N-Donor Ligands as Versatile Actinide Complexation Agents

Actinide coordination chemistry has attracted chemists´ interest for decades in terms of nuclear safety, catalysis, or radiopharmaceutical research. Especially the early actinides exhibit an unusually large range of oxidation states, which originates in the near degeneracy of the 5f, 6d and 7s energy levels. This offers unique chemical and physical properties, not yet fully understood. Schiff bases like the mixed N, O donor ligands of the salen (Bis(salicyliden)ethylendiamine) family are frequently chosen systems for complexation studies, because of their advantageous ability to stabilize a large number of metals including actinides, as well as their tuneable electronical and sterical properties. Based on pyrrole structural relatives of salen can be constructed, which only exhibit N-donor functionalities. This provides the possibility to investigate and compare the binding situation between early actinides and N atoms in different environments.
In this study a complex series with Th and U as well as the transuranic elements Np and Pu with the pyrrole-based ligand 1,2-ethylenediamine-N,N’-bis(1H-pyrrol-2-yl)methylene (H2pyren) H2L1 was synthesized. Characterization in solid state (via SC-XRD) shows an 8-fold coordination at the metal centre, where all N donors participate. Moreover, the An−Npyrrolide bond distance is in all cases shorter, compared to the An−Nimine bond. Quantum chemical calculations, including Pa(IV) prove not only the bond shortening, but also an increased bond strength for An−Npyrrolide over An−Nimine. Comparison to the salen system shows preference of the mixed N,O donor ligand over its pure N,N donor relative for Th as well as U-Pu. For Pa, however, the pyren ligand is preferred over salen, pointing to a special role of the f 1 electron configuration in actinide(IV) coordination chemistry. Synthesis of a mixed pyren salen complex shows a significant expansion of the 1H NMR shift range and altered signal position compared to the bis complexes. This indicates a possible reorientation of the easy axis of magnetization upon ligand exchange. Further investigations on the influence of ligand substitution is achived by synthesis of the mono pyren complex [UCl2(py)(pyren)] and comparison to the analogous salen complex.