Coordination chemistry of 2,2'-biphenylenedithiophosphinate and diphenyldithiophosphinate with U, Np, and Pu.

New members of the dithiophosphinic acid family of potential actinide extractants were prepared: heterocyclic 2,2'-biphenylenedithiophosphinic acids of stoichiometry HS2P(R2C12H6) (R = H or (t)Bu). The time- and atom-efficient syntheses afforded multigram quantities of pure HS2P(R2C12H6) in reasonable yields (∼60%). These compounds differed from other diaryldithiophosphinic acid extractants in that the two aryl groups were connected to one another at the ortho positions to form a 5-membered dibenzophosphole ring. These 2,2'-biphenylenedithiophosphinic acids were readily deprotonated to form S2P(R2C12H6)(1-) anions, which were crystallized as salts with tetraphenylpnictonium cations (ZPh4(1+); Z = P or As). Coordination chemistry between [S2P((t)Bu2C12H6)](1-) and [S2P(C6H5)2](1-) with U, Np, and Pu was comparatively investigated. The results showed that dithiophosphinate complexes of U(IV) and Np(IV) were redox stable relative to those of U(III), whereas reactions involving Pu(IV) gave intractable material. For instance, reactions involving U(IV) and Np(IV) generated An[S2P((t)Bu2C12H6)]4 and An[S2P(C6H5)2]4 whereas reactions between Pu(IV) and [S2P(C6H5)2](1-) generated a mixture of products from which we postulated a transient Pu(III) species based on UV-Vis spectroscopy. However, the trivalent Pu[S2P(C6H5)2]3(NC5H5)2 compound is stable and could be isolated from reactions between [S2P(C6H5)2](1-) and the trivalent PuI3(NC5H5)4 starting material. Attempts to synthesize analogous trivalent compounds with U(III) provided the tetravalent U[S2P(C6H5)2]4 oxidation product.

Structural and spectroscopic characterization of plutonyl(VI) nitrate under acidic conditions.

The plutonyl(VI) dinitrate complex [PuO(2)(NO(3))(2)(H(2)O)(2)]·H(2)O (1) has been structurally characterized by single-crystal X-ray diffraction and spectroscopically characterized by solid-state vis-NIR and Raman spectroscopies. Aqueous solution spectroscopic studies indicate only weak plutonyl(VI) nitrate complexation, with the mononitrate complex dominating and negligible dinitrate formation, even in concentrated nitric acid.

Plutonium(V/VI) Reduction by the Metal-Reducing Bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1.

We examined the ability of the metal-reducing bacteria Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1 to reduce Pu(VI) and Pu(V). Cell suspensions of both bacteria reduced oxidized Pu [a mixture of Pu(VI) and Pu(V)] to Pu(IV). The rate of plutonium reduction was similar to the rate of U(VI) reduction obtained under similar conditions for each bacteria. The rates of Pu(VI) and U(VI) reduction by cell suspensions of S. oneidensis were slightly higher than the rates observed with G. metallireducens. The reduced form of Pu was characterized as aggregates of nanoparticulates of Pu(IV). Transmission electron microscopy images of the solids obtained from the cultures after the reduction of Pu(VI) and Pu(V) by S. oneidensis show that the Pu precipitates have a crystalline structure. The nanoparticulates of Pu(IV) were precipitated on the surface of or within the cell walls of the bacteria. The production of Pu(III) was not observed, which indicates that Pu(IV) was the stable form of reduced Pu under these experimental conditions. Experiments examining the ability of these bacteria to use Pu(VI) as a terminal electron acceptor for growth were inconclusive. A slight increase in cell density was observed for both G. metallireducens and S. oneidensis when Pu(VI) was provided as the sole electron acceptor; however, Pu(VI) concentrations decreased similarly in both the experimental and control cultures.

Low-valent molecular plutonium halide complexes.

Treatment of plutonium metal with 1.5 equiv of bromine in tetrahydrofuran (thf) led to isolation of PuBr3(thf)4 (1), which is a new versatile synthon for exploration of non-aqueous Pu(III) chemistry. Adventitious water in the system resulted in structural characterization of the eight-coordinate complex [PuBr2(H2O)6][Br] (2). The crystal structure of PuI3(thf)4 (3) has been determined for the first time and is isostructural with UI3(thf)4. Attempts to form a bis(imido) plutonyl(VI) moiety ([Pu(NR)2](2+)) by oxidation of PuI3(py)4 with iodine and (t)BuNH2 resulted in crystallization of the Pu(III) complex [PuI2(thf)4(py)][I3] (4). Dissolution of a Pu(IV) carbonate with a HCl/Et2O solution in thf gave the mixed valent (III/IV) complex salt [PuCl2(thf)5][PuCl5(thf)] (5) as the only tractable product. Oxidation of Pu[N(SiMe3)2]3 with TeCl4 afforded the Pu(IV) complex Pu[N(SiMe3)2]3Cl (6), which may prove to be a useful entry route for investigation of organometallic/non-aqueous tetravalent plutonium chemistry.

