Interest in actinide-carbon bonds has persisted since actinide organometallics were first investigated for applications in isotope separation during the Manhattan Project. Transplutonium organometallics are rarely isolated and structurally characterized, likely owing to limited isotope inventories, a scarcity of suitable laboratory infrastructure, and intrinsic difficulties with the anaerobic conditions required. Herein, we report the discovery of an organometallic "berkelocene" complex prepared from 0.3 milligrams of berkelium-249. Single-crystal x-ray diffraction shows a tetravalent berkelium ion between two substituted cyclooctatetraene ligands, resulting in the formation of berkelium-carbon bonds. The coordination in berkelocene resembles that of uranocene, and calculations show that the berkelium 5f orbitals engage in covalent overlap with the delta-symmetry orbitals of the cyclooctatetraenide ligand pi system. Charge transfer from the ligands is diminished relative to uranocene and other actinocenes, which maximizes contributions from the stable, half-filled 5f7 configuration of tetravalent berkelium.

Amidate-based ligands (N-(tert-butyl)isobutyramide, ITA) bind κ2 to form homoleptic, 8-coordinate complexes with tetravalent 237Np (Np(ITA)4, 1-Np) and 242Pu (Pu(ITA)4, 1-Pu). These compounds complete an isostructural series from Th, U-Pu and allow for the direct comparison between many of the early actinides with stable tetravalent oxidation states by nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction (SCXRD). The molecular precursors are subjected to controlled thermolysis under mild conditions with the exclusion of exogenous air and moisture, facilitating the removal of the volatile organic ligands and ligand byproducts. The preformed metal-oxygen bond in the precursor, as well as the metal oxidation state, are maintained through the decomposition, forming fully stoichiometric, oxidation-state pure NpO2 and PuO2. Powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mapping supported the evaluation of these high-purity materials. This chemistry is applicable to a wide range of metals, including actinides, with accessible tetravalent oxidation states, and provides a consistent route to analytical standards of importance to the field of nuclear nonproliferation, forensics, and fundamental studies.

AbstractTargeted alpha therapy (TAT) pairs the specificity of antigen targeting with the lethality of alpha particles to eradicate cancerous cells. Actinium-225 [225Ac; t1/2 = 9.920(3) days] is an alpha-emitting radioisotope driving the next generation of TAT radiopharmaceuticals. Despite promising clinical results, a fundamental understanding of Ac coordination chemistry lags behind the rest of the Periodic Table due to its limited availability, lack of stable isotopes, and inadequate systems poised to probe the chemical behavior of this radionuclide. In this work, we demonstrate a platform that combines an 8-coordinate synthetic ligand and a mammalian protein to characterize the solution and solid-state behavior of the longest-lived Ac isotope, 227Ac [t1/2 = 21.772(3) years]. We expect these results to direct renewed efforts for 225Ac-TAT development, aid in understanding Ac coordination behavior relative to other +3 lanthanides and actinides, and more broadly inform this element’s position on the Periodic Table.

Curium's stable redox chemistry and ability to emit strong metal-based luminescence make it uniquely suitable for spectroscopic studies among the actinide series. Targeted ligand and coordination compound design can support both fundamental electronic structure studies and industrial safeguards with the identification of unique spectroscopic signatures. However, limited availability, inherent radioactive hazards, and arduous purifications have long inhibited such investigations of this element. A consolidated reprocessing procedure for curium has been developed for the milligram scale. The recovery of not only standard legacy curium samples but also hazardous legacy perchlorate containing curium samples was achieved, culminating in column chromatography utilizing the extraction resin DGA (N,N,N',N'-tetra-2-ethylhexyldiglycolamide, branched). Surprisingly, controlled elution of the Cm band from the extraction resin was followed through bright pink luminescence triggered by an inexpensive hand-held UV-vis lamp (380-400 nm). This observation inspired the design of an enantiopure, C2-symmetrical ligand bearing a chiral (trans-1,2-diaminocyclohexane) backbone with achiral DGA moieties (N,N,N',N'-tetra-n-octylacetamide), that enabled rarely observed curium circularly polarized luminescence upon metal chelation. These combined achievements should unlock more luminescence and circularly polarized luminescence studies of curium, and enable the recovery of many curium and other trivalent actinide samples.

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band EPR data were collected at <10 K and 77 K on ten U(II) complexes with 5f36d1 electron configurations and on Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K and U(III)-like spectra at 5 K. In contrast, U(II) complexes with a 5f4 configuration are EPR silent. Unlike U(II), the congener Nd(II) complex is EPR-silent although it possesses an analogous 4f35d1 configuration. The Ce(II) complex with a 4f15d1 configuration is also EPR silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. If the coupling is sufficiently strong, no signal is expected for ions with an even number of electrons, which is the observation for the lanthanide species. If the coupling is very weak, separate EPR signals for the f and d systems might be observed even in systems with an even number of electrons. The Coulomb interaction between 5f and 6d electrons is strong in trivalent uranium systems, which results in strong coupling between the 5f and 6d electrons. This coupling will be weaker in divalent uranium systems, but it is still expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration. Some potential sources of the EPR signal from the U(II) complexes are discussed.

