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

Rui Cao, Claudio Saracini, Jake W. Ginsbach, Matthew T. Kieber-Emmons, Maxime A. Siegler, Edward I. Solomon, Shunichi Fukuzumi, Kenneth D. Karlin

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52 Scopus citations

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.

Original languageEnglish
Pages (from-to)7055-7066
Number of pages12
JournalJournal of the American Chemical Society
Volume138
Issue number22
DOIs
StatePublished - 8 Jun 2016

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Publisher Copyright:
© 2016 American Chemical Society.

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