Plutonium Hybrid Materials: A Platform to Explore Assembly and Metal-Ligand Bonding.

We report the synthesis of five new hybrid materials containing the [PuCl6]2- anion and charge-balancing, noncovalent interaction donating 4-X-pyridinium (X = H, Cl, Br, I) cations. Single crystals of the title compounds were grown and harvested from acidic, chloride-rich, aqueous media, and their structures were determined via X-ray diffraction. Compounds 1-4, (4XPyH)2[PuCl6], and 5, (4IPyH)4[PuCl6]·2Cl, exhibit two distinct sheet-like structure types. Structurally relevant noncovalent interactions were tabulated from crystallographic data and verified computationally using electrostatic surface potential maps and the quantum theory of atoms in molecules (QTAIM). The strength of the hydrogen and halogen bonds was quantified using Kohn-Sham density functional theory, and a hierarchy of acceptor-donor pairings was established. The PuIV-Cl bonds were studied using QTAIM and natural localized molecular orbital (NLMO) analyses to delineate the underlying bond mechanism and hybrid atomic orbital contributions therein. The results of the PuIV-Cl bond analyses were compared across compositions via analogous treatments of previously reported [PuO2Cl4]2- and [PuCl3(H2O)5] molecular units. The Pu-Cl bonds are predominately ionic yet exhibit small varying degrees of covalent character that increases from [PuCl3(H2O)5] and [PuO2Cl4]2- to [PuCl6]2-, while the participation of the Pu-based s/d and f orbitals concurrently decreases and increases, respectively.

Measurement of local magnetic fields in actinide tetrafluorides.

Fluorine-19 magnetic shielding tensors have been measured in a series of actinide tetrafluorides (AnF4) by solid state nuclear magnetic resonance spectroscopy. Tetravalent actinide centers with 0-8 valence electrons can form tetrafluorides with the same monoclinic structure type, making these compounds an attractive choice for a systematic study of the variation in the electronic structure across the 5f row of the Periodic Table. Pronounced deviations from predictions based on localized valence electron models have been detected by these experiments, which suggests that this approach may be used as a quantitative probe of electronic correlations.

Crystallographic and Spectroscopic Characterization of Americium Complexes Containing the Bis[(phosphino)methyl]pyridine-1-oxide (NOPOPO) Ligand Platform.

The crystal structures of americium species containing a common multifunctional phosphine oxide ligand, reported for its ability to extract f elements from acidic solutions, namely, 2,6-[Ph2P(O)CH2]2C5H3-NO, L, were finally determined after over three decades of separations studies involving these species and their surrogates. The molecular compounds Am(L)(NO3)3, Am 1:1, and [Am(L)2(NO3)][2(NO3)], Am 2:1, along with their neodymium and europium analogues, were synthesized and characterized using single-crystal X-ray crystallography, attenuated total reflectance Fourier transform infrared spectroscopy, and luminescence spectroscopy to provide a comprehensive comparison with new and known analogous complexes.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants.

Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.

Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex.

We report the serendipitous discovery and magnetic characterization of a dysprosium bis(ammonia) metallocene complex, [(C5Me5)2Dy(NH3)2](BPh4) (1), isolated in the course of performing a well-established synthesis of the unsolvated cationic complex [(C5Me5)2Dy][(µ-Ph)2BPh2]. While side reactivity studies suggest that this bis(ammonia) species owes its initial incidence to impurities in the DyCl3(H2O)x starting material, we were able to independently prepare 1 and its tetrahydrofuran (THF) derivative, [(C5Me5)2Dy(NH3)(THF)](BPh4) (2), from the reaction of [(C5Me5)2Dy][(µ-Ph)2BPh2] with ammonia in THF. The low-symmetry complex 1 exhibits slow magnetic relaxation under zero applied direct-current (dc) field to temperatures as high as 46 K and notably exhibits an effective barrier to magnetic relaxation that is more than 150% greater than that previously reported for the [(C5Me5)2Ln][(µ-Ph)2BPh2] precursor. On the basis of fitting of the temperature-dependent relaxation data, magnetic relaxation is found to occur via Orbach, Raman, and quantum-tunneling relaxation processes, and the latter process can be suppressed by the application of a 1400 Oe dc field. Field-cooled and zero-field-cooled dc magnetic susceptibility measurements reveal a divergence at 4 K indicative of magnetic blocking, and magnetic hysteresis was observed up to 5.2 K. These results illustrate the surprises and advantages that the lanthanides continue to offer for synthetic chemists and magnetochemists alike.

Structure and Bonding Investigation of Plutonium Peroxocarbonate Complexes Using Cerium Surrogates and Electronic Structure Modeling.

