Lanthanum(III) and Uranyl(VI) Diglycolamide Complexes: Synthetic Precursors and Structural Studies Involving Nitrate Complexation

Stoichiometric Lanthanide Compounds with Diglycolamides: A Synthetic Approach toward Understanding Rare-Earth Speciation in Solution

A fundamental understanding of coordination chemistry across the lanthanide series is essential for explaining the chemical behavior of rare-earth metals in complex liquid-liquid extraction processes. Probing the exact bonding between the extractant and the metal is sometimes done through the synthesis of solid-state compounds that can serve as models for metal speciation in solution. In the case of diglycolamide (DGA), a commonly used neutral diamide extractant, extensive studies identify the stepwise formation of 1:1 [Ln(DGA)(H2O)6]3+, 1:2 [Ln(DGA)2(H2O)3]3+, and 1:3 [Ln(DGA)3]3+ complexes in solution. The crystallographic reports, however, exclusively show a 1:3 [Ln(DGA)3]3+ moiety in the solid state, while the structures of 1:1 and 1:2 complexes are yet to be isolated and comprehensively studied. In this work, we report the synthesis and characterization of three new families of stoichiometric N,N,N',N'-tetramethyldiglycolamide (TMDGA) compounds: [Ln(TMDGA)(H2O)5Cl]Cl2·2H2O, [Ln(TMDGA)2(H2O)3]Cl3·3H2O, and [Ln(TMDGA)3]Cl3·7H2O, where Ln = Nd, Eu, Gd, with Ln:TMDGA ratios of 1:1, 1:2, and 1:3, respectively. The compounds have been analyzed using vibrational spectroscopy (both Raman and FT-IR), as well as variable temperature fluorescence spectroscopy. A spectrophotometric titration of Eu(III) was performed to confirm the presence of the [Eu(TMDGA)n]3+ (n = 1, 2, and 3) species in solution and to compare the individual solid-state emission spectra to their respective analogues in solution.

Small Cyclic Diglycolamides: Tautomerism, Solvent Extraction and Coordination with f-Elements: One Strain to Rule Them All

The search for new effective extractants is an important task for the management of high-level liquid waste (HLW) generated during the reprocessing of spent nuclear fuel. Here, we synthesized a series of diglycolamides with cyclic substituents for the first time. We disclosed their coordination with f-element nitrates [La(NO3)3 and UO2(NO3)2] by SC-XRD study and complexation properties toward Am(III), Ln(III), and U(VI) during solvent extraction from nitric acid solutions. Using dynamic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations, the importance of tautomerism in the extraction properties of diglycolamides was shown.

Co-Crystallization of Plutonium(III) and Plutonium(IV) Diglycolamides with Pu(III) and Pu(IV) Hexanitrato Anions: A Route to Redox Variants of [PuIII,IV(DGA)3][PuIII,IV(NO3)6]x

N,N,N',N'-tetramethyl diglycolamide (TMDGA), a methylated variant of the diglycolamide extractants being proposed as curium holdback reagents in advanced used nuclear fuel reprocessing technologies, has been crystallized with plutonium, a transuranic actinide that has multiple accessible oxidation states. Two plutonium TMDGA complexes, [PuIII(TMDGA)3][PuIII(NO3)6] and[PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH, were crystallized through solvent diffusion of a reaction mixture containing plutonium(III) nitrate and TMDGA. The sample was then partially oxidized by air to yield [PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH. Single-crystal X-ray diffraction reveals that the multinuclear systems crystallize with hexanitrato anionic species, providing insight into the first solid-state isolation of the elusive trivalent plutonium hexanitrato species. Crystallography data show a change in geometry around the TMDGA metal center from Pu3+ to Pu4+, with the symmetry increasing approximately from C4v to D3h. These complexes provide a rare opportunity to investigate the bond metrics of plutonium in two different oxidation states with similar coordination environments. Further, these new structures provide insight into the potential chemical and structural differences arising from the radiation-induced formation of transient tetravalent curium oxidation states in used nuclear fuel reprocessing streams.

Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N',N',N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La³⁺, Nd³⁺, Eu³⁺, and Lu³⁺) were systematically investigated by using ¹H nuclear magnetic resonance (¹H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. ¹H NMR spectroscopic titration of La³⁺ and Lu³⁺ indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO₃^- and ClO₄^- media. The stability constants of NTAamide(Et) with Nd³⁺ and Eu³⁺ obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd³⁺ is lower than that with Eu³⁺ in the same anionic medium, while that of the same lanthanide complex is higher in ClO₄^- medium than in NO₃^- medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO₄^- and NO₃^- medium and the mixed one were determined to be [LnL₂(MeOH)](ClO₄)₃ (Ln = La, Nd, Eu, and Lu), [LaL₂(EtOH)₂][La(NO₃)₆], and [LaL₂(NO₃)](ClO₄)₂, separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln³⁺, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.

Solvation-Anionic Exchange Mechanism of Solvent Extraction: Enhanced U(VI) Uptake by Tetradentate Phenanthroline Ligands

Phenanthroline diamides (L) demonstrated a unique ability to extract uranium from nitric acid solutions into a polar organic solvent forming complexes of 1:2 stoichiometry as tight ion pairs {[UO₂LNO₃]⁺[UO₂(NO₃)₃]^-} by a novel extraction mechanism, which is a combination of two already well-known mechanisms: solvation and ion-pair anion exchange. A UV-vis study was used to confirm the formation of such complexes directly in the organic phase. Moreover, chemical synthesis and single crystal growth were performed to confirm unambiguously the structure of the complexes in the solid state.

Lanthanide(III) Complexes Based on an 18-Membered Macrocycle Containing Acetamide Pendants. Structural Characterization and paraCEST Properties

We report a detailed investigation of the coordination properties of macrocyclic lanthanide complexes containing a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane scaffold functionalized with four acetamide pendant arms. The X-ray structures of the complexes with the large Ln³⁺ ions (La and Sm) display 12- and 10-coordinated metal ions, where the coordination sphere is fulfilled by the six N atoms of the macrocycle, the four O atoms of the acetamide pendants, and a bidentate nitrate anion in the La³⁺ complex. The analogous Yb³⁺ complex presents, however, a 9-coordinated metal ion because one of the acetamide pendant arms remains uncoordinated. ¹H NMR studies indicate that the 10-coordinated form is present in solution throughout the lanthanide series from La to Tb, while the smaller lanthanides form 9-coordinated species. ¹H and ⁸⁹Y NMR studies confirm the presence of this structural change because the two species are present in solution. Analysis of the ¹H chemical shifts observed for the Tb³⁺ complex confirms its D₂ symmetry in aqueous solution and evidences a highly rhombic magnetic susceptibility tensor. The acetamide resonances of the Pr³⁺ and Tb³⁺ complexes provided sizable paraCEST effects, as demonstrated by the corresponding Z-spectra recorded at different temperatures and studies on tube phantoms recorded at 22 °C.

Understanding Selectivity of Mesoporous Silica-Grafted Diglycolamide-Type Ligands in the Solid-Phase Extraction of Rare Earths

