Valence-to-Core X-ray Emission Spectroscopy as a Probe of O-O Bond Activation in Cu₂ O₂ Complexes

Valence-to-Core (VtC) X-ray emission spectroscopy (XES) was used to directly detect the presence of an O-O bond in a complex comprising the [Cu^II₂ (μ-η² :η² -O₂ )]²⁺ core relative to its isomer with a cleaved O-O bond having a [Cu^III₂ (μ-O)₂ ]²⁺ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu^II₂ (μ-η² :η² -O₂ )]²⁺ core corresponds to the copper stabilized in-plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O-O bond activation in μ-η² :η² -O₂ species and highlight the potential of this method for time-resolved studies of reaction mechanisms.