Structural and Thermodynamic Study of the Complexes of Nd(III) with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetramethyl-3-oxa-glutaramide and the Acid Analogues

The thermodynamics of Nd­(III) complexes with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethyl-3-oxa-glutaramide (TMOGA, L^I), <i>N</i>,<i>N</i>-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 (<b>I</b>), Nd­(L^I)₃(NO₃)₃(H₂O)₂ (<b>II</b>), and Nd­(L^II)₃(H₂O)_7.5 (<b>III</b>), 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^III₃(NaClO₄)₂(H₂O)₆ (<b>I</b><b>V</b>). 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

Structural and Thermodynamic Study of the Complexes of Nd(III) with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetramethyl-3-oxa-glutaramide and the Acid Analogues

The thermodynamics of Nd­(III) complexes with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethyl-3-oxa-glutaramide (TMOGA, L^I), <i>N</i>,<i>N</i>-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 (<b>I</b>), Nd­(L^I)₃(NO₃)₃(H₂O)₂ (<b>II</b>), and Nd­(L^II)₃(H₂O)_7.5 (<b>III</b>), 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^III₃(NaClO₄)₂(H₂O)₆ (<b>I</b><b>V</b>). 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

Structural and Thermodynamic Study of the Complexes of Nd(III) with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetramethyl-3-oxa-glutaramide and the Acid Analogues

The thermodynamics of Nd­(III) complexes with <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethyl-3-oxa-glutaramide (TMOGA, L^I), <i>N</i>,<i>N</i>-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 (<b>I</b>), Nd­(L^I)₃(NO₃)₃(H₂O)₂ (<b>II</b>), and Nd­(L^II)₃(H₂O)_7.5 (<b>III</b>), 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^III₃(NaClO₄)₂(H₂O)₆ (<b>I</b><b>V</b>). 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

Complexation of Curium(III) with DTPA at 10–70 °C: Comparison with Eu(III)–DTPA in Thermodynamics, Luminescence, and Coordination Modes

Separation of trivalent actinides (An­(III)) from trivalent lanthanides (Ln­(III)) is a challenging task because of the nearly identical chemical properties of these groups. Diethylenetriaminepentaacetate (DTPA), a key reagent used in the TALSPEAK process that effectively separates An­(III) from Ln­(III), is believed to play a critical role in the An­(III)/Ln­(III) separation. However, the underlying principles for the separation based on the difference in the complexation of DTPA with An­(III) and Ln­(III) remain unclear. In this work, the complexation of DTPA with Cm­(III) at 10–70 °C was investigated by spectrophotometry, luminescence spectroscopy, and microcalorimetry, in conjunction with computational methods. The binding strength, the enthalpy of complexation, the coordination modes, and the luminescence properties are compared between the Cm­(III)–DTPA and Eu­(III)–DTPA systems. The experimental and computational data demonstrated that the difference between Cm­(III) and Eu­(III) in the binding strength with DTPA can be attributed to the stronger covalence bonding between Cm­(III) and the nitrogen donors of DTPA

Complexation of U(VI) with Dipicolinic Acid: Thermodynamics and Coordination Modes

Complexation of UO₂²⁺ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO₄. The stability constants (log β₁ and log β₂) for two successive complexes, UO₂L and UO₂L₂^2– where L^2– stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (Δ<i>H</i>₁ and Δ<i>H</i>₂) were measured to be −(6.9 ± 0.2) and −(28.9 ± 0.5) kJ·mol^–1 by microcalorimetry. The entropies of complexation (Δ<i>S</i>₁ and Δ<i>S</i>₂) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K^–1·mol^–1. The strong complexation of UO₂²⁺ with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na₂UO₂L₂(H₂O)₈(s) shows that, in the 1:2 UO₂²⁺/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO₂²⁺ via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO₂²⁺, little energy is required for the preorganization of the ligand, resulting in strong UO₂²⁺/DPA complexation

Complexation of U(VI) with Dipicolinic Acid: Thermodynamics and Coordination Modes

Complexation of UO₂²⁺ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO₄. The stability constants (log β₁ and log β₂) for two successive complexes, UO₂L and UO₂L₂^2– where L^2– stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (Δ<i>H</i>₁ and Δ<i>H</i>₂) were measured to be −(6.9 ± 0.2) and −(28.9 ± 0.5) kJ·mol^–1 by microcalorimetry. The entropies of complexation (Δ<i>S</i>₁ and Δ<i>S</i>₂) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K^–1·mol^–1. The strong complexation of UO₂²⁺ with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na₂UO₂L₂(H₂O)₈(s) shows that, in the 1:2 UO₂²⁺/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO₂²⁺ via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO₂²⁺, little energy is required for the preorganization of the ligand, resulting in strong UO₂²⁺/DPA complexation

Complexation of U(VI) with Dipicolinic Acid: Thermodynamics and Coordination Modes

Complexation of UO₂²⁺ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO₄. The stability constants (log β₁ and log β₂) for two successive complexes, UO₂L and UO₂L₂^2– where L^2– stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (Δ<i>H</i>₁ and Δ<i>H</i>₂) were measured to be −(6.9 ± 0.2) and −(28.9 ± 0.5) kJ·mol^–1 by microcalorimetry. The entropies of complexation (Δ<i>S</i>₁ and Δ<i>S</i>₂) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K^–1·mol^–1. The strong complexation of UO₂²⁺ with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na₂UO₂L₂(H₂O)₈(s) shows that, in the 1:2 UO₂²⁺/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO₂²⁺ via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO₂²⁺, little energy is required for the preorganization of the ligand, resulting in strong UO₂²⁺/DPA complexation

