Complexation of Lanthanides with N,N,N′,N′-Tetramethylamide

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

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Complexation of Lanthanides with N,N,N′,N′‑Tetramethylamide Derivatives of Bipyridinedicarboxylic Acid and Phenanthrolinedicarboxylic Acid: Thermodynamics and Coordination Modes Baihua Chen, Jun Liu, Lina Lv, Liang Yang, Shunzhong Luo, Yanqiu Yang,* and Shuming Peng*

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Institute of Nuclear Physics and Chemistry, CAEP, Mianyang, Sichuan 621900, China S Supporting Information *

ABSTRACT: The thermodynamics of Nd(III) and Eu(III) complexes with N,N,N′,N′-tetramethyl-2,2′-bipyridine-6,6′-dicarboxamide (TMBiPDA) and N,N,N′,N′-tetramethyl-1,10-phenanthroline-2,9-dicarboxamide (TMPhenDA) in CH3OH/10%(v)H2O solutions were studied. Stability constants and enthalpies of complexation were determined by absorption spectrophotometry, luminescence, and calorimetry. The stability constants of corresponding lanthanide complexes decrease in the order of TMPhenDA > TMBiPDA, while those of the corresponding ligand complexes with lanthanides decrease in the order of Nd(III) > Eu(III). The stepwise reactions for all 1:1 complexes as well as for the 1:2 Nd(III) complexes are driven by both enthalpy and entropy, while those for the 1:2 Eu(III) complexes are driven by entropy. The stronger affinity of TMPhenDA to Nd(III) and Eu(III) than that of TMBiPDA is predominantly arisen from its high preorganization. The spectra of the complexes in solutions are similar, implying that Nd(III) and Eu(III) coordinate with the two ligands in the same mode, which have been validated by 1H and 13C NMR titrations using La(III) as lanthanide tracer. The luminescence lifetimes of the Eu(III) complexes with TMBiPDA and TMPhenDA were evaluated by TRLFS. Structures of Nd(III)/TMPhenDA and Eu(III)/TMPhenDA complexes, identified by single-crystal X-ray diffractometry, show that ligand coordinates to metal in a tetradentate mode via two aromatic N-donors and two amide Odonors, and the central cation (Nd(III) or Eu(III)) is 10-coordinated by two whole TMPhenDA and two solvent (water or methanol) molecules. The M−O bond distances are almost identical, while the Nd−N bond distance is shorter than the Eu−O bond.

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dicarbollide in fluorinate diluents, was found to be selective to extract Am from lanthanides (SFAm/Eu ≤ 30).19,20 N,N′Diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline exhibited excellent extraction ability and high selectivity toward hexavalent (SFU/Eu ≤ 277), tetravalent (SFTh/Eu ≤ 2277), and trivalent (SFAm/Eu ≤ 67) actinides over lanthanides in high acidic solution (1.0 M HNO3), using cyclohexanone as diluent.20,21 The highly preorganized mixed N- and O-donor ligand like unsaturate δ-lactam-1,10-phenanthroline (BLPhen) effectively extracted Am(III) from 3 M HNO3 solution into dichloroethane with high selectivity for Am(III) over Eu(III) (SFAm/Eu ≤ 211).7 It is well-known that the complexation between ligand and metal ion is the chemical basis behind separation in liquid extraction, but limited studies on the complexation of such tetradentate N-O-O-N ligands with lanthanides/actinides in a homogeneous phase and the complex structures in solid have been reported. Borisova and co-workers found that the affinity of different 2,2′-bipyridyl-

INTRODUCTION The PUREX (Plutonium and URanium EXtraction) process is used worldwide to recover Pu and U from the spent nuclear fuel (SNF), while the remainder containing the minor actinides accounts for the major long-term radiotoxicity of the SNF.1 In the recent decades, great effort has been made to separate the minor actinides from the lanthanides and other fission products in the partitioning and transmutation (P&T) strategy.2 Grouped ActiNide Extraction (GANEX) has been proposed as an alternative to minor actinides separation, recovering uranium and all transuranium elements from the SNF.3,4 Being of enough complexation strength to actinides of various valences as well as high actinide selectivity over lanthanides, mixed N- and O-donor ligands are potential separation reagents for the GANEX process.5−7 Diamide derivatives of bipyridinedicarboxylate (BiPDA) and phenanthrolinedicarboxylate (PhenDA) are the most intensively investigated mixed N- and O-donor ligands.5−18 N,N′Diethyl-N,N′-phenyl-2,2′-bipyridine-6,6′-dicarboxamide, synergistic by the lipophilic anion source chlorinated cobalt © XXXX American Chemical Society

Received: February 25, 2019

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DOI: 10.1021/acs.inorgchem.9b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of TMBiPDA and TMPhenDA

