Speciation and unusual reactivity in PuO2+x

Pu L(3) XAFS measurements show that the excess oxygen in single phase PuO(2+)(x)() occurs as oxo groups with Pu-O distances of 1.83-1.91 A. This distance and the energy of the edge (via comparison with a large number of related compounds) are more consistent with a Pu(IV/V) than a Pu(IV/VI) mixture. Analogous to Pu(IV) colloids, although the Pu-Pu pair distribution remains single site even when it shows substantial disorder, the Pu-O distribution can display a number of additional shells at specific distances up to 3.4 A even in high fired materials when no oxo groups are present, implying intrinsic H(+)/OH(-)(/H(2)O). The number of oxo atoms increases when samples are equilibrated with humid air at ambient temperature, indicating that the Pu reactivity in this solid system differs notably from that of isolated complexes and demonstrating the importance of nanoscale cooperative phenomena and total free energy in determining its chemical properties.     

Preparation, stability, and structural characterization of plutonium(VII) in alkaline aqueous solution

A freshly prepared solution of Pu(VI) in 2 M NaOH was oxidized to Pu(VII), via ozonolysis, while simultaneously collecting X-ray absorption spectra. Analyses of the XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) data, acquired throughout the in situ experiments, show a dioxo coordination environment for Pu(VI), PuO(2)(2+), typical for it and the hexavalent actinyl species of U and Np, and its evolution into a tetraoxo-coordination environment for Pu(VII), PuO(4)(-), like that known for Np(VII). The EXAFS data provide average Pu-O distances of 1.79(1) and 1.88(1) ?, respectively. The second coordination shells, also fit as O atoms, provide Pu-O distances of 2.29-2.32 ? that are independent of the Pu oxidation state. The coordination numbers for the distant O atoms in sums with those for the nearest O atoms are consistent with 6-O environments for both Pu(VI) and Pu(VII) ions in accordance with their previously proposed speciation as [Pu(VI)O(2)(OH)(4)](2-) and [Pu(VII)O(4)(OH)(2)](3-), respectively. This solution speciation accounts precisely for the Pu(VI) and Pu(VII) coordination environments reported in various solid state structures. The Pu(VII) tetraoxo-dihydroxo anion was found to have a half-life of 3.7 h. Its instability is attributed to spontaneous reduction to Pu(VI) and not to a measurable extent of disproportionation. We found no direct evidence for Pu(VIII) in the X-ray data and, furthermore, the stoichiometry of the oxidation of Cr(III) by Pu is consistent with that expected for a valence-pure Pu(VII) preparation by ozonation and, in turn, stoichiometrically equivalent to the established Np(VII)/Cr(III) redox reaction.     

Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).     

Quantum chemical calculations of the redox potential of the Pu(VII)/Pu(VIII) couple

The redox potential of the Pu(VII)/Pu(VIII) couple was studied by density functional theory calculations. The spin-orbit effect was corrected at the CASSCF level. The redox potential (relative to the standard hydrogen potential) of the Pu(VII)/Pu(VIII) couple in alkaline solution was found to vary from 4.36 to 1.06 V depending on the number of Pu-O oxo bonds, coordination numbers, and coordination modes. The redox potential drops substantially as the number of Pu-O oxo bonds increases. Pu(VIII) may be synthesized in strong alkaline solution assuming that both Pu(VII) and Pu(VIII) exist in penta-oxo form, Pu (VII)O 5OH (4-) and Pu (VIII)O 5OH (3-), respectively. The Mulliken population of Pu in Pu(VII) and Pu(VIII) complexes are very similar, suggesting that the spin-orbit effect is rather small in Pu(VII) complexes and that when Pu(VII) is oxidized to Pu(VIII) the electron is stripped mainly from the ligand. Consequently, Pu(VIII) is in an unstable oxidation state and easily reduced back to Pu(VII) by the solvent water molecules. In acidic medium, the Pu(VII)/Pu(VIII) redox potential is too high to get the Pu(VIII) valence state.     

