Observation of a promethium complex in solution.

Lanthanide rare-earth metals are ubiquitous in modern technologies1-5, but we know little about chemistry of the 61st element, promethium (Pm)6, a lanthanide that is highly radioactive and inaccessible. Despite its importance7,8, Pm has been conspicuously absent from the experimental studies of lanthanides, impeding our full comprehension of the so-called lanthanide contraction phenomenon: a fundamental aspect of the periodic table that is quoted in general chemistry textbooks. Here we demonstrate a stable chelation of the 147Pm radionuclide (half-life of 2.62 years) in aqueous solution by the newly synthesized organic diglycolamide ligand. The resulting homoleptic PmIII complex is studied using synchrotron X-ray absorption spectroscopy and quantum chemical calculations to establish the coordination structure and a bond distance of promethium. These fundamental insights allow a complete structural investigation of a full set of isostructural lanthanide complexes, ultimately capturing the lanthanide contraction in solution solely on the basis of experimental observations. Our results show accelerated shortening of bonds at the beginning of the lanthanide series, which can be correlated to the separation trends shown by diglycolamides9-11. The characterization of the radioactive PmIII complex in an aqueous environment deepens our understanding of intra-lanthanide behaviour12-15 and the chemistry and separation of the f-block elements16.

Reactivity of a Chloride Decorated, Mixed Valent CeIII/IV38-Oxo Cluster.

A cerium-oxo nanocluster capped by chloride ligands, [CeIV38-nCeIIInO56-(n+1)(OH)n+1Cl51(H2O)11]10- (n = 1-24), has been isolated from acidic chloride solutions by using potassium counterions. The crystal structure was elucidated using single crystal X-ray diffraction. At the center of the cluster is a {Ce14} core that exhibits the same fluorite-type structure as bulk CeO2, with eight-coordinate Ce sites bridged by tetrahedral oxo anions. The {Ce14} is further surrounded by a peripheral shell of six tetranuclear {Ce4} subunits that are located on each of the faces of the core to yield the {Ce38} cluster. The surface of the cluster is capped by 51 bridging/terminal chloride ligands and 11 water molecules; the anionic cluster is charge balanced by potassium counterions that exist in the outer coordination sphere. While assignment of the Ce oxidation state by bond valence summation was ambiguous, Ce L3-edge X-ray absorption, X-ray photoelectron, and UV-vis-NIR absorption results were consistent with a CeIII/CeIV cluster. Systematic changes in the XANES and UV-vis-NIR absorption spectra over time pointed to reactivity of the cluster upon exposure to air. These changes were examined using single crystal X-ray diffraction, and a clear single-crystal-to-single-crystal transformation was captured; an overall loss of surface-bound chlorides and water molecules as well as new µ2-OH sites was observed on the cluster surface. This work provides a rare snapshot of metal oxide cluster reactivity. The results may hold implications for understanding the physical and chemical properties of ceria nanoparticles and provide insight into the behavior of other metal-oxo clusters of significant technological and environmental interest.

Characterizing Polyoxovanadate-Alkoxide Clusters Using Vanadium K-Edge X-Ray Absorption Spectroscopy.

A number of technologies would benefit from developing inorganic compounds and materials with specific electronic and magnetic exchange properties. Unfortunately, designing compounds with these properties is difficult because metal⋅⋅⋅metal coupling schemes are hard to predict and control. Fully characterizing communication between metals in existing compounds that exhibit interesting properties could provide valuable insight and advance those predictive capabilities. One such class of molecules are the series of Lindqvist iron-functionalized and hexavanadium polyoxovanadate-alkoxide clusters, which we characterized here using V K-edge X-ray absorption spectroscopy. Substantial changes in the pre-edge peak intensities were observed that tracked with the V 3d-electron count. The data also suggested substantial delocalization between the vanadium cations. Meanwhile, the FeIII cations were electronically isolated from the polyoxovanadate core.

Resonant inelastic X-ray scattering using a miniature dispersive Rowland refocusing spectrometer.

