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

Inorg Chem. 2021 Sep 20;60(18):14302-14309. doi: 10.1021/acs.inorgchem.1c01859. Epub 2021 Sep 9.

Abstract

A series of thorium(IV) complexes featuring the redox-active 4,6-di-tert-butyl-N-(2,6-di-isopropylphenyl)-o-iminobenzoquinone (dippiq) ligand family have been synthesized and characterized. The neutral iminoquinone ligand was used to generate Th(dippiq)Cl4(dme)2 (1-iq) and Th(dippiq)2Cl4 (2-iq), 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 (3-isq), which features a radical in each ligand. Further reduction furnishes the amidophenolate species, Th(dippap)3]K2(THF)2 (4-ap), which has the ligands in their dianionic form. Attempts to sequester the potassium ions with cryptand resulted in the [Th(dippap)3K][K(crypt)] (4-ap mono crypt) and [Th(dippap)3][K(crypt)]2 (4-ap crypt) species. A bis(amidophenolate) complex was accessed by incorporating bulky triphenylphosphine oxide (OPPh3) ligands to generate Th(dippap)2(OPPh)3 (5-ap). 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 3-isq 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.