Local structure and charge distribution in the UO(2)-U(4)O(9) system

Analysis of X-ray absorption fine structure spectra of UO(2+x) for x = 0-0.20 (UO(2)--U(4)O(9)) reveals that the adventitious O atoms are incorporated as oxo groups with U--O distances of 1.74 A, most likely associated with U(VI), that occur in clusters so that the UO(2) fraction of the material largely remains intact. In addition to the formation of some additional longer U--O bonds, the U sublattice consists of an ordered portion that displays the original U--U distance and a spectroscopically silent, glassy part. This is very different from previous models derived from neutron diffraction that maintained long U--O distances and high U--O coordination numbers. UO(2+x) also differs from PuO(2+x) in its substantially shorter An-oxo distances and no sign of stable coordination with H(2)O and its hydrolysis products.     

Theoretical study of the structural properties of plutonium(IV) and (VI) complexes

The structural properties of several plutonium(IV) and (VI) complexes have been examined in the gaseous and aqueous phases using Kohn-Sham density functional theory calculations with scalar relativistic effective core potentials and the polarizable continuum solvation model. The aquo and nitrate complexes of PuO(2)(2+) and Pu(4+) were considered in addition to the aquo-chloro complexes of PuO(2)(2+). The nitrate and chloro- complexes formed with triphenylphosphine oxide (TPPO) and tributylphosphate (TBP) respectively were also studied. The structural parameters of the plutonyl complexes were compared to their uranyl and neptunyl analogues. The bond lengths and vibrational frequencies of the plutonyl complexes can generally be computed with sufficient accuracy with the pure PBE density functional with shorter bond lengths being predicted by the B3LYP functional. The structural parameters of the [PuO(2)Cl(2)L(2)] systems formed with TPPO and TBP as well as the aqueous [PuO(2)Cl(2)(H(2)O)(3)] complex are matched to previous experimental results. Overall, the inclusion of ligands in the equatorial region results in significant changes in the stretching frequency of the plutonyl group. The structural features of the plutonyl (VI) systems are rather similar to those of their 5f(0) uranyl and 5f(1) neptunyl counterparts. For the Pu(IV) aquo and nitrate complexes, the average of the calculated Pu-OH(2) and Pu-O(nitrate) bond lengths are generally within 0.04 ? of the reported experimental values. Overall Kohn-Sham DFT can be used successfully in predicting the structures of this diverse set of Pu(VI) and Pu(IV) complexes.     

Effects of large halides on the structures of lanthanide(III) and plutonium(III) borates

Reactions of LnBr(3) or LnOI with molten boric acid result in formation of Ln[B(5)O(8)(OH)(H(2)O)(2)Br] (Ln = La-Pr), Nd(4)[B(18)O(25)(OH)(13)Br(3)], or Ln[B(5)O(8)(OH)(H(2)O)(2)I] (Ln = La-Nd). Reaction of PuOI with molten boric acid yields Pu[B(7)O(11)(OH)(H(2)O)(2)I]. The Ln(III) and Pu(III) centers in these compounds are found as nine-coordinate hula-hoop or 10-coordinate capped triangular cupola geometries where there are six approximately coplanar oxygen donors provided by triangular holes in the polyborate sheets. The borate sheets are connected into three-dimensional networks by additional BO(3) triangles and/or BO(4) tetrahedra that are roughly perpendicular to the layers. The room-temperature absorption spectrum of single crystals of Pu[B(7)O(11)(OH)(H(2)O)(2)I] shows characteristic f-f transitions for Pu(III) that are essentially indistinguishable from Pu(III) in other compounds with alternative ligands and different coordination environments.     