X-ray absorption spectroscopy (XAS) beamlines worldwide are steadily increasing their emphasis on full photon-in/photon-out spectroscopies, such as resonant inelastic X-ray scattering (RIXS), resonant X-ray emission spectroscopy (RXES) and high energy resolution fluorescence detection XAS (HERFD-XAS). In such cases, each beamline must match the choice of emission spectrometer to the scientific mission of its users. Previous work has recently reported a miniature tender X-ray spectrometer using a dispersive Rowland refocusing (DRR) geometry that functions with high energy resolution even with a large X-ray spot size on the sample [Holden et al. (2017). Rev. Sci. Instrum. 88, 073904]. This instrument has been used in the laboratory in multiple studies of non-resonant X-ray emission spectroscopy using a conventional X-ray tube, though only for preliminary measurements at a low-intensity microfocus synchrotron beamline. This paper reports an extensive study of the performance of a miniature DRR spectrometer at an unfocused wiggler beamline, where the incident monochromatic flux allows for resonant studies which are impossible in the laboratory. The results support the broader use of the present design and also suggest that the DRR method with an unfocused beam could have important applications for materials with low radiation damage thresholds and that would not survive analysis on focused beamlines.

Advancing Chelation Chemistry for Actinium and Other +3 f-Elements, Am, Cm, and La.

A major chemical challenge facing implementation of 225Ac in targeted alpha therapy-an emerging technology that has potential for treatment of disease-is identifying an 225Ac chelator that is compatible with in vivo applications. It is unclear how to tailor a chelator for Ac binding because Ac coordination chemistry is poorly defined. Most Ac chemistry is inferred from radiochemical experiments carried out on microscopic scales. Of the few Ac compounds that have been characterized spectroscopically, success has only been reported for simple inorganic ligands. Toward advancing understanding in Ac chelation chemistry, we have developed a method for characterizing Ac complexes that contain highly complex chelating agents using small quantities (µg) of 227Ac. We successfully characterized the chelation of Ac3+ by DOTP8- using EXAFS, NMR, and DFT techniques. To develop confidence and credibility in the Ac results, comparisons with +3 cations (Am, Cm, and La) that could be handled on the mg scale were carried out. We discovered that all M3+ cations (M = Ac, Am, Cm, La) were completely encapsulated within the binding pocket of the DOTP8- macrocycle. The computational results highlighted the stability of the M(DOTP)5- complexes.

Energy-Degeneracy-Driven Covalency in Actinide Bonding.

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2 g*/eg *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M-Cl orbital mixing, such that AnIV 6d - and Cl 3p-mixing decreased and metal 5f - and Cl 3p-orbital mixing increased across this series.

Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA-); Synthesis and Structure of MIVTTA4 (MIV = Zr, Hf, Ce, Th, U, Np, Pu) and MIII(TTA)4- (MIII = Ce, Nd, Sm, Yb).

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating microscopic amounts of transuranic actinides (i.e., Np and Pu) from macroscopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extraction-or find alternatives-because little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (MIV = Zr, Hf, Ce, Th, U, Np, and Pu vs MIII = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula MIV(TTA)4. Meanwhile, +3 metals formed anionic MIII(TTA)4- species. Characterization of these M(TTA)4x- ( x = 0, 1) compounds by UV-vis-NIR, IR, 1H and 19F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that NpIV(TTA)4 and PuIV(TTA)4 were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between MIV(TTA)4 and MIII(TTA)4- are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np IV to PuIV, HfIV, and ZrIV.

A Pseudotetrahedral Uranium(V) Complex.

