Stepwise protonation and electron-transfer reduction of a primary copper-dioxygen adduct

Ryan L. Peterson, Jake W. Ginsbach, Ryan E. Cowley, Munzarin F. Qayyum, Richard A. Himes, Maxime A. Siegler, Cathy D. Moore, Britt Hedman, Keith O. Hodgson, Shunichi Fukuzumi, Edward I. Solomon, Kenneth D. Karlin

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68 Scopus citations

Abstract

The protonation-reduction of a dioxygen adduct with [LCu I][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(O 2•-)]+) 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 × 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 [LCu II(O2•-)]+ (1). Along with results obtained from XAS and DFT calculations, hydrogen bonding of HOAc F 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 H 2O2, 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.

Original languageEnglish
Pages (from-to)16454-16467
Number of pages14
JournalJournal of the American Chemical Society
Volume135
Issue number44
DOIs
StatePublished - 6 Nov 2013

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