Stabilising pentavalent actinides--visible-near infrared and X-ray absorption spectroscopic studies of the utility of the [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Sb, Bi) structural type

We report the interaction between B-type tri-lacunary heteropolyoxotungstate anions and actinyl(V) cations in aqueous solution, yielding a greater understanding of the stability of the O≡An≡O(1+) linear dioxo actinide moiety. Previously we reported that B-α-[BiW(9)O(33)](9-) and B-α-[SbW(9)O(33)](9-) will react with NpO(2)(1+) to yield [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb). Single crystal structural characterisation of salts of these complexes revealed a core in which three Np(V) atoms interact with a central W(VI) atom through bridging oxo groups. These bridging oxygen atoms come from one of the two axial oxygens in O≡Np≡O(1+) and represent a highly unusual interaction for a discrete molecular species. In this study visible/near infra-red spectroscopy indicates that [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) could be readily stabilized in solution at near neutral pH for several months, with (NH(4))(14)Na(4)[(Np(3)W(4)O(15))(H(2)O)(39)BiW(9)O(33))(3)]·62H(2)O crystallising from solution in high yield. At lower pH and [BiW(9)O(33)](9-) : NpO(2)(1+) ratios additional Np(V) species could be observed in solution. Stabilization of [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) in solution proved more challenging, with several distinctive Np(V) near infra-red transitions observed in solution. Slow complexation kinetics and reduction to Np(IV) was also observed. High [SbW(9)O(33)](9-) : NpO(2)(1+) molar ratios and careful control of solution pH was required to prepare solutions in which [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) was the only neptunium containing species. In stark contrast to the NpO(2)(1+) chemistry, [BiW(9)O(33)](9-) readily oxidizes PuO(2)(1+) to PuO(2)(2+) yielding further evidence of the decreased stability of Pu(V)vs. Np(V). Np L(II)-edge XAFS measurement revealed very good agreement with single crystal diffraction data for the Np structural environment for [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb) in the solid state. There was also good agreement between coordination shells for [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) in the solid state and in solution, yielding further confirmation of the high stability of this particular cluster.     

Multidentate aryloxide and oxo-aryloxide complexes of antimony: synthesis and structural characterization of [eta4-N(o-C6H4O)3]Sb(OSMe2), {{[eta3-N(o-C6H4OH)- (o-C6H4O)2]Sb}2(mu2-O)}2 and {[eta3-PhN(o-C6H4O)2]Sb}4(mu3-O)2

Antimony compounds that feature multidentate aryloxide ligands, namely [eta4-N(o-C6H4O)3]Sb(OSMe2), {{[eta3-N(o-C6H4OH)(o-C6H4O)2]Sb}2(mu2-O)}2, and {[eta3-PhN(o-C6H4O)2]Sb}4(mu3-O)2 have been synthesized from N(o-C6H4OH)3 and PhN(o-C6H4OH)2 and structurally characterized by X-ray diffraction. While [eta4-N(o-C6H4O)3]Sb(OSMe2) exists as a discrete mononuclear species, the oxo complexes {{[eta3-N(o-C6H4OH)(o-C6H4O)2]Sb}2(mu2-O)}2 and {[eta3-PhN(o-C6H4O)2]Sb}4(micro3-O)2 are multinuclear. Specifically, the dinuclear fragment {[eta3-N(o-C6H4OH)(o-C6H4O)2]Sb}2(mu2-O)} exists in a dimeric form due to the bridging oxo ligand participating in an intermolecular hydrogen bonding interaction, while the dinuclear fragment {[eta3-PhN(o-C6H4O)2]Sb}2(mu-O) exists in a dimeric form due to the bridging oxo ligand serving as a donor to the antimony of a second fragment. The structures of {{[eta3-N(o-C6H4OH)(o-C6H4O)2]Sb}2(mu2-O)}2 and {[eta3-PhN(o-C6H4O)2]Sb}4(mu3-O)(2), therefore, indicate that an oxo ligand bridging two Sb(III) centers is sufficiently electron rich to serve as both an effective hydrogen bond acceptor and as a ligand for an additional Sb(III) center.     

