Stabilization of Pu(IV) in PuBr4(OPCy3)2 and Comparisons with Structurally Similar ThX4(OPR3)2 (R = Cy, Ph) Molecules.

The pursuit of a trivalent plutonium halide phosphine oxide compound, e.g., "PuBr3(OPR)3," instead led to the isolation of the tetravalent trans-PuIVBr4(OPCy3)2, PuBr/Cy, compound by spontaneous oxidation of PuIII. The donating nature of phosphine oxides has allowed the isolation and characterization of PuBr/Cy by crystallographic, multinuclear NMR, solid state, and solution phase UV-vis-NIR spectroscopic techniques. The presence of a putative plutonyl(VI) complex formulated as "trans-PuVIO2Br2(OPCy3)2" was also observed spectroscopically and tentatively by single-crystal X-ray diffraction as a cocrystal of PuBr/Cy. A series of trans-ThX4(OPCy3)2 (X = Cl, ThCl/Cy; Br, ThBr/Cy; I, ThI/Cy) complexes were synthesized for comparison to PuBr/Cy. The triphenylphosphine oxide, OPPh3, complexes, trans-AnI4(OPPh3)2 (An = Th, ThI/Ph; U, UI/Ph), were also synthesized for comparison, completing the series trans-UX4(OPPh3)2 (X = Cl, Br, I), UX/Ph. To enable the synthesis of ThI/Cy and ThI/Ph, a new nonaqueous thorium iodide starting material, ThI4(Et2O)2, was synthesized. The syntheses of organic solvent soluble ThI4L2 (L = Et2O, OPCy3, and OPPh3) are the first examples of crystallographically characterized neutral thorium tetraiodide materials beyond binary ThI4. To show the viability of ThI4(Et2O)2 as a starting material for organothorium chemistry, (C5Me4H)3ThI was synthesized and crystallographically characterized.

Expanding Transuranium Organoactinide Chemistry: Synthesis and Characterization of (Cp'3M)2(µ-4,4'-bpy) (M = Ce, Np, Pu).

Dinuclear, organometallic, transuranium compounds, (Cp'3M)2(µ-4,4'-bpy) (Cp'- = trimethylsilylcyclopentadienide, 4,4'-bpy = 4,4'-bipyridine, M = Ce, Np, Pu), reported herein provide a rare opportunity to probe the nature of actinide-carbon bonding. Significant splitting of the f-f transitions results from the unusual coordination environment in these complexes and leads to electronic properties that are currently restricted to organoactinide systems. Structural and spectroscopic characterization in the solid state and in solution for (Cp'3M)2(µ-4,4'-bpy) (M = Np, Pu) are reported, and their structural metrics are compared to a cerium analogue.

Synthesis and characterization of a bimetallic americium(III) pyrithionate coordination complex.

The aqueous reaction of sodium pyrithione, (Na)mpo, with 243AmCl3·nH2O yields a dimerized complex, [243Am(mpo)2(µ-O-mpo)(H2O)]2·3H2O. This compound is compared with isostructural lanthanide pyrithionates, where dimerization across the 4f-block is observed to be dependent upon the size of the cation. Unlike in most reported Am(III) UV-visible absorption spectra, [243Am(mpo)2(µ-O-mpo)(H2O)]2·3H2O shows significant splitting in the fingerprint excitations. This is attributed to a unique ligand-field environment, where the Am-mpo bonds possess different bonding compared to the Nd(III) analog because of increasing covalent interactions.

Cyclopentadienyl coordination induces unexpected ionic Am-N bonding in an americium bipyridyl complex.

Variations in bonding between trivalent lanthanides and actinides is critical for reprocessing spent nuclear fuel. The ability to tune bonding and the coordination environment in these trivalent systems is a key factor in identifying a solution for separating lanthanides and actinides. Coordination of 4,4'-bipyridine (4,4'-bpy) and trimethylsilylcyclopentadienide (Cp') to americium introduces unexpectedly ionic Am-N bonding character and unique spectroscopic properties. Here we report the structural characterization of (Cp'3Am)2(µ - 4,4'-bpy) and its lanthanide analogue, (Cp'3Nd)2(µ - 4,4'-bpy), by single-crystal X-ray diffraction. Spectroscopic techniques in both solid and solution phase are performed in conjunction with theoretical calculations to probe the effects the unique coordination environment has on the electronic structure.

Isolation and characterization of a californium metallocene.

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin-orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1-3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4-12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf-C bond. Analysis suggests the Cf-C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16-22.

Synthesis, characterization, and theoretical analysis of a plutonyl phosphine oxide complex.

