Electron Beam Irradiation as a General Approach for the Rapid Synthesis of Covalent Organic Frameworks under Ambient Conditions.

Crystalline porous materials such as covalent organic frameworks (COFs) are advanced materials to tackle challenges of catalysis and separation in industrial processes. Their synthetic routes often require elevated temperatures, closed systems with high pressure, and long reaction times, hampering their industrial applications. Here we use a traditionally unperceived strategy to assemble highly crystalline COFs by electron beam irradiation with controlled received dosage, contrasting sharply with the previous observation that radiation damages the crystallinity of solids. Such synthesis by electron beam irradiation can be achieved under ambient conditions within minutes, and the process is amendable for large-scale production. The intense and targeted energy input to the reactants leads to new reaction pathways that favor COF formation in nearly quantitative yield. This strategy is applicable not only to known COFs but also to new series of flexible COFs that are difficult to obtain using traditional methods.

Evidence of Alpha Radiolysis in the Formation of a Californium Nitrate Complex.

Well-characterized complexes of transplutonium elements are scarce because of the experimental challenges of working with these elements and the rarity of the isotopes. This leads to a lack of structural and spectroscopic data needed to understand the nature of chemical bonds in these compounds. In this work, the synthesis of Cf(DOPOq )2 (NO3 )(py) (DOPOq =2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate; py=pyridine) is reported, in which the nitrate anion is hypothesized to form through the α-radiolysis-induced reaction of pyridine and/or the ligand. Computational analysis of the electronic structure of the complex reveals that the CfIII -ligand interactions are largely ionic.

Examination of Molten Salt Reactor Relevant Elements Using Hydrothermal Synthesis.

The structural chemistry of elements relevant to the FLiBe molten salt reactor, Th, U, Np, and Zr, including Ce and Nd (as analogues for Pu and Am, respectively), have been examined using hydrothermal synthesis at 200 °C. These reactions serve to model the reaction of molten salts under hydrolysis conditions. The results show that U and Np formed LiAnF5, while Ce formed Li4CeF8. The source of U also controlled the crystal quality, where UO2 gave small crystals, while UO3·2H2O gave very large crystals. It is likely that Be incorporation was not observed because of the high solubility of [BeF4]2- in water. Zr formed a third product, Li6BeF4ZrF8, which features isolated [BeF4]2- and [ZrF8]4- units bridged by Li+. Additionally, Li2BeF4 was regularly isolated. When little to no alkali metal was included in the reaction, M3F12(H2O) was isolated for Np, U, and Ce.

Synthesis, Spectroscopy, and Theoretical Details of Uranyl Schiff-Base Coordination Complexes.

Two uranyl Schiff-base coordination complexes, UO2L(MeOH) and UO2Cl2(H2L) {L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes have been characterized by structural, spectroscopic, and theoretical analysis to offer an electronic structure basis to explain the bonding parameters and stability. Single-crystal X-ray analysis reveals that UO2L(MeOH) adopts the typical "soft taco confirmation" characteristic of uranyl salophen complexes, whereas UO2Cl2(H2L) features an unusual neutral ligand coordination that contains an internal hydrogen bond between the phenol and imine. Rate constants calculated from electrochemical experiments confirm a quasi-reversible UO22+/UO2+ couple. Single-configurational and multiconfigurational methods were used to explore the bonding in UO2L(MeOH) and UO2Cl2(H2L). For UO2Cl2(H2L), the U-Cl bond exhibits more covalent contributions than U-OL.

Structural Complexity and Magnetic Orderings in a Large Family of 3d-4f Heterobimetallic Sulfates.