Pu(Ⅳ)、Pu(Ⅴ)在不同质地砂土中的分配

Pu(Ⅲ)、Pu(Ⅳ)、Pu(Ⅴ)和Pu(Ⅵ)在自然水的pH和Eh(氧化还原电势)值范围内能够共存。 在自然环境中主要以Pu(Ⅴ)的PuO2(H2O)+n和Pu(Ⅳ)的Pu(OH)4形式存在。 采用静态法测定了Pu(Ⅳ)和Pu(Ⅴ)在砂土介质中的分配系数Kd，Pu(Ⅳ)和Pu(Ⅴ)在砂土中的分配系数范围分别为5.5×103~4×104 mL/g 和3.2×103～1.1×104 mL/g。 表明在砂土介质中Pu(Ⅳ)的分配系数(Kd)大于Pu(Ⅴ)的，并且分配系数随砂土介质中粘土矿物含量的增加及介质颗粒粒径减小而增大。     

Deviation between the chemistry of Ce(IV) and Pu(IV) and routes to ordered and disordered heterobimetallic 4f/5f and 5f/5f phosphonates

The heterobimetallic actinide compound UO(2)Ce(H(2)O)[C(6)H(4)(PO(3)H)(2)](2)·H(2)O was prepared via the hydrothermal reaction of U(VI) and Ce(IV) in the presence of 1,2-phenylenediphosphonic acid. We demonstrate that this is a kinetic product that is not stable with respect to decomposition to the monometallic compounds. Similar reactions have been explored with U(VI) and Ce(III), resulting in the oxidation of Ce(III) to Ce(IV) and the formation of the Ce(IV) phosphonate, Ce[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O, UO(2)Ce(H(2)O)[C(6)H(4)(PO(3)H)(2)](2)·H(2)O, and UO(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·H(2)O. In comparison, the reaction of U(VI) with Np(VI) only yields Np[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O and aqueous U(VI), whereas the reaction of U(VI) with Pu(VI) yields the disordered U(VI)/Pu(VI) compound, (U(0.9)Pu(0.1))O(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·H(2)O, and the Pu(IV) phosphonate, Pu[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O. The reactions of Ce(IV) with Np(VI) yield disordered heterobimetallic phosphonates with both M[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Ce, Np) and M[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O (M = Ce, Np) structures, as well as the Ce(IV) phosphonate Ce[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O. Ce(IV) reacts with Pu(IV) to yield the Pu(VI) compound, PuO(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·3H(2)O, and a disordered heterobimetallic Pu(IV)/Ce(IV) compound with the M[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Ce, Pu) structure. Mixtures of Np(VI) and Pu(VI) yield disordered heterobimetallic Np(IV)/Pu(IV) phosphonates with both the An[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Np, Pu) and An[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O (M = Np, Pu) formulas.     

Crystal engineering through face interactions between tetrahedral and octahedral building blocks: crystal structure of [epsilon-Al13O4(OH)24(H2O)12]2[V2W4O19]3(OH)2).27H2O

A new intercluster salt crystal [epsilon-Al13O4(OH)24(H2O)12]2[V2W4O19]3(OH)2).27H2O (1) was synthesized from the reaction of octahedral Lindqvist-type polyoxometalate [V2W4O19](4-) and truncated tetrahedral Keggin-type [epsilon-Al13O4(OH)24-H2O)(12)](7+) cluster ions. The crystal structure shows that the oppositely charged cluster ions are arranged alternately and have their contacting faces parallel to each other for maximal interactions, both electrostatic and hydrogen bonding. The face-to-face interaction mode of the clusters allows analysis of the crystal structure in an analogy to the bond directionality of conventional inorganic crystals. Therefore, the packing of clusters in 1 is that of As2O3 (Claudetite-II). With the bond directionality, the crystal has large one-dimensional channels with a cross-sectional area of 14.17 x 13.88 A(2) that are filled by lattice water and charge-balancing OH-.     

