The similar charge/ionic radius ratios of iron(III) and plutonium(IV) allow for the utilization of strong iron chelators for a basis of actinide ligand design. Ferrichrome, a tris-hydroxamate siderophore (biological ligand used primarily for iron uptake in bacteria), has been the inspiration the design of potent ligands for actinides. We are interested in two topologies for polyhydroxamate ligands. First, although ferrichrome is a cyclic peptide, in 70 years of research encompassing hundreds of reported analogs, almost no peptide analogs of it have ever been made/studied leaving many basic questions unanswered. For example, why are the three chelating arms in ferrichrome adjacent instead of symmetrically distributed around the cyclic peptide hexamer? Can larger, cyclic octamers be made that would make better ligands for f-block metals? Although cyclization of peptides remains challenging, peptoids (N-substituted glycine oligomers), allow for a straightforward modular synthesis of close ferrichrome analogs which incorporate hydroxamate-containing residues into a cyclic backbone. Second, EDTA is a powerful chelator but is marginally too small even for iron(III), leaving a coordinated water. A decadentate hydroxamate EDTA-analog that replaces all the carboxylates with hydroxamates may be even smaller for a lanthanide or actinide. In that case a larger homolog may function better, but at what length? We have synthesized several such ligands varying the side chain length (1-3 methylenes) and rigidity (with or without an aromatic spacer) and are characterizing their lanthanide and actinide complexes via potentiometric titrations.

Precise control of polyhydroxamate ligand topology for selective actinide coordination

254th National Meeting (2017)

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