The interplay of bond strength and covalency are examined in AnO2Cl2(OPcy3)2 (An = Pu, U) complexes. The synthesis of trans-PuO2Cl2(OPcy3)2, 1-Pu, has been carried out and confirmed by single crystal X-ray diffraction along with UV-vis-NIR, and 31P NMR spectroscopies. Theoretical analysis finds that despite a higher calculated covalency for the Pu-Cl interaction, the Pu-OPcy3 interaction is stronger due to the accumulation of electron density in the interatomic region. The coordination of equatorial ligands slightly decreases the strength of the PuOyl interactions relative to the free gas phase (PuO2)2+ ion.

An1.33T4Al8Si2 (An = Ce, Th, U, Np; T = Ni, Co): Actinide Intermetallics with Disordered Gd1+xFe4Si10-y Structure Type Grown from Metal Flux.

An1.33T4Al8Si2 (An = Ce, Th, U, Np; T = Ni, Co) were synthesized in metal flux reactions carried out in aluminum/gallium melts. In previous work, U1.33T4Al8Si2 (T = Co, Ni) analogues were formed by arc-melting U:T:Si and reacting this mixture in Al/Ga flux. However, in the current work, all compounds were synthesized by using AnO2 reactants, taking advantage of the ability of the aluminum in the flux to act as both solvent and reducing agent. While reactions with T = Co yielded hexagonal Gd1.33Fe4Si10-type quaternary phases for all An, reactions with T = Ni produced these compounds only with An = U and Np. For reactions with An = Ce and Th, the reactions led instead to the formation of AnNi3-xSixAl4-yGay phases, with the tetragonal KCu3S4 structure type. Attempts to synthesize plutonium analogues Pu1.33T4Al8Si2 were also unsuccessful, producing the previously reported PuCoGa5 and Pu2Ni5Si6 instead. Magnetic data collected on the neptunium analogues Np1.33T4Al8Si2 (T = Ni, Co) show antiferromagnetic coupling at low temperatures and indicate a tetravalent state for the Np ions.

Crystallographic characterization of (C5H4SiMe3)3U(BH4).

New syntheses have been developed for the synthesis of (borohydrido-κ3 H)tris-[η5-(tri-methyl-sil-yl)cyclo-penta-dien-yl]uranium(IV), [U(BH4)(C8H13Si)3] or Cp'3U(BH4) (Cp' = C5H4SiMe3) and its structure has been determined by single-crystal X-ray crystallography. This compound crystallized in the space group P and the structure features three η 5-coordinated Cp' rings and a κ 3-coordinated (BH4)- ligand.

Pronounced Pressure Dependence of Electronic Transitions for Americium Compared to Isomorphous Neodymium and Samarium Mellitates.

The mellitate ion is relevant in spent nuclear fuel processing and is utilized as a surrogate for studying the interactions of f elements with humic acids. A wealth of different coordination modes gives the potential for diverse structural chemistry across the actinide series. In this study, an americium mellitate, 243Am2[(C6(COO-)6](H2O)8·2H2O (1-Am), has been synthesized and characterized using structural analysis and spectroscopy at ambient and elevated pressures. 1-Am was then compared to isomorphous neodymium (1-Nd) and samarium (1-Sm) mellitates via bond-length analysis and pressure dependence of their Laporte-forbidden f → f transitions. Results show that the pressure dependence of the f → f transitions of 1-Am is significantly greater than that observed in 1-Nd and 1-Sm, with average shifts of 21.4, 4.7, and 3.6 cm-1/GPa, respectively. This greater shift found in 1-Am shows further evidence that the 5f orbitals are more affected than the 4f orbitals when pressure is applied to isostructural compounds.

Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes.

A pair of neptunium Schiff base coordination complexes, NpVIO2L(MeOH) and NpIVL2 {H2L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, 1H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that NpVIO2L(MeOH) and NpIVL2 are isomorphous with the previously reported UVIO2L(MeOH) and MIVL2 (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of NpVIO2L(MeOH) in situ or direct synthesis from a (NpVO2)+ source shows evidence of a pentavalent neptunyl (NpVO2L)xn- species as determined by UV/vis/NIR and 1H NMR spectroscopy. The synthesis of (NpVO2L)xn- directly from a (NpVO2)+ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np-L covalent interactions in NpIVL2 are similar to those in the NpVIO2L(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np-L bonds.

Structure and Characterization of an Americium Bis(O,O'-diethyl)dithiophosphate Complex.

