Spectroscopic and Computational Characterization of Diethylenetriaminepentaacetic Acid/Transplutonium Chelates: Evidencing Heterogeneity in the Heavy Actinide(III) Series

The chemistry of trivalent transplutonium ions (Am³⁺, Cm³⁺, Bk³⁺, Cf³⁺, Es³⁺…) is usually perceived as monotonic and paralleling that of the trivalent lanthanide series. Herein, we present the first extended X‐ray absorption fine structure (EXAFS) study performed on a series of aqueous heavy actinide chelates, extending past Cm. The results obtained on diethylenetriaminepentaacetic acid (DTPA) complexes of trivalent Am, Cm, Bk, and Cf show a break to much shorter metal–oxygen nearest‐neighbor bond lengths in the case of Cf³⁺. Corroborating those results, density functional theory calculations, extended to Es³⁺, suggest that the shorter Cf?O and Es?O bonds could arise from the departure of the coordinated water molecule and contraction of the ligand around the metal relative to the other [M^IIIDTPA(H₂O)]^2? (M=Am, Cm, Bk) complexes. Taken together, these experimental and theoretical results demonstrate inhomogeneity within the trivalent transplutonium series that has been insinuated and debated in recent years, and that may also be leveraged for future nuclear waste reprocessing technologies.     

Coordination Chemistry of Homoleptic Actinide(IV)–Thiocyanate Complexes

The synthesis, X‐ray crystal structure, vibrational and optical spectroscopy for the eight‐coordinate thiocyanate compounds, [Et₄N]₄[Pu^IV(NCS)₈], [Et₄N]₄[Th^IV(NCS)₈], and [Et₄N]₄[Ce^III(NCS)₇(H₂O)] are reported. Thiocyanate was found to rapidly reduce plutonium to Pu^III in acidic solutions (pH<1) in the presence of NCS^?. The optical spectrum of [Et₄N][SCN] containing Pu^III solution was indistinguishable from that of aquated Pu^III suggesting that inner‐sphere complexation with [Et₄N][SCN] does not occur in water. However, upon concentration, the homoleptic thiocyanate complex [Et₄N]₄[Pu^IV(NCS)₈] was crystallized when a large excess of [Et₄N][NCS] was present. This compound, along with its U^IV analogue, maintains inner‐sphere thiocyanate coordination in acetonitrile based on the observation of intense ligand‐to‐metal charge‐transfer bands. Spectroscopic and crystallographic data do not support the interaction of the metal orbitals with the ligand π system, but support an enhanced An^IV–NCS interaction, as the Lewis acidity of the metal ion increases from Th to Pu.     

Experimental and Quantum Mechanical Characterization of an Oxygen-Bridged Plutonium(IV) Dimer

We report the synthesis and characterization of K₄{[PuCl₂(NO₃)₃]₂(μ₂-O)}⋅H₂O, which contains the first known μ₂-oxo bridge between two Pu^IV metal centers. Adding to its uniqueness is the Pu−(μ₂-O) bond length of 2.04 Å, which is the shortest of other analogous compounds. The Pu−(μ₂-O)−Pu bridge is characterized by the mixing of s-, d-, and p-orbitals from Pu with the p-orbitals of O; the 5f-orbitals do not participate in bonding. Natural bond orbital analysis indicates that Pu and O interact through one 3c-2e σ_Pu-O-Pu and two 3c-2e π_Pu-O-Pu bonding orbitals and that the electron density is highly polarized on the μ₂-O. Bond topology properties analysis indicates that the Pu−(μ₂-O) bond shares both ionic and covalent character. Quantum mechanical calculations also show that the dimer has multiconfigurational ground states, where the nonet, septet, quintet, triplet, and singlet are close in energy. This work demonstrates the interplay between experimental and computational efforts that is required to understand the chemical bonding of Pu compounds.     

