Proton-Induced, Reversible Interconversion of a μ‑1,2-Peroxo and a μ‑1,1-Hydroperoxo Dicopper(II) Complex

The μ-1,2-peroxo dicopper­(II) complex (2) of a compartmental bis­(tetradentate) pyrazolate-based ligand is shown to convert, upon protonation, to the corresponding μ-1,1-hydro­peroxo dicopper­(II) complex (3). The transformation is cleanly reversed with base, and an apparent pKa = 22.2 ± 0.3 for the Cu2OOH unit in MeCN has been determined. The unprecedented stability of 3 (t1/2 = 9 h in nitrile solvents at room temperature, giving the hydroxo-bridged dicopper complex) has allowed for its structural characterization by X-ray diffraction. While the O–O bond length (1.462(3) Å) barely changes upon protonation from 2 to 3, the O–O stretching frequency is much higher in the hydro­peroxo complex 3 (860 cm–1). 3 mediates 2e– oxo transfer to the nucleophilic substrate PPh3 but is not activated for H-atom abstraction.