Peroxo and Superoxo Moieties Bound to Copper Ion: Electron-Transfer Equilibrium with a Small Reorganization Energy

Abstract

Oxygenation of [Cu2(UN-O(-))(DMF)](2+) (1), a structurally characterized dicopper Robin-Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O(-) as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [Cu(II)2(UN-O(-))(O2(•-))](2+) (2). The formation kinetics provide that kon = 9 × 10(-2) M(-1) s(-1) (-80 °C), ΔH(‡) = 31.1 kJ mol(-1) and ΔS(‡) = -99.4 J K(-1) mol(-1) (from -60 to -90 °C data). Complex 2 can be reversibly reduced to the peroxide species [Cu(II)2(UN-O(-))(O2(2-))](+) (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with Ket = 5.3 (-80 °C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (2(1,2)) and μ-1,1- (2(1,1)) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [Cu(II)2(UN-O(-))(μ-1,2-O2(•-))](2+) (2(1,2)) and μ-1,2-peroxo structures [Cu(II)2(UN-O(-))(O2(2-))](+) (3) having a small bond reorganization energy of 0.4 eV (λin). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 2(1,2) and 3 calculated in the context of Marcus theory.

Figure 1

Oxygen reduction and modeling chemistry. (a) Reduction of molecular oxygen in aqueous media. E°′ values are the reduction potentials at pH 7 versus NHE {Note: In the text, reduction potential values have been converted to versus SCE, as follows: E_SCE (V) = E_NHE (V) − 0.242 (V)}. (b) Model chemistry of mononuclear LCu^I and dinuclear LCu^I–Cu^IL centers and their reversible reactions with dioxygen. (c) The phenolate-bridged mixed-valent Cu^I–Cu^II complex [Cu^ICu^II(UN-O⁻)]²⁺ (1) (UN-OH = 2-(bis(2-(pyridin-2-yl)ethyl)amino)-6-((bis(2-(pyridin-2-yl)ethyl)amino)- methyl)phenol, UN-O⁻ is the corresponding phenolate) reacts with dioxygen to form a superoxide species, either 2^1,1 or 2^1,2, which are in rapid equilibrium (see text). As previously reported, oxygenation of the phenolate dicopper(I) complex [Cu^I₂(UN-O⁻)]⁺ gives the peroxide species [Cu^II₂(UN-O⁻)(O₂²⁻)]⁺ (3). At the center of attention in this report is the interconversion chemistry of 2^1,2 and 3 via the use of outer-sphere ferrocenium derived redox agents.

Figure 2

Displacement ellipsoid plot of 1, its Oxygenation Reaction and EPR Spectroscopy. (a) Displacement ellipsoid plot (50% probability level) of the cationic part of the mixed-valent complex [Cu^ICu^II(UN-O⁻)( DMF)]²⁺ (1) at 110(2) K, Cu–Cu = 3.65 Å, hydrogen atoms are omitted for clarity. (b) UV–vis spectral changes due to the formation of superoxide complex [Cu^II₂(UN-O⁻)(O₂^•−)]²⁺ (2) (λ_max = 404 nm) by oxygenation of 1 (λ_max = 350 nm) in DCM solution at −80 °C; time period ~1 h. (c) Frozen DCM solution (~77 K) EPR spectra of 1 and 2 (~9 GHz). Also, see the text.

Figure 3

(a) Resonance Raman spectra of 2 with 407 nm excitation; ¹⁶O₂ (blue), ¹⁸O₂ (red), mixed isotope (a 1:2:1 mixture of ¹⁶O₂:^16,18O₂:¹⁸O₂ green), 1/4(¹⁶O₂ + ¹⁸O₂) (purple), and 1/2(¹⁶O₂ + ¹⁸O₂) (orange) and Gaussian fits (individual transitions are gray for ¹⁶O₂ and ¹⁸O₂ and black for mixed isotope while the Gaussian sum for each spectrum is shown as a dashed curve). (b) Resonance Raman spectra of 3 with 530 nm excitation; ¹⁶O₂ (blue), ¹⁸O₂ (red), mixed isotope (a 1:2:1 mixture of ¹⁶O₂:^16,18O₂:¹⁸O₂ green), and 1/4(¹⁶O₂ + ¹⁸O₂) (purple) and Gaussian fits (individual transitions are gray for ¹⁶O₂ and ¹⁸O₂ and black for mixed isotope while the Gaussian sum for each spectrum is shown as a dashed curve).

Figure 4

UV–vis spectroscopy of the [Cu^II₂(UN-O⁻)(O₂²⁻)]⁺ (3) and [Cu^II₂(UN-O⁻)(O₂^•−)]²⁺ (2) interconversion as well as equilibrium for 3 converting to 2 with diphenylamine ferrocenium as oxidant. (a) E° values for the oxidants used to convert 3 to 2 and for the reductants used to convert 2 to 3. (b) Oxidation of 3 to 2 using dimethylferrocenium ion. (c) Diphenylamine ferrocenium as oxidant used to reach an equilibrium state between 3 and 2.

Figure 5

Time course for the electron transfer from 3 to Me2Fc⁺ to produce 2 in DCM solvent at −80 °C.

Figure 6

Plot of ln k_et vs T⁻¹ for the electron transfer from [Cu^II₂(UN-O⁻)(O₂²⁻)]⁺ (3) to Me₂Fc⁺ in DCM solvent.

Figure 7

Interconversion of the μ-1,2-peroxide 3, and the μ-1,2-superoxide 2^1,2, and equilibrium of μ-1,2-superoxide and μ-1,1-superoxide complexes.