Observation of a Cu II 2 (μ‐1,2‐peroxo)/Cu III 2 (μ‐oxo) 2 Equilibrium and its Implications for Copper–Dioxygen Reactivity

Synthesis of small‐molecule Cu 2 O 2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu II 2 (μ‐η 2 :η 2 ‐peroxo) and Cu III 2 (μ‐oxo) 2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu I ] + initially yields an “end‐on peroxo” species, that subsequently converts to the thermodynamically more stable “bis‐μ‐oxo” isomer ( K eq =3.2 at −90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end‐on peroxo species is in fact a result of an accessible bis‐μ‐oxo isomer, an electrophilic Cu 2 O 2 isomer in contrast to the nucleophilic reactivity of binuclear Cu II end‐on peroxo species. This study is the first report of the interconversion of an end‐on peroxo to bis‐μ‐oxo species in transition metal‐dioxygen chemistry.