Structural characterization of Pu[N(SiMe(3))(2)](3), a synthetically useful nonaqueous plutonium(III) precursor.

A rare, and synthetically versatile, nonaqueous plutonium complex, Pu[N(SiMe3)2]3 (1), has been structurally characterized by single-crystal X-ray diffraction for the first time and reveals significantly shorter agostic interactions compared to the cerium(III) analogue, indicating possible covalency differences.

Experimental and theoretical comparison of actinide and lanthanide bonding in M[N(EPR(2))(2)](3) complexes (M = U, Pu, La, Ce; E = S, Se, Te; R = Ph, iPr, H).

Treatment of M[N(SiMe3)2]3 (M = U, Pu (An); La, Ce (Ln)) with NH(EPPh2)2 and NH(EPiPr2)2 (E = S, Se), afforded the neutral complexes M[N(EPR2)2]3 (R = Ph, iPr). Tellurium donor complexes were synthesized by treatment of MI3(sol)4 (M = U, Pu; sol = py and M = La, Ce; sol = thf) with Na(tmeda)[N(TePiPr2)2]. The complexes have been structurally and spectroscopically characterized with concomitant computational modeling through density functional theory (DFT) calculations. The An-E bond lengths are shorter than the Ln-E bond lengths for metal ions of similar ionic radii, consistent with an increase in covalent interactions in the actinide bonding relative to the lanthanide bonding. In addition, the magnitude of the differences in the bonding is slightly greater with increasing softness of the chalcogen donor atom. The DFT calculations for the model systems correlate well with experimentally determined metrical parameters. They indicate that the enhanced covalency in the M-E bond as group 16 is descended arises mostly from increased metal d-orbital participation. Conversely, an increase in f-orbital participation is responsible for the enhancement of covalency in An-E bonds compared to Ln-E bonds. The fundamental and practical importance of such studies of the role of the valence d and f orbitals in the bonding of the f elements is emphasized.

Plutonium(IV) reduction by the metal-reducing bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1.

The bacterial reduction of actinides has been suggested as a possible remedial strategy for actinide-contaminated environments, and the bacterial reduction of Pu(VI/V) has the potential to produce highly insoluble Pu(IV) solid phases. However, the behavior of plutonium with regard to bacterial reduction is more complex than for other actinides because it is possible for Pu(IV) to be further reduced to Pu(III), which is relatively more soluble than Pu(IV). This work investigates the ability of the metal-reducing bacteria Geobacter metallireducens GS15 and Shewanella oneidensis MR1 to enzymatically reduce freshly precipitated amorphous Pu(IV) (OH)(4) [Pu(IV)(OH)(4(am))] and soluble Pu(IV)(EDTA). In cell suspensions without added complexing ligands, minor Pu(III) production was observed in cultures containing S. oneidensis, but little or no Pu(III) production was observed in cultures containing G. metallireducens. In the presence of EDTA, most of the Pu(IV)(OH)(4(am)) present was reduced to Pu(III) and remained soluble in cell suspensions of both S. oneidensis and G. metallireducens. When soluble Pu(IV)(EDTA) was provided as the terminal electron acceptor, cell suspensions of both S. oneidensis and G. metallireducens rapidly reduced Pu(IV)(EDTA) to Pu(III)(EDTA) with nearly complete reduction within 20 to 40 min, depending on the initial concentration. Neither bacterium was able to use Pu(IV) (in any of the forms used) as a terminal electron acceptor to support growth. These results have significant implications for the potential remediation of plutonium and suggest that strongly reducing environments where complexing ligands are present may produce soluble forms of reduced Pu species.

Redox behavior of cyclo[6]pyrrole in the formation of a uranyl complex.

A uranyl complex, the first metal complex to be formed from the cyclo[n]pyrrole series of expanded porphyrins, is formed when cyclo[6]pyrrole is treated with the uranyl cation under aerobic conditions. Spectroscopic, spectroelectrochemical, and electron spin resonance data of this species are consistent with the ligand in the complex being oxidized to an antiaromatic form.

Biological reduction of Np(V) and Np(V) citrate by metal-reducing bacteria.

