Program Pu Futures 2006
The coordination chemistry of plutonium remains relatively unexplored. Thus, the fundamental coordination chemistry of plutonium is being studied using simple multi-dentate ligands with the intention that the information gleaned from these studies may be used in the future to develop plutonium-specific sequestering agents. Towards this goal, hard Lewis-base donors are used as model ligands. Maltol, an inexpensive natural product used in the commercial food industry, is an ideal ligand because it is an all-oxygen bidentate donor, has a rigid structure, and is of small enough size to impose little steric strain, allowing the coordination preferences of plutonium to be the deciding geometric factor. Additionally, maltol is the synthetic precursor of 3,4-HOPO, a siderophore-inspired bidentate moiety tested by us previously as a possible sequestering agent for plutonium under acidic conditions. As comparisons to the plutonium structure, Ce(IV) complexes of the same and related ligands were examined as well. Cerium(IV) complexes serve as good models for plutonium(IV) structures because Ce(IV) has the same ionic radius as Pu(IV) (0.94 {angstrom}). Plutonium(IV) maltol crystals were grown out of a methanol/water solution by slow evaporation to afford red crystals that were evaluated at the Advanced Light Source at Lawrence Berkeley National Laboratory using single crystal X-ray diffraction. Cerium(IV) complexes with maltol and bromomaltol were crystallized via slow evaporation of the mother liquor to afford tetragonal, black crystals. All three complexes crystallize in space group I4{sub 1}/a. The Ce(IV) complex is isostructural with the Pu(IV) complex, in which donating oxygens adopt a trigonal dodecahedral geometry around the metal with the maltol rings parallel to the crystallographic S{sub 4} axis and lying in a non-crystallographic mirror plane of D{sub 2d} molecular symmetry (Fig 1). The metal-oxygen bonds in both maltol complexes are equal to within 0.04 {angstrom} for each oxygen type. In contrast to the maltol structures, the cerium(IV) bromomaltol complex arranges the maltol rings in a drastically different manner while maintaining the S{sub 4} crystallographic symmetry (Fig 2). The coordination geometry around the cerium remains a trigonal dodecahedron, but the chelating ligands span a different set of edges as in the maltol structures; the two-fold related bromomaltol ligands twist away from planarity, breaking the D{sub 2d} molecular symmetry. It is unlikely that steric interaction with a bromine on the same molecule would have caused the observed rearrangement, as there would be sufficient separation between them to accommodate their bulk in the geometry of the plutonium and cerium maltol complexes. The extended packing in the unit cell of both the plutonium and cerium maltol crystals indicates that pi stacking occurs throughout the lattice via the maltol rings with close contacts between rings of approximately 3.6 {angstrom}. Introduction of the bromine to this structure would disrupt the packing that would allow these interactions, causing the molecule to adopt the geometry present in the bromomaltol structure. In this unexpected arrangement the complex is still able to maintain some pi stacking with the maltol rings of adjacent molecules with a close contact of approximately 3.3 {angstrom}. Additionally, the bromine on each ligand is arranged such that its next closest contact is with a bromine 3.64 {angstrom} away on another molecule. Despite the different ligand geometry, the bromomaltol structure exhibits metal-oxygen bond distances that are within 0.06 {angstrom} of those in the maltol complexes.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 898020
- Report Number(s):
- UCRL-PROC-222342; TRN: US200706%%198
- Resource Relation:
- Conference: Presented at: Program for Pu Futures 2006, Pacific Grove, CA, United States, Jul 09 - Jul 13, 2006
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
43 PARTICLE ACCELERATORS
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ADVANCED LIGHT SOURCE
BROMINE
CERIUM
CHEMISTRY
EVAPORATION
FOOD INDUSTRY
GEOMETRY
MIRRORS
MONOCRYSTALS
OXYGEN
PLUTONIUM
PRECURSOR
SPACE GROUPS
SYMMETRY
X-RAY DIFFRACTION