission spectra of the Eu3+/glutaroimide-dioxime system (t = 25 °C, I = 1 mol/L NaClO4). CMo/CLo/CHo in mmol/L: 12.2/23.6/47.2 (2.5 mL), titrant: 50.6 mmol/L NaOH. (b) deconvoluted emission spectra of Eu3+ and Eu(HL)2+. Excitation wavelength: (395 ± 5 nm).

enthalpy and entropy driven. The large positive entropies of complexation probably result from the release of water molecules from the primary coordination spheres of Nd3+ or Eu3+ and the ligand, upon the complexation with the ligand. In particular, the degree of disorder in the primary hydration sphere is expected to increase significantly if charge neutralization accompanies the formation of complexes. Comparison of Binding Strength of Glutaroimidedioxime with Ln3+, UO22+, and Fe3+. The stability constants of glutaroimide-dioxime complexes with lanthanides (Nd3+ and Eu3+) are compared with those with UO22+ and Fe3+ in Table 2. For all complexes, the stability constants follow the order: Fe3+ > UO22+ > Eu3+ ≥ Nd3+. The order is in excellent agreement with the order of ionic potential, Z/r, where Z is the effective charge and r is the ionic radius of the ion. With the values of r (Å)18 as 0.55 (low-spin Fe3+), 0.73 (UO22+, CN = 8), 1.120 (Eu3+, CN = 9), and 1.163 (Nd3+, CN = 9), and the values of Z as +3 (Fe3+), + 3.2 (UO22+),19 + 3 (Eu3+), and +3 (Nd3+), the ionic potentials are calculated to be 5.4, 4.38, 2.68, and 2.56 for Fe3+, UO22+, Eu3+, and Nd3+, respectively. A linear correlation between log βM(HL) and Z/r in Figure 5 suggests that the interactions between glutaroimide-dioxime and these ions are predominantly ionic in nature. Of course, in addition to ionic binding, the Fe3+/glutaroimide-dioxime complexes may gain strength due to the larger participation of the d-orbitals of Fe3+ in bonding than the f-orbitals of UO22+ cation or the lanthanides.

and the spectra observed in Figure 3 are mostly from the nonbonded Eu3+ ions. Also, the excitation spectra of the Eu(III) complexes might be shifted to some extent so that the complexes are not efficiently excited at 395 nm. The quenching effect by the ligand precludes the use of the luminescence lifetime data to determine the hydration number and the coordination mode of the ligand in the Eu3+/H2L complex(es). Nonetheless, the luminescence spectra could be fitted with the HypSpec program based on eq 1. Similar to the spectrophotometric data for Nd3+, the best fit was obtained with the model including the formation of only one complex, Eu(HL)2+. The equilibrium constant for Eu(HL)2+ was determined to be (20.4 ± 0.2), in agreement with the value obtained by potentiometry (Table 2). Again, for the complexation systems involving complexes with different degree of protonation, H+-potentiometric data seem to be more informative than the data from luminescence or absorption spectroscopy. Enthalpy of Complexation by Microcalorimetry. Figure 4 shows representative calorimetric titrations of Nd3+/H2L and Eu3+/H2L (at different CL/CM ratios) with NaOH. The calculated enthalpies of complexation are summarized in Table 2. The results show that the enthalpies of complexation for all the three complexes, M(HL)2+, M(HL)L, and M(HL)2+, are exothermic and very similar for corresponding complexes of Nd3+ and Eu3+. The entropies of complexation, calculated from the enthalpies and Gibbs free energy accordingly, are all positive, indicating that the complexation processes are both E

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Figure 4. Representative calorimetric titrations of Nd3+/glutaroimide-dioxime (upper figures) and Eu3+/glutaroimide-dioxime (lower figures) (t = 25 °C, I = 1 mol/L NaClO4). Left figures: calorimetric thermograms; right figures: cumulative heat (O, experimental; --- fit; left y-axis) and speciation of Nd3+ or Eu3+ (solid lines, right y-axis) as a function of titrant volume. Experimental conditions (Vo = 0.75 mL, titrant: 50.6 mmol/L NaOH): total μmoles of M, L, and H): 5.22/16.34/31.68 (for Nd3+); 5.24/25.15/50.0 (for Eu3+).