Oxidized actinide species are often more mobile than reduced forms. Bioremediation strategies have been developed to exploit this chemistry and stabilize actinides in subsurface environments. We investigated the ability of metal-reducing bacteria Geobacter metallireducens and Shewanella oneidensis to enzymatically reduce Np(V) and Np(V) citrate, as well as the toxicity of Np(V) to these organisms. A toxic effect was observed for both bacteria at concentrations of > or = 4.0 mM Np(V) citrate. Below 2.0 mM Np(V) citrate, no toxic effect was observed and both Fe(III) and Np(V) were reduced. Cell suspensions of S. oneidensis were able to enzymatically reduce unchelated Np(V) to insoluble Np(IV)(s), but cell suspensions of G. metallireducens were unable to reduce Np(V). The addition of citrate enhanced the Np(V) reduction rate by S. oneidensisand enabled Np(V) reduction by G. metallireducens. The reduced form of neptunium remained soluble, presumably as a polycitrate complex. Growth was not observed for either organism when Np(V) or Np(V) citrate was provided as the sole terminal electron acceptor. Our results show that bacteria can enzymatically reduce Np(V) and Np(V) citrate, but that the immobilization of Np(IV) may be dependent on the abundance of complexing ligands.

An entry route into non-aqueous plutonyl coordination chemistry.

The Pu(VI) molecular complex, [PuO(2)Cl(2)(thf)(2)]2, is prepared by addition of a HCl/Et(2)O solution to a suspension of PuO(2)CO(3) in thf, yielding the first example of a precursor suitable for investigation of the non-aqueous chemistry of the plutonyl dioxo cation under inert atmospheric conditions.

Purification and characterization of rhodobactin: a mixed ligand siderophore from Rhodococcus rhodochrous strain OFS.

The siderophore produced by Rhodococcus rhodochrous strain OFS, rhodobactin, was isolated from iron-deficient cultures and purified by a combination of XAD-7 absorptive/partition resin column and semi-preparative HPLC. The siderophore structure was characterized using 1D and 2D (1)H, (13)C and (15)N NMR techniques (DQFCOSY, TOCSY, NOESY, HSQC and LR-HSQC) and was confirmed using ESI-MS and MS/MS experiments. The structural characterization revealed that the siderophore, rhodobactin, is a mixed ligand hexadentate siderophore with two catecholate and one hydroxamate moieties for iron chelation. We further investigated the effects of Fe concentrations on siderophore production and found that Fe limiting conditions (Fe concentrations from 0.1 microM to 2.0 microM) facilitated siderophore excretion. Our interests lie in the role that siderophores may have in binding metals at mixed contamination sites (containing metals/radionuclides and organics). Given the broad metabolic capacity of this microbe and its Fe scavenging ability, R. rhodochrous OFS may have a competitive advantage over other organisms employed in bioremediation.

Complexation of Pu(IV) with the natural siderophore desferrioxamine B and the redox properties of Pu(IV)(siderophore) complexes.

The bioavailability and mobility of Pu species can be profoundly affected by siderophores and other oxygen-rich organic ligands. Pu(IV)(siderophore) complexes are generally soluble and may constitute with other soluble organo-Pu(IV) complexes the main fraction of soluble Pu(IV) in the environment. In order to understand the impact of siderophores on the behavior of Pu species, it is important to characterize the formation and redox behavior of Pu(siderophore) complexes. In this work, desferrioxamine B (DFO-B) was investigated for its capacity to bind Pu(IV) as a model siderophore and the properties of the complexes formed were characterized by optical spectroscopy measurements. In a 1:1 Pu(IV)/DFO-B ratio, the complexes Pu(IV)(H2DFO-B)4+, Pu(IV)(H1DFO-B)3+, Pu(IV)(DFO-B)2+, and Pu(IV)(DFO-B)(OH)+ form with corresponding thermodynamic stability constants log beta1,1,2 = 35.48, log beta1,1,1 = 34.87, log beta1,1,0 = 33.98, and log beta1,1,-1 = 27.33, respectively. In the presence of excess DFO-B, the complex Pu(IV)H2(DFO-B)22+ forms with the formation constant log beta2,1,2 = 62.30. The redox potential of the complex Pu(IV)H2(DFO-B)22+ was determined by cyclic voltammetry to be E1/2 = -0.509 V, and the redox potential of the complex Pu(IV)(DFO-B)2+ was estimated to be E1/2 = -0.269 V. The redox properties of Pu(IV)(DFO-B)2+ complexes indicate that Pu(III)(siderophore) complexes are more than 20 orders of magnitude less stable than their Pu(IV) analogues. This indicates that under reducing conditions, stable Pu(siderophore) complexes are unlikely to persist.

