On the border between localization and delocalization: tris(iminoxolene)titanium(iv)

Abstract

The tris(aminophenol) ligand tris(4-methyl-2-(3′,5′-di-tert-butyl-2′-hydroxyphenylamino)phenyl)amine, MeClampH₆, reacts with Ti(OⁱPr)₄ to give, after exposure to air, the dark purple, neutral, diamagnetic complex (MeClamp)Ti. The compound is six-coordinate, with an uncoordinated central nitrogen (Ti–N = 2.8274(12) Å), and contains titanium(IV) and a doubly oxidized ligand, formally a bis(iminosemiquinone)-mono(amidophenoxide). The compound is unsymmetrical in the solid state, though the three ligands are equivalent on the NMR timescale in solution. Ab initio calculations indicate that the ground state is a multiconfigurational singlet, with a low-lying multiconfigurational triplet state. Variable-temperature NMR measurements are consistent with a singlet–triplet gap of 1200 ± 70 cm⁻¹, in good agreement with calculations. The distortion from threefold symmetry allows a low-lying, partially populated ligand-centered π nonbonding orbital to mix with largely occupied metal–ligand π bonding orbitals. The energetic accessibility of this distortion is inversely related to the strength of the metal–ligand π bonding interaction.