Incorporation of Technetium into Spinel Ferrites

Technetium (⁹⁹Tc) is a problematic fission product for the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate, the stable species in aerobic environments. One approach to preventing ⁹⁹Tc contamination is using sufficiently durable waste forms. We report the incorporation of technetium into a family of synthetic spinel ferrites that have environmentally durable natural analogs. A combination of X-ray diffraction, X-ray absorption fine structure spectroscopy, and chemical analysis reveals that Tc­(IV) replaces Fe­(III) in octahedral sites and illustrates how the resulting charge mismatch is balanced. When a large excess of divalent metal ions is present, the charge is predominantly balanced by substitution of Fe­(III) by M­(II). When a large excess of divalent metal ions is absent, the charge is largely balanced by creation of vacancies among the Fe­(III) sites (maghemitization). In most samples, Tc is present in Tc-rich regions rather than being homogeneously distributed

Influence of Pyrazolate vs <i>N</i>‑Heterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes

Two isostructural series of trigonal prismatic complexes, M­(Bp^Me)₃ and M­(Bc^Me)₃ (M = Y, Tb, Dy, Ho, Er, U; [Bp^Me]⁻ = dihydrobis­(methypyrazolyl)­borate; [Bc^Me]⁻ = 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 <i>N</i>-heterocyclic carbene complexes, M­(Bc^Me)₃, than for the isomeric pyrazolate complexes, M­(Bp^Me)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(Bc^Me)₃ complexes, even when dipolar interactions are largely absent. Notably, the dilute species U­(Bc^Me)₃ exhibits <i>U</i>_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 <i>N-</i>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

Impact of Natural Organic Matter on Uranium Transport through Saturated Geologic Materials: From Molecular to Column Scale

The risk stemming from human exposure to actinides via the groundwater track has motivated numerous studies on the transport of radionuclides within geologic environments; however, the effects of waterborne organic matter on radionuclide mobility are still poorly understood. In this study, we compared the abilities of three humic acids (HAs) (obtained through sequential extraction of a peat soil) to cotransport hexavalent uranium (U) within water-saturated sand columns. Relative breakthrough concentrations of U measured upon elution of 18 pore volumes increased from undetectable levels (<0.001) in an experiment without HAs to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect on U mobility was positively correlated with the hydrophobicity of organic matter and NMR-detected content of alkyl carbon, which indicates the possible importance of hydrophobic organic matter in facilitating U transport. Carbon and uranium elemental maps collected with a scanning transmission X-ray microscope (STXM) revealed uneven microscale distribution of U. Such molecular- and column-scale data provide evidence for a critical role of hydrophobic organic matter in the association and cotransport of U by HAs. Therefore, evaluations of radionuclide transport within subsurface environments should consider the chemical characteristics of waterborne organic substances, especially hydrophobic organic matter

Theory and X‑ray Absorption Spectroscopy for Aluminum Coordination Complexes – Al K‑Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR₂, and (BDI)AlX₂ Complexes

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)­Al, (BDI)­AlX₂, and (BDI)­AlR₂ coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal–ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)­AlX₂ systems with more electronegative X^1– ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)­AlH₂ the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core–Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)­AlH₂ are similar relative to the (BDI)­AlX₂ and (BDI)­AlMe₂ compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b₁ symmetry that is localized on Al and orthogonal to the (BDI)­Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)­AlX₂ and (BDI)­AlR₂. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds

Theory and X‑ray Absorption Spectroscopy for Aluminum Coordination Complexes – Al K‑Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR₂, and (BDI)AlX₂ Complexes

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)­Al, (BDI)­AlX₂, and (BDI)­AlR₂ coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal–ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)­AlX₂ systems with more electronegative X^1– ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)­AlH₂ the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core–Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)­AlH₂ are similar relative to the (BDI)­AlX₂ and (BDI)­AlMe₂ compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b₁ symmetry that is localized on Al and orthogonal to the (BDI)­Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)­AlX₂ and (BDI)­AlR₂. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds

Probing the Interfacial Interaction in Layered-Carbon-Stabilized Iron Oxide Nanostructures: A Soft X‑ray Spectroscopic Study

We have stabilized the iron oxide nanoparticles (NPs) of various sizes on layered carbon materials (Fe-oxide/C) that show excellent catalytic performance. From the characterization of X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), scanning transmission X-ray microscopy (STXM) and X-ray magnetic circular dichroism spectroscopy (XMCD), a strong interfacial interaction in the Fe-oxide/C hybrids has been observed between the small iron oxide NPs and layered carbon in contrast to the weak interaction in the large iron oxide NPs. The interfacial interaction between the NPs and layered carbon is found to link with the improved catalytic performance. In addition, the Fe <i>L</i>-edge XMCD spectra show that the large iron oxide NPs are mainly γ-Fe₂O₃ with a strong ferromagnetic property, whereas the small iron oxide NPs with strong interfacial interaction are mainly α-Fe₂O₃ or amorphous Fe₂O₃ with a nonmagnetic property. The results strongly suggest that the interfacial interaction plays a key role for the catalytic performance, and the experimental findings may provide guidance toward rational design of high-performance catalysts