Dissociation of Diglycolamide Complexes of Ln³⁺ (Ln = La–Lu) and An³⁺ (An = Pu, Am, Cm): Redox Chemistry of 4f and 5f Elements in the Gas Phase Parallels Solution Behavior

Tripositive lanthanide and actinide ions, Ln³⁺ (Ln = La–Lu) and An³⁺ (An = Pu, Am, Cm), were transferred from solution to gas by electrospray ionization as Ln­(L)₃³⁺ and An­(L)₃³⁺ complexes, where L = tetramethyl-3-oxa-glutaramide (TMOGA). The fragmentation chemistry of the complexes was examined by collision-induced and electron transfer dissociation (CID and ETD). Protonated TMOGA, HL⁺, and Ln­(L)­(L–H)²⁺ are the major products upon CID of La­(L)₃³⁺, Ce­(L)₃³⁺, and Pr­(L)₃³⁺, while Ln­(L)₂³⁺ is increasingly pronounced beyond Pr. A C–O_ether bond cleavage product appears upon CID of all Ln­(L)₃³⁺; only for Eu­(L)₃³⁺ is the divalent complex, Eu­(L)₂²⁺, dominant. The CID patterns of Pu­(L)₃³⁺, Am­(L)₃³⁺, and Cm­(L)₃³⁺ are similar to those of the Ln­(L)₃³⁺ for the late Ln. A striking exception is the appearance of Pu­(IV) products upon CID of Pu­(L)₃³⁺, in accord with the relatively low Pu­(IV)/Pu­(III) reduction potential in solution. Minor divalent Ln­(L)₂²⁺ and An­(L)₂²⁺ were produced for all Ln and An; with the exception of Eu­(L)₂²⁺ these complexes form adducts with O₂, presumably producing superoxides in which the trivalent oxidation state is recovered. ETD of Ln­(L)₃³⁺ and An­(L)₃³⁺ reveals behavior which parallels that of the Ln³⁺ and An³⁺ ions in solution. A C–O_ether bond cleavage product, in which the trivalent oxidation state is preserved, appeared for all complexes; charge reduction products, Ln­(L)₂²⁺ and Ln­(L)₃²⁺, appear only for Sm, Eu, and Yb, which have stable divalent oxidation states. Both CID and ETD reveal chemistry that reflects the condensed-phase redox behavior of the 4f and 5f elements

Tetrapositive Plutonium, Neptunium, Uranium, and Thorium Coordination Complexes: Chemistry Revealed by Electron Transfer and Collision Induced Dissociation

The Pu⁴⁺, Np⁴⁺, and U⁴⁺ ions, which have large electron affinities of ∼34.6, ∼33.6, and ∼32.6 eV, respectively, were stabilized from solution to the gas phase upon coordination by three neutral tetramethyl-3-oxa-glutaramide ligands (TMOGA). Both collision induced dissociation (CID) and electron transfer dissociation (ETD) of Pu­(TMOGA)₃⁴⁺ reveal the propensity for reduction of Pu­(IV) to Pu­(III), by loss of TMOGA⁺ in CID and by simple electron transfer in ETD. The reduction of Pu­(IV) is in distinct contrast to retention of Th­(IV) in both CID and ETD of Th­(TMOGA)₃⁴⁺, where only the C–O_ether bond cleavage product was observed. U­(TMOGA)₃⁴⁺ behaves similarly to Th­(TMOGA)₃⁴⁺ upon CID and ETD, while the fragmentation patterns of Np­(TMOGA)₃⁴⁺ lie between those of Pu­(TMOGA)₃⁴⁺ and U­(TMOGA)₃⁴⁺. It is notable that the gas-phase fragmentation behaviors of these exceptional tetrapositive complexes parallel fundamental differences in condensed phase chemistry within the actinide series, specifically the tendency for reduction from the IV to III oxidation states

Extraction of Lanthanide and Actinide Ions from Aqueous Mixtures Using a Carboxylic Acid-Functionalized Porous Aromatic Framework

Porous aromatic frameworks (PAFs) incorporating a high concentration of acid functional groups possess characteristics that are promising for use in separating lanthanide and actinide metal ions, as required in the treatment of radioactive waste. These materials have been shown to be indefinitely stable to concentrated acids and bases, potentially allowing for multiple adsorption/stripping cycles. Additionally, the PAFs combine exceptional features from MOFs and inorganic/activated carbons giving rise to tunable pore surfaces and maximum chemical stability. Herein, we present a study of the adsorption of selected metal ions, Sr²⁺, Fe³⁺, Nd³⁺, and Am³⁺, from aqueous solutions employing a carbon-based porous aromatic framework, BPP-7 (Berkeley Porous Polymer-7). This material displays high metal loading capacities together with excellent adsorption selectivity for neodymium over strontium based on Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. Based in part upon X-ray absorption spectroscopy studies, the stronger adsorption of neodymium is attributed to multiple metal ion and binding site interactions resulting from the densely functionalized and highly interpenetrated structure of BPP-7. Recyclability and combustibility experiments demonstrate that multiple adsorption/stripping cycles can be completed with minimal degradation of the polymer adsorption capacity