X-ray absorption spectroscopy and variable temperature magnetometry show evidence of 4f-orbital mixing in Cp′3Eu, which increases its magnetic susceptibility.

We report the synthesis and characterization of the macrocyclic californium derivative Na[Cf(H2O)(DOTA)] (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), 1-Cf, which was studied in comparison to its dysprosium counterpart, Na[Dy(H2O)(DOTA)], 1-Dy. Divergent spectroscopic and magnetic behaviors were observed between 1-Cf and 1-Dy. Based upon spectroscopic measurements, we propose that accessible 5f → 6d transitions (potentially operating in tandem with charge-transfer transitions) are the major contributors to the observed broadband photoluminescence in 1-Cf. Dc magnetic susceptibility data for 1-Cf revealed lower magnetic moments than those previously observed for 1-Dy and expected for an f9 free ion, which calculations suggest is the result of greater ligand field effects. Notably, 1-Cf displays slow magnetic relaxation on the time scale of ac susceptibility measurements, making it the first example of a californium-based single-molecule magnet. A side-by-side comparison of the ac susceptibility data reveals magnetic relaxation properties that widely differ between 1-Cf and 1-Dy. This divergent relaxation behavior is attributed mainly to the inherent difference in spin-orbit coupling between Dy3+ and Cf3+.

Syntheses of rare examples of U(V) methyl and U(IV) ylidecomplexesare reported. Reaction of the previously reported U(IV) imido complex[(C5Me5)(2)U(py)(NMes)] (py= pyridine, and Mes = 2,4,6-Me3C6H2) with CuI forms the U(V) complex [(C5Me5)(2)U(I)(NMes)]. Reaction of the iodo complex with MgMe2 produces the methyl complex [(C5Me5)(2)U(CH3)(NMes)]. The methyl complexwas reacted with CH2PPh3, surprisingly forming[(C5Me5)(2)U(CH2PPh3)(NMes)], a U(IV) ylide. This is formed from a disproportionationof a transient U(V) carbene, leading to the U(IV) ylide and a U(VI)bis(imido) complex, [(C5Me5)(2)U(NMes)(2)]. These complexes were characterized using spectroscopicmethods (nuclear magnetic resonance, infrared, and ultraviolet-visible-neaninfrared), SQUID magnetometry, and X-ray crystallography, and densityfunctional theory calculations are used to compare the U(V) methylwith the targeted U(V) carbene ligands.

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.

The reaction of 1 equiv of 1-azidoadamantane with [UIII(NR2)3] (R = SiMe3) in Et2O results in the formation of [UV(NR2)3(NAd)] (1, Ad = 1-adamantyl) in good yields. The electronic structure of 1, as well as those of the related U(V) complexes, [UV(NR2)3(NSiMe3)] (2) and [UV(NR2)3(O)] (3), were analyzed with EPR spectroscopy, SQUID magnetometry, NIR-visible spectroscopy, and crystal field modeling. This analysis revealed that, within this series of complexes, the steric bulk of the E2- (E═O, NR) ligand is the most important factor in determining the electronic structure. In particular, the increasing steric bulk of this ligand, on moving from O2- to [NAd]2-, results in increasing U═E distances and E-U-Namide angles. These changes have two principal effects on the resulting electronic structure: (1) the increasing U═E distances decreases the energy of the fσ orbital, which is primarily σ* with respect to the U═E bond, and (2) the increasing E-U-Namide angles increases the energy of fδ, due to increasing antibonding interactions with the amide ligands. As a result of the latter change, the electronic ground state for complexes 1 and 2 is primarily fφ in character, whereas the ground state for complex 3 is primarily fδ.