High-Resolution Imaging of Polymer Electrolyte Membrane Fuel Cell Cathode Layers by Soft X‑ray Spectro-Ptychography

Polymer electrolyte membrane fuel cells (PEMFCs) are a promising and sustainable alternative to internal combustion engines for automotive applications. Polymeric perfluorosulfonic acid (PFSA) plays a key role in PEMFCs as a proton conductor in the anode and cathode catalyst layers and in the electrolyte membrane. In this study, spectroscopic scanning coherent diffraction imaging (spectro-ptychography) and spectro-ptychographic tomography were used to quantitatively image PFSA ionomers in PEMFC cathodes in both two and three dimensions. We verify that soft X-ray ptychography gives significant spatial resolution improvement on soft matter polymeric materials. A two-dimensional spatial resolution of better than 15 nm was achieved. With better detectors and brighter and more coherent X-ray beams, radiation-sensitive PFSA ionomers will be visualized with acceptable levels of chemical and structural modification. This work is a step toward visualization of ionomers in PEMFC cathodes at high spatial resolution (presently sub-15 nm, but ultimately below 10 nm), which will be transformative with respect to optimization of PEMFCs for automotive use

High-Resolution Imaging of Polymer Electrolyte Membrane Fuel Cell Cathode Layers by Soft X‑ray Spectro-Ptychography

Polymer electrolyte membrane fuel cells (PEMFCs) are a promising and sustainable alternative to internal combustion engines for automotive applications. Polymeric perfluorosulfonic acid (PFSA) plays a key role in PEMFCs as a proton conductor in the anode and cathode catalyst layers and in the electrolyte membrane. In this study, spectroscopic scanning coherent diffraction imaging (spectro-ptychography) and spectro-ptychographic tomography were used to quantitatively image PFSA ionomers in PEMFC cathodes in both two and three dimensions. We verify that soft X-ray ptychography gives significant spatial resolution improvement on soft matter polymeric materials. A two-dimensional spatial resolution of better than 15 nm was achieved. With better detectors and brighter and more coherent X-ray beams, radiation-sensitive PFSA ionomers will be visualized with acceptable levels of chemical and structural modification. This work is a step toward visualization of ionomers in PEMFC cathodes at high spatial resolution (presently sub-15 nm, but ultimately below 10 nm), which will be transformative with respect to optimization of PEMFCs for automotive use

Synthesis and Characterization of Eight Compounds of the MU₈Q₁₇ Family: ScU₈S₁₇, CoU₈S₁₇, NiU₈S₁₇, TiU₈Se₁₇, VU₈Se₁₇, CrU₈Se₁₇, CoU₈Se₁₇, and NiU₈Se₁₇

The solid-state MU₈Q₁₇ compounds ScU₈S₁₇, CoU₈S₁₇, NiU₈S₁₇, TiU₈Se₁₇, VU₈Se₁₇, CrU₈Se₁₇, CoU₈Se₁₇, and NiU₈Se₁₇ were synthesized from the reactions of the elements at 1173 or 1123 K. These isostructural compounds crystallize in space group <i>C</i>_2<i>h</i>³ - <i>C</i>2<i>/m</i> of the monoclinic system in the CrU₈S₁₇ structure type. X-ray absorption near-edge structure spectroscopic studies of ScU₈S₁₇ indicate that it contains Sc³⁺, and hence charge balance is achieved with a composition that includes U³⁺ as well as U⁴⁺. The other compounds charge balance with M²⁺ and U⁴⁺. Magnetic susceptibility measurements on ScU₈S₁₇ indicate antiferromagnetic couplings and a highly reduced effective magnetic moment. Ab Initio calculations find the compound to be metallic. Surprisingly, the Sc–S distances are actually longer than all the other M–S interactions, even though the ionic radii of Sc³⁺, low-spin Cr²⁺, and Ni²⁺ are similar

Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, ⁵¹V and ¹³C Nuclear Magnetic Resonance Spectroscopy, and V L_3,2-Edge X‑ray Absorption Near-Edge Structure Spectroscopy

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl₂(N<i>t</i>Bu)­[NTMS­(N^<i>t</i>Bu)], <b>2</b>, with PMe₃, PEt₃, PMe₂Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>3</b>, VCl­(PEt₃)₂(N^<i>t</i>Bu)₂ <b>4</b>, VCl­(PMe₂Ph)₂(N^<i>t</i>Bu)₂ <b>5</b>, and VCl­(Py)₂(N^<i>t</i>Bu)₂ <b>6</b>. The halide series (Cl–I) was synthesized by use of TMSBr and TMSI to give VBr­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>7</b> and VI­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>8</b>. The phenyl derivative was obtained by reaction of <b>3</b> with MgPh₂ to give VPh­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>9</b>. These neutral complexes are compared to the previously reported cationic bisimides [V­(PMe₃)₃(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>10</b>, [V­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>11</b>, and [V­(DMAP)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray diffraction, ¹³C NMR, ⁵¹V NMR, and V L_3,2-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of ⁵¹V NMR shift, ¹³C NMR Δδ_αβ, and L₃-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV­(N^<i>t</i>Bu)­X₂ (Cp = C₅H₅^1–)