Evgeny V. Alekseev

NDTB‐1: A Supertetrahedral Cationic Framework That Removes TcO4− from Solution

A cubic thorium borate possesses a porous supertetrahedral cationic framework with extraframework borate anions. These anions are readily exchanged with a variety of environmental contaminants, especially those from the nuclear industry, including chromate and pertechnetate.

Cubic and rhombohedral heterobimetallic networks constructed from uranium, transition metals, and phosphonoacetate: new methods for constructing porous materials

Four heterobimetallic U(vi)/M(ii) (M = Mn, Co, Cd) carboxyphosphonates have been synthesized. M(2)[(UO(2))(6)(PO(3)CH(2)CO(2))(3)O(3)(OH)(H(2)O)(2)]·16H(2)O (M = Mn(ii), Co(ii), and Cd(ii)) adopt cubic three-dimensional network structures with large cavities approximately 16 Å in diameter that are filled with co-crystallized water molecules. [Cd(3)(UO(2))(6)(PO(3)CH(2)CO(2))(6)(H(2)O)(13)]·6H(2)O forms a rhombohedral channel structure with hydrated Cd(ii) within the channels. The cubic compound (Co) displays differential gas absorption with a surface area for CO(2) uptake of 40 m(2) g(-1) at 273 K, and no uptake of N(2) at 77 K.

Differentiating between trivalent lanthanides and actinides

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.

Recent progress in actinide borate chemistry

The use of molten boric acid as a reactive flux for synthesizing actinide borates has been developed in the past two years providing access to a remarkable array of exotic materials with both unusual structures and unprecedented properties. [ThB(5)O(6)(OH)(6)][BO(OH)(2)]·2.5H(2)O possesses a cationic supertetrahedral structure and displays remarkable anion exchange properties with high selectivity for TcO(4)(-). Uranyl borates form noncentrosymmetric structures with extraordinarily rich topological relationships. Neptunium borates are often mixed-valent and yield rare examples of compounds with one metal in three different oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Finally, americium borates show a dramatic departure from plutonium borates, and there are scant examples of families of actinides compounds that extend past plutonium to examine the bonding of later actinides. There are several grand challenges that this work addresses. The foremost of these challenges is the development of structure-property relationships in transuranium materials. A deep understanding of the materials chemistry of actinides will likely lead to the development of advanced waste forms for radionuclides present in nuclear waste that prevent their transport in the environment. This work may have also uncovered the solubility-limiting phases of actinides in some repositories, and allows for measurements on the stability of these materials.

Unusual structure, bonding and properties in a californium borate

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

Neptunium Diverges Sharply from Uranium and Plutonium in Crystalline Borate Matrixes: Insights into the Complex Behavior of the Early Actinides Relevant to Nuclear Waste Storage

In contrast to uranium and plutonium borates, neptunium borates are mixed-valent and simultaneously display three coordination environments and three oxidation states.

How are centrosymmetric and noncentrosymmetric structures achieved in uranyl borates

Four uranyl borates, UO(2)B(2)O(4) (UBO-1), alpha-(UO(2))(2)[B(9)O(14)(OH)(4)] (UBO-2), beta-(UO(2))(2)[B(9)O(14)(OH)(4)] (UBO-3), and (UO(2))(2)[B(13)O(20)(OH)(3)].1.25H(2)O (UBO-4), have been prepared from boric acid fluxes at 190 degrees C. UBO-3 and UBO-4 are centrosymmetric, whereas UBO-1 and UBO-2 are noncentrosymmetric (chiral and polar). These uranyl borates possess layered structures constructed from UO(8) hexagonal bipyramids, BO(3) triangles, and BO(4) tetrahedra. In the case of UBO-4, clusters of BO(3) triangles link the layers together to form open slabs with a thickness of almost 2 nm. The ability of uranyl borates to use very similar layers to yield both centrosymmetric and noncentrosymmetric layers is detailed in this work.

