David A. Costa

Synthesis and structure of N-heterocyclic carbene complexes of uranyl dichloride

Treatment of UO2Cl2(thf)3 in THF with two equivalents of 1,3-dimesitylimidazole-2-ylidene (IMes) or 1,3-dimesityl-4,5-dichloroimidazole-2-ylidene (IMesCl2) yields novel monomeric uranyl N-heterocyclic carbene complexes, representing the first examples of actinyl carbon bonds.

Chloroheptakis(dimethyl sulfoxide)­uranium(IV) trichloride

In the title complex, [UCl(C(2)H(6)OS)(7)]Cl(3), the uranium metal center is coordinated in a distorted bicapped trigonal prism geometry by seven O atoms from dimethyl sulfoxide ligands and by a terminal chloride ligand. Charge balance is maintained by three outer-sphere chloride ions per uranium(IV) metal center. Principle bond lengths include U-O 2.391 (2)-2.315 (2) A, U-Cl 2.7207 (9) A, and average S-O 1.540 (5) A.

Electrorecovery of Actinides at Room Temperature

There are a large number of purification and processing operations involving actinide species that rely on high-temperature molten salts as the solvent medium. One such application is the electrorefining of impure actinide metals to provide high purity material for subsequent applications. There are some drawbacks to the electrodeposition of actinides in molten salts including relatively low yields, lack of accurate potential control, maintaining efficiency in a highly corrosive environment, and failed runs. With these issues in mind we have been investigating the electrodeposition of actinide metals, mainly uranium, from room temperature ionic liquids (RTILs) and relatively high-boiling organic solvents. The RTILs we have focused on are comprised of 1,3-dialkylimidazolium or quaternary ammonium cations and mainly the {sup -}N(SO{sub 2}CF{sub 3}){sub 2} anion [bis(trif1uoromethylsulfonyl)imide {equivalent_to} {sup -}NTf{sub 2}]. These materials represent a class of solvents that possess great potential for use in applications employing electrochemical procedures. In order to ascertain the feasibility of using RTILs for bulk electrodeposition of actinide metals our research team has been exploring the electron transfer behavior of simple coordination complexes of uranium dissolved in the RTIL solutions. More recently we have begun some fundamental electrochemical studies on the behavior of uranium and plutonium complexes in the organic solvents N-methylpyrrolidone (NMP) and dimethylsulfoxide (DMSO). Our most recent results concerning electrodeposition will be presented in this account. The electrochemical behavior of U(IV) and U(III) species in RTILs and the relatively low vapor pressure solvents NMP and DMSO is described. These studies have been ongoing in our laboratory to uncover conditions that will lead to the successful bulk electrodeposition of actinide metals at a working electrode surface at room temperature or slightly elevated temperatures. The RTILs we have focused on thus far are based on 1,3-dialkylimidazolium or quaternary ammonium cations and {sup -}N(SO{sub 2}CF{sub 3}){sub 2} anions. Our results from XPS studies of e1ectrooxidized uranium metal surfaces indicate that uranium metal reacts with the anion from the RTIL, most likely through an initial f1uoride abstraction, forming decomposition products that inhibit the bulk electrodeposition of uranium metal. Similar results were found when the organic solvents were used with TBA[B(C{sub 6}F{sub 5}){sub 4}] as the supporting electrolyte, although the voltammetric data of uranium ions in these solutions is more encouraging in relation to electrodeposition of uranium metal. Preliminary results on the voltammetric behavior and bulk electrodeposition of plutonium species are also presented.

Lewis base binding affinities and redox properties of plutonium complexes

As part of the actinide molecular science competency development effort, the initial goal of this work is to synthesize and investigate several series of complexes, varying by actinide metal, ligand set, and oxidation state. We are examining the reactivity of plutonium and neptunium organometallic complexes to elucidate fundamental chemical parameters of the metals. These reactions will be compared to those of the known corresponding uranium complexes in order to recognize trends among the actinide elements and to document differences in chemical behavior.

Actinide chemistry in room temperature ionic liquids: actinide chemistry in RTIL systems (why?)

Room temperature ionic liquids (RTILs) have potential throughout the nuclear industry in the recovery and purification of actinide elements, as reactor components, as waste disposal forms, and potentially as media for the storage and/or separation of spent nuclear fuels. Due to their unique dissolution properties, RTILs can be used as substitutes for solvents currently used in the extraction of uranium from native ores, and in the dissolution and reprocessing of spent nuclear fuels. Research efforts in our laboratory focus on determining the chemical properties (i.e., solubility, complexation, redox properties, etc.) of actinide species in RTIL systems. We are currently involved in RTIL projects ranging from the spectroscopic characterization of actinide complexes by O17NMR, low temperature UV-Vis, and EXAFS, to the enhanced dissolution and separation of actinide oxides in room temperature ionic liquids.

Spectroscopy of UO2Cl42− in basic aluminum chloride: 1-ethyl-3-methylimidazolium chloride

In this study we focus on the spectroscopic characterization of UO2Cl42− in 40:60 AlCl3:EMIC using multiple techniques. A combination of absorption, emission, excitation, two-photon excitation, Fourier transform Raman, and time-resolved emission spectra for solution (298 K) and frozen glass (75 K) samples have been measured and analyzed in terms of the electronic and vibrational structures of the UO2Cl42− ion. The spectroscopic properties of UO2Cl42− in single crystals are well known, and the analyses of the spectra of UO2Cl42− in 40:60 AlCl3:EMIC rely heavily on single crystal studies. Results are also compared with absorption spectra from a previous study of UO2Cl42− in 48:52 AlCl3:EMIC (a basic ionic liquid). The results from multiple spectroscopic measurements and their comparison with previous results help provide a picture of the structure and environment of UO2Cl42− in 40:60 AlCl3:EMIC.