U(IV) chalcogenolates synthesized via oxidation of uranium metal by dichalcogenides.

Treatment of uranium metal with dichalcogenides in the presence of a catalytic amount of iodine in pyridine affords molecular U(IV) chalcogenolates that do not require stabilizing ancillary ligands. Oxidation of U(0) by PhEEPh yields monomeric seven-coordinate U(EPh)4(py)3 (E = S(1), Se(2)). The dimeric eight-coordinate complexes [U(EPh)2(mu2-EPh)2(CH3CN)2]2 (E = S(3), Se(4)) are obtained by crystallization from solutions of 1 and 2 dissolved in acetonitrile. Oxidation of U(0) by pySSpy and crystallization from thf yields nine-coordinate U(Spy)4(thf) (5). Incorporation of elemental selenium into the oxidation of U(0) by PhSeSePh results in the isolation of [U(py)2(SePh)(mu3-Se)(mu2-SePh)]4.4py (6), a tetrameric cluster in which each U(IV) ion is eight-coordinate and the U4Se4 core forms a distorted cube. The compounds were analyzed spectroscopically and the single-crystal X-ray structures of 1 and 3-6 were determined. The isolation of 1-6 represents six new examples of actinide chalcogenolates and allows insight into the nature of "hard" actinide ion-"soft" chalcogen donor interactions.

Iron(III) coordination properties of a pyoverdin siderophore produced by Pseudomonas putida ATCC 33015.

The iron complexation of a fluorescent green pyoverdin siderophore produced by the environmental bacterium Pseudomonas putida was characterized by solution thermodynamic methods. Pyoverdin binds iron through three bidentate chelate groups, a catecholate, a hydroxamate, and an alpha-hydroxycarboxylic acid. The deprotonation constants of the free pyoverdin and Fe(III)-pyoverdin complex were determined through a series of potentiometric and spectrophotometric experiments. The ferric complex of pyoverdin forms at very low pH (pH < 2), but full iron coordination does not occur until neutral pH. The calculated pM value of 25.13 is slightly lower than that for pyoverdin PaA (pM = 27), which coordinates iron by a catecholate and two hydroxamate groups. The redox potential of Fe-pyoverdin was found to be very pH sensitive. At high pH (approximately pH 9-11) where pyoverdin coordinates Fe in a hexadentate mode the redox potential is -0.480 V (NHE); however, at neutral pH where full Fe coordination is incomplete, the redox potential is more positive (E(1/2) = -0.395 V). The positive shift in the redox potential and the partial dissociation of the Fe-pyoverdin complex with pH decrease provides a path toward in vivo iron release.

Enhanced exopolymer production and chromium stabilization in Pseudomonas putida unsaturated biofilms.

Chromium-contaminated soils threaten surface and groundwater quality at many industrial sites. In vadose zones, indigenous bacteria can reduce Cr(VI) to Cr(III), but the subsequent fate of Cr(III) and the roles of bacterial biofilms are relatively unknown. To investigate, we cultured Pseudomonas putida, a model organism for vadose zone bioremediation, as unsaturated biofilms on membranes overlaying iron-deficient solid media either containing molecular dichromate from potassium dichromate (Cr-only treatment) or with deposits of solid, dichromate-coated hematite (Fe+Cr treatment) to simulate vadose zone conditions. Controls included iron-deficient solid medium and an Fe-only treatment using solid hematite deposits. Under iron-deficient conditions, chromium exposure resulted in lower cell yield and lower amounts of cellular protein and carbohydrate, but providing iron in the form of hematite overcame these toxic effects of Cr. For the Cr and Fe+Cr treatments, Cr(VI) was completely reduced to Cr(III) that accumulated on biofilm cells and extracellular polymeric substances (EPSs). Chromium exposure resulted in elevated extracellular carbohydrates, protein, DNA, and EPS sugars that were relatively enriched in N-acetyl-glucosamine, rhamnose, glucose, and mannose. The proportions of EPS protein and carbohydrate relative to intracellular pools suggested Cr toxicity-mediated cell lysis as the origin. However, DNA accumulated extracellularly in amounts far greater than expected from cell lysis, and Cr was liberated when extracted EPS was treated with DNase. These results demonstrate that Cr accumulation in unsaturated biofilms occurs with enzymatic reduction of Cr(VI), cellular lysis, cellular association, and extracellular DNA binding of Cr(III), which altogether can facilitate localized biotic stabilization of Cr in contaminated vadose zones.

