Five Nitrogen Oxidation States from Nitro to Amine: Stabilization and Reactivity of a Metastable Arylhydroxylamine Complex

Redox
noninnocent ligands enhance the reactivity of the metal they
complex, a strategy used by metalloenzymes and in catalysis. Herein,
we report a series of copper complexes with the same ligand framework,
but with a pendant nitrogen group that spans five different redox
states between nitro and amine. Of particular interest is the synthesis
of a unprecedented copper­(I)-arylhydroxylamine complex. While hydroxylamines
typically disproportionate or decompose in the presence of transition
metal ions, the reactivity of this metastable species is arrested
by the presence of an intramolecular hydrogen bond. Two-electron oxidation
yields a copper­(II)-(arylnitrosyl radical) complex that can dissociate
to a copper­(I) species with uncoordinated arylnitroso. This combination
of ligand redox noninnocence and hemilability provides opportunities
in catalysis for two-electron chemistry via a one-electron copper­(I/II)
shuttle, as exemplified with an aerobic alcohol oxidation.