Using a redox-active dioxophenoxazine
ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-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(DOPOq)2(DOPOsq), where two ligands are of the oxidized quinone
form (DOPOq) and the third is of the reduced semiquinone
(DOPOsq) 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.