Working solutions of metal ions were prepared by quantitative dilution of the stock solutions directly. Ligand solutions were prepared directly by dissolving the ligand with 0.1 M NaClO4 CH3OH/10%(v)H2O solution in a volumetric flask. The concentration of the ligand was calculated by weighing. TMBiPDA Synthesis. The synthetic route is shown in Scheme 1. 10 mL of SOCl2, 2.0 g (8.2 mmol) of H2BiPDA, and a drop of DMF were refluxed for 3 h. Excess SOCl2 was removed under reduced pressure. Solid residues were dissolved in 20 mL of CH2Cl2, and then 10 mL of dimethylamine (40 wt % in H2O) was added dropwise. The resulting solution was stirred overnight at room temperature, and then was poured into water (70 mL) and extracted with CH2Cl2 (2 × 150 mL). Combined organic extracts were washed with water (2 × 120 mL) and dried over Na2SO4, and then were purified by silica column chromatography (hexanes/EtOAc) to afford white TMBiPDA (80% yield). Anal. Calcd: C, 64.41; H, 6.08; N, 18.78; O, 10.73. 1H NMR (600 MHz, CDCl3): δ = 8.47 (d, 2H), 8.10 (t, 2H), 7.66 (d, 2H), 3.2 (s, 12H). 13C NMR (600 MHz, CDCl3): δ = 169.60, 154.21, 153.47, 138.62, 123.30, 121.88, 38.39, 34.85. TMPhenDA Synthesis. The synthetic route is shown in Scheme 1. 10 mL of SOCl2, 2.0 g (6.1 mmol) of PhenDA, and a drop of DMF were refluxed for 3 h. Excess SOCl2 was removed under reduced pressure. Solid residues were dissolved in CH2Cl2 (20 mL), and 10 mL dimethylamine (40 wt % in H2O) was added dropwise. The resulting solution was stirred overnight at room temperature, and then was poured into water (70 mL) and extracted with CH2Cl2 (2 × 150 mL). Combined organic extracts were washed with water (2 × 120 mL) and dried over Na2SO4, and then were purified by silica column chromatography (DCM/MeOH) to afford TMPhenDA (82% yield) as a light gray solid. Anal. Calcd: C, 67.07; H, 5.63; N, 17.38; O, 9.93. 1 H NMR (600 MHz, CDCl3): δ = 8.39 (d, 2H), 8.11 (d, 2H), 7.90 (s, 2H), 3.3 (s, 12H). 13C NMR (600 MHz, CDCl3): δ = 146.2, 158.3, 128.9, 119.4, 136.6, 128.6, 162.3, 38.3. Spectrophotometry. UV−vis absorption spectra of Nd 3+ solutions were collected in the wavelength region of 560−650 nm (0.2 nm interval) on a double beam Cary 6000i spectrophotometer (Agilent, USA) to determine the stability constants of Nd(III) complexes with TMBiPDA and TMPhenDA. Quartz cells (QS-6, Hellma, Germany) with a 10 mm optical path were used. The sample and reference cuvettes were wrapped in the double jacket holders, maintaining the temperature at (298.2 ± 0.1) K by circulating water. For a typical titration, 2.0 mL of Nd(III) solution was put in the sample cuvette, into which appropriate aliquots of the buret were added by a progammable injection pump (HAV E11, USA), and the solution was mixed thoroughly (more than 2 min) before the spectrum was collected. Pre-experiments showed that the complexation reactions were fast and the absorbance became stable within 1 min of mixing. Usually 16−21 spectra were recorded in each set of titration. The stability constants of the metal/L complexes were calculated by the nonlinear regression program HypSpec 2009.35 Luminescence Spectroscopy. Luminescence emission spectra of Eu(III) were recorded at 298.2 K on a Horiba Jobin Yvon IBH Fluorolog-3 fluorimeter. A sub-microsecond xenon flash lamp (Jobin

6,6′-dicarboxamide ligands to lanthanides decreased linearly with increasing atomic number of the lanthanide,22 and Hancock and co-workers observed that the affinity of 1,10phenanthroline-2,9-dicarboxamide to lanthanides had a similar but unobvious trend.23,24 However, Hancock and co-workers determined some 1:1 lanthanide and actinide complexes with PhenDA, but did not find a similar trend for the complexation strength.25,26 Moreover, Borisova and Hancock only detected the 1:1 lanthanide complexes with such tetradentate mixed Nand O-donor ligands in solutions,22−24 while the 1:2 lanthanide complexes had been identified by crystallography.7,17,27−31 Ogden and co-workers first detected the 1:2 complexes of Nd(III) and Am(III) with PhenDA in aqueous solution,14 and Makrliḱ even assumed the existence of the 1:3 complex to interpret the solvent extraction data.15 What’s more, none of the aforementioned studies analyzed the complexation and its changing trend along the lanthanide series from the thermodynamic origin. So far, the tetradentate mixed N- and O-donor ligands, such as BiPDA, PhenDA, and their amide derivatives, are less explored and need further investigations to understand the fundamental coordination behavior in solutions and solid. In our previous work, we investigated the complexation thermodynamics of BiPDA and PhenDA with U(VI)32 as well as PhenDA with Np(V)33 in solutions. In the present work, the stability constants and the enthalpy of the complexation between lanthanides (Nd and Eu) and N,N,N′,N′-tetramethyl2,2′-bipyridine-6,6′-dicarboxamide (TMBiPDA) and N,N,N′,N′-tetramethyl-1,10-phenanthroline-2,9-dicarboxamide (TMPhenDA) in CH3OH/10%(v)H2O solutions were determined by thermodynamic measurements including spectrophotometry and microcalorimetry. Time-resolved fluorescence spectroscopy (TRLFS), nuclear magnetic resonance (NMR), and single-crystal X-ray diffractometry were used to reveal the structural aspects of the complexes.

