Using Redox-Active Ligands to Generate Actinide Ligand Radical Species

Abstract

Using a redox-active dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-<i>tert</i>-butyl-1-oxo-1<i>H</i>-phenoxazine-9-olate), a family of actinide (U, Th, Np, and Pu) and Hf tris(ligand) coordination compounds was synthesized. The full characterization of these species using ¹H NMR spectroscopy, electronic absorption spectroscopy, SQUID magnetometry, and X-ray crystallography showed that these compounds are analogous and exist in the form M(DOPO^q)₂(DOPO^sq), where two ligands are of the oxidized quinone form (DOPO^q) and the third is of the reduced semiquinone (DOPO^sq) form. The electronic structures of these complexes were further investigated using CASSCF calculations, which revealed electronic structures consistent with metals in the +4 formal oxidation state and one unpaired electron localized on one ligand in each complex. Furthermore, f orbitals of the early actinides show a sizable bonding overlap with the ligand 2p orbitals. Notably, this is the first example of a plutonium-ligand radical species and a rare example of magnetic data being recorded for a homogeneous plutonium coordination complex.