A facile synthesis of an americium complex with a sulfur-donor ligand has been developed, allowing characterization of americium bonding from multiple perspectives via several techniques. Reaction of 243Am with S2P(OEt)2- yields the tetrakis complex [Am(S2P(OEt)2)4]- that can be crystallized as the tetraphenylarsonium salt. Structures obtained from single crystal X-ray diffraction show bond length discrepancies from the neodymium analogue consistent with the soft-donor bond enhancement common to actinides. Solid state optical spectroscopy confirms interaction of the ligand with 5f orbitals. 31P nuclear magnetic reflects the minor paramagnetism of Am(III). Computational investigations through CASSCF calculations, ligand-field density functional theory, and quantum chemical topological analysis allow a quantification of covalency or orbital interaction effects via total energy density and nephelauxetic parameters, both of which indicate greater covalency in the americium species than in the neodymium analogue or the americium aquo complex.

A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes.

An approach to obtaining substantial amounts of data from a hazardous starting material that can only be obtained and handled in small quantities is demonstrated by the investigation of a single small-scale reaction of cyclooctatetraene, C8H8, with a solution obtained from the reduction of Cp'3Pu (Cp' = C5H4SiMe3) with potassium graphite. This one reaction coupled with oxidation of a product has provided single-crystal X-ray structural data on three organoplutonium compounds as well as information on redox chemistry thereby demonstrating an efficient route to new reactivity and structural information on this highly radioactive element. The crystal structures were obtained from the reduction of C8H8 by a putative Pu(II) complex, (Cp'3PuII)1-, generated in situ, to form the Pu(III) cyclooctatetraenide complex, [K(crypt)][(C8H8)2PuIII], 1-Pu, and the tetra(cyclopentadienyl) Pu(III) complex, [K(crypt)][Cp'4PuIII], 2-Pu. Oxidation of the sample of 1-Pu with Ag(I) afforded a third organoplutonium complex that has been structurally characterized for the first time, (C8H8)2PuIV, 3-Pu. Complexes 1-Pu and 3-Pu contain Pu sandwiched between parallel (C8H8)2- rings. The (Cp'4PuIII)- anion in 2-Pu features three η5-Cp' rings and one η1-Cp' ring, which is a rare example of a formal Pu-C η1-bond. In addition, this study addresses the challenge of small-scale synthesis imparted by radiological and material availability of transuranium isotopes, in particular that of pure metal samples. A route to an anhydrous Pu(III) starting material from the more readily available PuIVO2 was developed to facilitate reproducible syntheses and allow complete spectroscopic analysis of 1-Pu and 2-Pu. PuIVO2 was converted to PuIIIBr3(DME)2 (DME = CH3OCH2CH2OCH3) and subsequently PuIIIBr3(THF)x, which was used to independently synthesize 1-Pu, 2-Pu, and 3-Pu.

Compression of curium pyrrolidine-dithiocarbamate enhances covalency.

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]- spectra, we observe smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.

Pressure-Induced Spectroscopic Changes in a Californium 1D Material Are Twice as Large as Found in the Holmium Analog.

In this study, the synthesis, characterization, and pressure response of a 1D californium mellitate (mellitate = 1,2,3,4,5,6-benzenehexacarboxylate) coordination polymer, Cf2(mell)(H2O)10·4H2O (Cf-1), are reported. The Cf-O lengths within the crystal structure are compared to its gadolinium (Gd-1) and holmium (Ho-1) analogs as well. These data show that the average Cf-O bond distance is slightly longer than the average Gd-O bond, consistent with trends in effective ionic radii. UV-vis-NIR absorption spectra as a function of pressure were collected using diamond-anvil techniques for both Cf-1 and Ho-1. These experiments show that the Cf(III) f → f transitions have a stronger dependence on pressure than that of the holmium analog. In the former case, the shift is nearly linear with applied pressure and averages 6.6 cm-1/GPa, whereas in the latter, it is <3 cm-1/GPa.

Examination of Molten Salt Reactor Relevant Elements Using Hydrothermal Synthesis.

The structural chemistry of elements relevant to the FLiBe molten salt reactor, Th, U, Np, and Zr, including Ce and Nd (as analogues for Pu and Am, respectively), have been examined using hydrothermal synthesis at 200 °C. These reactions serve to model the reaction of molten salts under hydrolysis conditions. The results show that U and Np formed LiAnF5, while Ce formed Li4CeF8. The source of U also controlled the crystal quality, where UO2 gave small crystals, while UO3·2H2O gave very large crystals. It is likely that Be incorporation was not observed because of the high solubility of [BeF4]2- in water. Zr formed a third product, Li6BeF4ZrF8, which features isolated [BeF4]2- and [ZrF8]4- units bridged by Li+. Additionally, Li2BeF4 was regularly isolated. When little to no alkali metal was included in the reaction, M3F12(H2O) was isolated for Np, U, and Ce.