Rare earth elements (REEs) and their compounds are essential for rapidly developing modern technologies. These materials are especially critical in the area of green/sustainable energy; however, only very high-purity fractions are appropriate for these applications. Yet, achieving efficient REE separation and purification in an economically and environmentally effective way remains a challenge. Moreover, current extraction technologies often generate large amounts of undesirable wastes. In that perspective, the development of selective, reusable, and extremely efficient sorbents is needed. Among numerous ligands used in the liquid-liquid extraction (LLE) process, the diglycolamide-based (DGA) ligands play a leading role. Although these ligands display notable extraction performance in the liquid phase, their extractive chemistry is not widely studied when such ligands are tethered to a solid support. A detailed understanding of the relationship between chemical structure and function (i.e., extraction selectivity) at the molecular level is still missing although it is a key factor for the development of advanced sorbents with tailored selectivity. Herein, a series of functionalized mesoporous silica (KIT-6) solid phases were investigated as sorbents for the selective extraction of REEs. To better understand the extraction behavior of these sorbents, different spectroscopic techniques (solid-state NMR, X-ray photoelectron spectroscopy, XPS, and Fourier transform infrared spectroscopy, FT-IR) were implemented. The obtained spectroscopic results provide useful insights into the chemical environment and reactivity of the chelating ligand anchored on the KIT-6 support. Furthermore, it can be suggested that depending on the extracted metal and/or structure of the ligand and its attachment to KIT-6, different functional groups (i.e., C═O, N-H, or silanols) act as the main adsorption centers and preferentially capture targeted elements, which in turn may be associated with the different selectivity of the synthesized sorbents. Thus, by determining how metals interact with different supports, we aim to better understand the solid-phase extraction process of hybrid (organo)silica sorbents and design better extraction materials.

Near-infrared photothermal conversion of stable radicals photoinduced from a viologen-based coordination polymer

A highly stable radical photoinduced from a viologen-based coordination polymer exhibited a remarkable near-infrared photothermal effect with good recyclability.

Highly Efficient N-Pivot Tripodal Diglycolamide Ligands for Trivalent f-Cations: Synthesis, Extraction, Spectroscopy, and Density Functional Theory Studies

A series of four N-pivot tripodal diglycolamide (DGA) ligands, where three DGA moieties are attached to the central N atom via spacers of different lengths and with varying alkyl substituents on the amidic nitrogen of DGA (L_I-L_IV), were studied for their extraction and complexation ability toward trivalent lanthanide/actinide ions, including solvent extraction, complexation using spectrophotometric titrations, and luminescence spectroscopic studies. Introduction of a methyl group on the amidic nitrogen atom gives rise to a 400 fold increase of the Eu distribution ( D) value [L_III (NMe) vs L_II (NH)] at 1 M HNO₃. Enlargement of the spacer length between the pivotal N atom and the DGA moieties with one carbon atom results in a 14 times higher D_Eu value [L_I (C3) vs L_II (C2)]. Slope analyses showed that Eu³⁺ was extracted as a bis-solvated species with all four ligands. The compositions of the Eu³⁺/L complexes were further confirmed by spectroscopic measurements, its formation constants following the order: L_III > L_IV > L_I > L_II. Luminescence spectroscopy and electrospray ionization mass spectrometry revealed that all four ligands form [Eu(L)₂(NO₃)₃] complexes. Density functional theory and thermodynamic parameters corroborated the existence of [Eu(L)₂(NO₃)₃] complexes.

Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f Elements by Diastereomers of a Methylated Diglycolamide

When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me₂ -TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X-ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter-ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal-ligand complexation mechanisms, which will promote the development of more efficient separation techniques.

Enhanced free energy of extraction of Eu3+ and Am3+ ions towards diglycolamide appended calix[4]arene: insights from DFT-D3 and COSMO-RS solvation models

Density functional theory in conjunction with COSMO and COSMO-RS solvation models employing dispersion correction (DFT-D3) has been applied to gain an insight into the complexation of Eu³⁺/Am³⁺ with diglycolamide (DGA) and calix[4]arene appended diglycolamide (CAL4DGA) in ionic liquids by studying structures, energetics, thermodynamics and population analysis. The calculated Gibbs free energy for both Eu³⁺ and Am³⁺ ions with DGA was found to be smaller than that with CAL4DGA. The entropy of complexation was also found to be reduced to a large extent with DGA compared to complexation with CAL4DGA. The solution phase free energy was found to be negative and was higher for Eu³⁺ ion. The entropy of complexation was not only found to be further reduced but also became negative in the case of DGA alone. Though the entropy was found to be negative it could not outweigh the high negative enthalpic contribution. The same trend was observed in the solution where the free energy of extraction, ΔG, for Eu³⁺ ions was shown to be higher than that for Am³⁺ ions towards free DGA. But the values of ΔG and ΔΔG(= ΔG_Eu-ΔG_Am) were found to be much higher with CAL4DGA (-12.58 kcal mol^-1) in the presence of nitrate ions compared to DGA (-1.69 kcal mol^-1) due to enhanced electronic interaction and positive entropic contribution. Furthermore, both the COSMO and COSMO-RS models predict very close values of ΔΔΔG (= ΔΔG_CAL4DGA - ΔΔG_nDGA), indicating that both solvation models could be applied for evaluating the metal ion selectivity. The value of the reaction free energy was found to be higher after dispersion correction. The charge on the Eu and Am atoms for the complexes with DGA and CAL4DGA indicates the charge-dipole type interaction leading to strong binding energy. The present theoretical results support the experimental findings and thus might be of importance in the design of functionalized ligands.

