Actinide Coordination Chemistry
The use of actinides in power production, as ballast in ships and airplanes, in ceramics, in radiation shielding, and as a heat and fuel sources in space exploration has resulted in a growing list of potential environmental and health concerns. Close to one-third of the world's electrical power is produced using nuclear fuels, and the challenge of limiting the environmental and health effects from contamination, be it accidental or sabotage, is of increasing concern. Complicating the problem is our relatively limited understanding (in comparison to transition metals) of the solution and coordination behavior of the actinides.
The similarities in the chemical properties and the biological transport and distribution properties of Fe(III) and Pu(IV) explain much of the behavior of plutonium in vivo. In particular, Fe(III) and Pu(IV) have similar charge to ionic size ratios and hydrolysis properties. These similarities inspired a biomimetic approach to the development of preorganized multidentate sequestering agents for Pu(IV) based on the chelating units found in siderophores, naturally occurring highly selective Fe(III) sequestering agents.
Gorden, A. E. V.; Xu, J.; Raymond, K. N.; Durbin, P. W. "The Rational Design of Sequestering Agents for Plutonium and other Actinide Elements." Chem. Rev. 2003, 103, 4207-4282.
The aim of this project is to continue to study the coordination and solution behaviors of ligands for actinide decorporation and selective extraction based on siderophores. This project is the result of a collaborative effort in inorganic chemistry, synthetic organic chemistry, and actinide biology, and incorporates the design, synthesis, structural and thermodynamic characterization of ligands and their metal complexes with actinides and lanthanides and the evaluation of these compounds in metal ion separations. We use a variety of characterization techniques including spectrophotometric titrations, extractions, Extended X-ray Absorbance Fine Structure and X-ray diffraction.
Actinide Decorporation Agents
Effective chelation therapy has been shown to reduce acute radiation damage, chemical toxicity, and long-term radiation effects in patients who have taken actinides internally. Since 1978, in collaboration with Patricia Durbin of Lawrence Berkeley National Laboratory, the Raymond group has produced and evaluated over 60 multidentate ligands consisting of chelating subunits based on those found in siderophores attached to a variety of molecular backbones. Biological evaluation of the efficacy and toxicity of these ligands has enabled us to develop our understanding the relationships underlying the overall effectiveness of a ligand for actinide chelation in vivo (e.g., denticity, binding group acidity, backbone flexibility, solubility). Several ligands containing catecholate (CAM) or hydroxypyridinonate (HOPO) binding groups have also been found to chelate other actinides and identified as agents suitable for in vivo chelation of Am(III), Np(IV) or U(VI).
Guilmette, R. A.; Hakimi, R.; Durbin, P. W.; Xu, J.; Raymond, K. N. "Competitive Binding of Pu and Am with bone mineral and novel chelating agents." Rad. Prot. Dosim. 2003, 105, 527-534.
Xu, J.; Durbin, P. W.; Kullgren, B.; Ebbe, S. N.; Uhlir, L. C.; Raymond, K. N. "Synthesis and Initial Evaluation for In Vivo Chelation of Pu(IV) of a Mixed Octadentate Spermine-Based Ligand Containing 4-Carbamoyl-3-hydroxy-1-methyl-2(1H)-pyridinone and 6-Carbamoyl-1-hydroxy-2(1H-pyridinone)." J. Med. Chem. 2002, 45, 3963-3971.
Actinide Coordination Chemistry
While several siderophore-inspired ligands have been tested for in vivo chelation of Pu(IV), little is known about the detailed coordination behavior of these ligands with actinides. To elucidate this chemical behavior, crystals of HOPO ligands and their analogs bound to various actinides are being grown and analyzed by single crystal X-ray diffraction. Complexes of Ce(IV) and Th(IV) are also being investigated as models for Pu(IV). This work complements potentiometric and spectrophotometric titrations of these ligands in solution.
Crystal structure of Pu(1,2-HOPO)4
The coordination chemistry of the linear uranyl cation (UO22+) is being investigated in a similar fashion; multidentate ligands incorporating HOPO and terephalamide (TAM) chelating moieties are being synthesized and studied crystallographically. These efforts seek to develop strongly-binding ligands that have the shape-recognition properties ideal for selective sequestering of linear actinyl cations in the presence of spherical cations, whether in radioactive waste or biological decorporation applications. Research of this type is fundamental to our understanding of the coordination chemistry and speciation behavior of the actinides, which will be of significance in the development of actinide sequestering agents, waste treatment methods, and long-term strategies for nuclear waste repositories.
Crystal structure of UO2(ophen-bis-Me-3,2-HOPO)(DMSO)
Szigethy, G.; Xu, J.; Gorden, A. E. V.; Teat, S. J.; Shuh, D. K.; Raymond, K. N. "Surprising Coordination Geometry Differences in Ce(IV)- and Pu(IV)-Maltol Complexes," Eur. J. Inorg. Chem. 2008, 2143-2147.
Gorden, A. E. V.; Xu, J.; Szigethy, G.; Oliver, A.; Shuh, D. K.; Raymond, K. N. "Characterization of a Mixed Salt of 1-Hydroxypyridin-2-one Pu(IV) Complexes." J. Am. Chem. Soc., 2007, 129, 6674-6675.
Gramer, C. J.; Raymond, K. N. "Characterization of 2,3-Dihydroxyterephthalamides as M(IV) Chelators." Inorg. Chem. 2004, 43, 6397-6402.
Xu, J.; Whisenhunt Jr., D. W.; Veeck, A. C.; Uhlir, L. C.; Raymond, K. N. "Thorium(IV) Complexes of Bidentate Hydroxypyridinonates." Inorg. Chem. 2003, 42, 2665-2674.
Xu, J.; Raymond, K. N. "Uranyl Sequestering Agents: Correlation of Properties and Efficacy with Structure for UO22+ Complexes of Linear Tetradentate 1-Methyl-3-hydroxy-2(1H)-pyridinone Ligands." Inorg. Chem. 1999, 38, 308-315.
Environmental Remediation and Extractants
Many of the radioactive wastes present at DOE long-term storage sites are large mixtures of metals with actinides in solution. The selective removal of the actinides, Cs, and Sr, the predominant radioactive elements in these waste streams, would concentrate the volume of high-level waste and reduce the amount of waste requiring disposal in a geological repositories. Ligands incorporating terephthalamide and hydroxypyridonate chelating subunits have been tested as actinide extractants under acidic waste conditions and could perhaps also be useful in the development of extraction agents of other metals.
Veeck, A. C.; White, D. J.; Whisenhunt Jr., D. W.; Xu, J.; Gorden, A. E. V.; Romanovski, V.; Hoffman, D. C.; Raymond, K. N. "Hydroxypyridinone Extraction Agents for Pu(IV)." Sep. Sci. and Technol. 2004 39, 1332-1337.
Xu, J.; Gorden, A. E. V.; Raymond, K. N. "Octadentate 2,3-Dihydroxybenzamide and 2,3-Dihydroxyterephthalamide Ligands for the Chelation of Actinides." Eur. J. Org. Chem. 2004, 3244-3253.
Gramer, C. J.; Raymond, K. N.; Jarvinen, G. D.; Schroeder, N. C.; Robison, T. W.; Smith, B. F. "The Removal of Dilute Concentrations of Pu(IV) from Waste Streams Using 2,3-Dihydroxyterephthalamide-Functionalized PEI and Polymer Filtration." Sep. Sci. and Technol. 2003, 39, 321-339.Video (Thphi22)