Abstract
Currently, the leading approach for extraction of uranium from seawater is selective sorption of UO22+ ions onto a poly(acrylamidoxime) fiber. Amidoxime functional groups are the most studied extractant moieties for this application, but are not perfectly selective, and understanding how these groups (and more generally the oxime groups) interact with UO22+ and competing ions in seawater is an important step in designing better extractants. We have started a new research programme aiming at in-depth studies of the uranyl-oxime/amidoxime interactions and we report here our first results which cover aspects of the coordination chemistry of 2-pyridyl ketoximes towards UO22+. Detailed synthetic investigations of various UO22+/mepaoH and UO22+/phpaoH reaction systems (mepaoH is methyl 2-pyridyl ketoxime and phpaoH is phenyl 2-pyridyl ketoxime) have provided access to the complexes [UO2(mepao)2(MeOH)2]{[UO2(NO3)(mepao)(MeOH)2]}2 (1), [UO2(mepao)2(MeOH)2] (2), [(UO2)2(O2)(O2CMe)2(mepaoH)2] (3) and [UO2(phpao)2(MeOH)2] (4). The peroxido group in 3, which was isolated without the addition of external peroxide sources, probably arises from a bis(aquo)- and/or bis(hydroxido)-bridged diuranyl precursor in solution followed by photochemical oxidation of the bridging groups. The UVI atom in the [UO2(NO3)(mepao)(MeOH)2] molecules of 1 (1A) is surrounded by one nitrogen and seven oxygen atoms in a very distorted hexagonal bipyramidal geometry; two oxygen atoms from the terminal MeOH ligands, two oxygen atoms from the bidentate chelating nitrato group, and the oxygen and nitrogen atoms from the η2 oximate group of the 1.110 (Harris notation) mepao− ligand define the equatorial plane. This plane consists of two terminal MeOH ligands and two η2 oximate groups in the [UO2(mepao)2(MeOH)2] molecule (1B) of 1. The structure of the [UO2(mepao)2(MeOH)2] molecule that is present in 2 is very similar to the structure of the corresponding molecule in 1. The structure of the dinuclear molecule that is present in 3 consists of two {UO2(O2CMe)(mepaoH)}+ units bridged by a η2:η2:μ O22− group. The equatorial plane of each uranyl site is composed of the pyridyl and oxime nitrogen atoms of a 1.011 mepaoH ligand, the oxygen atoms of an almost symmetrically coordinated bidentate chelating MeCO2− group and the two oxygen atoms of the peroxido groups. The core molecular structure of 4 is similar to that of 2, the only difference being the presence of 1.110 phpao− ligands in the former instead of mepao− groups in the latter. The free pyridyl nitrogen atoms of mepao− and phpao− ligands of 1, 2 and 4 are acceptors of intramolecular H bonds from the ligated MeOH oxygen atoms. H-bonding and π–π stacking interactions build interesting supramolecular networks in the crystal structures of the four complexes. Compounds 1–4 are the first structurally characterized uranyl complexes with 2-pyridyl aldoximes or ketoximes as ligands. IR data are discussed in terms of the coordination modes of the ligands in the complexes. 1H NMR data in DMSO-d6 suggest that the complexes decompose in solution. The ESI(−) MS spectrum of 4 dissolved in the NH4(O2CMe) buffer is indicative of the presence of [UO2(O2CMe)3]−, [UO2(O2CMe)2(phpao)]−, [UO2(O2CMe)(phpao)2]− and [UO2(phpao)3]− species. A common structural motif of the complexes containing the anionic mepao− (1, 2) and phpao− (4) ligands is that the deprotonated oximate group prefers to bind in the η2 fashion forming a 3-membered chelating ring in spite of the presence of a pyridyl nitrogen atom, whose coordination would be expected to lead to 5- or 6-membered chelating rings.
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