Scheme 2. Labeling of Hydrogen and Carbon Atoms in H2L and La(HL)2+

Figure 5. Equilibrium constants of M(HL) complexes (M = Nd3+, Eu3+, UO22+, and Fe3+) vs ionic potential (Z/r).

the 1H signals are consistently shifted downfield as the solution pH is decreased from 12.6 (solution a) to 11.6 (solution b) and further to 3.2 (solution c). This is consistent with the increase in the degree of protonation of the ligand, since the dominant species of the ligand changes from L2− (87%, solution a) to HL− (70%, solution b) and further to H2L(aq) (93%, solution c). The protonation of the functional groups in glutaroimidedioxime deshields the protons on the proximate alkyl groups. (iii) In the presence of La3+ (Figure 6 right, solutions A, B, C, and D), the 1H signals all downfield shifted from those for the free deprotonated ligand, L2− (Figure 6 left, solution a), indicating the interactions between the ligand and the La3+. Also, only two 1H signals are observed though the solutions contain a mixture of ligand and metal/ligand complexes in different percentages. [Note: The speciation of solutions A−D (relative to total ligand concentration): (A) 9.0% H2L(aq), 9.0% HL−, 82% LaHL2(aq); (B) 28% H2L, 9.0% LaHL2+, 19% LaHL2(aq), 44% La(HL)2+; (C) 55% H2L, 28% LaHL2+, 17%

Coordination Modes in Ln3+/H2L Complex. NMR Data. To facilitate the discussions of the NMR data, the hydrogen and carbon atoms in glutaroimide-dioxime (H2L) and the La3+/ H2L complex are labeled as shown in Scheme 2. Figure 6 shows the 1H NMR spectra of glutaroimide-dioxime in the absence and presence of La3+. Several features of the 1H NMR spectra are noted: (i) two 1H signals are observed for all samples (at different pH values, with or without La3+), corresponding to the H(3) (a “quintet”) and H(2,2′) (a “triplet”) atoms (see Scheme 2). The fact that the multiplicity of the signals is the same in the free ligand and the La3+/H2L complex(es) suggests that the equivalency of the H(2,2′) atoms remains unchanged in the La3+/H2L complex(es). In other words, the ligand coordinates to the metal ion (La3+) in a symmetrical mode, very probably as that shown in Scheme 2. (ii) In the absence of La3+ (Figure 6 left, solutions a, b, and c), F

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Inorganic Chemistry

Figure 6. 1H NMR spectra of glutaroimide-dioxime in the absence (left figure) and presence (right figure) of La3+. Left figure: (a) pH = 12.6, [L] = 13 mmol/L; (b) pH = 11.6, [L] = 15 mmol/L, (c) pH = 3.2, [L] = 13 mmol/L. Right figure: (A) pH = 10.7, [L] = 15 mmol/L, [La] = 6.0 mmol/L; (B) pH = 7.5, [L] = 14 mmol/L, [La] = 5.9 mmol/L; (C) pH = 6.6, [L] = 14 mmol/L, [La] = 5.9 mmol/L; (D) pH = 6.9, [L] = 14 mmol/L, [La] = 16 mmol/L.

La(HL)2+; (D) 5.0% H2L, 76% LaHL2+, 8.0% La(HL)2+]. The fact that the 1H signals of ligand in its different species (free ligand and complexes) could not be resolved implies that the ligand exchange between the species is fast on the 1H NMR time scale so that only the “average” chemical shifts are observed. Similar discussions on the symmetrical coordination mode in the La3+/H2L complex(es) can be made from the 13C NMR spectra shown in Figure 7. In fact, the 13C NMR data show that the equivalencies of C(1,1′) and C(2,2′) atoms remain unchanged for samples with or without La3+. However, differing from the 1H spectra, the 13C spectra show a new set of three 13 C signals which are noted as (C1/1′)*, (C2/2′)*, and (C3)* in Figure 7 right. This new set is assigned to the carbons of the ligand in the 1:1 complex (La(HL)2+), as its appearance and intensity change are consistent with the speciation variation of this complex from sample A to D. Also, the shifting of the peaks agrees with the proposed coordination mode in Scheme 2 as described below. The 13C spectra in Figure 7 indicate that, upon the formation of the 1:1 complex, the chemical shift of the C(1,1′) atoms is downfield displaced to a large extent ((C1/1′)*: ∼9 ppm, compared to that in spectrum 7b), and chemical shifts of the C(2,2′) and C(3) atoms are upfield displaced to a small extent ((C2/2′)*: ∼3.2 ppm; (C3)*: ∼0.3 ppm). These displacements can be attributed to the variations of the electron density on the carbon atoms induced by the complexation. In the structure of the trident complex La(HL)2+ in Scheme 2, the large positive charge of La3+ could significantly reduce the electron densities on C(1,1′) atoms due to their