Mononuclear oxovanadium complexes of tris(2-pyridylmethyl)amine

Mononuclear oxovanadium(IV) and dioxovanadium(V) complexes of tris(2-pyridylmethyl)amine (tpa) have been prepared for the first time. Crystal structure determinations of three oxovanadium(IV) complexes, [VO(SO4)(tpa)], [VOCl(tpa)]PF6, or [VOBr(tpa)]PF6, and a dioxovanadium(v) complex [V(O)2(tpa)]PF6 disclosed that the tertiary nitrogen of the tpa ligand always occupies the trans-to-oxo site. The structures of an oxo-peroxo complex [VO(O2)(tpa)]Cl that was prepared previously and of a mu-oxo vanadium(III) complex [{VCl(tpa)}2(mu-O)](PF6)2 have also been determined. The tertiary nitrogen is located at a trans site to the peroxo and chloride ligands, respectively. The total sums of the four V-N bond lengths from the tpa ligand are remarkably similar among the six complexes, indicating that the vanadium oxidation states become less influential in tpa bonding due primarily to the coordination of electron-donating oxo ligand(s). Absorption spectra of [VOCl(tpa)]+ in acetonitrile showed a significant change upon addition of p-toluenesulfonic acid and HClO4, but not on addition of benzoic acid. Protonation at the oxo ligand by the former two acids is suggested. Cyclic voltammetric studies in acetonitrile verified the proton-coupled redox behavior of the V(III)/V(IV) process involving the oxo ligand for the first time. From the dependence of the added p-toluenesulfonic acid to the CV, redox potentials for the following species have been estimated: [V(IV)OCl(tpa)]+/[V(III)OCl(tpa)](E1/2=-1.59 V vs. Fc+/Fc), [V(IV)(OH)Cl(tpa)]2+/[V(III)(OH)Cl(tpa)]+(Epc=-1.34 V), [V(IV)(OH2)Cl(tpa)]3+/[V(III)(OH2)Cl(tpa)]2+(Epa=-0.49 V), and [V(IV)Cl2(tpa)]2+/[V(III)Cl2(tpa)]+(E1/2=-0.89 V). The reduction of [V(V)(O)2(tpa)]+ in 0.05 M [(n-Bu)4N]PF6 acetonitrile showed a major irreversible reduction wave V(V)/(IV) at -1.48 V. The metal reduction potentials of the oxovanadium(IV) and dioxovanadium(V) species are very close, reinforcing the significant influence of the oxo ligand(s).     

Pu(VI) hydrolysis: further evidence for a dimeric plutonyl hydroxide and contrasts with U(VI) chemistry

A significant fraction of plutonium that is soluble in environmental waters and other aqueous solutions can be present as complexes of plutonyl, PuO2(2+). Few thermodynamic data are available for this ion, representing a problematic gap in plutonium chemistry and in the forecasting of radionuclide behavior under contamination and nuclear repository conditions. To address this need and more accurately determine the stoichiometry and stability of the basic hydrolytic products, we completed complimentary potentiometric and spectrophotometric studies of plutonium(VI) hydrolysis over the concentration range of 10(-2) to 10(-5) M Pu(VI). Dinuclear hydroxide species (PuO2)2(OH)2(2+) and (PuO2)2(OH)4(0)(aq) with hydrolysis constants log beta(2,2) = -7.79 +/- 0.20 and log beta(4,2) = -19.3 +/- 0.5 are indicated in all experiments of millimolar Pu(VI), 0.10 M NaNO3 solutions at 25 degrees C. At lower Pu(VI) concentrations, at and below 10(-4) M, the monomeric species PuO2OH+ and PuO2(OH)2(0)(aq) form with hydrolysis constants of log beta(1,1) = -5.76 +/- 0.07 and log beta(2,1) = -11.69 +/- 0.05, respectively. Distinct optical absorbance bands at 842 and 845 nm are reported for the mononuclear and dinuclear first hydrolysis species. Standard hydrolysis constants at zero ionic strength were calculated from the experimentally determined constants using the specific ion interaction theory. The Pu(VI) hydrolysis species and constants are compared with results from previous studies for plutonium and uranium. Major differences between uranyl and plutonyl hydrolysis are described.     

