Restricted Speciation and Supramolecular Assembly in the 5f Block

Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster

Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U60 Ox30 *, which captures and stabilizes oxygen-based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U60 Ox30 * was characterized by single crystal X-ray diffraction as well as infrared (IR), Raman, UV-Vis-NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification of the free radicals present in U60 Ox30 * was done via room temperature solid and solution state X-band EPR studies using spin trapping methods. The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was definitive for identifying the free radicals in U60 Ox30 *, which are hydroxyl radicals (⋅OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) further confirmed the radicals were oxygen based, and deuteration experiments showed that the origin of the free radicals was from the decomposition of H2 O2 in water. These results demonstrate that highly oxidizing species such as the ⋅OH radical can be stabilized in UPCs, which alters our understanding of the role of free radicals present in spent nuclear fuel.

(NH4)2[UO2Cl4]·2H2O, a new uranyl tetra-chloride with ammonium charge-balancing cations

A new uranyl tetra-chloride salt with chemical formula, (NH4)2[UO2Cl4]·2H2O, namely, di-ammonium uranyl tetra-chloride dihydrate, 1, was prepared and crystallized via slow evaporation from a solution of 2 M hydro-chloric acid. As confirmed by powder X-ray diffraction, the title compound crystallizes with an ammonium chloride impurity that formed as a result of the breakdown of a triazine precursor. The (UO2Cl4)2- dianion is charge balanced by ammonium cations, while an extensive hydrogen-bond network donated from structural water mol-ecules stabilize the overall assembly. Compound 1 adds to the extensive collection of actinyl tetra-chloride salts, but it represents the first without an alkali cation for purely inorganic compounds. Diffuse reflectance and luminescence spectra show typical absorption and emission behavior, respectively, of uranyl materials.

A novel symmetric pyrazine (pyz)-bridged uranyl dimer [UO2Cl3(H2O)(Pyz)0.5]22-: synthesis, structure and computational analysis

Herein we report on the synthesis of (HPyz⁺)₂[UO₂Cl₃(H₂O)(Pyz)_0.5]₂·2H₂O which features a novel pyrazine-bridged uranyl dimer, [UO₂Cl₃(H₂O)(Pyz)_0.5]₂^2-. A rigorous computational and experimental analysis of this compound was performed to fully explore the influence of coordination on the electronic structure and potential charge-transfer characteristics of this dimer, revealing a delocalized π-system across the bridging pyrazine and the axial components of both uranyl centers. Electrostatic surface potentials, used to rationalize the observed assembly, indicate a decreased basicity of the uranyl oxo versus [UO₂Cl₄]^2-, and signify a lessened capacity for the terminal -yl oxos of the [UO₂Cl₃(H₂O)(Pyz)_0.5]₂^2- dimer to participate in supramolecular assembly. A combined density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) analysis further evidenced an increase in UO bond strengths within the dimer, which is supported by a blue shift in the characteristic Raman-active uranyl symmetric stretch (ν₁) with respect to the more typically observed [UO₂Cl₄]^2-.

Bimetallic uranyl/cobalt(II) isothiocyanates: structure, property and spectroscopic analysis of homo- and heterometallic phases

We report the synthesis and characterization of a family of UO22+/Co2+ isothiocyanate materials containing [UO2(NCS)5]3- and/or [Co(NCS)4]2- building units charged balanced by tetramethylammonium cations and assembled via SS or SOyl non-covalent interactions (NCIs), namely (C4H12N)3[UO2(NCS)5], (C4H12N)2[Co(NCS)4], and (C4H12N)5[Co(NCS)4][UO2(NCS)5]. The homometallic uranyl phase preferentially assembles via SS interactions, whereas in the heterometallic phase SOyl interactions are predominant. The variation in assembly mode is explored using electrostatic surfaces potentials, revealing that the pendant -NCS ligands of the [Co(NCS)4]2- anion is capable of outcompeting those of the [UO2(NCS)5]3- anion. Notably, the heterometallic phase displays atypical blue shifting of the uranyl symmetric stretch in the Raman spectra, which is in contrast to many other compounds featuring non-covalent interactions at uranyl oxygen atoms. A combined experimental and computational (density functional theory and natural bond orbital analyses) approach revealed that coupling of the uranyl symmetric stretch with isothiocyanate modes of equatorial -NCS ligands was responsible for the atypical blue shift in the heterometallic phase.

Controlling the Reduction of Chelated Uranyl to Stable Tetravalent Uranium Coordination Complexes in Aqueous Solution

The solution-state interactions between octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) chelating ligands and uranium were investigated and characterized by UV-visible spectrophotometry and X-ray absorption spectroscopy (XAS), as well as electrochemically via spectroelectrochemistry (SEC) and cyclic voltammetry (CV) measurements. Depending on the selected chelator, we demonstrate the controlled ability to bind and stabilize U^IV, generating with 3,4,3-LI(1,2-HOPO), a tetravalent uranium complex that is practically inert toward oxidation or hydrolysis in acidic, aqueous solution. At physiological pH values, we are also able to bind and stabilize U^IV to a lesser extent, as evidenced by the mix of U^IV and U^VI complexes observed via XAS. CV and SEC measurements confirmed that the U^IV complex formed with 3,4,3-LI(1,2-HOPO) is redox inert in acidic media, and U^VI ions can be reduced, likely proceeding via a two-electron reduction process.

Structural, spectroscopic, and computational evaluations of cation–cation and halogen bonding interactions in heterometallic uranyl hybrid materials

A route for systematically accessing the oxo atoms of the linear uranyl (UO₂²⁺) cation via cation–cation and halogen bonding interactions is detailed, and interaction strengths are probed via structural, vibrational, and computational means.

Combinatorial design of multimeric chelating peptoids for selective metal coordination

The combinatorial synthesis of a new library of tetrameric peptoid ligands is introduced, enabling coordination and characterization of f-block metals.

Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)₂(CAM)₂ and 3,4,3-LI(CAM)₂(1,2-HOPO)₂. Coordination-based luminescence studies were carried out with Eu³⁺ and Tb³⁺ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu³⁺ stability constants log β₁₁₀ ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.

Halogen bonding in UiO-66 frameworks promotes superior chemical warfare agent simulant degradation

Herein, a series of halogenated UiO-66 derivatives was synthesized and analyzed for the breakdown of the chemical warfare agent simulant dimethyl-4-nitrophenyl phosphate (DMNP) to analyze ligand effects. UiO-66-I degrades DMNP at a rate four times faster than the most active previously reported MOFs. MOF defects were quantified and ruled out as a cause for increased activity. Theoretical calculations suggest the enhanced activity of UiO-66-I originates from halogen bonding of the iodine atom to the phosphoester linkage allowing for more rapid hydrolysis of the P-O bond.

Reductive activation of neptunyl and plutonyl oxo species with a hydroxypyridinone chelating ligand

Neptunyl(vi) and plutonyl(vi) oxo-activation with reduction to tetravalent hydroxides was investigated in gas and condensed phases, and by density functional theory.