A series of uranium amides were synthesized from N, N, N-cyclohexyl(trimethylsilyl)lithium amide [Li][N(TMS)Cy] and uranium tetrachloride to give U(NCySiMe3) x(Cl)4- x, where x = 2, 3, or 4. The diamide was isolated as a bimetallic, bridging lithium chloride adduct ((UCl2(NCyTMS)2)2-LiCl(THF)2), and the tris(amide) was isolated as the lithium chloride adduct of the monometallic species (UCl(NCyTMS)3-LiCl(THF)2). The tetraamide complex was isolated as the four-coordinate pseudotetrahedron. Cyclic voltammetry revealed an easily accessible reversible oxidation wave, and upon chemical oxidation, the UV amido cation was isolated in near-quantitative yields. The synthesis of this family of compounds allows a direct comparison of the electronic structure and properties of isostructural UIV and UV tetraamide complexes. Spectroscopic investigations consisting of UV-vis, NIR, MCD, EPR, and U L3-edge XANES, along with density functional and wave function calculations, of the four-coordinate UIV and UV complexes have been used to understand the electronic structure of these pseudotetrahedral complexes.

Covalency in Americium(III) Hexachloride.

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficultly in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers ( Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954 , 76 , 1461 ) when it was proposed that covalency from 5f-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding in-arguably-one of the most difficult systems to study, the Am-Cl interaction within AmCl63-. We observed both 5f- and 6d-orbital mixing with the Cl-3p orbitals; however, contributions from the 6d-orbitals were more substantial. Comparisons with the isoelectronic EuCl63- indicated that the amount of Cl 3p-mixing with EuIII 5d-orbitals was similar to that observed with the AmIII 6d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that AmIII 5f-orbital covalency was more substantial than 4f-orbital mixing for EuIII.

Electronic Structure and Properties of Berkelium Iodates.

The reaction of 249Bk(OH)4 with iodate under hydrothermal conditions results in the formation of Bk(IO3)3 as the major product with trace amounts of Bk(IO3)4 also crystallizing from the reaction mixture. The structure of Bk(IO3)3 consists of nine-coordinate BkIII cations that are bridged by iodate anions to yield layers that are isomorphous with those found for AmIII, CfIII, and with lanthanides that possess similar ionic radii. Bk(IO3)4 was expected to adopt the same structure as M(IO3)4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller ZrIV cation. BkIII-O and BkIV-O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO3)4 show evidence for doping with BkIII in these crystals. In addition to luminescence from BkIII in the Bk(IO3)4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249Bk (t1/2 = 320 d) causes oxidation of BkIII and only BkIV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO3)3 and Bk(IO3)4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin-orbit coupling (CASSCF), and by a full-model Hamiltonian with spin-orbit coupling and Slater-Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in BkIV that does not strictly adhere to Russel-Saunders coupling and Hund's Rule even though it possesses a half-filled 5f 7 shell. Multiple factors contribute to the asymmetry that include 5f electrons being present in microstates that are not solely spin up, spin-orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the BkIV-O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f7 ions such as GdIII or CmIII.

Polyoxovanadate-Alkoxide Clusters as a Redox Reservoir for Iron.

Inspired by the multielectron redox chemistry achieved using conventional organic-based redox-active ligands, we have characterized a series of iron-functionalized polyoxovanadate-alkoxide clusters in which the metal oxide scaffold functions as a three-dimensional, electron-deficient metalloligand. Four heterometallic clusters were prepared through sequential reduction, demonstrating that the metal oxide scaffold is capable of storing up to four electrons. These reduced products were characterized by cyclic voltammetry, IR, electronic absorption, and 1H NMR spectroscopies. Moreover, Mössbauer and X-ray absorption spectroscopies suggest that the redox events involve primarily the vanadium ions, while the iron atoms remained in the 3+ oxidation state throughout the redox series. In this sense, the vanadium portion of the cluster mimics a conventional organic-based redox-active ligand bound to an iron(III) ion. Magnetic coupling within the hexanuclear cluster was characterized using SQUID magnetometry. Overall, the results suggest extensive electronic delocalization between the metal centers of the cluster core. These results demonstrate the ability of electronically flexible, reducible metal oxide supports to function as redox-active reservoirs for transition-metal centers.

Magnetic circular dichroism of UCl6- in the ligand-to-metal charge-transfer spectral region.

