Bis(iminoxolene)iridium complexes over four oxidation states: from sodium–iridium bonding to ligand-centered radicals

Abstract

The novel iminoquinone N-(2,6-dibromophenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone (Briq), with electron-withdrawing and only mildly bulky 2,6-dibromophenyl substituents, is metalated by the iridium(I) complex {(coe)₂IrCl}₂ to give an inseparable mixture of (Briq)₂Ir, (Briq)₂IrCl, and (Briq)₂IrCl₂. This mixture can be funneled into a single pure compound either by exhaustive oxidation with iodobenzene dichloride to give (Briq)₂IrCl₂, or by exhaustive reduction by sodium naphthalenide to give (Briq)₂IrNa(THF)₂. The latter compound can be oxidized by one electron to give four-coordinate (Briq)₂Ir, or undergo two-electron oxidative addition reactions with iodine or iodomethane to yield (Briq)₂IrI or (Briq)₂IrCH₃. Structural data in the four different oxidation states indicate that each one-electron redox step is delocalized across the metal and iminoxolene ligand, with the redox changes becoming progressively more ligand-centered as the overall oxidation state increases. In the solid state, (Briq)₂IrNa(THF)₂ has a covalent bond between sodium and iridium (d = 2.9754(8) Å). The iridium–sodium bond partially heterolyzes in solution (ΔG° = 1.3 kcal mol⁻¹ in THF-d₈), and the four-coordinate anion has been characterized in the solid as the cobaltocenium salt. The iridium–sodium bond dissociation free energy is 74.3 kcal mol⁻¹.