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.