Herein, we report the synthesis and structural characterization of K8[(CO3)3Pu]2(µ-η2-η2-O2)2·12H2O. This is the second Pu-containing addition to the previously studied alkali-metal peroxocarbonate series M8[(CO3)3A]2(µ-η2-η2-O2)2·xH2O (M = alkali metal; A = Ce or Pu; x = 8, 10, 12, or 18), for which only the M = Na analogue has been previously reported when A = Pu. The previously reported crystal structure for Na8[(CO3)3Pu]2(µ-η2-η2-O2)2·12H2O is not isomorphous with its known Ce analogue. However, a new synthetic route to these M8[(CO3)3A]2(µ-η2-η2-O2)2·12H2O complexes, described below, has produced crystals of Na8[(CO3)3Ce]2(µ-η2-η2-O2)2·12H2O that are isomorphous with the previously reported Pu analogue. Via this synthetic method, the M = Na, K, Rb, and Cs salts of M8[(CO3)3Ce]2(µ-η2-η2-O2)2·xH2O have also been synthesized for a systematic structural comparison with each other and the available Pu analogues using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory calculations. The Ce salts, in particular, demonstrate subtle differences in the peroxide bond lengths, which correlate with Raman shifts for the peroxide Op-Op stretch (Op = O atoms of the peroxide bridges) with each of the cations studied: Na+ [1.492(3) Å/847 cm-1], Rb+ [1.471(1) Å/854 cm-1], Cs+ [1.474(1) Å/859 cm-1], and K+ [1.468(6) Å/870 cm-1]. The trends observed in the Op-Op bond distances appear to relate to supermolecular interactions between the neighboring cations.

Record High Single-Ion Magnetic Moments Through 4f(n)5d(1) Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]⁻.

The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 µB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets.

Cocrystallization of (µ-S2)2- and (µ-S)2- and formation of an [η2-S3N(SiMe3)2] ligand from chalcogen reduction by (N2)2- in a bimetallic yttrium amide complex.

The reactivity of the (N(2))(2-) complex {[(Me(3)Si)(2)N](2)Y(THF)}(2)(µ-η(2):η(2)-N(2)) (1) with sulfur and selenium has been studied to explore the special reductive chemistry of this complex and to expand the variety of bimetallic rare-earth amide complexes. Complex 1 reacts with elemental sulfur to form a mixture of compounds, 2, that is the first example of cocrystallized complexes of (S(2))(2-) and S(2-) ligands. The crystals of 2 contain both the (µ-S(2))(2-) complex {[(Me(3)Si)(2)N](2)Y(THF)}(2)(µ-η(2):η(2)-S(2)) (3) and the (µ-S)(2-) complex {[(Me(3)Si)(2)N]2Y(THF)}(2)(µ-S) (4), respectively. Modeling of the crystal data of 2 shows a 9:1 ratio of 3:4 in the crystals of 2 obtained from solutions that have 1:1 to 4:1 ratios of 3/4 by (1)H NMR spectroscopy. The addition of KC(8) to samples of 2 allows for the isolation of single crystals of 4. The [S(3)N(SiMe(3))(2)](-) ligand was isolated for the first time in crystals of [(Me(3)Si)(2)N](2)Y[η(2)-S(3)N(SiMe(3))(2)](THF) (5), obtained from the mother liquor of 2. In contrast to the sulfur chemistry, the (µ-Se(2)(2-) analogue of 3, namely, {[(Me(3)Si)(2)N](2)Y(THF)}(2)(µ-η(2):η(2)-Se(2)) (6), can be cleanly synthesized in good yield by reacting 1, with elemental selenium. The (µ-Se)(2-) analogue of 4, namely, {[(Me(3)Si)(2)N]2Y(THF)}(2)(µ-Se) (7), was synthesized from Ph(3)PSe.

Influence of an inner-sphere K+ ion on the magnetic behavior of N2(3-) radical-bridged dilanthanide complexes isolated using an external magnetic field.

The synthesis and full magnetic characterization of a new series of N2(3-) radical-bridged lanthanide complexes [{(R2N)2(THF)Ln}2(µ3-η(2):η(2):η(2)-N2)K] [1-Ln; Ln = Gd, Tb, Dy; NR2 = N(SiMe3)2] are described for comprehensive comparison with the previously reported series [K(18-crown-6)(THF)2]{[(R2N)2(THF)Ln]2(µ-η(2):η(2)-N2)} (2-Ln; Ln = Gd, Tb, Dy). Structural characterization of 1-Ln crystals grown with the aid of a Nd2Fe13B magnet reveals inner-sphere coordination of the K(+) counterion within 2.9 Å of the N2(3-) bridge, leading to bending of the planar Ln-(N2(3-))-Ln unit present in 2-Ln. Direct current (dc) magnetic susceptibility measurements performed on 1-Gd reveal antiferromagnetic coupling between the Gd(III) centers and the N2(3-) radical bridge, with a strength matching that obtained previously for 2-Gd at J ∼ -27 cm(-1). Unexpectedly, however, a competing antiferromagnetic Gd(III)-Gd(III) exchange interaction with J ∼ -2 cm(-1) also becomes prominent, dramatically changing the magnetic behavior at low temperatures. Alternating current (ac) magnetic susceptibility characterization of 1-Tb and 1-Dy demonstrates these complexes to be single-molecule magnets under zero applied dc field, albeit with relaxation barriers (Ueff = 41.13(4) and 14.95(8) cm(-1), respectively) and blocking temperatures significantly reduced compared to 2-Tb and 2-Dy. These differences are also likely to be a result of the competing antiferromagnetic Ln(III)-Ln(III) exchange interactions of the type quantified in 1-Gd.