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EXPERIMENTAL SECTION

Chemicals. All chemicals were reagent grade or higher. Boiled Milli-Q water was used to prepare all solutions. Spectrographic reagents of methanol (Sigma-Aldrich), Eu2O3, and Nd2O3 (99.99%, Alfa Aesar) were used without further purification. Ligands of TMBiPDA and TMPhenDA were synthesized in the lab, as described in the next section, and were dried under vacuum for 24 h at room temperature prior to use. Stock solutions of metal ions were prepared by dissolving Eu2O3 or Nd2O3 with 1 M HClO4, and the excess oxides were removed by filtration. The concentration of metal ions (Nd3+ or Eu3+) was determined by EDTA titration as described in the literature.34 B

DOI: 10.1021/acs.inorgchem.9b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement for [Nd(TMPhenDA)2(H2O)2](ClO4)3·CH3OH and [Eu(TMPhenDA)2(H2O)(CH3OH)](ClO4)3·CH3OH empirical formula formula weight temp/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 reflns collected independent reflns data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ (I)] final R indexes [all data] largest diff. peak/hole/e Å−3

[Nd(TMPhenDA)2(H2O)2](ClO4)3·CH3OH

[Eu(TMPhenDA)2(H2O)(CH3OH)](ClO4)3·CH3OH

C37H44Cl3N8NdO19 1155.39 293.15 triclinic P1̅ 11.5962(8) 13.1912(11) 15.9186(13) 83.528(7) 82.356(6) 72.248(7) 2291.8(3) 2 1.674 1.393 1170 0.35 × 0.3 × 0.2 Mo Kα (λ = 0.71073 Å) 6.048−52.744 −14 ≤ h ≤ 14, −16 ≤ k ≤ 16, −19 ≤ l ≤ 19 18840 9361 [Rint = 0.0273, Rsigma = 0.0492] 9361/0/629 1.055 R1 = 0.0382, wR2 = 0.0923 R1 = 0.0476, wR2 = 0.0985 0.95/−0.72

C38H49Cl3EuN8O19 1179.14 296.15 triclinic P1̅ 12.0033(17) 13.1428(19) 17.421(3) 74.727(5) 83.628(5) 63.778(4) 2378.3(6) 2 1.634 1.570 1178.0 0.33 × 0.25 × 0.15 Mo Kα (λ = 0.71073 Å) 4.48−55.856 −15 ≤ h ≤ 15, −17 ≤ k ≤ 17, −22 ≤ l ≤ 22 105273 11186 [Rint = 0.0323, Rsigma = 0.0167] 11186/468/701 1.122 R1 = 0.0444, wR2 = 0.1376 R1 = 0.0503, wR2 = 0.1517 2.45/−1.68

Figure 1. Representative spectrophotometric titrations of Nd(III) with TMBiPDA (a) and TMPhenDA (b) in CH3OH/10%(v)H2O (T = 298.2 K, I = 0.1 M NaClO4). (Upper): spectra of Nd(III) normalized to its initial concentration; (Lower): molarity absorptivities of Nd(III) (black) and its complexes (red for NdL, and blue for NdL2). Initial solutions in cuvette: (a) V0 = 2.0 mL, CNd0 = 18.1 mM; (b) V0 = 2.0 mL, CNd0 = 19.0 mM. Titrant: (a) 0.1 M TMBiPDA, (b) 0.1 M TMPhenDA.

