Dioxygen activation at a single copper site: Structure, bonding, and mechanism of formation of 1:1 Cu-O₂ adducts

To evaluate the fundamental process of O₂ activation at a single copper site that occurs in biological and catalytic systems, a detailed study of O₂ binding to Cu(I) complexes of β-diketiminate ligands L (L¹ = backbone Me; L² = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for L²CuO₂ was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(III) - peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of L¹Cu(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of L¹Cu(THF) and (b) direct, bimolecular oxygenation of L¹Cu(RCN) via an associative process.