The synthesis of a large family of heterobimetallic lanthanide copper sulfates was realized via stoichiometric hydrothermal reactions among Ln2O3, CuO, and H2SO4, giving rise to four distinct phases, namely Ln2Cu(SO4)2(OH)4 (Ln = Sm-Ho) (LnCuSO4-1), Ln4Cu(SO4)2(OH)10 (Ln = Tm-Lu) (LnCuSO4-2), LnCu(SO4)(OH)3 (Ln = Nd-Gd, except Pm) (LnCuSO4-3), and LnCu(SO4)(OH)3 (Ln = Dy-Lu) (LnCuSO4-4), with completely different topologies. The passage from LnCuSO4-1 and LnCuSO4-3 to LnCuSO4-2 and LnCuSO4-4 across the 4f series, respectively, can be ascribed to the effect of lanthanide contraction, which progressively induces shrinking of the Ln-O distance, reduction in the Ln coordination number, and eventually structural transitions. The incorporation of identical 3d-4f metal ions into different spin-lattices, in conjunction with substitution of diverse Ln3+ cations within the same spin-lattice, gives rise to tunable magnetic properties varying from ferromagnetic ordering in GdCuSO4-3 and HoCuSO4-4 to antiferromagnetic ordering in YbCuSO4-4, and to paramagnetic correlations found in GdCuSO4-1 and YbCuSO4-2.

Employing Lewis Acidity to Generate Bimetallic Lanthanide Complexes.

With the advent of lanthanide-based technologies, there is a clear need to advance the fundamental understanding of 4f-element chelation chemistry. Herein, we contribute to a growing body of lanthanide chelation chemistry and report the synthesis of bimetallic 4f-element complexes within an imine/hemiacetalate framework, Ln2TPTOMe [Ln = lanthanide; TPTOMe = tris(pyridineimine)(Tren)tris(methoxyhemiacetalate); Tren = tris(2-aminoethylamine)]. These products are generated from hydrolysis and methanolysis of the cage ligand tris(pyridinediimine)bis(Tren) (TPT; Tadanobu et al. Chem. Lett. 1993, 22 (5), 859-862) likely facilitated by inductive effects stemming from the Lewis acidic lanthanide cations. These complexes are interesting because they result from imine cleavage to generate two metal binding sites: one pocketed site within the macrocycle and the other terminal site capping a hemiacetalate moiety. A clear demarcation in reactivity is observed between samarium and europium, where the lighter and larger lanthanides generate a mixture of products, Ln2TPTOMe and LnTPT. Meanwhile, the heavier and smaller lanthanides generate exclusively bimetallic Ln2TPTOMe. The cleavage reactivity to form Ln2TPTOMe was extended beyond methanol to include other primary alcohols.

Trivalent f-Element Squarates, Squarate-Oxalates, and Cationic Materials, and the Determination of the Nine-Coordinate Ionic Radius of Cf(III).

The synthesis, structure, and solid-state UV-vis-NIR spectroscopy of four new f-element squarates, M2(C4O4)3(H2O)4 (M = Eu, Am, Cf) and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O, four new cationic lanthanide squarate chlorides, [M4(C4O4)5(H2O)12]Cl2·5H2O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M2(C4O4)2(C2O4)(H2O)4 (M = Am, Cf), are presented. All of the metal centers are trivalent. Single-crystal X-ray diffraction analysis reveals that M2(C4O4)3(H2O)4 and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O have a two-dimensional sheet structure constructed from MO7(H2O)2 monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO6(H2O)2 square-antiprismatic (CN = 8) metal centers, respectively, whereas M2(C4O4)2(C2O4)(H2O)4 have a three-dimensional (3D) structure constructed from MO7(H2O)2 monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M4(C4O4)5(H2O)12]Cl2·5H2O have a 3D structure constructed from two crystallographically unique MO5(H2O)3 square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M2(C4O4)3(H2O)4 (M = La-Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf 3+ was determined to be 1.127 ± 0.003 Å. Finally, computational analysis of the americium and californium complexes M2(C4O4)3(H2O)4 and M2(C4O4)2(C2O4)(H2O)4 reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.

Structural and Spectroscopic Investigation of Two Plutonium Mellitates.