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DOI: 10.1021/acs.inorgchem.9b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Thermodynamics for the Nd(III) and Eu(III) Complexes with TMBiPDA and TMPhenDA (T = 298.2 K, I = 0.1 M NaClO4 in CH3OH/10%(v)H2O) ligand TMBiPDA

TMPhenDA

log β

reaction 3+

3+

Nd + L = NdL Nd3+ + 2L = NdL23+ Eu3+ + L = EuL3+ Eu3+ + 2L = EuL23+ Nd3+ + L = NdL3+ Nd3+ + 2L = NdL23+ Eu3+ + L = EuL3+ Eu3+ + 2L = EuL23+

4.45 7.03 4.39 6.14 5.91 10.35 5.82 8.86

± ± ± ± ± ± ± ±

0.02 0.02 0.01 0.02 0.04 0.10 0.06 0.09

ΔG (kJ·mol−1)

ΔH (kJ·mol−1)

−(25.4 −(40.1 −(25.1 −(35.1 −(29.6 −(59.1 −(33.2 −(50.6

−(5.8 −(8.5 −(6.6 −(3.9 −(2.18 −(3.1 −(2.04 0.68

± ± ± ± ± ± ± ±

0.1) 0.1) 0.1) 0.1) 0.2) 0.6) 0.3) 0.5)

± ± ± ± ± ± ± ±

0.2) 0.3) 0.1) 0.1) 0.09) 0.1) 0.04) 0.06

ΔS (J·K−1·mol−1) 66 106 62 105 92 187 105 172

± ± ± ± ± ± ± ±

4 6 2 3 2 5 3 5

Figure 2. Representative luminescence tiatrations of Eu(III) with TMBiPDA (a) and TMPhenDA (b) in CH3OH/10%(v)H2O (T = 298.2 K, I = 0.1 M NaClO4). (Upper): emission spectra of Eu(III) (relative intensity) normalized to its initial concentration; (Lower): deconvoluted emission spectra of Eu3+ (black), [EuL]3+ (red), and [EuL2]3+ (blue). Initial solutions in cuvette: (a) V0 = 2.00 mL, CEu0 = 4.00 mM; (b) V0 = 2.00 mL, CEu0 = 4.00 mM. Titrant: (a) 50.00 mM TMBiPDA, (b) 50.00 mM TMPhenDA. Yvon, 5000XeF) was used as the light source, coupled with a doublegrating excitation monochromator for spectra module (Horiba Jobin Yvon IBH, TBX-04-D) equipped with a fast-rise-time photomultiplier tube, and a wideband width preamplifier as well as a picosecond constant-fraction discriminator was used as the detector. The luminescence emission spectra were obtained in the wavelength region of 550−720 nm (0.2 nm interval, 4 nm bandwidth) by excitation at 394 nm (2 nm bandwidth). The luminescence decay was obtained through a Nano-LED with an excitation of 390 ± 10 nm, and the decay curve was followed at an emission wavelength of 614 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. Microcalorimetry. Calorimetric titrations were conducted at 298.2 K with an isothermal microcalorimeter (TAM III, TA Instruments-Waters LLC, USA) to determine the enthalpy of complexation. The performance of the calorimeter was tested by measuring the enthalpy of protonation of tris(hydroxymethyl)aminomethane (THAM). The value, (−47.7 ± 0.3) kJ·mol−1, was obtained at 298.2 K and is in excellent agreement with the literature.36 0.750 mL of a solution containing M(III)/HClO4 (M = Nd or Eu) was placed in the calorimetric cell and then titrated with a solution of

ligand (TMBiPDA or TMPhenDA). Multiple titrations with different concentrations of the reagents were performed to reduce the uncertainty. In a typical titration, n additions of 0.005−0.01 mL of titrant were made (n = 25−50) through a 0.250 mL syringe, resulting in n experimental values of the heat generated in the titration cell (Qex,j, j = 1 − n). These values were corrected for the heats of titrant dilution (Qdil,j) that were measured in a separate run. The net reaction heat at the jth point (Qr,j) was obtained from the difference: Qr,j = Qex,j − Qdil,j. The value of Qr,j is a function of the concentrations of the reactants (CM and CL), the equilibrium constants, and the enthalpies of the reactions that occurred in the titration. A least-squares minimization program, HypDeltaH,37 was used to calculate the reaction enthalpies (ΔH). The corresponding entropies of complexation (ΔS) were calculated from the expression ΔG = ΔH − TΔS, where ΔG = −RT ln β. Crystallization. [Nd(TMPhenDA)2(H2O)2](ClO4)3·CH3OH (I): 2.5 mL of 0.02 M Nd(III) was added to 1.1 mL of 0.10 M TMPhenDA (L/M = 2.2) solution using CH3OH/10%(v)H2O as solvent. The mixture solution was shaken thoroughly and evaporated at room temperature. Lavender crystals appeared about a week later. [Eu(TMPhenDA)2(H2O)(CH3OH)](ClO4)3·CH3OH (II): 0.17 mL of a stock solution of Eu(III) (0.295 M) was evaporated at about 353 K D