Sequestered plutonium: [Pu(IV){5LIO(Me-3,2-HOPO)}2]--the first structurally characterized plutonium hydroxypyridonate complex

The first single-crystal X-ray diffraction analysis of a hydroxypyridonate plutonium(IV) complex is presented, that of the tetradentate ligand 5LIO(Me-3,2-HOPO) with Pu(IV). The [Pu(IV){5LIO(Me-3,2-HOPO)}(2)] complex crystallizes in the space group Pna2(1) with the asymmetric unit cell containing two unique eight-coordinate plutonium complexes and one perchlorate anion. According to shape measure analysis, the geometry of both Pu centers is closest to a bicapped trigonal prism (C(2v) symmetry, for Pu 1: S(C(2v))=13.48 degrees , S(D(4d))=15.43 degrees , S(D(4d))=16.10 degrees ). The average bond length for the Pu--O(phenolic) is 2.31(4) A, whereas the Pu--O(amide) distances are slightly longer, averaging 2.40(2) A. The preparative chemistry of this compound and the implications of the structure are discussed.     

Electron Spin Relaxation Rates for High-Spin Fe(III) in Iron Transferrin Carbonate and Iron Transferrin Oxalate

To optimize simulations of CW EPR spectra for high-spin Fe(III) with zero-field splitting comparable to the EPR quantum, information is needed on the factors that contribute to the line shapes and line widths. Continuous wave electron paramagnetic resonance (EPR) spectra obtained for iron transferrin carbonate from 4 to 150 K and for iron transferrin oxalate from 4 to 100 K did not exhibit significant temperature dependence of the line shape, which suggested that the line shapes were not relaxation determined. To obtain direct information concerning the electron spin relaxation rates, electron spin echo and inversion recovery EPR were used to measure T(1) and T(m) for the high-spin Fe(III) in iron transferrin carbonate and iron transferrin oxalate between 5 and 20-30 K. For comparison with the data for the transferrin complexes, relaxation times were obtained for tris(oxalato)ferrate(III). The relaxation rates are similar for the three complexes and do not exhibit a strong dependence on position in the spectrum. Extrapolation of the observed temperature dependence of the relaxation rates to higher temperatures gives values consistent with the conclusion that the CW line shapes are not relaxation determined up to 150 K.     

Binding of Chromium(III) to Transferrin Could Be Involved in Detoxification of Dietary Chromium(III) Rather than Transport of an Essential Trace Element

Cr^III binding to transferrin (Tf; the main Fe^III transport protein) has been postulated to mediate cellular uptake of Cr^III to facilitate a purported essential role for this element. Experiments using HepG2 (human hepatoma) cells, which were chosen because of high levels of the transferrin receptor, showed that Cr^III binding to vacant Fe^III‐binding sites of human Tf effectively blocks cellular Cr^III uptake. Through bio‐layer interferometry studies of the Tf cycle, it was found that both exclusion and efflux of Cr₂^IIITf from cells was caused by 1) relatively low Cr₂Tf affinity to cell‐surface Tf receptors compared to Fe₂Tf, and 2) disruption of metal release under endosomal conditions and post‐endosomal Tf dissociation from the receptor. These data support mounting evidence that Cr^III is not essential and that Tf binding is likely to be a natural protective mechanism against the toxicity and potential genotoxicity of dietary Cr through blocking Cr^III cellular accumulation.     

Insight of the Metal–Ligand Interaction in f-Element Complexes by Paramagnetic NMR Spectroscopy

