End-On Copper(I) Superoxo and Cu(II) Peroxo and Hydroperoxo Complexes Generated by Cryoreduction/Annealing and Characterized by EPR/ENDOR Spectroscopy

In this report, we investigate the physical and chemical properties of monocopper Cu(I) superoxo and Cu(II) peroxo and hydroperoxo complexes. These are prepared by cryoreduction/annealing of the parent [LCuI(O-2)](+) Cu(I) dioxygen adducts with the tripodal, N-4-coordinating, tetradentate ligands L = (PV)tmpa, (DMM)tmpa, TMG(3)tren and are best described as [LCuII(O-2(center dot-))](+) Cu(II) complexes that possess end-on (eta(1)-O-2(center dot-)) superoxo coordination. Cryogenic gamma-irradiation (77 K) of the EPR-silent parent complexes generates mobile electrons from the solvent that reduce the [LCuII(O-2(center dot-))](+) within the frozen matrix, trapping the reduced form fixed in the structure of the parent complex. Cryoannealing, namely progressively raising the temperature of a frozen sample in stages and then cooling back to low temperature at each stage for examination, tracks the reduced product as it relaxes its structure and undergoes chemical transformations. We employ EPR and ENDOR (electron-nuclear double resonance) as powerful spectroscopic tools for examining the properties of the states that form. Surprisingly, the primary products of reduction of the Cu(II) superoxo species are metastable cuprous superoxo [LCuI(O-2(center dot-))](+) complexes. During annealing to higher temperatures this state first undergoes internal electron transfer (IET) to form the end-on Cu(II) peroxo state, which is then protonated to form Cu(II)-OOH species. This is the first time these methods, which have been used to determine key details of metalloenzyme catalytic cycles and are a powerful tools for tracking PCET reactions, have been applied to copper coordination compounds.

Automated summary

In this study, the physical and chemical properties of monocopper Cu(I) superoxo and Cu(II) peroxo and hydroperoxo complexes were investigated. The complexes were prepared using cryoreduction/annealing technique and characterized using EPR and ENDOR spectroscopy.