We present a combined ab initio theoretical and experimental study of the magnetic circular dichroism (MCD) spectrum of the octahedral UCl6- complex ion in the UV-Vis spectral region. The ground state is an orbitally non-degenerate doublet E5/2u and the MCD is a -term spectrum caused by spin-orbit coupling. Calculations of the electronic spectrum at various levels of theory indicate that differential dynamic electron correlation has a strong influence on the energies of the dipole-allowed transitions and the envelope of the MCD spectrum. The experimentally observed bands are assigned to dipole-allowed ligand-to-metal charge transfer into the 5f shell, and 5f to 6d transitions. Charge transfer excitations into the U 6d shell appear at much higher energies. The MCD-allowed transitions can be assigned via their signs of the -terms: Under Oh double group symmetry, E5/2u → E5/2g transitions have negative -terms whereas E5/2u → F3/2g transitions have positive -terms if the ground state g-factor is negative, as it is the case for UCl6-.

Covalency-Driven Dimerization of Plutonium(IV) in a Hydroxamate Complex.

The reaction of formohydroxamic acid [NH(OH)CHO, FHA] with Pu(III) should result in stabilization of the trivalent oxidation state. However, slow oxidation to Pu(IV) occurs, which leads to formation of the dimeric plutonium(IV) formohydroxamate complex Pu2(FHA)8. In addition to being reductants, hydroxamates are also strong π-donor ligands. Here we show that formation of the Pu2(FHA)8 dimer occurs via covalency between the 5f orbitals on plutonium and the π* orbitals of FHA(-) anions, which gives rise to a broad and intense ligand-to-metal charge-transfer feature. Time-dependent density functional theory calculations corroborate this assignment.

Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates.

The reaction of Ce(III) or Pu(III) with 1,10-phenanthroline-2,9-dicarboxylic acid (PDAH2) results in the formation of new f-element coordination complexes. In the case of cerium, Ce(PDA)(H2O)2Cl·H2O (1) or [Ce(PDAH)(PDA)]2[Ce(PDAH)(PDA)] (2) was isolated depending on the Ce/ligand ratio in the reaction. The structure of 2 is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a Ce(III) cation bound by one PDA(2-) dianionic ligand and one PDAH(-) monoanionic ligand, both of which are tetradentate. Bridging by the carboxylate moieties leads to either [Ce(PDAH)(PDA)]2 dimers or [Ce(PDAH)(PDA)]1∞ helical chains. For plutonium, Pu(PDA)2 (3) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH2, Pu(III) is oxidized to Pu(IV), generating 3. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in 1 and 2 from UV-vis-near-IR spectra invoked the use of Ce L3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of Ce(III) in both compounds. The bond distances and coordination numbers are also consistent with these assignments. 3 contains 8-coordinate Pu(IV), whereas the cerium centers in 1 and 2 are 9- and/or 10-coordinate, which correlates with the increased size of Ce(III) versus Pu(IV). Taken together, these data provide an example of a system where the differences in the redox behavior between these f elements creates more complex chemistry with cerium than with plutonium.

Why is uranyl formohydroxamate red?

The complexation of UO2(2+) by formohydroxamate (FHA(-)) creates solutions with dark red coloration. The inherent redox activity of formohydroxamate leads to the possibility that these solutions contain U(V) complexes, which are often red. We demonstrate that the reaction of U(VI) with formohydroxamate does not result in reduction, but rather in formation of the putative cis-aquo UO2(FHA)2(H2O)2, whose polymeric solid-state structure, UO2(FHA)2, contains an unusually bent UO2(2+) unit and a highly distorted coordination environment around a U(VI) cation in general. The bending of the uranyl cation results from unusually strong π donation from the FHA(-) ligands into the 6d and 5f orbitals of the U(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features.

Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes.

