Mononuclear Uranium Research Articles

Mononuclear Uranium and Heterobimetallic Actinide (An=Th, U) Alkoxides with Divalent Group 14 Elements (MII=Ge, Sn, Pb)

AbstractWhen compared to main group and lanthanide elements, the heterobimetallic alkoxides of actinide elements with metals other than alkali metals are elusive. We report here for the first time alkoxide derivatives containing early actinides and the divalent group 14 elements. For this purpose in this work, co‐alcoholysis reactions of silyl amides of divalent group 14 elements (MII=Ge, Sn, Pb) and actinides (AnIV=Th, U) were studied that resulted in the formation of [AnM(OtBu)6] (1‐AnM) (AnIV=Th, MII=Ge, Sn, Pb; AnIV=U, MII=Ge, Sn). A 2‐electron redox reaction occurred in the UIV‐PbII couple, which generated nearly quantitatively a UV alkoxide species and elemental lead. Precise adjustments of the stoichiometry in this reaction allowed to synthesize [U(OtBu)5Py] (3), amongst others in high yield. 3 and the UIV alkoxide [U(OtBu)4(Py)2] (4) described in this work represent first mononuclear UIV or UV containing alkoxides. First promising attempts to use [U(OtBu)5Py] (3) as a starting material in co‐alcoholysis reactions with divalent group 14 silylamides resulted in the formation of heterobimetallic UV‐MII alkoxides [UM(OtBu)7] (MII=Ge, Sn, Pb) amongst others. All compounds described in this study were thoroughly characterized by multi‐nuclear NMR and IR spectroscopic investigations, single crystal X‐ray diffraction data, and CHN‐ analysis.

Mixed-Ligand Uranyl Squarate Coordination Polymers: Structure Regulation and Redox Activity.

The electron-rich squarate ion (C4O42-, SA2-) possesses electronic delocalization over the entire molecule and good redox activity, and the functionalization of metal-organic complexes with the SA2- group is desirable. In this work, a mixed-ligand method is used to construct novel uranyl squarate coordination polymers utilizing 4,4'-bipyridine (bpy), 4,4'-bipyridine-N,N'-dioxide (bpydo), 1,10-phenanthroline (phen), 4,4'-vinylenedipyridine (vidpy), and in situ formed oxalate (OA2-) as ancillary ligands. Seven mixed-ligand uranyl compounds, [(UO2)(OH)(SA)](Hbpy) (1), [(UO2)(H2O)(SA)2](H2bpy) (2), (UO2)(H2O)(SA)(bpydo)·2H2O (3), (UO2)(H2O)(SA)(phen)·H2O (4), (UO2)(OH)(SA)0.5(phen)·H2O (5), [(UO2)(SA)(OA)0.5](Hphen) (6), and [(UO2)(SA)(OA)0.5](Hvidpy) (7), with varying crystal structures were synthesized under hydrothermal conditions. Compound 1, together with bpy molecules filling in the interlayer space as template agents, has a two-dimensional (2D) network structure, while 2 gives a one-dimensional (1D) chain based on mononuclear uranium units. Compound 3 shows a neutral 2D network through the combined linkage of SA2- and bpydo. Both 4 and 5 have a similar chain-like structure due to the capping effect of phen motifs, while phen molecules in 6 act as templating agents after protonation. Similar to 6, compound 7 has a "sandwich-like" structure in which the Hvidpy motifs locate in the voids of layers of 2D uranyl-squarate networks. The redox properties of typical mixed-ligand uranyl-squarate compounds, 1, 4, and 5 with high phase purity, are characterized using cyclic voltammetry. All three of these uranyl coordination compounds show anode peaks (Ea) at 0.777, 0.804, and 0.760 V, respectively, which correspond to the oxidation process of SA2- → SA. Meanwhile, cathodic peaks (Ec) at -0.328, -0.315, and -0.323 V corresponding to the reduction process of U(VI) → U(V) are also observed. The results reveal that all three of these uranyl coordination compounds show good redox activity and, most importantly, the interplay between two different redox-active motifs of SA2- organic linker and uranyl node. This work enriches the library of redox-active uranyl compounds and provides a feasible mixed-ligand method for regulating the synthesis of functional actinide compounds.

