Unprecedented direct cupric-superoxo conversion to a bis-μ-oxo dicopper(III) complex and resulting oxidative activity

Investigations of small molecule copper-dioxygen chemistry can and have provided fundamental insights into enzymatic processes (e.g., copper metalloenzyme dioxygen binding geometries and their associated spectroscopy and substrate reactivity). Strategically designing copper-binding ligands has allowed for insight into properties that favor specific (di)copper-dioxygen species. Herein, the tetradentate tripodal TMPA-based ligand (TMPA = tris((2-pyridyl)methyl)amine) possessing a methoxy moiety in the 6-pyridyl position on one arm (^OCH3TMPA) was investigated. This system allows for a trigonal bipyramidal copper(II) geometry as shown by the UV-vis and EPR spectra of the cupric complex [(^OCH3TMPA)Cu^II(OH₂)](ClO₄)₂. Cyclic voltammetry experiments determined the reduction potential of this copper(II) species to be -0.35 V vs. Fc^+/0 in acetonitrile, similar to other TMPA-derivatives bearing sterically bulky 6-pyridyl substituents. The copper-dioxygen reactivity is also analogous to these TMPA-derivatives, affording a bis-μ-oxo dicopper(III) complex, [{(^OCH3TMPA)Cu^III}₂(O^2-)₂]²⁺, upon oxygenation of the copper(I) complex [(^OCH3TMPA)Cu^I](B(C₆F₅)₄) at cryogenic temperatures in 2-methyltetrahydrofuran. This highly reactive intermediate is capable of oxidizing phenolic substrates through a net hydrogen atom abstraction. However, after bubbling of the precursor copper(I) complex with dioxygen at very low temperatures (-135 °C), a cupric superoxide species, [(^OCH3TMPA)Cu^II(O₂ ^•-)]⁺, is initially formed before slowly converting to [{(^OCH3TMPA)Cu^III}₂(O^2-)₂]²⁺. This appears to be the first instance of the direct conversion of a cupric superoxide to a bis-μ-oxo dicopper(III) species in copper(I)-dioxygen chemistry.