The aqueous reaction of mellitic acid (H6mell) with 242PuBr3·nH2O forms two plutonium mellitates, 242Pu2(mell)(H2O)9·H2O (Pu-1α) and 242Pu2(mell)(H2O)8·2H2O (Pu-1ß). These compounds are compared to the isomorphous lanthanide mellitates with similar ionic radii via bond length analysis. Both plutonium compounds form three-dimensional metal-organic frameworks, with Pu-1α having two unique metal centers and Pu-1ß having one. All plutonium metal centers exhibit nine-coordinate geometries. Our results show metal-oxygen bond lengths for plutonium significantly shorter than those of the previously reported lanthanum and herein reported cerium analogues, consistent with the nine-coordinate ionic radii. Clear Laporte-forbidden 5f → 5f transitions are observed in the ultraviolet-visible-near-infrared spectra and are assigned to trivalent plutonium. However, there is a distinct color difference between the two plutonium compounds.

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations.

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML3]3+, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML3]3+ in the presence of H2O yielded a protonated ligand [L(H)]+ and hydroxide [ML2(OH)]2+ or hydrate [ML(L-H)(H2O)]2+, where (L-H)- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am3+ versus Ln3+, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL3]3+ versus [LaL3]3+ and for both product isomers, [ML2(OH)]2+ and [ML(L-H)(H2O)]2+. More facile removal of a ligand from [AmL3]3+ by CID does not necessarily contradict stronger Am3+-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding.

Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides.

An isostructural family of f-element compounds (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra- tert-butyl-1-oxo-1 H-phenoxazin-9-olate) was prepared. This family, of the form M(DOPOq)3, represents the first nonaqueous isostructural series, including the later actinides berkelium and californium. The lanthanide derivatives were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by X-ray crystallography and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calculations were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approximation (ZORA) method, the same CASSCF analysis using experimental structures versus UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. This shows that UDFT-ZORA combined with CASSCF could be a useful first approximation to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.

Contemporary Chemistry of Berkelium and Californium.

The merging of small-scale syntheses and rapid crystallization methods have provided access to crystalline samples of berkelium (Z=97) and californium (Z=98) coordination complexes and compounds that can be interrogated with a suite of spectroscopic tools and structural elucidation approaches that have come online over the last 20 years. The combination of this experimental data with relativistic theoretical methods that capture the effects of spin-orbit coupling and scalar relativistic effects have allowed us to understand the electronic structure of berkelium and californium compounds at a level of detail that was not previously possible. The harbinger of this new era of post-curium chemistry was the synthesis and characterization of [Cf{B6 O8 (OH)5 }]. This compound possesses a structure type that is distinct from earlier actinide borates, a reduction in coordination number for californium, contracted Cf-O bond lengths, a substantially reduced magnetic moment with respect to the calculated free-ion moment and, most importantly, vibronically coupled broadband photoluminescence. Ligand-field analysis also showed that the splitting of the ground state was larger than typically found in the f-block elements, and when taken together places its overall electronic structure as a hybrid of d- and f-block components. The discovery of the unusual properties of this compound has led to the development of large families of 4f and 5f coordination complexes, in an effort to uncover the underlying origin of the electronic structure oddities, and whether there really is a sharp onset of these changes at californium. This in turn pushed the development of far more challenging berkelium chemistry (from a radiologic standpoint) because the half-life of the isotopes decreases from 351 years for 249 Cf to 330 days for 249 Bk. This short review details some of the chemistry that has been reported over the last 15 years, and its consequences for understanding the periodic table.

Origin of Selectivity of a Triazinyl Ligand for Americium(III) over Neodymium(III).

[M(EtBTP)3 ][BPh4 ]3 ⋅3 CH3 CN (M=Nd, Am; EtBTP=2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine) have been synthesized from reactions of MCl3 ⋅n H2 O with EtBTP in acetonitrile followed by anion metathesis. Structural analysis reveals that these compounds contain M3+ cations bound by tridentate EtBTP ligands to create a tricapped trigonal prismatic geometry around the metal centers. Collection of high-resolution, single-crystal X-ray diffraction data also allowed reduction in bond lengths esd's, such that a slight contraction of Δ=0.0158(18) Šin the Am-N versus Nd-N bond lengths was observed, even though these cations ostensibly have matching ionic radii. Theoretical evaluation revealed enhanced metal-ligand bonding through back donation in the [Am(EtBTP)3 ]3+ complex that is absent in [Nd(EtBTP)3 ]3+ .

