The NMR techniques: a powerful method to study solution structure of new agents for actinides/lanthanides extraction

audience: researcher, studen

Nuclear Magnetic Resonance and Nuclear Waste Reprocessing.

audience: researcher, studen

NMR studies on radioactive actinide complexes. A completely foolish idea?

The complete separation of actinides and lanthanides by solvent extraction is an important step in the reprocessing of nuclear wastes. The trivalent ions of these two families of f elements have very similar properties and it is only recently that an effective separation method has been developed. It is assumed that this separation can be achieved because of small differences in the covalency of the coordination bonds formed by the f ions but this has not been proved so far. We embarked into the first NMR study of the actinide complexes with the hope of better unravelling their solution structures and of clarifying the role of covalency. Nuclear magnetic resonance has nearly never been used for investigating actinide ions and their complexes not only because of their radioactivity and their toxicity but also because of the lack of dedicated spectrometers. It was also assumed that paramagnetism would cause excessive line broadenings. Many difficulties had thus to be overcome and the first step in our work was to show that well-resolved NMR spectra of actinide complexes (U to Cm) could indeed be obtained. Relatively narrow resonances have been observed for a variety of ions in different oxidation states provided their complexes are stable, symmetric and rigid. We used several advanced NMR techniques in order to fully characterize the actinide ions and their chelates in water or in organic solvents. The dispersion of the longitudinal relaxation time T1 of solvent nuclei with the magnetic field (NMRD) yields information on the magnetic properties and on the dynamic behaviour of paramagnetic species; 17O NMR allows the measurement of the water exchange times and 1H and 13C spectra yield information on the solution structures of the complexes. It will be shown that the paramagnetic shifts induced by the actinide ions originate from both a through space dipolar contribution and a through bonds contact contribution. The latter gives access to delocalized unpaired electron densities that are directly related to the covalency of the metal-ligand bonds. However, electron densities can only be obtained after separation of the two contributions. This could be accomplished thanks to variable temperature studies. In another approach, new ligands with rigid aliphatic substituents have been synthesized and contact contribution were deduced with the assumption that delocalization does not proceed to the 1H nuclei most removed from the metal center. This study has been extended to actinides in the trivalent, tetravalent and hexavalent states and new ligands have been synthesized in order to increase the magnetic anisotropy while keeping the symmetry and the rigidity needed to simplify the NMR analyses

Nuclear magnetic resonance applied to actinide ions and their complexes. In search of covalency effects

ACSPE

A gadolinium triacetic monoamide DOTA derivative with a methanethiosulfonate anchor group. Relaxivity properties and conjugation with albumin and thiolated particles

The gadolinium(III) complex with a new DOTA-based ligand bearing a methanethiosulfonate group (MTS) was synthesized and its relaxivity properties were investigated. MTS-ADO3A is a triacid DOTA derivative with an amide arm substituted by an ethylmethanethiosulfonate function. This ligand was obtained in two steps: tri-tert-butyl 2,2′,2″-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetate was reacted with S-(2-aminoethyl)methanesulfonothioate and the tert-butyl groups were removed with trifluoroacetic acid. The Gd(III) MTS–ADO3A complex readily formed disulfide bonds with albumin (BSA) in its native and reduced forms and with thiolated silica particles. Four- to five-fold relaxivity increases at 20 MHz were measured on the isolated adducts. The EuMTS-ADO3A chelate was found to be monohydrated by fluorescence and the relaxivity parameters of the Gd(III) complex were obtained by 17O NMR and by measuring the nuclear magnetic relaxation dispersion between 0.01 and 80 MHz. The water exchange time τm is increased upon forming disulfide bonds with macromolecules and particles and the relaxivity gains of all the complexes are limited by the τm factor. Forming covalent or hydrophobic/electrostatic bonds with BSA seems to bring about similar relaxivity changes but the covalent BSA adducts can be isolated and their properties can be directly studied. The addition of dithiothreitol or glutathione leads to the removal of the metal chelates from the macromolecules, as indicated by the relaxation times reverting to their values before binding. It is thus expected that the chelate will stay in the body long enough for imaging but will still be excreted through the kidneys