title: Stepwise protonation and electron-transfer reduction of a primary copper-dioxygen adduct.
abstract: The protonation–reduction of a dioxygen adduct with [LCu(I)][B(C6F5)4], cupric superoxo complex [LCu(II)(O2(•–))]+ (1) (L = TMG3tren (1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine)) has been investigated. Trifluoroacetic acid (HOAcF) reversibly associates with the superoxo ligand in ([LCu(II)(O2(•–))]+) in a 1:1 adduct [LCu(II)(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 × 10(5) M(–1) (−130 °C) and ΔH° = −6.9(7) kcal/mol, ΔS° = −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 [LCu(II)(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 [LCu(II)(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.
authors.forename: Ryan L
authors.initials: RL
authors.lastname: Peterson
authors.forename: Jake W
authors.initials: JW
authors.lastname: Ginsbach
authors.forename: Ryan E
authors.initials: RE
authors.lastname: Cowley
authors.forename: Munzarin F
authors.initials: MF
authors.lastname: Qayyum
authors.forename: Richard A
authors.initials: RA
authors.lastname: Himes
authors.forename: Maxime A
authors.initials: MA
authors.lastname: Siegler
authors.forename: Cathy D
authors.initials: CD
authors.lastname: Moore
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004579:Electron Transport; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory
chemical_list: D009942:Organometallic Compounds; D011522:Protons; D003300:Copper; D010100:Oxygen