Stepwise Protonation and Electron-Transfer Reduction of a Primary Copper–Dioxygen Adduct

The
protonation–reduction of a dioxygen adduct with [LCuI]­[B­(C6F5)4], cupric superoxo
complex [LCuII(O2•–)]+ (1) (L = TMG3tren (1,1,1-tris­[2-[N2-(1,1,3,3-tetramethylguanidino)]­ethyl]­amine))
has been investigated. Trifluoroacetic acid (HOAcF) reversibly
associates with the superoxo ligand in ([LCuII(O2•–)]+) in a 1:1 adduct [LCuII(O2•–)­(HOAcF)]+ (2), as characterized by UV–visible,
resonance Raman (rR), nuclear magnetic resonance (NMR), and X-ray
absorption (XAS) spectroscopies, along with density functional theory
(DFT) calculations. Chemical studies reveal that for the binding of
HOAcF with 1 to give 2, Keq = 1.2 × 105 M–1 (−130 °C) and ΔHo =
−6.9(7) kcal/mol, ΔSo = −26(4)
cal mol–1 K–1). Vibrational (rR)
data reveal a significant increase (29 cm–1) in vO–O (= 1149 cm–1) compared
to that known for [LCuII(O2•–)]+ (1). Along with results obtained from
XAS and DFT calculations, hydrogen bonding of HOAcF to
a superoxo O-atom in 2 is established. Results from NMR
spectroscopy of 2 at −120 °C in 2-methyltetrahydrofuran
are also consistent with 1/HOAcF = 1:1 formulation
of 2 and with this complex possessing a triplet (S = 1) ground state electronic configuration, as previously
determined for 1. The pre-equilibrium acid association
to 1 is followed by outer-sphere electron-transfer reduction
of 2 by decamethylferrocene (Me10Fc) or octamethylferrocene
(Me8Fc), leading to the products H2O2, the corresponding ferrocenium salt, and [LCuII(OAcF)]+. Second-order rate constants for electron transfer
(ket) were determined to be 1365 M–1 s–1 (Me10Fc) and 225
M–1 s–1 (Me8Fc) at
−80 °C. The (bio)­chemical relevance of the proton-triggered
reduction of the metal-bound dioxygen-derived fragment is discussed.