We report the synthesis and characterization of a new family of pentavalent uranyl amide complexes, supported also by photoluminescence and theoretical investigations. These studies reveal for the first time that the UV-visible emission of uranyl(V) is an admixture of charge transfer transitions accompanied by vibronic coupling of the quartet excited state with uranyl oxo and amide vibrations, thereby offering new insights into the electronic structure of the reactive uranyl(V)

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

We report the preparation of a range of alkali metal uranyl(VI) tri- bis(silyl)amide complexes [{M(THF) x}{(μ-O)U(O)(N″)3}] (1M) (N″ = {N(SiMe3)2}-, M = Li, Na, x = 2; M = K, x = 3; M = K, Rb, Cs, x = 0) containing electrostatic alkali metal uranyl-oxo interactions. Reaction of 1M with 2,2,2-cryptand or 2 equiv of the appropriate crown ether resulted in the isolation of the separated ion pair species [U(O)2(N″)3][M(2,2,2-cryptand)] (3M, M = Li-Cs) and [U(O)2(N″)3][M(crown)2] (4M, M = Li, crown = 12-crown-4 ether; M = Na-Cs, crown = 15-crown-5 ether). A combination of crystallographic studies and IR, Raman and UV-vis spectroscopies has revealed that the 1M series adopts contact ion pair motifs in the solid state where the alkali metal caps one of the uranyl-oxo groups. Upon dissolution in THF solution, this contact is lost, and instead, separated ion pair motifs are observed, which is confirmed by the isolation of [U(O)2(N″)3][M(THF) n] (2M) (M = Li, n = 4; M = Na, K, n = 6). The compounds have been characterized by single crystal X-ray diffraction, multinuclear NMR spectroscopy, IR, Raman, and UV-vis spectroscopies, and elemental analyses.

A series of cyclometallated and functionalised NHC gold(I) and gold(III) complexes, many of which feature chiral ligands, and their application to A3-coupling reactions is presented. Gold(III) complexes were found to be particularly effective catalysts for the coupling in a range of solvents, however no asymmetric induction was obtained when using chiral gold complexes and the rate of product formation was found to be similar even when using different ligand systems. In-situ NMR analysis of these reactions indicates that decomposition of the catalyst occurs during the course of the reaction while TEM studies revealed the presence of gold nanoparticles in crude reaction mixtures. Taken together these data suggest that the gold nanoparticles, rather than the intact gold complexes, could be the catalytically active species, and if so this may have significant implications for other gold-catalysed systems.

Molecular uranium complexes are the most widely studied in actinide chemistry, and make a significant and growing contribution to inorganic and organometallic chemistry. However, reliable computational procedures to accurately describe the properties of such systems are not yet available. In this contribution, 18 experimentally characterized molecular uranium compounds, in oxidation states ranging from III to VI and with a variety of ligand environments, are studied computationally using density functional theory. The computed geometries and vibrational frequencies are compared with X-ray crystallographic, and infra-red and Raman spectroscopic data to establish which computational approach yields the closest agreement with experiment. NMR parameters and UV–vis spectra are studied for three and five closed-shell U(VI) compounds respectively. Overall, the most robust methodology for obtaining accurate geometries is the PBE functional with Grimme's D3 dispersion corrections. For IR spectra, different approaches yield almost identical results, which makes the PBE functional with Grimme's D3 dispersion corrections the best choice. However, for Raman spectra the dependence on functional is more pronounced and no clear recommendation can be made. Similarly, for 1H and 13C NMR chemical shifts, no unequivocal recommendation emerges as to the best choice of density functional, although for spin-spin couplings, the LC-ωPBE functional with solvent corrections is the best approach. No form of time-dependent density functional theory can be recommended for the simulation of the electronic absorption spectra of uranyl (VI) compounds; the orbitals involved in the transitions are not calculated correctly, and the energies are also typically unreliable. Two main approaches are adopted for the description of relativistic effects on the uranium centres: either a relativistic pseudopotential and associated valence basis set, or an all-electron basis set with the ZORA Hamiltonian. The former provides equal, if not better, agreement with experiment vs all-electron basis set calculations, for all properties investigated.