closeness to the coordination sites, deshielding the C(1,1′) nuclei and resulting in a large downfield displacement of their chemical shifts. As to the chemical shifts of C(2,2′) and C(3) atoms, the small upfield displacements in the complex could result from two opposing effects: on one hand, the coordination of La3+ could reduce the electron densities on C(2,2′) and C(3), causing a deshielding effect on C(2,2′) and C(3) atoms, but to a lesser extent than that on C(1,1′) atoms due to the larger distances of the former to the coordination sites. On the other hand, the electrons of carbon-bound hydrogen atoms on C(2,2′) and C(3) atoms could also be pulled toward C(2,2′) and C(3) through a δ-bond due to the La3+ coordination, resulting in a shielding effect. It is probably because the shielding effect is slightly stronger than the deshielding effect that net upfield shifts are observed for the C(2,2′) and C(3) atoms in the complex. In brief, the 1H and 13 C NMR data agree with the speciation calculated with the thermodynamic constants determined in this work and supported the symmetrical coordination mode in the glutaroimide-dioxime complex(es) with the lanthanide ions. Crystal Structure of Eu3+/H2L Complex. In contrast to the crystals of mononuclear UO22+ and Fe3+ complexes with glutaroimide-dioxime,1,2 crystals of [Eu4(HL)6(H2O)6ClO4)6]· 8.5H2O were obtained and the structure is shown in Figure 8. The asymmetric unit is a one-dimensional chain that contains four Eu atoms in two different coordination environments noted as EuI and EuII (Figure 8). The EuI center is coordinated by one water molecule and two whole ligand molecules, while the EuII center has two water molecules and only one whole G

DOI: 10.1021/acs.inorgchem.5b02653 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 7. 13C NMR spectra of glutaroimide-dioxime in the absence (left figure) and presence (right figure) of La3+. The concentrations of samples are identical to those for the 1H NMR spectra in Figure 6.

Figure 8. Crystal structure of [Eu4(HL)6(H2O)6ClO4)6]·8.5H2O(s). Unbound water molecules, perchlorate ions, and hydrogen atoms are omitted for clarity. Two different Eu3+ sites are labeled as I and II, respectively.

ligand. The remaining coordination sites are filled by bridging atoms from neighboring groups. The chains form an infinite coordination polymer with perchlorate ions and water molecules in a complex hydrogen bonded network between the chains. Similar to the UO2(HL)2 and Fe(HL)Cl2 complexes,1,2 the singly deprotonated glutaroimide-dioxime ligand, HL−, coordinates to the Eu3+ center in a tridentate mode with two oxime oxygen atoms and one imide nitrogen atom. In the complexes with the singly deprotonated ligand HL−, the imide group is deprotonated and the protons on the oxime groups are relocated from the oxygen to the nitrogen atom, resulting in a

conjugated system with delocalized electron density on the ligand (−O−N−C−N−C−N−O−) that forms strong complexes with the metal ions including UO22+, Fe3+, and Eu3+/ Nd3+. Among the three complexes (Eu, U, and Fe), the bond distances of Eu−O (from 2.363(4) to 2.542(4) Å) and Eu−N (from 2.462(4) to 2.518(5) Å) are comparable to those of U− O and U−N (2.429(3)−2.535(3) Å and 2.563(3) Å),1 but much longer than those of Fe−O and Fe−N (2.027(1)− 2.069(1) and 2.004(1)−2.030(1) Å),2 even after the difference in the ion radii is taken into consideration. The trend in the bond distances is consistent with the thermodynamic trend in H

