Interactions of Bis(2,4,4-trimethylpentyl)dithiophosphinate with Trivalent Lanthanides in a Homogeneous Medium: Thermodynamics and Coordination Modes

Complexation of trivalent lanthanides with a sulfur-bearing ligand, bis­(2,4,4-trimethylpentyl) dithiophosphinate, was studied in ethanol under identical conditions by optical spectroscopy, microcalorimetry, luminescence lifetime measurement, and extended X-ray absorption fine structure (EXAFS). Three successive complexes, LnL²⁺, LnL₂⁺, and LnL₃, where Ln and L denote the trivalent lanthanide and the dithiophosphinate ligand, respectively, formed in the solution. In contrast to the general findings that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, the complexation strength between Ln­(III) and dithiophosphinate first increases from La­(III) to Nd­(III) and then decreases gradually toward heavier Ln­(III) across the lanthanide series. This trend agrees well with the results of solvent extraction using the same ligand as an extractant. The complexation is driven by highly positive entropies and opposed by endothermic enthalpies. The enthalpies of complexation become less endothermic from lighter to heavier Ln­(III), suggesting that less energy is required for desolvation for the complexation of heavier Ln­(III). EXAFS study shows that, from lighter to heavier Ln­(III), the number of sulfur atoms in the primary coordination sphere decreases while the number of oxygen atoms increases, which confirms that fewer solvent molecules are desolvated from heavier Ln­(III) during the complexation process. A correlation between the thermodynamics trends and the coordination modes has thereby been well established

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

Scientific Basis for Efficient Extraction of Uranium from Seawater. I: Understanding the Chemical Speciation of Uranium under Seawater Conditions

In recent years, the prospective recovery of uranium from seawater has become a topic of interest owing to the increasing demand for nuclear fuel worldwide and because of efforts to find sustainable alternatives to terrestrial mining for uranium. To date, the most advanced and promising method of extracting and concentrating uranium from seawater involves the use of polymeric sorbents containing the amidoxime binding moiety. Among a number of different moieties investigated, glutaroimide-dioxime is the most promising one, forming very stable complexes with U­(VI) even in the presence of carbonate. To properly assess the affinity of uranium toward the amidoxime substrates, a comprehensive knowledge of the aqueous chemical equilibria of uranium is required. With this aim, in this paper we review the chemical equilibria of uranium (as UO₂²⁺) in seawater, focusing on the solution equilibria leading to the formation of the stable complexes, M_<i>m</i>(UO₂)­(CO₃)₃^{(2<i>m</i>–4)}(aq) (M = Ca or Mg, <i>m</i> = 0–2). These binary and ternary species dominate the chemistry of uranium in seawater and have recently been the object of study in several papers in the literature. The solubility equilibria of UO₂²⁺ in seawater leading to the formation of the known minerals, including Liebigite, Ca₂(UO₂)­(CO₃)₃·10H₂O­(cr), Swartzite, CaMg­(UO₂)­(CO₃)₃·12H₂O­(cr), Bayleyite Mg₂(UO₂)­(CO₃)₃·18H₂O­(cr), and Andersonite, Na₂Ca­(UO₂)­(CO₃)₃·6H₂O­(cr), are also critically reviewed. Newly calculated values of the solubility products (log <i>K</i>⁰_<i>s</i>) for these solid compounds are presented based on the currently proposed speciation model that includes the most recent aforementioned data for the aqueous speciation of UO₂²⁺. Based on these data, simulated speciation diagrams are calculated, both at zero ionic strength and in seawater-like media. In combination with the speciation data for uranium with glutaroimide-dioxime, these models provide a better, more comprehensive picture of the chemical equilibria of U­(VI) in seawater while also providing useful tools to help assess the feasibility of its recovery through amidoxime-based collection systems

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