The reaction of (248)CmCl3 with excess 2,6-pyridinedicarboxylic acid (DPA) under mild solvothermal conditions results in crystallization of the tris-chelate complex Cm(HDPA)3 · H2O. Approximately half of the curium remains in solution at the end of this process, and evaporation of the mother liquor results in crystallization of the bis-chelate complex [Cm(HDPA)(H2DPA)(H2O)2Cl]Cl·2H2O. (248)Cm is the daughter of the α decay of (252)Cf and is extracted in high purity from this parent. However, trace amounts of (249,250,251)Cf are still present in all samples of (248)Cm. During the crystallization of Cm(HDPA)3 · H2O and [Cm(HDPA)(H2DPA)(H2O)2Cl]Cl · 2H2O, californium(III) spontaneously separates itself from the curium complexes and is found doped within crystals of DPA in the form of Cf(HDPA)3. These results add to the growing body of evidence that the chemistry of californium is fundamentally different from that of earlier actinides.

Metastable charge-transfer state of californium(iii) compounds.

Among a series of anomalous physical and chemical properties of Cf(iii) compounds revealed by recent investigations, the present work addresses the characteristics of the optical spectra of An(HDPA)3·H2O (An = Am, Cm, and Cf), especially the broadband photoluminescence from Cf(HDPA)3·H2O induced by ligand-to-metal charge transfer (CT). As a result of strong ion-ligand interactions and the relative ease of reducing Cf(iii) to Cf(ii), a CT transition occurs at low energy (<3 eV) via the formation of a metastable Cf(ii) state. It is shown that the systematic trend in CT transitions of the lanthanide series is not paralleled by actinide elements lighter than Cf(iii), and californium represents a turning point in the periodicity of the actinide series. Analyses and modeling of the temperature-dependent luminescence dynamics indicate that the metastable Cf(ii) charge-transfer state undergoes radiative and non-radiative relaxations. Broadening of the CT transition arises from strong vibronic coupling and hole-charge interactions in the valence band. The non-radiative relaxation of the metastable CT state results from a competition between phonon-relaxation and thermal tunneling that populates the excited states of Cf(iii).

Chirality and polarity in the f-block borates M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, and Cf).

The reactions of trivalent lanthanides and actinides with molten boric acid in high chloride concentrations result in the formation of M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, Cf). This cubic structure type is remarkably complex and displays both chirality and polarity. The polymeric borate network forms helical features that are linked via two different types of nine-coordinate f-element environments. The f-f transitions are unusually intense and result in dark coloration of these compounds with actinides.

Synthesis and spectroscopy of new plutonium(III) and -(IV) molybdates: comparisons of electronic characteristics.

Synthesis of a plutonium(III) molybdate bromide, PuMoO4Br(H2O), has been accomplished using hydrothermal techniques in an inert-atmosphere glovebox. The compound is green in color, which is in stark contrast to the typical blue color of plutonium(III) complexes. The unusual color arises from the broad charge transfer (CT) spanning from approximately 300 to 500 nm in the UV-vis-near-IR spectra. Repeating the synthesis with an increase in the reaction temperature results in the formation of a plutonium(IV) molybdate, Pu3Mo6O24(H2O)2, which also has a broad CT band and red-shifted f-f transitions. Performing an analogous reaction with neodymium produced a completely different product, [Nd(H2O)3][NdMo12O42]·2H2O, which is built of Silverton-type polyoxometallate clusters.

Structure and bonding of a radium coordination compound in the solid state.

The structure and bonding of radium (Ra) is poorly understood because of challenges arising from its scarcity and radioactivity. Here we report the synthesis of a molecular Ra2+ complex using 226Ra and the organic ligand dibenzo-30-crown-10, and its characterization in the solid state by single-crystal X-ray diffraction. The crystal structure of the Ra2+ complex shows an 11-coordinate arrangement comprising the 10 donor O atoms of dibenzo-30-crown-10 and that of a bound water molecule. Under identical crystallization conditions, barium (Ba2+) yielded a 10-coordinate 'Pac-Man'-shaped structure lacking water. Furthermore, the bond distance between the Ra centre and the O atom of the coordinated water is substantially longer than would be predicted from the ionic radius of Ra2+ and by analogy with Ba2+, supporting greater water lability in Ra2+ complexes than in their Ba2+ counterparts. Barium often serves as a non-radioactive surrogate for radium, but our findings show that Ra2+ chemistry cannot always be predicted using Ba2+.