Study of innovative 1,10 phenanthroline derivatives for An partitioning from nuclear waste : viable synthesis paths and screening of extraction behaviour

Abstract: the activity here presented is focused on the study of innovative ligands for the partitioning of minor actinides from nuclear waste within the overall framework of the EU project ACSEPT. The work has been focused more on the development of viable synthesis paths for the considered ligands, searching to comprehend and address the critical issues that have been encountered. We found that the first synthesis path attempted for the lipophilic molecule 2OXA4CO-PHEN[2,9-bis(2-oxapropyl-tert-butylketone)-1,10-phenanthroline] is not viable, for unexpected reactivity problems. The precursor neocuproine has been firstly oxidized to PHEN-dialdehyde and then reduced to PHEN-dialcohol. The final Williamson ether synthesis of the latter with the primary halide 1-bromopinacolone has not given results in any of the different tried procedures. Williamson pilot syntheses with different organic halides showed that the system is unexpectedly not reactive in case primary halides are used, while it is in case of halides giving a (benzyl-) carbocation stabilized by resonance. A valid synthesis path can be thus found by reacting the higher active PHEN-Br (carbocation stabilized by resonance) with 1-hydroxypinacolone. Then the second molecule considered is the hydrophilic complexant PHEN-6OH [N2,N9-bis (1,3-dihydroxy-2- (hydroxymethyl) propan-2-yl) -1,10-phenanthroline-2,9-dicarboxamide]. The attempted synthesis with different coupling agents have highlighted the need to protect the hydroxyl groups from the reactant TRIS to avoid esteric side reaction to take place. Due to the lack of equipment for performing especially the deprotection step (hydrogenation on palladium catalyst) the molecule has been completed months later through a collaboration with Prof. Casnati’s group, in Università degli Studi di Parma. Anyway, qualitative extraction tests have been performed on the highly impure products from the attempted syntheses. The results hint for interesting extraction properties for this class of complexants, confirmed in later studies by the rigorous tests on the finally synthesized PHEN-6OH. Finally, other screening tests on reaction intermediates to gather information on possible extracting properties (within the 1-10 phenanthroline derivatives family) highlighted the interesting features of 2,9-bis(hydroxymethil)-1,10-phenanthroline (PHEN-dialcohol). The molecule has shown even an SF > 20 with a DAm < 1. Further studies are the most encourage considering also the extreme synthesis simplicity and low cost, important for a possible industrial application.