Formation and Fragmentation Chemistry of Tripositive Ln(TMGA)33+ Complexes in the Gas Phase

Electrospray ionization (ESI) of LnCl₃ (Ln = La-Lu except Pm) and TMGA (tetramethyl glutaramide) mixtures resulted in the formation of gas-phase Ln(TMGA)₃³⁺ complexes, where tripositive lanthanide cation was coordinated by three neutral TMGA ligands. Collision induced dissociation (CID) was employed to investigate the fragmentation chemistry of these tripositive complexes. Ln(TMGA)₂(TMGA- 45)³⁺ resulting from C_carbonyl-N bond cleavage of TMGA and hydrogen transfer is the major CID product for all Ln(TMGA)₃³⁺ except Eu(TMGA)₃³⁺ which predominantly forms divalent Eu^II(TMGA)₂²⁺ complex via loss of TMGA⁺. Analogous Yb^II(TMGA)₂²⁺ and Sm^II(TMGA)₂²⁺ complexes arising from charge reduction were also observed, in competition with the formation of charge conserving Yb^III(TMGA)(TMGA-H)²⁺ and Sm^III(TMGA)(TMGA-H)²⁺ products. The yield of these charge reducing products follows their reduction potentials in condensed phase. In addition to Ln(TMGA)₃³⁺, tripositive ions such as Ln(TMGA)₄³⁺ and Ln(TMGA)₂³⁺ were experimentally identified as well. While the former was observed along with Ln(TMGA)₃³⁺ during ESI, the latter was observed upon CID of Ln(TMGA)₃³⁺, suggesting two TMGA molecules can stabilize Ln³⁺ in the gas phase. Graphical Abstract ᅟ.

Surface Engineering of PAMAM-SDB Chelating Resin with Diglycolamic Acid (DGA) Functional Group for Efficient Sorption of U(VI) and Th(IV) from Aqueous Medium

A novel chelating resin obtained via growth of PAMAM dendron on surface of styrene divinyl benzene resin beads, followed by diglycolamic acid functionalization of the dendrimer terminal. Batch experiments were conducted to study the effects of pH, nitric acid concentration, amount of adsorbent, shaking time, initial metal ion concentration and temperature on U(VI) and Th(IV) adsorption efficiency. Diglycolamic acid terminated PAMAM dendrimer functionalized styrene divinylbenzene chelating resin (DGA-PAMAM-SDB) is found to be an efficient candidate for the removal of U(VI) and Th(IV) ions from aqueous (pH >4) and nitric acid media (>3M). The sorption equilibrium could be reached within 60min, and the experimental data fits with pseudo-second-order model. Langmuir sorption isotherm model correlates well with sorption equilibrium data. The maximum U(VI) and Th(IV) sorption capacity onto DGA-PAMAMG₅-SDB was estimated to be about 682 and 544.2mgg^-1 respectively at 25°C. The interaction of actinides and chelating resin is reversible and hence, the resin can be regenerated and reused. DFT calculation on the interaction of U(VI) and Th(IV) ions with chelating resin validates the experimental findings.