Syntheses and structures of [UO2(L)5](ClO4)2 and [U(L′)4(H2O)4](ClO4)4 (L is dimethylformamide, L′ is N,N-dimethylcarbamide)

The reaction of aqueous solutions of uranyl perchlorate with selected organic amides was studied in the dark and under the sunlight. The complexes [UVIO2(C3H7NO)5](ClO4)2 (I) and [UIV(C3H8N2O)4(Н2О)4](ClO4)4 (II), where C3H7NO is N,N-dimethylformamide (Dmfa) and C3H8N2O is N,N-dimethylcarbamide (a-Dmur), were studied by X-ray diffraction. Complex II and the complex UIV(s-Dmur)4(Н2О)4(ClO4)4 (III), where s-Dmur is N,N'-dimethylcarbamide, were studied by IR spectroscopy. Crystals I and II are composed of mononuclear [UO2(Dmfa)5]2+ and [U(Dmur)4(Н2О)4]4+ groups as uranium-containing structural units belonging to the crystal-chemical groups AМ 7 1 (А = UVI, М 1 = O2– and Dmfa) and AМ 8 1 (А = UIV, М 1 = Dmur and Н2О) of uranium complexes, respectively. The mononuclear uranium- containing complexes in the crystals of U(IV) and U(VI) perchlorates were found to obey the 14 neighbors rule.

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond.

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (Ad,MeArO)3tacn3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((Ad,MeArO)3tacn)U-SH] (2) is obtained from the reaction of H2S with the uranium(iii) starting material [((Ad,MeArO)3tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.

Molecular and electronic structure of dinuclear uranium bis-μ-oxo complexes with diamond core structural motifs.

In a multiple-bond metathesis reaction, the triazacyclononane (tacn)-anchored methyl- and neopentyl (nP)-substituted tris(aryloxide) U(III) complex [(((nP,Me)ArO)3tacn)U(III)] (1) reacts with mesityl azide and CO2 to form mesityl isocyanate and the dinuclear bis(μ-oxo)-bridged U(V)/U(V) complex [{(((nP,Me)ArO)3tacn)U(V)}2(μ-O)2] (3). This reaction proceeds via the mononuclear U(V) imido intermediate [(((nP,Me)ArO)3tacn)U(V)(NMes)] (2), which has been synthesized and fully characterized independently. The dimeric U(V) oxo species shows rich redox behavior: complex 3 can be reduced by one and two electrons, respectively, yielding the mixed-valent U(IV)/U(V) bis(μ-oxo) complex [K(crypt)][{(((nP,Me)ArO)3tacn)U(IV/V)}2(μ-O)2] (7) and the U(IV)/U(IV) bis(μ-oxo) complex K2[{(((nP,Me)ArO)3tacn)U(IV)}2(μ-O)2] (6). In addition, complex 3 can be oxidized to provide the mononuclear uranium(VI) oxo complexes [(((nP,Me)ArO)3tacn)U(VI)(O)eq(OTf)ax] (8) and [(((nP,Me)ArO)3tacn)U(VI)(O)eq]SbF6 (9). The unique series of bis(μ-oxo) complexes also shows notable magnetic behavior, which was investigated in detail by UV/vis/NIR and EPR spectroscopy as well as SQUID magnetization studies. In order to understand possible magnetic exchange phenomena, the mononuclear terminal oxo complexes [(((nP,Me)ArO)3tacn)U(V)(O)(O-pyridine)] (4) and [(((nP,Me)ArO)3tacn)U(V)(O)(O-NMe3)] (5) were synthesized and fully characterized. The magnetic study revealed an unusually strong antiferromagnetic exchange coupling between the two U(V) ions in 3. Examination of the (18)O-labeled bis(μ-oxo)-bridged dinuclear complexes 3, 6, and 7 allowed for the first time the unambiguous assignment of the vibrational signature of the [U(μ-O)2U] diamond core structural motif.

Multimetallic Cooperativity in Uranium-Mediated CO2 Activation

The metal-mediated redox transformation of CO2 in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris(tert-butoxy)siloxide ligands induces a large cooperative effect in the reduction of CO2. Specifically, the ion pair complex [K(18c6)][U(OSi(O(t)Bu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O(t)Bu)3)4(μ-κ(2):κ(1)-CO3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(O(t)Bu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(O(t)Bu)3)4K], 3, thus providing a remarkable example of two-electron transfer in U(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO2.

