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.