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.
All molecular crystals contain defects, yet the effects these defects have on the material properties they exhibit are poorly understood. Here, the relationship between mechanical properties and ...defect concentration is established through nanoindentation studies on single crystals of uric acid (UA) with two different types of substitutional defects. Defects are intentionally created by preparing UA-dye crystals with two different dyes chosen to span either one or more than one molecular layer when included in the host matrix. The included dye concentrations in materials prepared from several well-defined growth conditions are then spectroscopically quantified. Sector-specific nanoindentation measurements on UA-dye crystals with dye concentrations ranging from 0.01 to 1 wt % illustrate two competing effects. UA-dye crystals with the lowest dye concentrations exhibited Young’s moduli reductions of up to ∼50% compared to undoped crystals, with defects causing elastic softening. With progressively higher concentrations of defects spanning multiple UA layers, a second competing material stiffening effect is activated. At the concentrations necessary for material strengthening to occur, twinning and other morphological changes are also observed, suggesting that the macroscopic changes are likely related to efforts to reduce lattice strain. The magnitude of the property changes realized illustrates the potential of defect engineering to tune mechanical properties and may be especially beneficial in systems where increased plasticity is desired.
The solution‐state interactions of plutonium and berkelium with the octadentate chelator 3,4,3‐LI(1,2‐HOPO) (343‐HOPO) were investigated and characterized by X‐ray absorption spectroscopy, which ...revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X‐ray absorption near‐edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white‐line position energies for Pu‐343‐HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk‐343‐HOPO results were found to mirror the XANES behavior of Bk(III)‐DTPA. Extended X‐ray absorption fine structure results revealed An—OHOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343‐HOPO via density functional theory calculations. Pu(III)‐ and Bk(III)‐343‐HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)‐, Cm(III)‐, Cf(III)‐, and Es(III)‐343‐HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series.
X‐ray absorption spectroscopy was used to probe the interactions between an octadentate hydroxypyridinone chelator and two transuranic elements in microgram quantities – plutonium and berkelium – within buffered solutions. Despite the precedence for chelation‐driven stabilization of the tetravalent oxidation state of actinides with this ligand, in situ reductive decomposition yielded plutonium(III) and berkelium(III) coordination complexes.
Examinations of uranium(IV) complexation by aliphatic dicarboxylates, ranging from the three-carbon malonate to the six-carbon adipate, yielded six novel phases. Single crystals of the compounds ...were prepared through either evaporation or solvent layering, and the structures were determined via single-crystal X-ray diffraction. The compounds were further characterized via IR and Raman spectroscopies. For the phases synthesized via solvent layering, correlations between the solution- and solid-state speciation were probed using UV–visible absorption spectroscopy. In the presence of malonate (MA), U(MA)2(H2O)3 n (U-1a) was isolated. As the crystals were not suitable for structure determination, reactions of MA with thorium were pursued. These efforts yielded polymorphs, Th-1a and Th-1b; in both An-1a (An = Th, U) and Th-1b, the An(IV) metal centers are bridged via the MA ligands into 2D sheets. Upon increasing the length of the aliphatic backbone, monodentate and bridging bidentate binding modes are observed. Two distinct ligand-bridged extended networks that vary in metal ion nuclearity were observed for succinate (SA): UCl2(SA)(H2O)2 n (U-2) and U6O4(OH)4(SA)4(H2O)10·4Cl·mH2O (U-3). Whereas U-2 consists of mononuclear U(IV) atoms that are bridged through the organic carboxylate into a 3D network, U-3 is built from hexanuclear U(IV)-hydroxo-oxo clusters that are further bridged into a 3D network. From glutarate (GA) ligand systems, two phases were isolated including UCl2(HGA)2(H2O)2 n (U-4) that consists of ligand-bridged one-dimensional chains and U2Cl6(HGA)2(H2O)2 n (U-5), which is built from chloride-bridged {U2Cl2} dimers that are further linked via GA into two-dimensional sheets. Finally, in the presence of adipate (AA), U6O4(OH)4(AA)4(H2O)8·4Cl·7(H2O) (U-6) is observed; the structure consists of hexameric U6O4(OH)412+ clusters that are propagated into one-dimensional chains via the AA and uranium-bound water molecules that directly link adjacent clusters. Taken together, this work provides a systematic examination of the influence of the identity of the dicarboxylate backbones on actinide structural chemistry and highlights the role that the organic ligand plays in stabilizing unique tetravalent actinide structural units.
The transplutonium elements (atomic numbers 95-103) are a group of metals that lie at the edge of the periodic table. As a result, the patterns and trends used to predict and control the physics and ...chemistry for transition metals, main-group elements and lanthanides are less applicable to transplutonium elements. Furthermore, understanding the properties of these heavy elements has been restricted by their scarcity and radioactivity. This is especially true for einsteinium (Es), the heaviest element on the periodic table that can currently be generated in quantities sufficient to enable classical macroscale studies
. Here we characterize a coordination complex of einsteinium, using less than 200 nanograms of
Es (with half-life of 275.7(5) days), with an organic hydroxypyridinone-based chelating ligand. X-ray absorption spectroscopic and structural studies are used to determine the energy of the L
-edge and a bond distance of einsteinium. Photophysical measurements show antenna sensitization of Es
luminescence; they also reveal a hypsochromic shift on metal complexation, which had not previously been observed in lower-atomic-number actinide elements. These findings are indicative of an intermediate spin-orbit coupling scheme in which j-j coupling (whereby single-electron orbital angular momentum and spin are first coupled to form a total angular momentum, j) prevails over Russell-Saunders coupling. Together with previous actinide complexation studies
, our results highlight the need to continue studying the unusual behaviour of the actinide elements, especially those that are scarce and short-lived.