DOI: 10.1021/acs.inorgchem.9b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Representative calorimetric titrations of Nd(III) (a) and Eu(III) (b) with TMBiPDA in CH3OH/10%(v)H2O (T = 298.2 K, I = 0.1 M NaClO4). Top: thermogram; bottom: total reaction heat (left y axis, (◊) experimental, (- -) calculated) and speciation of metal ions (right y axis, (black) M3+, (red) [ML]3+, and (green) [ML2]3+). Initial solutions: (a) V0 = 0.75 mL, CNd0 = 10.05 mM; (b) V0 = 0.75 mL, CEu0 = 10.19 mM. Titrant: 99.20 mM TMBiPDA. nearly dry, and then 1.1 mL of TMPhenDA solution (0.10 M, CH3OH as solvent) was added. The mixture solution was shaken thoroughly and evaporated at room temperature, and crystals appeared about a week later. X-ray Diffraction. Representative crystals were mounted on the goniometer and crystallographic data were collected on an Xcalibur (Agilent Technologies, USA) X-ray single-crystal diffractometer at 293 K. The XRD data indicate that compound I is a 1:2 Nd(III)/ TMPhenDA complex, and compound II is a 1:2 Eu(III)/TMPhenDA compound. Detailed crystallographic data for compounds I and II are given in Table 1, and details of the structural information have been deposited with the Cambridge Structural Database (CCDC 1899204, 1899205).38

TMPhenDA systems (upper row), as well as the spectra of the Nd(III) complexes (lower row), are very similar but different intensities, suggesting that for TMBiPDA and TMPhenDA complex NdL as well as NdL2 having similar structures.40,41 Stability Constants of Eu(III) Complexes. The stability constants of Eu(III) complexes were determined by luminescence titrations of Eu(III) by ligand (TMBiPDA or TMPhenDA) solutions, collecting the emission spectra of Eu(III) in the wavelength region of 560−720 nm. The spectrum of Eu(III) in this range contains features originating from electronic transitions from the lowest excited 5D0 to the ground state manifold, 7F1 (590.6 nm, magnetic dipole), 7F2 (614.6 nm, electric dipole), and 7F4 (697.6 nm, electric dipole).42 Figure 2 shows representative sets of emission spectra of Eu(III) titrated by TMBiPDA (Figure 2a) and TMPhenDA (Figure 2b). The amplitude of the 5D0 → 7F2 transition and the shape of the 5D0 → 7F4 transition, which are all sensitive to the environment of Eu3+, changed dramatically along with the addition of ligands. On the other hand, the relative intensities of the 5D0 → 7F1 and 7F2 emissions (7F1/7F2) are very sensitive to the detailed nature of the ligand environment, reflecting the hypersensitive character of the 5D0 → 7F2 transition.42,43 For examples as shown in Figure 2, the 7 F1/7F2 ratio for the Eu(III)/TMBiPDA titration decreased from 1.37 at the beginning to 0.26 at the end, and a similar 7 F1/7F2 ratio change (from 1.36 to 0.26) was observed for the Eu(III)/TMPhenDA titration system. All the observations indicated the complexation of Eu3+ with ligands which replaced solvent molecules (water/methanol) in the inner coordination sphere of Eu3+, and the nonexistence of an inversion center for

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RESULTS AND DISCUSSION Thermodynamic Parameters. Stability Constants of Nd(III) Complexes. Figure 1 shows the representative sets of absorption spectra of the hypersensitive 4I9/2 → 2G7/2, 4G5/2 transitions of Nd(III) for the titrations of Nd(III)/TMBiPDA (Figure 1a) and Nd(III)/TMPhenDA (Figure 1b). Significant changes were observed with the addition of ligands. The absorption bands of Nd(III) were generally red-shifted, with a new strong absorption band around 582 nm and a weak band around 594 nm appearing successively, suggesting the successive complexation of Nd(III) with ligands.39 Factor analysis with the HypSpec 2009 program indicated the presence of three absorption species in the titration system, i.e., free Nd3+, NdL, and NdL2 (L = TMBiPDA and TMPhenDA). Accordingly, the stability constants were calculated and are summarized in Table 2. As shown in Figure 1, spectrum changes of the Nd(III)/TMBiPDA and Nd(III)/ E

DOI: 10.1021/acs.inorgchem.9b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Luminescence lifetime of Eu3+ solutions titrated by TMBiPDA (a) and TMPhenDA (b) (T = 298.2 K, I = 0.1 M NaClO4). (Upper): luminescence lifetime vs the volume of titrant; (Lower): speciation of Eu(III), free Eu3+ (black), [EuL]3+ (red), and [EuL2]3+ (blue). Initial solutions in cuvette: (a) V0 = 2.00 mL, CEu0 = 4.00 mM; (b) V0 = 2.00 mL, CEu0 = 3.75 mM. Titrant: (a) 50.00 mM TMBiPDA, 1.00 mL added, (b) 50.00 mM TMPhenDA, 0.32 mL added.

