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

Abstract

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 ΔH° = − 6.9(7) kcal/mol, ΔS° = − 26(4) cal/mol•K). Vibrational (rR) data reveal a significant increase (29 cm−1) in νO-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. NMR spectroscopy of 2 at −120 °C in 2-methyltetrahydrofuran are also consistent with 1/HOAcF = 1:1 formulation 2 and that this complex possesses 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.

Copper(I)‐Dioxygen Adducts and Copper Enzyme Mechanisms

“…Addition of reducing agent decamethylferrocene (Me 10 Fc; E red =−0.52 V vs. Fc + /Fc in MeTHF), or stronger reducing agent cobaltocene (CoCp 2 ; E red = −1.345 V vs. Fc + /Fc in MeTHF), exhibited no reactivity with [Cu II (TMG 3 tren)(O 2 •− )] + at −130°C. [57] Upon addition of 10 equiv. of trifluoroacetic acid (TFA) with no reductant present, the superoxide complex changed to a new intermediate with UV-Vis features at λ max (ε, M −1 cm −1 ): 330 (5200), 549 (370), 670 (170), 985 (430) nm.…”

Transition Metal Complexes and the Activation of Dioxygen

“…It was suggested on the basis of kinetic studies that this reaction proceeds not via hydride transfer, but by rate-limiting hydrogen atom abstraction from substrate by the Cu(II)-O À 2 moiety. The protonation/reduction of 80 and 84 ( Figure 39) also has been studied [297,298]. In the reaction of 80 to yield H 2 O 2 with CF 3 CO 2 H, initial formation of a hydrogen-bonded Cu(II)-O À 2 ÁÁÁHO 2 CCF 3 adduct was proposed on the basis of spectroscopy and theory results [297].…”

“…The protonation/reduction of 80 and 84 ( Figure 39) also has been studied [297,298]. In the reaction of 80 to yield H 2 O 2 with CF 3 CO 2 H, initial formation of a hydrogen-bonded Cu(II)-O À 2 ÁÁÁHO 2 CCF 3 adduct was proposed on the basis of spectroscopy and theory results [297]. Subsequent reduction by alkyl-substituted ferrocenes yields H 2 O 2 and a Cu(II) product.…”

Ether formation on the tridentate Schiff base ligands of copper(II) complexes

“…Less common are hydroxylation reactions that occur on the non-aromatic parts instead of aromatic parts of the ligands of copper complexes. This type of reaction [48][49][50][51][52] has been suggested to be a model system of the reactions of copper monooxygenase enzymes such as dopamine β-monooxygenase.…”

“…[51] have suggested the possibility of Cu(II)-OOH and The suggested structure of the minor product of the reaction of CuSalser·imid + D-galactose (1 : 2) after sonication in MeO/NaOH/air for 2 h.Cu(II)L Cu(I)LCu(II)L.O 2 Cu(II)L-OH Cu(II)L-The proposed scheme for the reaction of CuL·imid + D-galactose in R 1 OH/NaOH/air after sonication. Cu(II)-O .…”