Bonding Trends Traversing the Tetravalent Actinide Series: Synthesis, Structural, and Computational Analysis of An(IV)((Ar)acnac)(4) Complexes (An = Th, U, Np, Pu; (Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3))

A series of tetravalent An(IV) complexes with a bis-phenyl β-ketoiminate N,O donor ligand has been synthesized with the aim of identifying bonding trends and changes across the actinide series. The neutral molecules are homoleptic with the formula An((Ar)acnac)(4) (An = Th (1), U (2), Np (3), Pu (4); (Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)) and were synthesized through salt metathesis reactions with actinide chloride precursors. NMR and electronic absorption spectroscopy confirm the purity of all four new compounds and demonstrate stability in both solution and the solid state. The Th, U, and Pu complexes were structurally elucidated by single-crystal X-ray diffraction and shown to be isostructural in space group C2/c. Analysis of the bond lengths reveals shortening of the An-O and An-N distances arising from the actinide contraction upon moving from 1 to 2. The shortening is more pronounced upon moving from 2 to 4, and the steric constraints of the tetrakis complexes appear to prevent the enhanced U-O versus Pu-O orbital interactions previously observed in the comparison of UI(2)((Ar)acnac)(2) and PuI(2)((Ar)acnac)(2) bis-complexes. Computational analysis of models for 1, 2, and 4 (1a, 2a, and 4a, respectively) concludes that both the An-O and the An-N bonds are predominantly ionic for all three molecules, with the An-O bonds being slightly more covalent. Molecular orbital energy level diagrams indicate the largest 5f-ligand orbital mixing for 4a (Pu), but spatial overlap considerations do not lead to the conclusion that this implies significantly greater covalency in the Pu-ligand bonding. QTAIM bond critical point data suggest that both U-O/U-N and Pu-O/Pu-N are marginally more covalent than the Th analogues.     

Sorption of Pu(VI) onto TiO2

The sorption of Pu(VI) onto TiO(2) was studied as a function of pH (2-10) and Pu concentration (10(-8)-10(-4) M) under an N(2) atmosphere, in 0.016 and 0.1 M NaClO(4). A batch-wise method was used, in which pH was measured in separate experimental containers after removal of a sample to determine the amount of Pu that had been sorbed. As Pu is radioactive, it was used as a tracer and measured by liquid scintillation counting. No ionic strength dependence was discerned, which was taken as an indication of inner sphere complex formation. In the interval of pH 2-7 the system could be described by the formation of two positively charged surface complexes using a 1-pK Stern model. Sorption of the plutonyl ion (PuO(2)(2+)) and the first hydrolysis species (PuO(2)(OH)(+)) was estimated using FITEQL to logK(1)=6.9 and logK(2)=1.4, respectively.     

Charge distribution and local structure and speciation in the UO2+x and PuO2+x binary oxides for x?0.25

The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO_2+x (0.00?x?0.20) and PuO_2+x/PuO_2+x−y(OH)_2y·zH₂O have been determined by U/Pu L_III XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75 Å) U-O distances consistent with U(VI) concomitant with a large range of U displacements that reduce the apparent number of U neighbors and (2) that the UO₂ fraction remains intact implying that these O defects interact to form clusters and give the heterogeneous structure consistent with the diffraction patterns. The PuO_2+x system, which does not show a separate phase at its x=0.25 endpoint, also displays (1) oxo groups at longer 1.9 Å distances consistent with Pu(V+δ), (2) a multisite Pu-O distribution even when x is close to zero indicative of the formation of stable species with H₂O and its hydrolysis products with O²⁻, and (3) a highly disordered, spectroscopically invisible Pu-Pu component. The structure and bonding in AnO_2+x are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements.     

A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: synthesis,structure, and redox activity

The concerned diols (general abbreviation, H(2)L) are catechol (H(2)L(1)) and its 3,5-Bu(t)(2) derivative (H(2)L(2)). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H(2)L with VO(xsal)(H(2)O) or VO(xsal)(OMe)(HOMe), where xsal(2-) is the diionized salicylaldimine of glycine (x = g), L-alanine (x = a), or L-valine (x = v). The reaction of VO(acac)(2) with H(2)L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL(1)), 2a, and VO(vsal)(HL(2)), 2f, the HL(-) ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal(2-) coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O...O, 2.60 A) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L(2)).H(2)O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O.O, 2.84 A). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single (51)V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H(2)L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O(2) to 2 is proposed to occur via hydrogen bonding with chelated HL(-).     

