title: 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.
authors.affiliation: Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Rui
authors.initials: R
authors.lastname: Cao
authors.affiliation: Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.|Department of Chemistry and Nano Science, Ewha Womans University , Seoul 120-750, Korea.
authors.forename: Claudio
authors.initials: C
authors.lastname: Saracini
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Jake W
authors.initials: JW
authors.lastname: Ginsbach
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Kieber-Emmons
authors.affiliation: Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Maxime A
authors.initials: MA
authors.lastname: Siegler
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
authors.affiliation: Faculty of Science and Engineering, ALCA, SENTAN, Japan Science and Technology Agency (JST), Meijo University , Nagoya, Aichi 468-0073, Japan.|Department of Chemistry and Nano Science, Ewha Womans University , Seoul 120-750, Korea.
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.affiliation: Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis* / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D004579:Electron Transport; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010545:Peroxides / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013481:Superoxides / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D056831:Coordination Complexes; D010545:Peroxides; D013481:Superoxides; D003300:Copper; D010100:Oxygen