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.

Uranium(III)/(IV) nitrile adducts including UI4(N[triple bond]CPh)4, a synthetically useful uranium(IV) complex.

The synthesis of complexes used to elucidate an understanding of fundamental An(III) and An(IV) coordination chemistry requires the development of suitable organic-soluble precursors. The reaction of oxide-free uranium metal turnings with 1.3 equivalents of elemental iodine in acetonitrile provided the U(III)/U(IV) complex salt, [U(N[triple bond]CMe)9][UI6][I] (1), in which the U(III) cation is surrounded by nine acetonitrile molecules in a tricapped trigonal prismatic arrangement, a [UI6]2- counterion, and a noncoordinating iodide. The U-N distances for the prismatic and capping nitrogens are 2.55(3) and 2.71(5) A, respectively. The same reaction performed in benzonitrile afforded crystalline UI4(N[triple bond]CPh)4 (3) in 78% isolated yield. In the solid state, 3 shows an eight-coordinate U(IV) atom in a "puckered" square antiprismatic geometry with U-N and U-I distances of 2.56(1) and 3.027(1) A, respectively. This benzonitrile UI4 adduct is a versatile U(IV) synthon that is soluble in methylene chloride, benzonitrile, and tetrahydrofuran, and moderately soluble in toluene and benzene, but decomposes in benzonitrile at 198 degrees C to [UI(N[triple bond]CPh)8][UI]6 (4), a U(III)/U(IV) salt analogous to 1. A toluene slurry of 3 treated with 2.2 equiv of Cp*MgCl.THF (Cp* = pentamethylcyclopentadienide) provided Cp*2UI2(N[triple bond]CPh) (5) in low yields. Single-crystal X-ray structure determination shows that the iodide ligands in 5 are in a rare cis configuration with an acute I-U-I angle of 83.16(7) degrees . Treatment of a methylene chloride solution of 3 with KTp* (Tp* = hydridotris(3,5-dimethylpyrazolylborate)) formed green TpUI3 (6) which was converted to yellow Tp*UI3(N[triple bond]CMe) (7) by rinsing with acetonitrile. Addition of 2.2 equiv of KTp* to a toluene solution of 3 followed by heating at 95 degrees C, filtration, and crystallization led to the isolation of the dinuclear species [Tp*UI(dmpz)]2[mu-O] (9) (dmpz = 3,5-dimethylpyrazolide), presumably formed by hydrolytic cleavage of excess KTp* by adventitious water. The Tp* complexes 6, 7, and 9 were characterized by single-crystal X-ray diffraction, NMR, FT-IR, and optical absorbance spectroscopies.

Homoleptic uranium(III) imidodiphosphinochalcogenides including the first structurally characterised molecular trivalent actinide-Se bond.

Preparation of U[N(EPPh2)2]3 (E = S or Se) by treating U[N(SiMe3)2]3 with three equivalents of NH(EPPh2)2 is described together with presentation of a hitherto unreported molecular crystal structure containing An(III)-Se bonds (An = actinide).

Actinide and metal toxicity to prospective bioremediation bacteria.

Bacteria may be beneficial for alleviating actinide contaminant migration through processes such as bioaccumulation or metal reduction. However, sites with radioactive contamination often contain multiple additional contaminants, including metals and organic chelators. Bacteria-based bioremediation requires that the microorganism functions in the presence of the target contaminant, as well as other contaminants. Here, we evaluate the toxicity of actinides, metals and chelators to two different bacteria proposed for use in radionuclide bioremediation, Deinococcus radiodurans and Pseudomonas putida, and the toxicity of Pu(VI) to Shewanella putrefaciens. Growth of D. radiodurans was inhibited at metal concentrations ranging from 1.8 microM Cd(II) to 32 mM Fe(III). Growth of P. putida was inhibited at metal concentrations ranging from 50 microM Ni(II) to 240 mM Fe(III). Actinides inhibited growth at mM concentrations: chelated Pu(IV), U(VI) and Np(V) inhibit D. radiodurans growth at 5.2, 2.5 and 2.1 mM respectively. Chelated U(VI) inhibits P. putida growth at 1.7 mM, while 3.6 mM chelated Pu(IV) inhibits growth only slightly. Pu(VI) inhibits S. putrefaciens growth at 6 mM. These results indicate that actinide toxicity is primarily chemical (not radiological), and that radiation resistance does not ensure radionuclide tolerance. This study also shows that Pu is less toxic than U and that actinides are less toxic than other types of metals, which suggests that actinide toxicity will not impede bioremediation using naturally occurring bacteria.