Isolation of a Square-Planar Th(III) Complex: Synthesis and Structure of [Th(OC₆H₂^tBu₂‑2,6-Me-4)₄]^1–

Reduction of Th­(OC6H2tBu2-2,6-Me-4)4 using either KC8 or Li in THF forms a new example of a crystallographically characterizable Th­(III) complex in the salts [K­(THF)5(Et2O)]­[Th­(OC6H2tBu2-2,6-Me-4)4] and [Li­(THF)4]­[Th­(OC6H2tBu2-2,6-Me-4)4]. Surprisingly, in each structure the four aryloxide ligands are arranged in a square-planar geometry, the first example of this coordination mode for an f element complex. The Th­(III) ion and four oxygen donor atoms are coplanar to within 0.05 Å with O–Th–O angles of 89.27(8) to 92.02(8)° between cis ligands. The ligands have Th–O–C­(ipso) angles of 173.9(2) to 178.6(4)°, and the aryl rings make angles of 58.5 to 65.1° with the ThO4 plane. The effect of the eight tert-butyl substituents in generating the unusual structure through packing and/or dispersion forces is discussed. EPR spectroscopy reveals an axial signal consistent with a metal-based radical in a planar complex. DFT calculations yield a C4-symmetric structure that accommodates a low-lying SOMO of 6dz2 character with 7s Rydberg admixture.