the Eu(III) complexes.42 Factor analysis with the HypSpec 2009 program indicated that three fluorescent species existed in the titration system, assigned to L, EuL, and EuL2. The stability constants of complexes EuL and EuL2 were calculated and are listed in Table 2. The deconvoluted emission spectra are depicted in Figure 2 (lower row). A successive blue shift of the 5D0 → 7F4 transitions has been distinguished, from 615.0 nm for free Eu3+, to 614.2 nm/613.0 nm for complex EuL (L = TMBiPDA/TMPhenDA), and to 612.8 nm/612.0 nm for complex EuL2. Enthalpy of Complexation by Microcalorimetry. Figure 3 is the representative calorimetric titrations of Nd(III) (a) and Eu(III) (b) with TMBiPDA at 298.2 K. The calorimetric titrations of Nd(III) and Eu(III) with TMPhenDA were similar and are shown in Figure S1 in the Supporting Information (SI). Multiple titrations were performed with different concentrations of CM0 (M = Nd or Eu) and the same titrant solution of ligand TMBiPDA or TMPhenDA. Using the calorimetric data in conjunction with the stability constants obtained by spectrophotometry, we calculated the enthalpies, as well as the entropies of complexation, accordingly, which are summarized in Table 2. Thermodynamic Trends. The binding strengths of lanthanides with TMBiPDA and TMPhenDA (denoted by log βML) in this work are weaker than those with other ligand analogues bearing different substituents on the amide N atoms reported in the literatures, as shown in Figure S2 and Table S1 in the SI.22,24 We argue that this is the comprehensive result of a variety of effects, such as substituents, solvents, and ionic strength. A common trend is observed that the ligands form complexes with decreasing stability across the lanthanide series. Moreover, the distribution coefficients also decreased with increasing stomic number, when lanthanides were

extracted by N,N′-dialkyl-N,N′-diaryl-2,2′-bipyridyl-6,6′-dicarboxamides or N,N′-dialkyl-N,N′-diaryl-1,10-phenanthroline2,9-dicarboxamides.18,44,45 One possible explanation should be the radius selectivity of ligands like TMBiPDA and TMPhenDA for cations participating in complexation.24,46 The larger Nd(III) matches better the configuration of coordination atoms than the smaller Eu(III).47 In an energetic viewpoint, the formation reactions for ML complexes are exothermic and driven by enthalpy and entropy. It is interesting that the stepwise reactions for [NdL2]3+ complexes ([NdL]3+ + L ⇔ [NdL2]3+) are exothermic but endothermic for [EuL2]3+. In fact, the determined complexation enthalpy by microcalorimetry consists of two parts: (1) the released coordination enthalpy of metal with donors, and (2) the consumed desolvation enthalpy of metal ions in solution. On one hand, the solvation enthalpy (ΔHsolv) of Eu(III) is larger than that of Nd(III),48,49 such as ΔHsolv,Nd = −3445 kJ/mol and ΔHsolv,Eu = −3535 kJ/mol in aqueous solution, which means that Eu(III) consumes more desolvation enthalpy than Nd(III) when complexing with ligand. On the other hand, steric hindrance makes it more difficult for the second ligand molecule than the first one to approach metal ions (especially Eu(III) with a smaller radius), which can be validated by the fact that log K2 (log K2 = log βML2 − log βML, 1.75/2.58 (Eu/Nd) for ligand TMBiPDA and 3.04/5.16 for ligand TMPhenDA) is smaller than log K1 (log K1 = log βML). As the consequence, the enthalpies of the stepwise reactions for EuL2 are less than those for other complex species (NdL2, NdL, and EuL), and cannot compensate the desolvation enthalpy of Eu(III), which results in the endothermic nature of the stepwise reactions for EuL2 (2.70 kJ/mol for Eu(TMBiPDA)2 and 2.72 kJ/mol for Eu(TMPhenDA)2). F

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Figure 5. 1H NMR (left) and 13C NMR (right) spectra of TMBiPDA (40 mM) in the absence and presence of La3+ in CD3OD/10%(v)D2O.