Tailored Perovskite Waste Forms for Plutonium Trapping.

Perovskite ceramics have been extensively studied as host matrixes for radionuclide entrapment for nuclear waste disposal. As an expansion of these investigations, cerium, neodymium, and plutonium were incorporated into a perovskite phase, ACu3FeTi3O12 (A = Nd, Ce, Pu), using sol-gel methods under oxidizing and reducing atmospheres. The targeted materials contained varying levels of Ce3+ and Nd3+ on the A site, yielding potential compositions of Nd1- xCe xCu3FeTi3O12 ( x = 0, 0.1, 0.2, 0.3, 0.4, 0.8). However, interrogation of these materials shows that the maximum Ce3+ loading is achieved near x ≈ 0.2. A single composition with plutonium was targeted, Nd0.9Pu0.1Cu3FeTi3O12, in order to properly model more realistic loading levels for a repository-destined material. These compounds were characterized using powder X-ray diffraction with Rietveld refinements of the structures and by a variety of spectroscopic techniques. The data suggest that, in order to achieve Pu3+ substitution onto the A sites in the Nd0.9Pu0.1Cu3FeTi3O12, a reducing atmosphere must be employed. Otherwise, the redox activity of plutonium results in substitution onto multiple sites in the material as well as the formation of secondary phases such as TiO2.

High-Pressure Studies of Cesium Uranyl Chloride.

The pressure behavior of crystalline Cs2UO2Cl4 has been explored using a diamond anvil cell. The uranyl fluorescence intensity decreases dramatically with increasing pressure. Using the O-U-O symmetric stretching frequency, an apparent linear decrease in bond length with increasing pressure was observed. A linear decrease in fluorescence intensity with increasing pressure was attributed to a large growth in the nonradiative relaxation, likely attributed to increased relaxation through phonon modes. Quantum theory of atoms in molecules calculations and ab initio wave function methods (CASSCF) support the U≡O bond in UO22+ being highly sensitive to the bond distance, but negligibly affected by the U-Cl bond length.

Structure, Spectroscopy, and Theoretical Analysis of Zero- and Three-Dimensional Lithium Plutonium Fluorides: Li4PuF8 and LiPuF5.

The reaction of 242PuO2 with HF and LiF under hydrothermal conditions results in the formation of Li4PuF8 and LiPuF5. These compounds were structurally characterized using single-crystal X-ray diffraction, and UV-vis-near-IR absorption spectroscopy was employed to confirm the oxidation state of the plutonium in the compounds as 4+. The structure of Li4PuF8 consists of [PuF8]4- anions that adopt a bicapped trigonal-prismatic geometry with approximate C2v symmetry. These molecules are bridged by Li+ cations. In contrast, LiPuF5 forms a dense three-dimensional network constructed from [PuF9]5- units that are bridged by F- anions. The Pu4+ cations are found within tricapped trigonal prisms. Extensive theoretical analysis of the electronic and bonding interactions is included with a comparison between the results derived from complete-active-space self-consistent-field at different levels of theory, quantum theory of atoms in molecules, interacting quantum atom, natural localized molecular orbital, and Wiberg bond order analyses. Covalent interactions in these compounds are examined, and intramolecular trends in covalent and electrostatic interactions are discussed.

Electrochemical Studies of Selected Lanthanide and Californium Cryptates.

