Designing the Ideal Uranyl Ligand: a Sterically-Induced Speciation Change in Complexes with Thiophene-Bridged Bis(3-hydroxy-N-methylpyridin-2-one)

Designing the ideal uranyl ligand: a sterically-induced speciation change in complexes with thiophene-bridged, bis(3-hydroxy-N-methyl-pyridin-2-one) 1 Geza Szigethy, Kenneth N. Raymond* Department of Chemistry, University of California at Berkeley, Berkeley, CA, 94720-1460, USA. Chemical Science Division, Glenn T. Seaborg Center, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA RECEIVED DATE (will be automatically inserted after manuscript is accepted) Structural characterization of a mononuclear uranyl complex with a tetradentate, thiophene-linked bis(N- methyl-3-hydroxy-pyridin-2-one) ligand reveals the most planar coordination geometry yet observed with this ligand class. Introduction of ethylsulfanyl groups onto the thiophene linker disrupts this planar, conjugated ligand arrangement, resulting in the formation of dimeric (UO 2 ) 2 L 2 species in which each ligand spans two uranyl centers. Relative energy calculations reveal this tendency toward dimer formation is the result of steric interference between ethylsulfanyl substitutents and linking amides. While nuclear power is attractive as a carbon-free energy source, the safe use of this technology requires both a low risk of contamination of environmental or biological systems with radioactive elements and the ability to deal with such contamination if it occurs. 2 Ligands that can efficiently chelate and remove actinides from the environment or in vivo are being developed. 3 Because uranium is the feed stock material of most nuclear power sources and is the most abundant naturally-occurring actinide, uranium chelation is of particular interest. Uranium in oxidizing conditions and in vivo typically adopts a hexavalent oxidation state, in which it exists as a linear, dioxo dication (uranyl, UO 22+ ) 4 that is poorly decorporated by polyaminocarboxylic acids. 3 Unlike transition metal dioxo species, the uranyl cation maintains linearity to within a couple degrees in all of its coordination complexes, relegating coordinative variation to an equatorial plane perpendicular to the O=U=O vector. Exceptions to this behavior typically involve bulky ligands (e.g. Cp 5 or large NCN or NPN ligands 6 ) in which the uranyl cation may deviate more than 11° from linearity and coordinating atoms distort out of the equatorial coordination plane. The apical oxo moieties are essentially non-reactive and are typically only observed to interact with Lewis acids in the solid state and in appropriately designed macrocyclic systems. 7-11 These properties make the uranyl cation a challenging target for selective chelation. Recent work in our laboratory towards developing uranyl-specific chelators has focused on the use of poly- bidentate, oxygen-donating ligands incorporating synthetic analogs to siderophore chelating moieties, which are known to form high-affinity complexes with hard Lewis- acidic f-elements. 12 Xu et al. demonstrated that 3-hydroxy- N-methyl-pyridin-2-one (Me-3,2-HOPO) ligands bind the uranyl cation at four points of an equatorial pentagonal plane completed by solvent molecule coordination. 13 Chelator orientations about the uranyl are seen to depend strongly on the length of the linear ligand linker. In these complexes an intramolecular N amide -H∙∙∙O phenolate hydrogen- bonding interaction is responsible for stabilization of the deprotonated and metal-chelated ligands 14 and is optimized in ligands utilizing short, flexible linkers. 13 Figure 1. Bis-Me-3,2-HOPO ligands 3,4-thiophene-Me-3,2-HOPO (L 1 H 2 ) and 2,5-bis-ethylsulfanyl-3,4-thoiophene-Me-3,2-HOPO (2). To explore the structural effect of linker rigidity, the uranyl complexes with two bis-Me-3,2-HOPO ligands incorporating short, rigid linkers [3,4-thiophene-Me-3,2- HOPO (L 1 H 2 ) and 2,5-bis-ethylsulfanyl-3,4-thoiophene- Me-3,2-HOPO (L 2 H 2 ), Figure 1] were synthesized. The uranyl complex with L 1 is expected to exhibit a severely restricted coordination geometry, while that with L 2 is intended to explore the effect of 2,5-disubstitution on the thiophene ring such as may be employed in attaching solubilizing groups or linkers to L 1 (some degree of substituent torsion such as described by Lai et al. is expected). 15 In both cases, the short, relatively inflexible