(TMPhenDA)]3+) = (2.31 ± 0.10) × 10−4 s, and τ([Eu(TMPhenDA)2]3+) = (3.30 ± 0.10) × 10−4 s, respectively. NMR Data. Using the diamagnetic La(III) as the representative of Ln(III),55 we conducted the 1H NMR and 13 C NMR analysis of TMBiPDA solutions (CD3OD/ 10%(v)D2O as solvent) with various M/L ratios (0, 0.5, 1.0, and 4.0 respectively), as shown in Figure 5. Three singals at 8.48, 8.05, and 7.65 ppm are depicted in the 1H NMR spectra, cooresponding to the aromatic H(3/3′), H(4/4′), and H(5/ 5′), respectively, on the bipyridyl moiety. All the 1H NMR signals were broadened and shifted downfield and noted as H(3/3′)*, H(4/4′)*, and H(5/5′)* when M/L = 0.5, indicating the existence of complex [LaL2]3+. When M/L = 1.0, a new set of signals (H(3/3′)**, H(4/4′)**, and H(5/ 5′)**) appeared in the downfield, which were assigned to complex [LaL]3+. Along with the ratio of M/L increasing to 4.0, the locations of the new signals were the same as those when M/R = 1.0, while their profiles were similar to those of free ligand. The fact that the multiplicity of the aromatic 1H signals is the same in the free ligand and the [LaL]3+ complex suggests that the equivalency of the aromatic H atoms remains unchanged in the [LaL]3+ complex. In other words, the ligand coordinates to La3+ in a symmetrical (O-N-N-O) mode. Similar discussions on the symmetrical coordination mode in the La3+/L complexes can be achieved from the 13C NMR spectra shown in Figure 5 (right). In the absence of La3+(M/L = 0.0), six 13C signals were observed in the range of 172−122 ppm, assigned to the C(7/7′), C(2/2′), C(6/6′), C(5/5′), C(4/4′), and C(3/3′), respectively. When La3+ was added (M/L = 0.5), the 13C signals of C(7/7′), C(2/2′), C(5/5′), C(4/4′), and C(3/3′) shifted downfield a little for the coordination of La3+ which reduced the electron density on the carbon atoms. As to the chemical shifts of C(6/6′) atoms, the 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(6/6′), causing a deshielding effect on C(6/6′) atoms; on the other hand, the electrons of the N- and O-donors nearby could also be pulled toward C(6/ 6′) due to the La3+ coordination, resulting in a shielding effect. It is probably because the shielding effect is stronger than the deshielding effect that the net upfield shift was observed for the C(6/6′) atoms in the complex. Similar to what was observed in the 1H NMR spectra, when M/L = 1.0, another set of 13C

As shown in Table 2, the binding strength of M(III) (M = Nd or Eu) with TMPhenDA (represented by log β) is stronger than that with TMBiPDA. We attribute it to the wellpreorganization of PhenDA and its stronger basicity (the protonation constant of TMPhenDA is larger than that of TMBiPDA; see Figure S3 in the SI), which favor its complexation with metal ions.12,46 The values of −ΔH for the M(III)/TMPhenDA complexes are smaller than the corresponding values for the M(III)/TMBiPDA complexes (see Table 2), suggesting that the enhanced affinity of TMPhenDA is driven by entropy. We argue that the good preorganization of TMPhenDA should be the predominant contribution to its stronger complexation than TMBiPDA.50,51 Coordination Modes in Ln(III)/L Complex. Luminescence Lifetimes of Eu3+/L Species. The lifetime of Eu3+ in CH3OH/10%(v)H2O determined in this work is τ = 1.37 × 10−4 s, in good agreement with Tanaka’s result (τ = 1.37 × 10−4 s).52 The longer lifetime of Eu3+ in CH3OH/10%(v)H2O than that in aqueous solution (τ = 1.10 × 10−4 s) is originated from the coordination of methanol in the inner solvation sphere.53 Assuming the solvation number of Eu3+ is 9,49 about 3.0 methanol molecules are in the inner solvation sphere. This fact is also reflected in the smaller transition ratio 7F1/7F2 (1.37) than that of the aquo ions (7F1/7F2 = 2.0, for [Eu (H2O)9]3+ in D3h symmetry).54 The luminescence decay curves of the Eu3+/TMBiPDA and Eu3+/TMPhenDA titrations are supplied in Figure S4a and Figure S4b, respectively, in the SI. Fitting the decay curves monoexponentially, we obtained the overall lifetime of the Eu species, as shown in Figure 4 and Table S2 in the SI. The overall lifetime increased along with the addition of ligand, suggesting the successive replacement of methanol/water molecules in the inner coordination sphere of Eu3+ by TMBiPDA or TMPhenDA. The overall lifetime increased abruptly by twice (the upper row of Figure 4), coinciding perfectly with the formation of complexes [EuL]3+ and [EuL2]3+ (the lower row of Figure 4) in the titration system. Fitting the decay curves bis-exponentially, we evaluated the value of τ([EuL]3+) by averaging the longer lifetimes of the first four samples, as well as the value of τ([EuL2]3+) by averaging the longer lifetimes of the last four samples. They are τ([Eu(TMBiPDA)]3+) = (2.31 ± 0.10) × 10−4 s, τ([Eu(TMBiPDA)2]3+) = (4.54 ± 0.10) × 10−4 s, τ([EuG

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Figure 6. Crystal structures of [Nd(TMPhenDA)2(H2O)2](ClO4)3·CH3OH (I) and [Eu(TMPhenDA)2(H2O)(CH3OH)](ClO4)3·CH3OH (II). Unbound water molecules, methanol molecule, perchlorate ions and hydrogen atoms are omitted for clarity. Neodymium is colored yellow, europium green, oxygen red, nitrogen blue, and carbon gray.