The magnetic properties of Ln^III and An^III complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometries of these complexes, a crystallographic study by single-crystal X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) in solution was performed. Several methods to separate the paramagnetic shifts observed in the NMR spectra were applied to these complexes. Methods using a number of nuclei of the dipicolinate ligands revealed an abrupt change in the geometries of the complexes and a metal–ligand interaction in the middle of the lanthanide series. A study of the variation of the paramagnetic shifts with temperature demonstrated that higher-order terms of the dipolar and contact contributions are required, especially for the lightest Ln^III and almost all the studied An^III. Bleaney's parameters <S_z>_a and relating to the contact and dipolar terms, respectively, were deduced from experimental data and compared with the results of ab initio calculations. Quite a good agreement was found for the temperature dependencies of <S_z>_a and . However, the values obtained from cation magnetic anisotropy calculations showed some discrepancies with the values derived from Bleaney's equation defined for Ln^III. Other parameters, such as the crystal field parameter and the hyperfine constants F_i obtained from the experimental data of the [An(ethyl-dpa)₃]³⁻ complexes (ethyl-dpa=4-ethyl-2,6-dipicolinic acid), are at odds with the assumptions underlying Bleaney's theory.     

Backbonding in Thorium(IV) and Uranium(IV) Diarsenido Complexes with tBuNC and CO

The coordination of tBuNC and CO with the diarsenido complexes (C₅Me₅)₂An(η²-As₂Mes₂), An=Th, U, has been investigated. For the first time, a comparison between isostructural complexes of Th^IV and U^IV has been possible with CO; density functional calculations indicated an appreciable amount of π backbonding that originates from charge transfer from an actinide-arsenic sigma bond. The calculated CO stretching frequencies in the Th^IV and U^IV diarsenido complexes are consistent with the experimental measurements, both show large shifts to lower frequency. We demonstrate that the π backbonding is crucial to explaining the red shifts of CO frequency upon An^IV complex formation. Interestingly, this interaction essentially correlates to the parallel orientation of π*(C−O) orbitals relative to the An−As bond.     

稀土等离子与铁生物调控

本文在简介转铁蛋白、转铁蛋白受体的铁（Ⅲ）调控作用研究进展的基础上,综述了稀土离子、镓与转铁蛋白的作用及稀土离子、镓代谢与铁（Ⅲ）调控系统的关系。     

Binding of erbium(III) and holmium(III) to native and chemically modified alfalfa biomass: a spectroscopic investigation

Traditional treatment methods used to clean-up heavy metal contamination of soils and waters are cost intensive whereas more cost effective methods need to be developed. The use of plant materials to remediate heavy contamination has been studied for the past two decades. This technique has shown much promise for many of the common heavy metal contaminants, but few studies have focused on the lanthanide series elements. By investigating the binding and interactions of the lanthanide elements to alfalfa biomass, a more complete understanding of the binding mechanisms and the interactions of heavy metals with biomaterials can be obtained. Different chemical functional groups on the alfalfa biomass, carboxyl, amino, sulfur, and ester groups, were modified to investigate the binding mechanisms of erbium(III) and holmium(III). Batch experiments were performed with native and chemically modified alfalfa biomass suggesting that the carboxyl groups play a major role in the binding of erbium(III) and holmium(III) to the alfalfa biomass. In addition, X-ray absorption spectroscopy (XAS) studies corroborated the data obtained from the batch experiments.     

Actinide incorporation in a zirconia based pyrochlore (Nd1.8An0.2)Zr2O7+x (An=Th, U, Np, Pu, Am)

Actinides (thorium, uranium, neptunium, plutonium, and americium) were infiltrated into a porous Nd_1.8Zr₂O_6.7 matrix, prepared by gel-supported precipitation. (Nd_1.8An_0.2)Zr₂O_7+x pyrochlores were formed after sintering in Ar/H₂ and the pyrochlore structure remains during oxidation at 800 °C in air. X-ray diffraction reveals a linear relationship between the pyrochlore lattice parameter and the ionic radii of the actinides. EXAFS measurements on actinide L₃-edge show a split shell of nearest neighbour oxygen atoms similar to that surrounding of Nd. The actinide-oxygen bond distances decrease with the actinide ionic radii, which verifies that these actinides adopt the Nd site in the (Nd_1.8An_0.2)Zr₂O_7+x pyrochlore. The oxidation susceptibility of Np is related to the availability of oxygen vacancies and in contrast to stabilised zirconia Np(V) can be obtained in zirconia based pyrochlore.