DOI: 10.1021/acs.inorgchem.5b02653 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(5) Hay, B. P.; Clement, O.; Sandrone, G.; Dixon, D. A. Inorg. Chem. 1998, 37, 5887−5894. (6) Rao, L. Recent International R&D activities in the extraction of uranium from seawater, Lawrence Berkeley National Laboratory Report, LBNL-4034E, Berkeley, CA, 2010. (7) Kim, J.; Oyola, Y.; Tsouris, C.; Hexel, C. R.; Mayes, R. T.; Janke, J. C.; Dai, S. Ind. Eng. Chem. Res. 2013, 52, 9433−9440. (8) Tian, G.; Teat, S. J.; Rao, L. Dalton Trans. 2013, 42, 5690−5696. (9) Sun, X.; Tian, G.; Xu, C.; Rao, L.; Vukovic, S.; Kang, S. O.; Hay, B. P. Dalton Trans. 2014, 43, 551−557. (10) Kim, J.; Tsouris, C.; Oyola, Y.; Janke, C. J.; Mayes, R. T.; Dai, S.; Gill, G.; Kuo, L.-J.; Wood, J.; Choe, K.-Y.; Schneider, E.; Lindner, H. Ind. Eng. Chem. Res. 2014, 53, 6076−6083. (11) Gran, G. Analyst 1952, 77, 661−671. (12) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739−1753. (13) Wilson, E. W.; Smith, D. F. Anal. Chem. 1969, 41, 1903−1903. (14) Gans, P.; Sabatini, A.; Vacca, A. J. Solution Chem. 2008, 37, 467− 476. (15) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (16) Jorgensen, C. K. Absorption Spectra and Chemical Bonding in Complexes; Pergamon Press: London, 1962. (17) Gorller-Walrand, C.; Binnemans, K. Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr.; Eyring, L., Eds.; Elsevier: Amsterdam, 1998; Vol. 25, pp 101−264. (18) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 1st ed.; Pergamon Press: Hong Kong, 1984. (19) Choppin, G. R.; Rao, L. F. Radiochim. Acta 1984, 37, 143−146.

the binding strength and can be attributed mainly to the difference in the ion potential, Z/r, that is discussed in a previous section.

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CONCLUSIONS Nd and Eu3+ form fairly strong complexes with glutaroimidedioxime in aqueous solutions at moderate acidity and basicity (pCH ∼ 3−8). The complexation is both enthalpy and entropy driven. The binding strength of glutaroimide-dioxime with lanthanides and other cations (UO22+ and Fe3+) correlates very well with the charge density of the cations, implying that the binding is predominantly ionic in nature. Similar to the coordination mode in the complexes with UO22+ and Fe3+, the singly deprotonated ligand, HL−, coordinates to the Eu3+ center in a tridentate mode with the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. In contrast to the mononuclear complexes of UO 2 2+ and Fe 3+ , the glutaroimide-dioxime ligand in the lanthanide complex tends to bridge the Eu3+ centers to form a coordination polymer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02653. A file (Euliga.cif) describing the crystal structure of [Eu4(HL)6(H2O)6ClO4)6]·8.5H2O(s) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS The thermodynamic measurements and crystallographic work were supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-05CH11231 at LBNL. The synthesis of glutaroimide-dioxime and the NMR experiments were supported by the Fuel Resources Program, Fuel Cycle Research and Development Program, Office of Nuclear Energy of the U.S. DOE, under contract no. DE-AC02-05CH11231 at LBNL. Single-crystal X-ray diffraction data were collected and analysed at the Advanced Light Source (ALS). ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, U.S. DOE, under contract no. DE-AC02-05CH11231. S. A. Ansari acknowledges the Indo-US Science & Technology Forum (IUSSTF) for awarding a fellowship to support the experimental work at LBNL. The authors thank C. J. Leggett of LBNL for synthesizing and checking the purity of the glutaroimide-dioxime ligand.

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REFERENCES

(1) Tian, G.; Teat, S. J.; Zhang, Z. Y.; Rao, L. Dalton Trans. 2012, 41, 11579−11586. (2) Sun, X.; Xu, C.; Tian, G.; Rao, L. Dalton Trans. 2013, 42, 14621− 14627. (3) Ansari, S. A.; Bhattacharyya, A.; Zhang, Z.; Rao, L. Inorg. Chem. 2015, 54, 8693−8698. (4) Leggett, C. J.; Rao, L. Polyhedron 2015, 95, 54−59. I

DOI: 10.1021/acs.inorgchem.5b02653 Inorg. Chem. XXXX, XXX, XXX−XXX