Two isostructural series of trigonal prismatic complexes, M(BpMe)3 and M(BcMe)3 (M = Y, Tb, Dy, Ho, Er, U; BpMe− = dihydrobis(methypyrazolyl)borate; BcMe− = dihydrobis(methylimidazolyl)borate) are ...synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb3+, Dy3+, and U3+ metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(BcMe)3, than for the isomeric pyrazolate complexes, M(BpMe)3. Further, investigation of magnetically dilute samples containing 11–14 mol % of Tb3+, Dy3+, or U3+ within the corresponding Y3+ complex matrix reveals thermally activated relaxation is favored for the M(BcMe)3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(BcMe)3 exhibits U eff ≈ 33 cm–1, representing the highest barrier yet observed for a U3+ molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and 1H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers.
Thorium-227 (227Th) is an α-emitting radionuclide that has shown preclinical and clinical promise for use in targeted α-therapy (TAT), a type of molecular radiopharmaceutical treatment that harnesses ...high energy α particles to eradicate cancerous lesions. Despite these initial successes, there still exists a need for bifunctional chelators that can stably bind thorium in vivo. Toward this goal, we have prepared two macrocyclic chelators bearing 1,2-hydroxypyridinone groups. Both chelators can be synthesized in less than six steps from readily available starting materials, which is an advantage over currently available platforms. The complex formation constants (log βmlh) of these ligands with Zr4+ and Th4+, measured by spectrophotometric titrations, are greater than 34 for both chelators, indicating the formation of exceedingly stable complexes. Radiolabeling studies were performed to show that these ligands can bind 227ThTh4+ at concentrations as low as 10–6 M, and serum stability experiments demonstrate the high kinetic stability of the formed complexes under biological conditions. Identical experiments with zirconium-89 (89Zr), a positron-emitting radioisotope used for positron emission tomography (PET) imaging, demonstrate that these chelators can also effectively bind Zr4+ with high thermodynamic and kinetic stability. Collectively, the data reported herein highlight the suitability of these ligands for use in 89Zr/227Th paired radioimmunotheranostics.
Ultrafine 5 nm ceria isotropic nanoparticles were prepared using the rapid chemical precipitation approach from cerium(III) nitrate and ammonium hydroxide aqueous solutions. The as-prepared ...nanoparticles were shown to contain predominantly Ce(IV) species. The solubility of nanocrystalline CeO2 at several pH values was determined using ICP-MS and radioactive tracer methods. Phase composition of the ceria samples remained unchanged upon partial dissolution, while the shape of the particles changed dramatically, yielding nanorods under neutral pH conditions. According to X-ray absorption spectroscopy investigation of the supernatant, Ce(III) was the main cerium species in solution at pH < 4. Based on the results obtained, a reductive dissolution model was used for data interpretation. According to this model, the solubility product for ceria nanoparticles was determined to be log K sp = −59.3 ± 0.3 in 0.01 M NaClO4. Taken together, our results show that the pH dependence of ceria anti- and pro-oxidant activity can be related to the dissolution of CeO2 in aqueous media.
Homoleptic uranium(IV) amidate complexes have been synthesized and applied as single‐source molecular precursors for the chemical vapor deposition of UO2 thin films. These precursors decompose by ...alkene elimination to give highly crystalline phase‐pure UO2 films with an unusual branched heterostructure.
UO2 nanotree: Volatile uranium(IV) amidate complexes are used as single‐source molecular precursors to uranium oxide films. Chemical vapor deposition (CVD) of these single‐source precursors yields crystalline, phase‐pure UO2 films with a fir tree‐like microstructure and a high surface area.
Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO2 2+) for over half a century and have exhibited exceptional extractability and selectivity toward the ...uranyl ion due to the presence of the phosphoryl group (OP). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO2 2+ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO2 2+ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σ- and π-type interactions with U 5f and 6d orbitals in the UO2Cl2(TRPO)2 complexes. Although both the axial (Oyl) and equatorial (Oeq) oxygen atoms in the UO2Cl2(TRPO)2 complexes contribute to the X-ray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by Oeq and is assigned to the transition from Oeq 1s orbitals to the unoccupied molecular orbitals of 1b 1u + 1b 2u + 1b 3u symmetries resulting from the σ- and π-type mixing between U 5f and Oeq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of Oeq 2p character in these orbitals for the four UO2Cl2(TRPO)2 complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U–TRPO bonding interactions in the UO2Cl2(TRPO)2 complexes. The covalent bonding interactions between TRPO and UO2 2+, especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.
Lanthanides in the trivalent oxidation state are typically described using an ionic picture that leads to localized magnetic moments. The hierarchical energy scales associated with trivalent ...lanthanides produce desirable properties for e.g., molecular magnetism, quantum materials, and quantum transduction. Here, we show that this traditional ionic paradigm breaks down for praseodymium in the tetravalent oxidation state. Synthetic, spectroscopic, and theoretical tools deployed on several solid-state Pr
-oxides uncover the unusual participation of 4f orbitals in bonding and the anomalous hybridization of the 4f
configuration with ligand valence electrons, analogous to transition metals. The competition between crystal-field and spin-orbit-coupling interactions fundamentally transforms the spin-orbital magnetism of Pr
, which departs from the J
= 1/2 limit and resembles that of high-valent actinides. Our results show that Pr
ions are in a class on their own, where the hierarchy of single-ion energy scales can be tailored to explore new correlated phenomena in quantum materials.
Molecular qubits offer an attractive basis for quantum information processing, but challenges remain with regard to sustained coherence. Qubits based on clock transitions offer a method to improve ...the coherence times. We propose a general strategy for identifying molecules with high-frequency clock transitions in systems where a d electron is coupled to a crystal-field singlet state of an f configuration, resulting in an
= ±1/2 ground state with strong hyperfine coupling. Using this approach, a 9.834 GHz clock transition was identified in a molecular Pr complex, K(crypt)Cp'
Pr
, leading to 3-fold enhancements in
relative to other transitions in the spectrum. This result indicates the promise of the design principles outlined here for the further development of f-element systems for quantum information applications.
Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)3 (2) with KC8 yields the anionic bis(arene) complex K2.2.2cryptandU(TDA)2 (3) (TDA = ...N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I2 to form the dinuclear U(III)–U(III) triiodide species K2.2.2cryptand(UI(TDA)2)2(μ-I) (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex K2.2.2cryptandU(η7-C7H7)(TDA)2(THF) (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion K2.2.2cryptandU(TDA)4 (1-crypt) and U(COT)2, while the addition of COT to complex 2 instead yields the dinuclear U(IV)–U(IV) inverse sandwich complex U(TDA)32(μ-η8:η3-C8H8) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)4 (4) with KC8 gives the mono(arene) complex K2.2.2cryptandTh(TDA)3(THF) (5). The C–C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.
In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between ...the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be “core-like” and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.
Complexes featuring lanthanide–ligand multiple bonds are rare and highly reactive. They are important synthetic targets to understand 4f/5d-bonding in comparison to d-block and actinide congeners. ...Herein, the isolation and characterization of a bridging cerium(IV)-nitride complex: (TriNOx)Ce(Li2μ-N)Ce(TriNOx)BArF 4 is reported, the first example of a molecular cerium-nitride. The compound was isolated by deprotonating a monometallic cerium(IV)-ammonia complex: CeIV(NH3)(TriNOx)BArF 4. The average CeN bond length of (TriNOx)Ce(Li2μ-N)Ce(TriNOx)BArF 4 was 2.117(3) Å. Vibrational studies of the 15N-isotopomer exhibited a shift of the CeNCe asymmetric stretch from ν = 644 cm–1 to 640 cm–1, and X-ray spectroscopic studies confirm the +4 oxidation state of cerium. Computational analyses showed strong involvement of the cerium 4f shell in bonding with overall 16% and 11% cerium weight in the σ- and π-bonds of the CeNCe fragment, respectively.
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