Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States

Structural, spectroscopic, and reactivity
studies are presented for an electron transfer series of copper hydroxide
complexes supported by a tridentate redox-active ligand. Single crystal
X-ray crystallography shows that the mononuclear [CuOH]1+ core is stabilized via intramolecular H-bonds between the H-donors
of the ligand and the hydroxide anion when the ligand is in its trianionic
form. This complex undergoes two reversible oxidation processes that
produce two metastable “high-valent” CuOH species, which
can be generated by addition of stoichiometric amounts of 1e– oxidants. These CuOH species are characterized by an array of spectroscopic
techniques including UV–vis absorption, electron paramagnetic
resonance (EPR), and X-ray absorption spectroscopies (XAS), which
together indicate that all redox couples are ligand-localized. The
reactivity of the complexes in their higher oxidation states toward
substrates with modest O–H bond dissociation energies (e.g.,
4-substitued-2,6-di-tert-butylphenols) indicates
that these complexes act as 2H+/2e– oxidants,
differing from the 1H+/1e– reactivity
of well-studied [CuOH]2+ systems.