Bonding Changes in Plutonium(III) and Americium(III) Borates

Interest in trivalent actinide (e.g. Pu, Am, and Cm) borates stems from their potential formation in the geological repository for nuclear defense waste known as the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico, USA. A similar repository is being considered in Germany. In this salt deposit the concentration of borate species in intergranular brines can be as high as 166 ppm. Studies of the complexation of Nd by borates in solution have been performed indicating that borate is the primary complexant in WIPP for trivalent cations. Furthermore, WIPP is selfsealing, and once closed will be saturated with hydrogen and methane, making it a highly reducing environment that will favor lower oxidation states for plutonium. There are few systems that have been studied that extend between plutonium and americium for which detailed structural information is available. The differences in bonding between plutonium and americium in the same oxidation state (e.g. Pu and Am) is expected to be so small that the ligand set that binds these two cations has to be exquisitely sensitive to the differences in bonding between plutonium and americium to detect any divergence. In fact, the few systems where corresponding compounds are known such has the halides and triflates do not respond to the changes in bonding between plutonium and americium by exhibiting substantially different structures; although the actinide contraction is certainly found between these two elements. The formation of different polyborate networks is profoundly affected by numerous factors including minute changes in the pH value, reaction temperature, stoichiometry, cation size, and counterions. We have recently explored the syntheses, structures, spectroscopy, and stability of actinide borates from thorium to plutonium. These investigations have uncovered a pertechnetate-selective anion exchange material, 7] spectacularly complex acentric topologies in uranyl borates, 15,17] mixed-valency in neptunium borates, and new coordination environments in plutonium borates. 15,16] It occurred to us that as these studies progressed that Pu and Am polyborates might be an ideal system for observing changes in bonding that occur across the actinide series when all other variables are held constant except for the identity of the actinides because the polyborate network should be hyper-responsive to subtle changes in the metal centers. Herein we report the syntheses, structures, coordination chemistry, and spectroscopy of new Pu and Am borates that achieve the goal of observing substantially different structures between these two elements. The reactions of PuCl3 and AmCl3 with molten boric acid at 240 8C under strictly anaerobic conditions leads to the formation of Pu[B4O6(OH)2Cl] and Pu2[B13O19(OH)5Cl2(H2O)3], and Am[B9O13(OH)4]·H2O, respectively. Pu[B4O6(OH)2Cl] forms blue crystals that can exceed 1 mm in size. Pu2[B13O19(OH)5Cl2(H2O)3] forms smaller more lightly colored blue crystals. Am[B9O13(OH)4]·H2O is isolated as large pink tablets. Pictures of these crystals can be found in the Supporting Information. The crystals are not degraded by oxygen or water. Curiously the crystals of Pu[B4O6(OH)2Cl] are extremely hard and cannot be cut with steel tools. This might be a function of the multiple ways in which the structure is cross-linked. These crystals were used directly for structural and spectroscopic investigations. Single-crystal X-ray diffraction experiments on all three compounds yielded models for the structures with low residuals. There are very few single-crystal structures known for americium compounds. 4b, 19] Crystals of Am[B9O13(OH)4]·H2O are of remarkable quality, and the residuals for the model of the structure are the lowest reported for an americium compound, providing very precise metrics for the structure. All three compounds form dense, three-dimensional structures shown in Figure 1. Pu2[B13O19(OH)5Cl2(H2O)3] and Am[B9O13(OH)4]·H2O both contain similar polyborate sheets as shown in Figure 2b,c. These sheets contain an unusual unit of three BO4 tetrahedra that share a common corner. These clusters share corners with BO3 triangles to create sheets with triangular holes where the An (An = Pu, Am) cations reside. For comparison, this sheet topology can be also found in the Ln borate systems, Ln[B8O11(OH)5] (Ln = La–Nd) and Ln[B9O13(OH)4] (Ln = Pr–Eu). Pu[B4O6(OH)2Cl] possesses a very different sheet topology that lacks the clusters, and only contains cornersharing BO3 and BO4 units (Figure 2 a). Once again there are triangular holes to house the Pu cations. These layers are [*] M. J. Polinski, S. Wang, Prof. Dr. T. E. Albrecht-Schmitt Department of Civil Engineering and Geological Sciences and Department of Chemistry and Biochemistry University of Notre Dame, Notre Dame, IN 46556 (USA) E-mail: talbrec1@nd.edu

Surprising coordination for plutonium in the first plutonium(III) borate.

The first plutonium(III) borate, Pu(2)[B(12)O(18)(OH)(4)Br(2)(H(2)O)(3)]·0.5H(2)O, has been prepared by reacting plutonium(III) with molten boric acid under strictly anaerobic conditions. This compound contains a three-dimensional polyborate network with triangular holes that house the plutonium(III) sites. The plutonium sites in this compound are 9- and 10-coordinate and display atypical geometries.

Crystal Chemistry of the Potassium and Rubidium Uranyl Borate Families Derived from Boric Acid Fluxes

The reaction of uranyl nitrate with a large excess of molten boric acid in the presence of potassium or rubidium nitrate results in the formation of three new potassium uranyl borates, K(2)[(UO(2))(2)B(12)O(19)(OH)(4)].0.3H(2)O (KUBO-1), K[(UO(2))(2)B(10)O(15)(OH)(5)] (KUBO-2), and K[(UO(2))(2)B(10)O(16)(OH)(3)].0.7H(2)O (KUBO-3) and two new rubidium uranyl borates Rb(2)[(UO(2))(2)B(13)O(20)(OH)(5)] (RbUBO-1) and Rb[(UO(2))(2)B(10)O(16)(OH)(3)].0.7H(2)O (RbUBO-2). The latter is isotypic with KUBO-3. These compounds share a common structural motif consisting of a linear uranyl, UO(2)(2+), cation surrounded by BO(3) triangles and BO(4) tetrahedra to create an UO(8) hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO(3) triangles extend from the polyborate layers and are directed approximately perpendicular to the sheets. All of these compounds adopt layered structures. With the exception of KUBO-1, the structures are all centrosymmetric. All of these compounds fluoresce when irradiated with long-wavelength UV light. The fluorescence spectrum yields well-defined vibronically coupled charge-transfer features.