The construction of lanthanide(III) chelates that exhibit superior photophysical properties holds great importance in biological and materials science. One strategy to increase the luminescence properties of lanthanide(III) chelates is to hinder competitive non-radiative decay processes through perfluorination of the chelating ligands. Here, the synthesis of two families of heavily fluorinated lanthanide(III) β-diketonate complexes bearing monodentate perfluorinated tris phenyl phosphine oxide ligands have been prepared through a facile one pot reaction [Ln(hfac)3{(ArF)3PO}(H2O)] and [Ln(F7-acac)3{(ArF)3PO}2] (where Ln = Sm3+, Eu3+, Tb3+, Er3+ and Yb3+). Single crystal X-ray diffraction analysis in combination with photophysical studies have been performed to investigate the factors responsible for the differences in the luminescence lifetimes and intrinsic quantum yields of the complexes. Replacement of both bound H2O and C–H oscillators in the ligand backbone has a dramatic effect on the photophysical properties of the complexes, particularly for the near infra-red emitting ion Yb3+, where a five fold increase in luminescence lifetime and quantum yield is observed. The complexes [Sm(hfac)3{(ArF)3PO}(H2O)] (1), [Yb(hfac)3{(ArF)3PO}(H2O)] (5), [Sm(F7-acac)3{(ArF)3PO}2] (6) and [Yb(F7-acac)3{(ArF)3PO}2] (10) exhibit unusually long luminescence lifetimes and attractive intrinsic quantum yields of emission in fluid solution (ΦLn = 3.4% (1); 1.4% (10)) and in the solid state (ΦLn = 8.5% (1); 2.0% (5); 26% (6); 11% (10)), which are amongst the largest values for this class of compounds to date.

The redox chemistry of the actinide elements plays a central role in many aspects of nuclear fission technology including the reprocessing of spent fuel, safe disposal strategies and in the ability to reliably predict the mobility of actinides in natural and engineered environmental conditions. In both aqueous and non-aqueous conditions, the redox chemistry of the actinides can be complicated and diverse and speciation is governed by both the actinide in question and many environmental factors. Although, historically, actinyl(VI) and (V) ions have been the subject of the most in depth research, the study of actinide ions in the +IV oxidation state (principally for U, Np and Pu) is inherently important in governing speciation in all aspects of the nuclear fuel cycle. Importantly, reactions involving reduction, disproportionation and re-oxidation tend to involve actinide ions in the +IV oxidation state leading to complex systems particularly in aqueous solution that control the solubility and migratory behaviour of actinide containing species. In this review, we focus on recent developments in the coordination and redox chemistry of the actinides involving actinide(IV) species in terms of fundamental coordination chemical studies, mineral chemistry, biogeochemistry and the implications of hydrolysis chemistry on the chemical and physical behaviour of actinide(IV) ions in the natural and engineered environment.

The reaction of Li(2)(C(14)H(18))(TMEDA)(x) with UCl(4) yields U(eta(8)-C(14)H(18))(2), (UPn*(2); Pn* = C(14)H(18)) an analogue of CePn*(2) and U{eta(8)-C(8)H(4)(1,4-Si(i)Pr(3))(2)}(2). The UPn*(2) molecule is structurally characterised via a variety of techniques, its magnetism is probed in the solution and solid phase and the redox properties are investigated using cyclic voltammetry. During this study it was shown to be reducible and the reduced species reacted with N(2)to form a stable complex. An analogous complex was not found under Ar.

Arsenic(V), as the arsenate (AsO4)(3-) ion and its conjugate acids, is strongly sorbed to iron(III) oxides (alpha-Fe2O3), oxide hydroxides (alpha-,gamma-FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO4 tetrahedra and surface FeO6 polyhedra include bidentate corner-sharing (C-2), bidentate edge-sharing (E-2) and monodentate corner-sharing (V-1). We predicted the relative energies and geometries of AsO4-FeOOH surface complexes using density functional theory calculations on analogue Fe-2(OH)(2)(H2O)(n)AsO2(OH)(2)(3+) and Fe-2(OH)(2)(H2O)(n)AsO4+ clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (V-1) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO4)(3-) sorbed to hematite (alpha-Fe2O3), goethite(alpha-FeOOH), lepidocrocite(gamma-FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 Angstrom that have been attributed by previous investigators to result from E-2 and C-2 complexes. However, we show that the peak near 2.85 Angstrom appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 Angstrom As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (C-2) complex. We find no evidence for monodentate (V-1) complexes; this agrees with the predicted high energies of such complexes. Copyright (C) 2003 Elsevier Ltd.