Structural and Spectroscopic Investigation of Two Plutonium Mellitates.

The aqueous reaction of mellitic acid (H6mell) with 242PuBr3·nH2O forms two plutonium mellitates, 242Pu2(mell)(H2O)9·H2O (Pu-1α) and 242Pu2(mell)(H2O)8·2H2O (Pu-1ß). These compounds are compared to the isomorphous lanthanide mellitates with similar ionic radii via bond length analysis. Both plutonium compounds form three-dimensional metal-organic frameworks, with Pu-1α having two unique metal centers and Pu-1ß having one. All plutonium metal centers exhibit nine-coordinate geometries. Our results show metal-oxygen bond lengths for plutonium significantly shorter than those of the previously reported lanthanum and herein reported cerium analogues, consistent with the nine-coordinate ionic radii. Clear Laporte-forbidden 5f → 5f transitions are observed in the ultraviolet-visible-near-infrared spectra and are assigned to trivalent plutonium. However, there is a distinct color difference between the two plutonium compounds.

Probing a variation of the inverse-trans-influence in americium and lanthanide tribromide tris(tricyclohexylphosphine oxide) complexes.

The synthesis, characterization, and theoretical analysis of meridional americium tribromide tris(tricyclohexylphosphine oxide), mer-AmBr3(OPcy3)3, has been achieved and is compared with its early lanthanide (La to Nd) analogs. The data show that homo trans ligands display significantly shorter bonds than the cis or hetero trans ligands. This is particularly pronounced in the americium compound. DFT along with multiconfigurational CASSCF calculations show that the contraction of the bonds relates qualitatively with overall covalency, i.e. americium shows the most covalent interactions compared to lanthanides. However, the involvement of the 5p and 6p shells in bonding follows a different order, namely cerium > neodymium ∼ americium. This study provides further insight into the mechanisms by which ITI operates in low-valent f-block complexes.

Synthesis, Spectroscopy, and Theoretical Details of Uranyl Schiff-Base Coordination Complexes.

Two uranyl Schiff-base coordination complexes, UO2L(MeOH) and UO2Cl2(H2L) {L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes have been characterized by structural, spectroscopic, and theoretical analysis to offer an electronic structure basis to explain the bonding parameters and stability. Single-crystal X-ray analysis reveals that UO2L(MeOH) adopts the typical "soft taco confirmation" characteristic of uranyl salophen complexes, whereas UO2Cl2(H2L) features an unusual neutral ligand coordination that contains an internal hydrogen bond between the phenol and imine. Rate constants calculated from electrochemical experiments confirm a quasi-reversible UO22+/UO2+ couple. Single-configurational and multiconfigurational methods were used to explore the bonding in UO2L(MeOH) and UO2Cl2(H2L). For UO2Cl2(H2L), the U-Cl bond exhibits more covalent contributions than U-OL.

Electronic, Magnetic, and Theoretical Characterization of (NH4)4UF8, a Simple Molecular Uranium(IV) Fluoride.

The simple system of tetraammonium octafluorouranate is employed to derive a fundamental understanding of the uranium-fluorine interaction. The structure is composed of isolated molecules, enabling a detailed examination of the U4+ ( f2) ion. Characterization of single-crystals by X-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is combined with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method (IQA) is used to examine the nature of the U-F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) in [UF8]4-. For the structural and theoretical reasons discussed herein, it is proposed that the structure of (NH4)4UF8 may be appropriately employed as a benchmark compound for future theoretical characterization of U(IV).

Experimental and Theoretical Comparison of Transition-Metal and Actinide Tetravalent Schiff Base Coordination Complexes.

A series of homoleptic tetravalent transition-metal and actinide Schiff-base coordination complexes, ML2 {M = Zr, Hf, Th, U; L = N, N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes create the opportunity for comparing a series of complexes containing metal cations in the formal IV+ oxidation state by structural, spectroscopic, and theoretical analysis that also incorporate the previously reported Ce(IV) and Pu(IV) analogues. X-ray crystallographic analysis reveals that all complexes are isomorphous and feature a co-facial ligand geometry. TD-DFT and other quantum mechanical methods were used to explore bonding differences across between the complexes, and resulting calculated absorbance spectra for ML2 are in good agreement with the experimental data. The computational results also suggest that U(IV) and Pu(IV) analogs have more covalent character in their bonding than found with the other metal cations reported here.