Five novel tetravalent thorium (Th) compounds that consist of Th(H2O) x Cl y structural units were isolated from acidic aqueous solutions using a series of nitrogen-containing heterocyclic hydrogen ...(H) bond donors. Taken together with three previously reported phases, the compounds provide a series of monomeric ThIV complexes wherein the effects of noncovalent interactions (and H-bond donor identity) on Th structural chemistry can be examined. Seven distinct structural units of the general formulas Th(H2O) x Cl8–x x−4 (x = 2, 4) and Th(H2O) x Cl9–x x−5 (x = 5–7) are described. The complexes range from chloride-deficient Th(H2O)7Cl22+ to chloride-rich Th(H2O)2Cl62– species, and theory was used to understand the relative energies that separate complexes within this series via the stepwise chloride addition to an aquated Th cation. Electronic structure theory predicted the reaction energies of chloride addition and release of water through a series of transformations, generally highlighting an energetic driving force for chloride complexation. To probe the role of the counterion in the stabilization of these complexes, electrostatic potential (ESP) surfaces were calculated. The ESP surfaces indicated a dependence of the chloride distribution about the Th metal center on the pK a of the countercation, highlighting the directing effects of noncovalent interactions (e.g., Hbonding) on Th speciation.
Organic ligands with carboxylate functionalities have been shown to affect the solubility, speciation, and overall chemical behavior of tetravalent metal ions. While many reports have focused on ...actinide complexation by relatively simple monocarboxylates such as amino acids, in this work we examined Th(IV) and U(IV) complexation by 4-hydroxybenzoic acid in water with the aim of understanding the impact that the organic backbone has on the solution and solid state structural chemistry of thorium(IV) and uranium(IV) complexes. Two compounds of the general formula An6O4(OH)4(H2O)6(4-HB)12·nH2O An = Th (Th-1) and U (U-1); 4-HB = 4-hydroxybenzoate were synthesized via room-temperature reactions of AnCl4 and 4-hydroxybenzoic acid in water. Solid state structures were determined by single-crystal X-ray diffraction, and the compounds were further characterized by Raman, infrared, and optical spectroscopies and thermogravimetry. The magnetism of U-1 was also examined. The structures of the Th and U compounds are isomorphous and are built from ligand-decorated oxo/hydroxo-bridged hexanuclear units. The relationship between the building units observed in the solid state structure of U-1 and those that exist in solution prior to crystallization as well as upon dissolution of U-1 in nonaqueous solvents was investigated using small-angle X-ray scattering, ultraviolet–visible optical spectroscopy, and dynamic light scattering. The evolution of U solution speciation as a function of reaction time and temperature was examined. Such effects as well as the impact of the ligand on the formation and evolution of hexanuclear U(IV) clusters to UO2 nanoparticles compared to prior reported monocarboxylate ligand systems are discussed. Unlike prior reported syntheses of Th and U(IV) hexamers where the pH was adjusted to ∼2 and 3, respectively, to drive hydrolysis, hexamer formation with the HB ligand appears to be promoted only by the ligand.
Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with ...different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu
and Eu
ions in CaCl
, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K
to Na
to Ca
, the coordination number (CN) of Cl
in the first solvation shell increases from 5.6 (Eu
) and 5.9 (Eu
) in KCl to 6.9 (Eu
) and 7.0 (Eu
) in CaCl
. This coordination change is validated by the EXAFS measurements, in which the CN of Cl
around Eu increases from 5 in KCl to 7 in CaCl
. Our simulation shows that the fewer Cl
ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu
/Eu
are related to the rigidity of their first coordination shell of Cl
: the more rigid the first coordination shell is, the slower the solute cations diffuse.
Uranium(IV) complexation by 2-furoic acid (2-FA) was examined to better understand the effects of ligand identity and reaction conditions on species formation and stability. Five compounds were ...isolated: UCl
(2-FA)
(H
O)
(1), U
Cl
O
(THF)
(2-FA)
⋅2 THF (2), U
O
(OH)
(H
O)
(2-FA)
⋅7 THF⋅H
O (3), U
O
(OH)
(H
O)
(2-FA)
⋅8.76 H
O (4), and U
Cl
O
(OH)
(H
O)
⋅m H
O⋅n THF (5). The structures were determined by single-crystal X-ray diffraction and further characterized by Raman, IR, and optical absorption spectroscopy. The thermal stability and magnetic behavior of the compounds were also examined. Variations in the synthetic conditions led to notable differences in the structural units observed in the solid state. At low H
O/THF ratios, a tetranuclear oxo-bridged U
O
core was isolated. Aging of this solution resulted in the formation a U
oxo cluster capped by chloro and water ligands. However, at increasing water concentrations only hexanuclear units were observed. In all cases, at temperatures of 100-120 °C, UO
nanoparticles formed.
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