Dimensionality selection in a molecule-based magnet.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S=1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbor coupling strength.

Varying the Lewis base coordination of the Y2N2 core in the reduced dinitrogen complexes {[(Me3Si)2N]2(L)Y}2(µ-η2:η2-N2) (L = benzonitrile, pyridines, triphenylphosphine oxide, and trimethylamine N-oxide).

The effect of the neutral donor ligand, L, on the Ln(2)N(2) core in the (N═N)(2-) complexes, [A(2)(L)Ln](2)(µ-η(2):η(2)-N(2)) (Ln = Sc, Y, lanthanide; A = monoanion; L = neutral ligand), is unknown since all of the crystallographically characterized examples were obtained with L = tetrahydrofuran (THF). To explore variation in L, displacement reactions between {[(Me(3)Si)(2)N](2)(THF)Y}(2)(µ-η(2):η(2)-N(2)), 1, and benzonitrile, pyridine (py), 4-dimethylaminopyridine (DMAP), triphenylphosphine oxide, and trimethylamine N-oxide were investigated. THF is displaced by all of these ligands to form {[(Me(3)Si)(2)N](2)(L)Y}(2)(µ-η(2):η(2)-N(2)) complexes (L = PhCN, 2; py, 3; DMAP, 4; Ph(3)PO, 5; Me(3)NO, 6) that were fully characterized by analytical, spectroscopic, density functional theory, and X-ray crystallographic methods. The crystal structures of the Y(2)N(2) cores in 2-5 are similar to that in 1 with N-N bond distances between 1.255(3) Å and 1.274(3) Å, but X-ray analysis of the N-N distance in 6 shows it to be shorter: 1.198(3) Å.

Phase field model of uranium carbide solidification through a combined KKS and orientation field approach.

A Phase Field model is developed combining the Orientation Field approach to modeling solidification with the Kim, Kim, Suzuki method of modeling binary alloys. These combined methods produce a model capable of simulating randomly oriented second phase dendrites with discrete control of the solid-liquid interface energy and thickness. The example system of carbon in a liquid uranium (U) melt is used as a test for the model. The formation of uranium carbide within a liquid U melt is simulated for isothermal conditions and compares well with experiments.

2,6-Diiminopiperidin-1-ol: an overlooked motif relevant to uranyl and transition metal binding on poly(amidoxime) adsorbents.

Glutardiamidoxime, a structural motif on sorbents used in uranium extraction from seawater, was discovered to cyclize in situ at room temperature to 2,6-diimino-piperidin-1-ol in the presence of uranyl nitrate. The new diimino motif was also generated when exposed to competing transition metals Cu(ii) and Ni(ii). Multinuclear µ-O bridged U(vi), Cu(ii), and Ni(ii) complexes featuring bound diimino ligands were isolated.

Raman spectroscopy of the N-N bond in rare earth dinitrogen complexes.

Raman spectra have been collected on single crystals of over 20 different rare earth complexes containing reduced dinitrogen ligands to determine if these data will correlate with periodic properties or relative stability. Four types of complexes were examined: [(C5Me5)2Ln]2(µ-η(2):η(2)-N2), 1-Ln, [(C5Me4H)2(THF)Ln]2(µ-η(2):η(2)-N2), 2-Ln, [(C5H4Me)2Ln]2(µ-η(2):η(2)-N2), 3-Ln, and {[(Me3Si)2N]2(THF)Ln}2(µ-η(2):η(2)-N2), 4-Ln. Although each of the complexes contains a side-on bound dinitrogen ligand that is formally (N2)(2-), the N-N bond distances determined by X-ray crystallography range from 1.088(12) to 1.305(6) Å. In the 4-Ln series (Ln = Gd, Tb, Dy, Ho, Er and Tm), the 1.26-1.31 Å N-N distances do not follow any periodic trends, but the Raman stretching frequencies for Gd-Tm were found to decrease regularly with decreasing atomic number and increasing Lewis acidity of the metal. Similar correlations can be seen with the late metals in complexes of the other series, 1-Ln, 2-Ln and 3-Ln, but exceptions exist, particularly for the larger metals. Comparisons between the several types of complexes as a function of ligand were more complicated and variations in stretching frequency as a function of L in the {[(Me3Si)2N]2Y(L)}2(µ-η(2):η(2)-N2) substituted versions of 4-Y did not give trends consistent with bond distances or Gutmann donor numbers.