Investigations of the Electronic Structure of Arene-Bridged Diuranium Complexes

The electronic structure of the arene-bridged complex (μ-toluene)U2(N[tBu]Ar)4 (1a2-μ-toluene, Ar = 3,5-C6H3Me2) has been studied in relation to a variety of mononuclear uranium amide complexes, and their properties have been discussed comparatively. The syntheses, molecular structures (X-ray crystal structures and solution behavior based on variable-temperature NMR spectroscopic data), and corresponding spectroscopic (X-ray absorption near-edge structure and UV–vis–near-IR absorption) and magnetic properties are presented and interpreted with reference to results of density functional theory (DFT) and complete active space self-consistent field with corrections from second-order perturbation theory (CASSCF/CASPT2) calculations performed on model compounds. While the mononuclear compounds display expected electronic and magnetic properties for uranium complexes, 1a2-μ-toluene shows complicated properties in contrast. XANES spectroscopy, X-ray crystallography, and both density functional and CASSCF/CASPT2 ...

Computerized magnetic studies on d, f, d–d, f–f, and d– S, f– S systems under varying ligand and magnetic fields

A computer program is introduced which analyses the magnetic susceptibility of mononuclear and exchange-coupled homodinuclear d N –d N and f N –f N systems as well as heterodinuclear d N – S and f N – S complexes ( S standing for a pure spin centre with S= 1 2 ,1, 7 2 ) taking into consideration interelectronic repulsion, spin-orbit coupling, ligand field effect, isotropic exchange interactions, and applied magnetic field. The program is applied to interpret the paramagnetism of K 6Cu 12U 2S 15 (with magnetically active mononuclear uranium) and to predict the field-dependent magnetic susceptibility of the heterodinuclear complex 3d 2–4f 7.

Structural characterization of a ternary Fe(III)-U(VI)-citrate complex

Summary Citric acid, a naturally occurring hydroxycarboxylic acid, forms bidentate, tridentate, dinuclear, or polymeric species, depending on the metal. In the presence of iron and uranium a ternary Fe: U: citric acid complex is formed. We determined the molecular structure of the ternary complex in both the aqueous and solid phase. Fourier transform infrared spectroscopy (FTIR) showed the presence of uni- and bi-dentate bonding of the citric acid to the metals, and the involvement of the α-hydroxyl group. Analysis of molecular fragments generated from time-of-flight secondary ion mass spectroscopy (TOF-SIMS) confirmed the bonding of terminal carboxylate groups of citric acid to uranium and iron. Extended X-ray absorption fine structure (EXAFS) analysis revealed the dinuclear nature of the Fe bonding to citrate and mononuclear uranium species with citrate. Coordination of a sodium atom to the iron was also noted. The proposed empirical formula for the complex is [NaFe2(μ-O)(μ-citrato)(OH)2(H2O)(C6H4O7)bis(UO2)(C6H5O7)]7-. The structure consists of a dinuclear ferric ion core coordinated through an oxy (μ-O) and a carboxylate (μ-citrato) group of citric acid. The mononuclear uranyl ions are coordinated in bidentate fashion to the central carboxylate groups of citric acid and tridentate coordinated to citric acid.

Uranium tris-aryloxide derivatives supported by triazacyclononane: engendering a reactive uranium(III) center with a single pocket for reactivity.

The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U(III)(NCCH(3))] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U(IV)(N(3))] and [((ArO)(3)tacn)U(V)(NSi(CH(3))(3))]. Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center. DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via pi-bonding interaction, involving uranium f-orbitals and the axial acetonitrile and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely ionic in nature. The magnetism of the series of complexes with an [N3O3-N(ax)] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.

Radionuclide Sorption Modeling Using the Minteqa2 Speciation Code

ABSTRACTThe MINTEQA2 database has been updated and expanded to include radionuclide data from the most recent release of the EQ3/6 database. Comparison of U(VI)-speciation predicted using the old and new MINTEQA2 databases indicates several significant differences, including the introduction of neutral and anionic species at neutral to alkaline pH. In contrast, comparison of results calculated by EQ3 and MINTEQA2, both using Nuclear Energy Agency (NEA) uranium data, reveals only small differences that are likely due to differences in calculated activity coefficients.With the new database, MINTEQA2 was used to model U(VI)-goethite sorption data from the literature with the Triple-Layer Model (TLM). Values were independently fixed forall but one of the model parameters. The parameter optimization code FITEQL was then used to determine binding constants for mononuclear uranium complexes (UO2(OH)n2−n). The surface complex MOH2-UO2(OH)4− produced a very good fit of the sorption data, which was not significantly improved by the use of two or more surface complexes.