Syntheses, characterizations and properties of [Mo2O2S2]-based oxothiomolybdenum wheels incorporating bisphosphonate ligands

We report the syntheses and characterizations of the first polyoxothiometalate complexes isolated from the reaction of the oxothiocationic [Mo(V)(2)O(2)S(2)](2+) precursor and bisphosphonate ligands H(2)O(3)PCR(OH)PO(3)H(2) (R = C(4)H(5)N(2), zoledronic acid; R = C(3)H(6)NH(2), alendronic acid). [(Mo(2)O(2)S(2)(H(2)O))(4)(O(3)PC(O)(C(4)H(6)N(2))PO(3))(4)](8-) (Mo(8)S(8)(Zol)(4)) and [(Mo(2)O(2)S(2)(H(2)O))(4)(O(3)PC(O)(C(3)H(6)NH(3))PO(3))(4)](8-) (Mo(8)S(8)(Ale)(4)) contain four Mo(V) dimers connected via bisphosphonate ligands. These compounds offer a unique opportunity to compare the structures and properties of cyclic compounds obtained with [Mo(2)O(2)S(2)](2+) and with [Mo(2)O(4)](2+). The oxothio compounds appear less stable in solution than the oxo analogue, confirming the higher lability and versatility of [Mo(2)O(2)S(2)]-based compounds compared to [Mo(2)O(4)]-based POMs. Multinuclear and multidimensional solid-state NMR studies were carried out to complement X-ray diffraction analysis. Information on short-range interactions, dynamic behaviors, and local disorder within the crystalline materials are therefore reported. Furthermore, the electrocatalytic properties of Mo(8)S(8)(Ale)(4) and of the analogous [(Mo(2)O(4)(H(2)O))(4)(O(3)PC(O)(C(3)H(6)NH(3))PO(3))(4)](8-) (Mo(8)O(8)(Ale)(4)) immobilized onto the surface of a glassy carbon electrode were studied, thus evidencing the ability of [Mo(2)O(2)S(2)]-based cycles to promote the reduction of protons into hydrogen, whereas the oxo analogue appeared inactive.     

Cerium(IV), neptunium(IV), and plutonium(IV) 1,2-phenylenediphosphonates: correlations and differences between early transuranium elements and their proposed surrogates

The in situ hydrothermal reduction of Np(VI) to Np(IV) and Pu(VI) to Pu(IV) in the presence of 1,2-phenylenediphosphonic acid (PhP2) results in the crystallization of Np[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O (NpPhP2) and Pu[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (PuPhP2), respectively. Similar reactions have been explored with Ce(IV) resulting in the isolation of the Ce(IV) phenylenediphosphonate Ce[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (CePhP2). Single crystal diffraction studies reveal that although all these three compounds all crystallize in the triclinic space group P1?, only PuPhP2 and CePhP2 are isotypic, whereas NpPhP2 adopts a distinct structure. In the cerium and plutonium compounds edge-sharing dimers of MO(8) polyhedra are bridged by the diphosphonate ligand to create one-dimensional chains. NpPhP2 also forms chains. However, the NpO(8) units are monomeric. The protonation of the ligands is also different in the two structure types. Furthermore, the NpO(8) polyhedra are best described as square antiprisms (D(4d)), whereas the CeO(8) and PuO(8) units are trigonal dodecahedra (D(2d)). Bond-valence parameters of R(o) = 1.972 and b = 0.538 have been derived for Np(4+) using a combination of the data reported in this work with that available in crystallographic databases. The UV-vis-NIR absorption spectra of NpPhP2 and PuPhP2 are also reported and used to confirm the tetravalent oxidation states.