Isotopic signature of selected lanthanides for nuclear activities profiling using cloud point extraction and ICP-MS/MS

The presence of fission products, which include numerous isotopes of lanthanides, can impact the isotopic ratios of these elements in the environment. A cloud point extraction (CPE) method was used as a preconcentration/separation strategy prior to measurement of isotopic ratios of three lanthanides (Nd, Sm, and Eu) by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS). To minimise polyatomic interference, the combination of interferents removal by CPE, reaction/collision cell conditions in He and NH3 mode and tandem quadrupole configuration was investigated and provided optimal results for the determination of isotopic ratio in environmental samples. Isotopic ratios were initially measured in San Joaquin soil (NIST-2709a), an area with little contamination of nuclear origin. Finally, samples collected from three sites with known nuclear activities (Fangataufa Lagoon in French Polynesia, Chernobyl and the Ottawa River near Chalk River Laboratory) were analysed and all exhibited altered isotopic ratios for (143/145)Nd, (147/149)Sm, and (151/153)Eu. These results demonstrate the potential of CPE and ICP-MS/MS for the detection of altered isotopic ratio in environmental samples collected in area subjected to nuclear anthropogenic contamination. The detection of variations in these isotopic ratios of fission products represents the first application of CPE in nuclear forensic investigations of environmental samples.

The separation mechanism of Am(iii) from Eu(iii) by diglycolamide and nitrilotriacetamide extraction reagents using DFT calculations

DFT calculated stabilization energies reproduced the experimental separation behavior of Am³⁺ from Eu³⁺ using diglycolamide and nitrilotriacetamide ligands.

A fluorescence study on the complexation of Sm(iii), Eu(iii) and Tb(iii) with tetraalkyldiglycolamides (TRDGA) in aqueous solution, in solid state, and in solvent extraction

Trivalent lanthanide ions form 1 : 1, 1 : 2, and 1 : 3 complexes with tridentate ligand TMDGA in 1 M H/NaNO₃ and form 1 : 3 extracted complexes with DMDODGA during solvent extraction.

Complexation thermodynamics of diglycolamide with f-elements: solvent extraction and density functional theory analysis

Optimized complexes of Lu³⁺ with TMDGA in 1 : 1, 1 : 2 and 1 : 3 stoichiometric ratios and plots of theoretically calculated stepwise binding energy.

Thermodynamics of biphasic lanthanide extraction by tripodal diglycolamide: a solution calorimetry study

Isothermal titration calorimetry has been used for the direct measurement of the enthalpy of extraction (ΔH_extr) of metal ions in a solvent extraction process.

A structural and spectrophotometric study on the complexation of Am(III) with TMOGA in comparison with the extracted complex of DMDOOGA

Complexation of Am(iii) with tetramethyl-3-oxa-glutaramide (TMOGA, L(I)) is studied by spectrophotometric titrations and single crystal X-ray diffraction. Three successive complex species, [AmL(I)](3+), [AmL](3+), and [AmL](3+), have been identified and their stability constants are calculated to be 3.71 ± 0.012, 5.95 ± 0.021, and 6.93 ± 0.034 respectively, from the absorption spectra collected from the titrations of Am(iii) with L(I) at 25 °C in 1 M NaNO3. Single crystals of AmL(ClO4)3 have been grown from a HClO4 solution containing Am(3+) and L(I). The crystal structure of AmL(ClO4)3 shows that Am(iii) is coordinated by nine oxygen atoms from three L(I) ligands. The deconvoluted UV-Vis absorption spectrum of [AmL](3+) in aqueous solution is nearly identical to the diffusion reflectance spectrum of AmL(ClO4)3 in the solid state, indicating that the coordination geometry of the complexes is nearly the same. In addition, to provide parallels to solvent exaction, the extracted Am(iii) complex with N,N'-dimethyl-N,N'-dioctyl-3-oxa-glutaramide (DMDOOGA, L(II)) is also prepared and studied using spectrophotometry. The similarity in UV-Vis absorption of the extracted complex of Am(iii) with L(II) and [AmL](3+) suggests that the Am(iii) ion is also coordinated by three tridentate L(II) ligands existing as [AmL](3+) in the organic phase of solvent extraction.