We have identified important chemical reactions that control the fate of metal-contaminated estuarine sediments if they are left undisturbed (in situ) or if they are dredged. We combined information on the molecular bonding of metals in solids from X-ray absorption spectroscopy (XAS) with thermodynamic and kinetic driving forces obtained from dissolved metal concentrations to deduce the dominant reactions under reduced and oxidized conditions. We evaluated the in situ geochemistry of metals (cadmium, chromium, iron, lead, manganese and zinc) as a function of sediment depth (to 100 cm) from a 60 year record of contamination at the Alameda Naval Air Station, California. Results from XAS and thermodynamic modeling of porewaters show that cadmium and most of the zinc form stable sulfide phases, and that lead and chromium are associated with stable carbonate, phosphate, phyllosilicate, or oxide minerals. Therefore, there is minimal risk associated with the release of these trace metals from the deeper sediments contaminated prior to the Clean Water Act (1975) as long as reducing conditions are maintained. Increased concentrations of dissolved metals with depth were indicative of the formation of metal HS- complexes. The sediments also contain zinc, chromium, and manganese associated with detrital iron-rich phyllosilicates and/or oxides. These phases are recalcitrant at near-neutral pH and do not undergo reductive dissolution within the 60 year depositional history of sediments at this site. The fate of these metals during dredging was evaluated by comparing in situ geochemistry with that of sediments oxidized by seawater in laboratory experiments. Cadmium and zinc pose the greatest hazard from dredging because their sulfides were highly reactive in seawater. However, their dissolved concentrations under oxic conditions were limited eventually by sorption to or co-precipitation with an iron (oxy)hydroxide. About 50% of the reacted CdS and 80% of the reacted ZnS were bonded to an oxide-substrate at the end of the 90-day oxidation experiment. Lead and chromium pose a minimal hazard from dredging because they are bonded to relatively insoluble carbonate, phosphate, phyllosilicate, or oxide minerals that are stable in seawater. These results point out the specific chemical behavior of individual metals in estuarine sediments, and the need for direct confirmation of metal speciation in order to constrain predictive models that realistically assess the fate of metals in urban harbors and coastal sediments.

Green rust (Fe4(II)Fe2(III)(OH)12SO4 · 3H2O) is an intermediate phase in the formation of iron (oxyhydr)oxides such as goethite, lepidocrocite and magnetite; current thinking is that it occurs in many soil and sediment systems. Green rust has been shown to reduce sorbed selenate and nitrate and, therefore, might presumeably reduce sorbed arsenate to the more toxic and mobile As(III) species. We have investigated the mechanism of As(V) sorption onto green rust and its fate during oxidation of green rust to lepidocrocite. EXAFS spectroscopy was used to determine the As speciation and coordination environment. We find that As(V) is not reduced to the more mobile and toxic As(III) form following equilibrium with green rust for 24 h. It remains adsorbed as (AsO4)3− by forming inner-sphere surface complexes. The same result is obtained whether As(V) is added prior to or after green rust nucleation. Two different inner sphere surface complexes are resolved: one results from edge-sharing between AsO4 and FeO6 polyhedra while the second results from and double-corner sharing between AsO4 tetrahedra and adjacent FeO6 polyhedra. During the oxidation of green rust to lepidocrocite, the (AsO4)3− remains preferentially bound to green rust and only sorbs onto lepidocrocite when all of the green rust has been oxidized. Sorption onto lepidocrocite occurs via an inner-sphere complex resulting from bidentate corner sharing between AsO4 tetrahedra and adjacent FeO6 octahedra.

Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.