The reactivity of two metallocene aryloxide U(III) complexes, [(C5Me5)(2)U(O-Ar)], Ar = 4-(BuC6H4)-Bu-t, 1; Ar = 2,6-Bu-t(2)-4-CH3C6H2 (BHT), 3, with CO2 and CS2 has been investigated. The reaction of 1 with CO2 produces a bridging oxo complex, [{(C5Me5)(2)(4-(BuC6H4)-Bu-t-O)U}(2)(mu(2)-O)], 4, while 3 with CO2 results in reductive disproportionation to form the bridging carbonate species, [{(C5Me5)(2)(BHT)U}(2)(mu(2)-kappa(2):eta(1)-CO3)], 5. The difference in reactivity can be attributed to the steric properties of the ligand because the reaction of 3 with an oxo-delivering agent yields a U(V) terminal oxo complex, [(C5Me5)(2)(BHT)U=O], 6. Reduction of CS2 to form a bridging (CS2)(2-) ligand, [{(C5Me5)(2)((BuC6H4)-Bu-t-O)U}(2)(mu(2)-CS2)], 7, is observed with 1, while the reaction of 3 with CS2 also produces a bridging (CS2)(2-) reduced ligand complex, followed by C-H bond activation of a methyl group from one (C5Me5)1- ring, [(C5Me5)(2)(BHT)U{mu(2)-C(H)S-2}U(C5Me4CH2)(C5Me5)(BHT)], 8. All compounds are characterized by NMR and IR spectroscopy, and their solid-state structures are determined by X-ray crystallography.

Technetium-99 is a prevalent fission product from nuclear waste. The long half-life (211,000 yr) and environmental mobility of pertechnetate (TcO4-) render Tc particularly challenging to isolate and stabilize. Here we present two approaches for development of potential wasteforms using titanium dioxide, TiO2. Approach 1 is a low temperature chemical synthesis of TiO2 doped with Tc(IV) from TcO4- intended to mimic the Tc waste stream from the UREX family of separations and removes 98.5 % of the Tc, mainly present as edge-shared Tc(IV) pairs. Approach 2 utilizes TiO2 to photocatalytically reduce TcO4- to Tc(IV) stabilized on the surface of or within the TiO2 lattice. The %Tc removed from solution and adsorbed to TiO2 is pH dependent, with the maximum Tc(IV) adsorbed at pH 3-4 as either TcO2 or edge-sharing Tc(IV) octahedra. The Tc(IV)-TiO2 composites materials formed by both approaches are suitable for consolidation into a dense wasteform by Hot Isostatic Pressing (HIPing).

This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2- t BuNO)C6H4CH2)3N]3- (TriNO x3-) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce LIII absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce[double bond, length as m-dash]O vs. Ce[double bond, length as m-dash]N) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

We report the synthesis and characterization of the trinuclear 4d-4f compounds [Co(C5Me5)2][(C5Me5)2Ln(μ-S)2Mo(μ-S)2Ln(C5Me5)2], 1-Ln (Ln = Y, Gd, Tb, Dy), containing the highly polarizable MoS43- bridging unit. UV-Vis-NIR diffuse reflectance spectra and DFT calculations of 1-Ln reveal a low-energy metal-to-metal charge transfer transition assigned to charge transfer from the singly occupied 4dz2 orbital of MoV to the empty 5d orbitals of the lanthanides (4d in the case of 1-Y), mediated by sulfur-based 3p orbitals. Electron paramagnetic resonance spectra collected for 1-Y in a tetrahydrofuran solution show large 89Y hyperfine coupling constants of A⊥ = 23 MHz and A|| = 26 MHz, indicating the presence of significant yttrium-localized unpaired electron density. Magnetic susceptibility data support similar electron delocalization and ferromagnetic Ln-Mo exchange for 1-Gd, 1-Tb, and 1-Dy. This ferromagnetic exchange gives rise to an S = 15/2 ground state for 1-Gd and one of the largest magnetic exchange constants involving GdIII observed to date, with JGd-Mo = +16.1(2) cm-1. Additional characterization of 1-Tb and 1-Dy by ac magnetic susceptibility measurements reveals that both compounds exhibit slow magnetic relaxation. Although a Raman magnetic relaxation process is dominant for both 1-Tb and 1-Dy, an extracted thermal relaxation barrier of Ueff = 68 cm-1 for 1-Dy is the largest yet reported for a complex containing a paramagnetic 4d metal center. Together, these results provide a potentially generalizable route to enhanced nd-4f magnetic exchange, revealing opportunities for the design of new nd-4f single-molecule magnets and bulk magnetic materials.

Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT2Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT2Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet-triplet gap, is -0.29 eV using this model. This gap was also from the temperature independent paramagnetism of COT2Ce, which yielded a value of -0.32 eV.

Spin-vibronic coupling leads to spin relaxation in paramagnetic molecules, and an understanding of factors that contribute to this phenomenon is essential for designing next-generation spintronics technology, including single-molecule magnets and spin-based qubits, wherein long-lifetime magnetic ground states are desired. We report spectroscopic and magnetic characterization of the isoelectronic and isostructural series of homoleptic zerovalent transition metal triad M(CNDipp)6 (M = V, Nb, Ta; CNDipp = 2,6-diisopropylphenyl isocyanide) and show experimentally the significant increase in spin relaxation rate upon going from V to Nb to Ta. Correlated electronic calculations and first principle spin-phonon computations support the role of spin-orbit coupling in modulating spin-phonon relaxation. Our results provide experimental evidence that increasing magnetic anisotropy through spin-orbit coupling interactions leads to increased spin-vibronic relaxation, which is detrimental to long spin lifetime in paramagnetic molecules.

