The electronic structure and the chemical state in Am binary oxides and Am-doped UOFormula: see text were studied by means of X-ray absorption spectroscopy at shallow Am core (4d and 5d) edges. In ...particular, the Am 5f states were probed and the nature of their bonding to the oxygen states was analyzed. The interpretation of the experimental data was supported by the Anderson impurity model (AIM) calculations which took into account the full multiplet structure due to the interaction between 5f electrons as well as the interaction with the core hole. The sensitivity of the branching ratio of the Am Formula: see text and Formula: see text X-ray absorption lines to the chemical state of Am was shown using Am binary oxides as reference systems. The observed ratio for Am-doped UOFormula: see text suggests that at least at low Am concentrations, americium is in the Am(III) state in the UOFormula: see text lattice. To confirm the validity of the applied AIM approach, the analysis of the Am 4f X-ray photoelectron spectra of AmOFormula: see text and AmFormula: see textOFormula: see text was also performed which revealed a good agreement between experiment and calculations. As a whole, AmOFormula: see text can be classified as the charge-transfer compound with the 5f occupancy (Formula: see text) equal to 5.73 electrons, while AmFormula: see textOFormula: see text is rather a Mott-Hubbard system with Formula: see text = 6.05.
Two isostructural series of trigonal prismatic complexes, M(BpMe)3 and M(BcMe)3 (M = Y, Tb, Dy, Ho, Er, U; BpMe− = dihydrobis(methypyrazolyl)borate; BcMe− = 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 N-heterocyclic carbene complexes, M(BcMe)3, than for the isomeric pyrazolate complexes, M(BpMe)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(BcMe)3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(BcMe)3 exhibits U 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 N-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.
Abstract
Predictable stereoselective formation of supramolecular assembly is generally believed to be an important but complicated process. Here, we show that point chirality of a ligand decisively ...influences its supramolecular assembly behavior. We designed three closely related chiral ligands with different point chiralities, and observe their self-assembly into europium (Eu) tetrametallic tetrahedral cages. One ligand exhibits a highly diastereoselective assembly into homochiral (either ΔΔΔΔ or ΛΛΛΛ) Eu tetrahedral cages whereas the two other ligands, with two different approaches of loosened point chirality, lead to a significant breakdown of the diastereoselectivity to generate a mixture of (ΔΔΔΔ and ΛΛΛΛ) isomers. The cages are highly emissive (luminescence quantum yields of 16(1) to 18(1)%) and exhibit impressive circularly polarized luminescence properties (|
g
lum
|: up to 0.16). With in-depth studies, we present an example that correlates the nonlinear enhancement of the chiroptical response to the nonlinearity dependence on point chirality.
Ultrafine 5 nm ceria isotropic nanoparticles were prepared using the rapid chemical precipitation approach from cerium(III) nitrate and ammonium hydroxide aqueous solutions. The as-prepared ...nanoparticles were shown to contain predominantly Ce(IV) species. The solubility of nanocrystalline CeO2 at several pH values was determined using ICP-MS and radioactive tracer methods. Phase composition of the ceria samples remained unchanged upon partial dissolution, while the shape of the particles changed dramatically, yielding nanorods under neutral pH conditions. According to X-ray absorption spectroscopy investigation of the supernatant, Ce(III) was the main cerium species in solution at pH < 4. Based on the results obtained, a reductive dissolution model was used for data interpretation. According to this model, the solubility product for ceria nanoparticles was determined to be log K sp = −59.3 ± 0.3 in 0.01 M NaClO4. Taken together, our results show that the pH dependence of ceria anti- and pro-oxidant activity can be related to the dissolution of CeO2 in aqueous media.
Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations
. Biological systems use ...accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N
, through backbonding interactions
, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates
. Here, we report a metal-organic framework featuring exposed vanadium(II) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N
capacities and selectivities for the removal of N
from mixtures with CH
, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents.
Lanthanide borosilicate glasses containing Pu and the nonradioactive analog element Ce were subjects of an x‐ray absorption spectroscopy investigation to quantify the +3/+4 ratio of these ...redox‐sensitive elements. The data show that the dominant oxidation states are +4 for Pu and +3 for Ce. The data also indicate that the reductive potential of glasses can be quantified from a solution chemistry method adapted to glass chemistry, although allowances must be made for glass composition. These data can therefore be used to formulate glass compositions and processing schedules that lead to a controlled oxidation state of Pu in melts. Furthermore, the data show that Ce is a poor analog for Pu behavior in melts and that the suitability of surrogates can be assessed by the evaluation approach presented here. The method demonstrated in this paper can be used to estimate the oxidation states of a range of multi‐valent elements as functions of temperature and composition with data from only a single redox couple.
Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric ...field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO₂ as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO₂ is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be twice that of the Th 5f states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.
Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute ...a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An–Cl covalency in AnCl6 2– (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g */eg *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amount of M–Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
Synthetic routes to salts containing uranium bis-imido tetrahalide anions U(NR)2X42– (X = Cl–, Br–) and non-coordinating NEt4 + and PPh4 + countercations are reported. In general, these compounds can ...be prepared from U(NR)2I2(THF) x (x = 2 and R = t Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl–, the U(NMe)22+ cation also reacts with Br– to form stable NEt42U(NMe)2Br4 complexes. These materials were used as a platform to compare electronic structure and bonding in U(NR)22+ with UO22+. Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U–Cl bonding interactions in PPh42U(N t Bu)2Cl4 and PPh42UO2Cl4. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7–10% per U–Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.
The complexation of americium(
iii
) with nitrate was studied at temperatures from 10 to 85 °C in 1 M HNO
3
-HClO
4
by spectrophotometry. The 1 : 1 complex species, AmNO
3
2+
, was identified and the ...stability constants were calculated from the absorption spectra recorded for titrations at several temperatures. Specific ion interaction theory (SIT) was used for ionic strength corrections to obtain the stability constants of AmNO
3
2+
at infinite dilution and variable temperatures. The absorption spectra of Am(
iii
) in diluted HClO
4
were also reviewed, and the molar absorptivity of Am(
iii
) at around 503 nm and 813 nm was re-calibrated by titrations with standardized DTPA solutions to determine the concentration of Am(
iii
).
The complexation of americium(
iii
) with nitrate was studied at temperatures from 10 to 85 °C in 1 M HNO
3
-HClO
4
by spectrophotometry.
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