Observation of a CuII2(μ-1,2-peroxo)/CuIII2(μ-oxo)2 Equilibrium and its Implications for Copper-Dioxygen Reactivity

Synthesis of small‐molecule Cu2O2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the CuII2(μ‐η2:η2‐peroxo) and CuIII2(μ‐oxo)2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)CuI]+ initially yields an “end‐on peroxo” species, that subsequently converts to the thermodynamically more stable “bis‐μ‐oxo” isomer (Keq=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 Cu2O2 isomer in contrast to the nucleophilic reactivity of binuclear CuII 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. A copper system has been uncovered in which an end‐on peroxo [Cu2(O2)] core is in equilibrium with a bis‐μ‐oxo [Cu2(O)2] core. Calibration of computational methods to experimental thermodynamic results suggest that the electrophilic reactivity patterns recently attributed to an end‐on‐peroxo CuII2 species are, in fact, likely a result of an accessible bis‐μ‐oxo CuIII2 core, a structural type long associated with electrophilic reactivity.