signals were observed which were noted as C(7/7′)**, C(6/ 6′)**, C(5/5′)**, C(4/4′)**, and C(3/3′)**, respectively, which was the only one set of 13C signals when M/L = 4.0. In brief, the 1H and 13C NMR data agree with each other, confirming the formation of [LaL]3+ and [LaL2]3+ complexes and supporting the symmetrical (O-N-N-O) coordination mode between TMBiPDA and lanthanides. Crystal Structures of M(III)/TMPhenDA Complexes. Structures of the 1:2 M(III)/TMPhenDA (M = Nd and Eu) complexes in solid compounds, [Nd(TMPhenDA)2(H2O)2](ClO 4 ) 3 ·CH 3 OH (I) and [Eu(TMPhenDA) 2 (H 2 O)(CH3OH)](ClO4)3·CH3OH (II), are shown in Figure 6. Compounds I and II were the perchlorate salts of Nd(III)/ Eu(III) complexes with TMPhenDA, crystallized in the triclinic space group P1̅. The Nd(III) (or Eu(III)) cation was 10-coordinated with two whole ligand molecules and two solvent (water or methanol) molecules, conforming most closely to a dodecahedron. Each TMPhenDA molecule chelated with Nd (or Eu) in a tetradentate fashion through two aromatic N-donors and two carbonyl O-donors. Three perchlorate counterions were in the outer coordination sphere of Nd/Eu. Crystals were formed by accumulating of the zerodimensional complex [Nd(TMPhenDA)2(H2O)2](ClO4)3· CH3OH (or [Eu(TMPhenDA)2(H2O)(CH3OH)](ClO4)3· CH3OH by weak electrostatic interactions such as hydrogen bonds and π−π interactions. In solid the coordination mode (O-N-N-O) in solid between TMPhenDA molecules and a central Nd(III)/Eu(III) cation illustrates that, in solutions, ligand TMPhenDA (as well as TMBiPDA) interacts with lanthanide ions most probably in the (O-N-N-O) coordination mode also. The average Nd/Eu−OL distance (2.54(1)/2.49(1) Å) is shorter than that of the Nd/Eu−NL (2.65(1)/2.70(1) Å) probably because the basicity of the O-donor is “harder” than that of the N-donor which makes the Nd/Eu−OL bond stronger than the Nd/Eu−NL bond. The same observation has been found in other lanthanide/actinide complexes with mixed N- and O-donor ligands.7,18,56,57 Taking into account the radius difference between Nd(III) and Eu(III) (1.175 and 1.120 Å, respectively),58 the distance difference between the Nd−OL and the Eu−OL is in the error, while the Nd−NL is shorter than the Eu−NL. That is to say that Nd−NL is more stable than Eu−N L , validating that complex [Nd(TMPhenDA)2]3+ is more stable than [Eu(TMPhenDA)2]3+ in solution (see Table 2).

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CONCLUSIONS

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ASSOCIATED CONTENT

The complexation of Nd(III) and Eu(III) with two tetradentate mixed N- and O-donor ligands, TMBiPDA and TMPhenDA, was studied in solution and in solid. Varieties of analytical techniques, such as absorption spectroscopy, TRLFS, and NMR, have confirmed that, in solution, ligand TMPhenDA as well as TMBiPDA forms ML and ML2 complexes with Nd(III) and Eu(III) through the same (ON-N-O) coordination mode. 1:1 and 1:2 M(III)/L complexes were identified in 0.1 M NaClO4 CH3OH/10%(v)H2O solutions. The stability constants of the corresponding complexes decrease in the following orders: TMPhenDA > TMBiPDA, and Nd(III) > Eu(III). The stepwise reactions for 1:1 complexes and the 1:2 Nd(III) complexes are driven by both enthalpy and entropy, while those for the 1:2 Eu(III) complexes are driven by entropy only. The single-crystal X-ray diffraction data show that Nd/Eu coordinates with two whole ligand molecules and two solvent (water/methanol) molecules, and each TMBiPDA/TMPhenDA molecule coordinates with the central Nd/Eu atom through two aromatic N-donors and two amide O-donors. The M−O bond distances are almost identical, but the Nd−N bond distances are shorter than the Eu−O bond.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00545. Determination of the protonation constants of ligands, calorimetric results of M(III)/TMPhenDA (M = Nd and Eu) complexation system, decay curves and the overall lifetimes of the Eu(III)/L (L = TMBiPDA and TMPhenDA) titration system (PDF) Accession Codes

CCDC 1899204 and 1899205 contain 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. H

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (S.P.). ORCID

Yanqiu Yang: 0000-0001-6241-5234 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 11675156, U1830202) and the Science Challenge Project of China (TZ2016004).

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REFERENCES

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