Efforts to quantitatively reduce CfIII → CfII in solution as well as studies of its cyclic voltammetry have been hindered by its scarcity, significant challenges associated with manipulating an unusually intense γ emitter, small reaction scales, the need for nonaqueous solvents, and its radiolytic effects on ligands and solvents. In an effort to overcome these impediments, we report on the stabilization of CfII by encapsulation in 2.2.2-cryptand and comparisons with the readily reducible lanthanides, Sm3+, Eu3+, and Yb3+. Cyclic voltammetry measurements suggest that CfIII/II displays electrochemical behavior with characteristics of both SmIII/II and YbIII/II. The °E1/2 values of -1.525 and -1.660 V (vs Fc/Fc+ in tetrahydrofuran (THF)) for [Cf(2.2.2-crypt)]3+/2+ and [Sm(2.2.2-crypt)]3+/2+, respectively, are similar. However, the ΔE values upon complexation by 2.2.2-cryptand for CfIII/II more closely parallels YbIII/II with postencapsulation shifts of 705 and 715 mV, respectively, whereas the shift of SmIII/II (520 mV) mirrors that of EuIII/II (524 mV). This suggests more structural similarities between CfII and YbII in solution than with SmII that likely originates from more similar ionic radii and local coordination environments, a supposition that is corroborated by crystallographic and extended X-ray absorption fine structure measurements from other systems. Competitive-ion binding experiments between EuIII/II, SmIII/II, and YbIII/II were also performed and show less favorable binding by YbIII/II. Connectivity structures of [Ln(2.2.2-cryptand)(THF)][BPh4]2 (Ln = EuII, SmII) are reported to show the important role that THF plays in these redox reactions.

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex.

The reaction of SmI2 with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu4N][BPh4], allows for the isolation of [SmII(DB30C10)][BPh4]2 as bright-red crystals in good yield. Exposure of [Sm(DB30C10)]2+ to solvents containing trace water results in the conversion to the dinuclear SmIII complex, Sm2(DB30C10)(OH)2I4. Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with SmII to a twisted conformation with SmIII. The optical properties of [SmII(DB30C10)][BPh4]2 exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Examination of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) shows that the ground state of SmII in [SmII(DB30C10)]2+ is a 4f6 state with low-lying 4f55d1 states, where the latter states have been lowered in energy by ∼12 000 cm-1 with respect to the free ion. The decacoordination of the SmII cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of SmII.

Electronic, Magnetic, and Theoretical Characterization of (NH4)4UF8, a Simple Molecular Uranium(IV) Fluoride.

The simple system of tetraammonium octafluorouranate is employed to derive a fundamental understanding of the uranium-fluorine interaction. The structure is composed of isolated molecules, enabling a detailed examination of the U4+ ( f2) ion. Characterization of single-crystals by X-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is combined with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method (IQA) is used to examine the nature of the U-F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) in [UF8]4-. For the structural and theoretical reasons discussed herein, it is proposed that the structure of (NH4)4UF8 may be appropriately employed as a benchmark compound for future theoretical characterization of U(IV).

[Am(C5 Me4 H)3 ]: An Organometallic Americium Complex.

We report the small-scale synthesis, isolated yield, single-crystal X-ray structure, 1 H NMR solution spectroscopy /solid-state UV/Vis-nIR spectroscopy, and density functional theory (DFT)/ab initio wave function theory calculations on an Am3+ organometallic complex, [Am(C5 Me4 H)3 ] (1). This constitutes the first quantitative data on Am-C bonding in a molecular species.

Uncovering the Origin of Divergence in the CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family through Examination of the Chemical Bonding in a Molecular Cluster and by Band Structure Analysis.

A series of f-block chromates, CsM(CrO4)2 (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the AmIII derivatives and their lanthanide analogs. While all compounds form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the LnIII compounds. In order to probe the origin of these differences, the electronic structure of α-CsSm(CrO4)2, α-CsEu(CrO4)2, and α-CsAm(CrO4)2 were studied using both a molecular cluster approach featuring hybrid density functional theory and QTAIM analysis and by the periodic LDA+GA and LDA+DMFT methods. Notably, the covalent contributions to bonding by the f orbitals were found to be more than twice as large in the AmIII chromate than in the SmIII and EuIII compounds, and even larger in magnitude than the Am-5f spin-orbit splitting in this system. Our analysis indicates also that the Am-O covalency in α-CsAm(CrO4)2 is driven by the degeneracy of the 5f and 2p orbitals, and not by orbital overlap.