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 [CuII2(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 [CuII2(UN-O−)(O22−)]+ (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- (21,2) and μ-1,1- (21,1) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [CuII2(UN-O−)(μ-1,2-O2•−)]2+ (21,2) and μ-1,2-peroxo structures [CuII2(UN-O−)(O22−)]+ (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 21,2 and 3 calculated in the context of Marcus theory.

Direct Determination of Electron‐Transfer Properties of Dicopper‐Bound Reduced Dioxygen Species by a Cryo‐Spectroelectrochemical Approach.

“…Recent works have shown the possibility to indirectly estimate the redox potential of copper–oxygen species by using chemical oxidants . Though attractive, this method requires the use of a large series of chemical reagents, and leads to an approximate value of the standard potential.…”

“…For that purpose, we have developed new cryo‐electrochemical and UV/Vis‐NIR cryo‐spectroelectrochemical setups. Here, we have focused on the unsymmetrical [Cu 2 (UN‐O − )] 2+/+ complexes 1 2+ and 1 + , which react with dioxygen and lead to the discrete superoxo and peroxo species 2 2+ and 3 + , respectively (Scheme ) . These are only stable at low temperatures ( T <200 K) and the direct determination of their thermodynamic and kinetic redox properties, coupled to their authentic UV/Vis‐NIR spectroscopic features, requires a novel cryo‐spectroelectrochemical approach.…”

Activation of dioxygen by copper metalloproteins and insights from model complexes

“…In 1996 and 1997, the groups of Tolman and Stack published on the spectroscopic and structural features of dicopper(III) bis- μ -oxide complexes, where dioxygen has been fully reduced by four electrons and the O-O bond has been cleaved [20–23]. Peroxodicopper(II) complexes supported by phenolate-bridging ligands have also been described [24–27]. Recently, Meyer and coworkers published the crystal structure of a peroxodicopper(II) complex ligated by a pyrazolate-based binucleating ligand [28].…”

Is Marijuana Beneficial for Prevention and Treatment of Diabetes?

“…Reactive oxygen species are often generated by molecular oxygen binding to redox active metals such as copper and iron in the human body as side product for energy production. Formation and reactivity of reactive oxygen species are well characterized in ligand copper oxygen model chemistry, for example [29][30][31] . Inoguchi and coworkers [32] reported that high glucose level significantly increases reactive oxygen species production in cultured smooth muscle cells (SMCs) and endothelial cells (ECs) detected by electron spin resonance spectroscopy.…”