Reversible Double Oxidation and Protonation of the Non-Innocent Bridge in a Nickel(II) Salophen Complex

Substitution
on the aromatic bridge of a nickel­(II) salophen complex
with electron-donating dimethylamino substituents creates a ligand
with three stable, easily and reversibly accessible oxidation states.
The one-electron-oxidized product is characterized as a nickel­(II)
radical complex with the radical bore by the central substituted aromatic
ring, in contrast to other nickel­(II) salen or salophen complexes
that oxidize on the phenolate moieties. The doubly oxidized product,
a singlet species, is best described as having an iminobenzoquinone
bridge with a vinylogous distribution of bond lengths between the
dimethylamino substituents. Protonation of the dimethylamino substituents
inhibits these redox processes on the time scale of cyclovoltammetry,
but electrolysis and chemical oxidation are consistent with deprotonation
occurring concomitantly with electron transfer to yield the mono-
and dioxidized species described above.