Reaction of [K(DME)][Th{N(R)(SiMe2 CH2)}2(NR2)] (R = SiMe3) with 1 equiv. of [U(NR2)3(NH2)] (1) in THF, in the presence of 18-crown-6, results in formation of a bridged uranium-thorium nitride complex, [K(18-crown-6)(THF)2][(NR2)3UIV(μ-N)ThIV(NR2)3] (2), which can be isolated in 48% yield after work-up. Complex 2 is the first isolable molecular mixed-actinide nitride complex. Also formed in the reaction is the methylene-bridged mixed-actinide nitride, [K(18-crown-6)][K(18-crown-6)(Et2O)2][(NR2)2U(μ-N)(μ-κ2-C,N-CH2SiMe2NR)Th(NR2)2]2 (3), which can be isolated in 34% yield after work-up. Complex 3 is likely generated by deprotonation of a methyl group in 2 by [NR2]-, yielding the new μ-CH2 moiety and HNR2. Reaction of 2 with 0.5 equiv. of I2 results in formation of a UV/ThIV bridged nitride, [(NR2)3UV(μ-N)ThIV(NR2)3] (4), which can be isolated in 42% yield after work-up. The electronic structure of 4 was analyzed with EPR spectroscopy, SQUID magnetometry, and NIR-visible spectroscopy. This analysis demonstrated that the energies of 5f orbitals of 4 are largely determined by the strong ligand field exerted by the nitride ligand.

Understanding how interactions between the f orbitals and ligand orbitals in lanthanide and actinide systems affect their physical properties is the central issue in f-element chemistry. A wide variety of approaches including both theoretical and experimental tools have been used to study these relationships. Among the most widely used tools has been crystal field theory (CFT), which bridges theory and experiment in that it is a model based largely on atomic theory that is parametrized using experimental data. Crystal field theory is quite accurate for the lanthanides, due in part to the highly contracted nature of the 4f orbitals. For actinides, crystal field theory is less accurate, potentially due to the treatment of orbital mixing. In CFT, orbital mixing is handled implicitly by allowing the electron repulsion parameters (Slater Fk parameters) and the spin-orbit coupling constant to vary. As a result, orbital mixing in CFT is isotropic in that the Fk parameters and the spin-orbit coupling constant affect all f orbitals equally. This approximation works well for the lanthanides due to the limited degree of orbital mixing in these complexes. In actinide complexes, the 5f orbitals have greater overlap with the ligand orbitals, and this approximation is less accurate than in the lanthanides. Here, we report a modification of CFT that includes the effect of orbital mixing on electron repulsion and spin-orbit coupling for each f orbital. The model is applied to the tetravalent uranium hexahalide dianions and PrCl63- for which the energies of many low-lying excited states are known. The new model generally fits the data as well the traditional CFT although with fewer parameters. However, the new model does not fit the data better than the more complex CFT models of Faucher and co-workers. The results of the model show in detail how changes in overlap and orbital energies influence the energies of the bonding and antibonding orbitals.

Four simulated waste streams relevant to the vitrification of Hanford nuclear waste were studied to evaluate the removal kinetics of technetium-99 (Tc) and co-mingled Cr(VI) during treatment with solid ferrous hydroxide (Fe(OH)(2)(s)). Simulants treated with Fe(OH)(2)(s) were reacted for 24 h and sub-sampled periodically to monitor Tc and Cr removal. Solution sample analysis during the reaction was coupled with solid phase characterization, for example, X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), to establish influence of the solid phase product on Tc and Cr removal rates. Based on these results, the majority of Tc and Cr removal occurs within the first 5 min of simulant contact with Fe(OH)(2)(s). However, the order in which Tc and Cr are completely removed from each simulant depends on the simulant chemistry, the preferred reduction pathway, and the solid phase product, for example, magnetite (Fe3O4) versus goethite (alpha-FeOOH). Low pH and low Cr concentrations favor rapid Tc removal, with XRD and XAS confirming that Fe3O4 readily incorporates reduced Tc(IV) into its structure. High pH, high Cr concentrations, and the presence of other co-mingled constituents favor heterogeneous removal of Tc early in the reaction (<1 h) with removal rates often faster than those determined for Cr. At reaction times of >1 h, Tc removal slows as homogeneous removal of Cr begins to dominate, concurrent with an increase in the formation of FeOOH as the solid phase reaction product. These results suggest that for complex, high pH waste streams, Tc removal within the first hour after Fe(OH)(2)(s) treatment is necessary for complete Tc reduction and improved mineral immobilization.