Elucidation of Thorium Redox-Active Ligand Complexes: Evidence for a Thorium-Tri(radical) Species

Abstract

A series of thorium(IV) complexes featuring the redox-active 4,6-di-<i>tert</i>-butyl-<i>N</i>-(2,6-di-isopropylphenyl)-<i>o</i>-iminobenzoquinone (^dippiq) ligand family have been synthesized and characterized. The neutral iminoquinone ligand was used to generate Th(^dippiq)Cl₄(dme)₂ (<b>1-iq</b>) and Th(^dippiq)₂Cl₄ (<b>2-iq</b>), both of which show dative bonds between the thorium(IV) ion and the ligands. One electron reduction of the ligand forms the unique tris(iminosemiquinone) complex, Th(^dippisq)₃Cl (<b>3-isq</b>), which features a radical in each ligand. Further reduction furnishes the amidophenolate species, Th(^dippap)₃]K₂(THF)₂ (<b>4-ap</b>), which has the ligands in their dianionic form. Attempts to sequester the potassium ions with cryptand resulted in the [Th(^dippap)₃K][K(crypt)] (<b>4-ap mono crypt</b>) and [Th(^dippap)₃][K(crypt)]₂ (<b>4-ap crypt</b>) species. A bis(amidophenolate) complex was accessed by incorporating bulky triphenylphosphine oxide (OPPh₃) ligands to generate Th(^dippap)₂(OPPh)₃ (<b>5-ap</b>). Spectroscopic and structural characterization of each derivative established the +4 oxidation state for thorium with redox chemistry occurring at the ligands rather than the thorium ion. The reported <b>3-isq</b> complex is unprecedented as it is the first tri(radical) thorium complex with the highest reported magnetic moment for a thorium species as characterized by SQUID magnetometry.