Samantha K. Cary

Emergence of californium as the second transitional element in the actinide series

A break in periodicity occurs in the actinide series between plutonium and americium as the result of the localization of 5f electrons. The subsequent chemistry of later actinides is thought to closely parallel lanthanides in that bonding is expected to be ionic and complexation should not substantially alter the electronic structure of the metal ions. Here we demonstrate that ligation of californium(III) by a pyridine derivative results in significant deviations in the properties of the resultant complex with respect to that predicted for the free ion. We expand on this by characterizing the americium and curium analogues for comparison, and show that these pronounced effects result from a second transition in periodicity in the actinide series that occurs, in part, because of the stabilization of the divalent oxidation state. The metastability of californium(II) is responsible for many of the unusual properties of californium including the green photoluminescence.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state

Bonding to berkelium A geographical theme prevailed in the recent naming of the heaviest chemical elements. The choices brought to mind berkelium (Bk) and californium (Cf), the names chosen for elements 97 and 98 over half a century ago. Silver et al. now revisit the chemistry of Bk, which has proven fiercely challenging to study over the years on account of its vigorous radioactive decay. Synthetic crystallized Bk borate and dipicolinate compounds structurally resembled Cf analogs in the solid state but manifested distinct electronic and magnetic characteristics stemming from spin-orbit coupling effects. Science, this issue p. 888 Experiments and theory probe the coordination chemistry of a highly radioactive heavy element. INTRODUCTION Developing the chemistry of late actinides is hindered by the lack of availability of isotopes, the need for specialized research facilities, and the nuclear instability of the elements. Berkelium represents one of the last elements that can be prepared on a milligram scale in nuclear reactors. However, its only available isotope, 249Bk, has a half-life of only 320 days, which has greatly curtailed the expansion of its chemistry and fundamental exploration of how large relativistic and spin-orbit coupling effects alter its electronic structure. Furthermore, data gathered from Bk(III) in aqueous media suggest that its coordination may be different from that of earlier actinides. However, a single-crystal structure of a berkelium compound has remained elusive, leaving unanswered whether these structural changes occur in the solid state. RATIONALE This work focuses on characterizing two distinct berkelium compounds on the milligram scale. In particular, the goal was to obtain crystals of these compounds that could be used in structure determinations and physical property measurements. Two compounds were selected: a coordination complex of dipicolinate and a borate. Dipicolinate complexation occurs with most other lanthanides and actinides in the +3 oxidation state, facilitating comparisons across the series to discern periodic trends. In the borate family, the structural frameworks are hypersensitive to the nature of the bonding at the metal center and are rearranged accordingly. Modeling the experimental data using a variety of computational techniques allows us to deconvolute the role of covalent bonding and spin-orbit coupling in determining the electronic properties of berkelium. RESULTS Experiments with milligram quantities of 249Bk were choreographed for 6 months before the arrival of the isotope because the total quantity used in the studies was 13 mg, which corresponds to a specific activity of 21 Ci. Although this isotope is a low-energy β emitter, it decays to 249Cf at a rate of about 1.2% per week, and the latter produces hard γ radiation that represents a serious external hazard. In addition, the samples described in this work undergo about 1012 decays per second. This rapid decomposition necessitated the development of techniques for swiftly preparing and encapsulating samples and for collecting all structural and spectroscopic data within 24 hours of crystal formation. After this preparation, the single-crystal structures of Bk(III)tris(dipicolinate) and Bk(III) borate were determined. The latter compound has the same topology as that of californium(III) (Cf) and contains an eight-coordinate BkO8 unit. This reduction in coordination number is consistent with previous solution-phase x-ray absorption measurements and indicates that a drop in coordination number in the actinide series from nine to eight begins at berkelium. The magnetic and optical properties of these samples were also measured. The red luminescence from Bk(III) was similar in nature to that of curium(III) and is primarily based on an f-f transition. The ingrowth of the broad green luminescence from Cf(III), which is caused by a ligand-to-metal charge transfer, was shown to be distinct in nature from that originating from Bk(III). Ligand-field, density functional theory, and wave-function calculations were used to understand the spectroscopic features and revealed that the single largest contributor to the unexpected electronic properties of Bk(III) is spin-orbit coupling. This effect mixes the first excited state with the ground state and causes a large deviation from a pure Russell-Saunders state. The reduction in the measured magnetic moment for these samples from that calculated for an f8 electron configuration is primarily attributable to this multiconfigurational ground state. CONCLUSION The crystallographic data indicate that Bk(III) shares more structural similarities with Cf(III) than with Cm(III). However, ligand-field effects are more similar between Bk(III) and Cm(III). Terbium (Tb), in the lanthanide series, represents the closest analog of Bk because the trivalent cations possess 4f8 and 5f8 configurations, respectively. Spin-orbit coupling in Bk(III) creates mixing of the first excited state (5G6) with the ground state. In contrast, the ground state of the Tb(III)tris(dipicolinate) contains negligible contributions of this type. An overall conclusion from this study is that spin-orbit coupling plays a large role in determining the ground state of late actinide compounds. Crystal structure of a berkelium coordination compound. The central Bk(III) ion is coordinated by three monoprotonated dipicolinate ligands in tridentate O,N,O fashion. Bk, yellow; C, gray; N, blue; O, red; H, white. Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f7 configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium’s only available isotope, 249Bk, has hindered in-depth studies of the element’s coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium’s complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium’s multiconfigurational ground state.

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.

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.

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 Seaborgs 1954 hypothesis that AmIII 5f-orbital covalency was more substantial than 4f-orbital mixing for EuIII.

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.

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.

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 Hunds 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.

Unfolding the physics of URu2Si2 through silicon to phosphorus substitution.

The heavy fermion intermetallic compound URu2Si2 exhibits a hidden-order phase below the temperature of 17.5 K, which supports both anomalous metallic behavior and unconventional superconductivity. While these individual phenomena have been investigated in detail, it remains unclear how they are related to each other and to what extent uranium f-electron valence fluctuations influence each one. Here we use ligand site substituted URu2Si2-xPx to establish their evolution under electronic tuning. We find that while hidden order is monotonically suppressed and destroyed for x≤0.035, the superconducting strength evolves non-monotonically with a maximum near x≈0.01 and that superconductivity is destroyed near x≈0.028. This behavior reveals that hidden order depends strongly on tuning outside of the U f-electron shells. It also suggests that while hidden order provides an environment for superconductivity and anomalous metallic behavior, its fluctuations may not be solely responsible for their progression.

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.