Selective recovery of rare earth elements using chelating ligands grafted on mesoporous surfaces

Novel hybrid sorbents have been designed for the selective extraction of rare earth elements (REEs). The tunning of the ligand bite angle and the grafting of these organic molecules on a silica support allow for selective discrimination of REE ions.

The selectivity of diglycolamide (TODGA) and bis-triazine-bipyridine (BTBP) ligands in actinide/lanthanide complexation and solvent extraction separation – a theoretical approach

Charge transfer from the ligand to valence hybrid (mainly d) metal orbitals nearly equally stabilizes cationic Eu^III and Am^III TODGA complexes.

An europium(iii) diglycolamide complex: insights into the coordination chemistry of lanthanides in solvent extraction

A cationic europium(iii) complex with three neutral diglycolamide ligands (for Rn-C₈H₁₇) was prepared as the bismuth tetrachloride salt revealing a 9-O inner-sphere environment with 6 distant carbonyl (C_a) and 6 ether (C_b) carbon atoms in the Eu L₃-edge EXAFS.

Extraction and structural studies of an unexplored monoamide, N,N'-dioctyl, α-hydroxy acetamide with lanthanide(III) and actinide(III) ions

A monoamide, N,N'-dioctyl, α-hydroxy acetamide, shows unusual extraction properties towards trivalent lanthanide and actinide ions above 3 M HNO3. The extracted ions could be quantitatively back extracted using 0.5 M HNO3. This amide shows negligible extraction towards Sr(II) and Ru(III) ions, making it advantageous over other reported extractants. The structures of Sm(III) and Eu(III) nitrate compounds show that the metal ion is surrounded by three of the ligands, one nitrate and one water molecule. The ligand acts as a neutral bidentate ligand and bonds through the amido and hydroxyl oxygen atoms.

Structural and thermodynamic study of the complexes of Nd(III) with N,N,N',N'-tetramethyl-3-oxa-glutaramide and the acid analogues

The thermodynamics of Nd(III) complexes with N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA, L(I)), N,N-dimethyl-3-oxa-glutaramic acid (DMOGA, HL(II)), and oxydiacetic acid (ODA, H2L(III)) in aqueous solutions was studied. Stability constants, enthalpies, and entropies of complexation were determined by spectrophotometry, potentiometry, and calorimetry. The stability constants of corresponding Nd(III) complexes decrease in the following order: Nd(III)/L(III) > Nd(III)/L(II) > Nd(III)/L(I). For all complexes, the enthalpies of complexation are negative and the entropies of complexation are positive, indicating that the complexation is driven by both enthalpy and entropy. Furthermore, from L(III) to L(II), and to L(I), the enthalpy of complexation becomes more exothermic and the entropy of complexation less positive, suggesting that the substitution of a carboxylate group with an amide group on the ligands enhances the enthalpy-driven force but weakens the entropy-driven force of the complexation with Nd(III). Crystal structures of three 1:3 Nd(III) complexes, Nd(L(I))3(ClO4)3 (I), Nd(L(I))3(NO3)3(H2O)2 (II), and Nd(L(II))3(H2O)7.5 (III), were determined by single-crystal X-ray diffraction and compared with the structure of a 1:3 Nd(III)/L(III) complex in the literature, Na3NdL(III)3(NaClO4)2(H2O)6 (IV). In all four structures, the ligands are tridentate and Nd(III) is nine-coordinated with similar distorted tricapped trigonal prism geometry by three ether oxygen atoms capped on the three faces of the prism, and six oxygen atoms from the ketone group or carboxyl group at the corners. The absorption spectra of Nd(III) in solutions showed very similar patterns as Nd(III) formed successive 1:1, 1:2, and 1:3 complexes with L(I), L(II), and L(III), respectively, implying that the Nd(III) complexes with the three ligands have similar coordination geometries in aqueous solutions, as observed in the solids.