Hydrogen Atom Abstraction Thermodynamics of a μ-1,2-Superoxo Dicopper(II) Complex

Abstract

Pyrazolate-based μ-1,2-peroxo dicopper(II) complex 1 undergoes clean 1e oxidation at low potential (-0.59 V vs Fc/Fc) to yield the rather stable μ-1,2-superoxo dicopper(II) complex 3, which was characterized by spectroscopic methods (ν̃(O-O) = 1070 cm, Δ(O-O) = -59 cm) and analyzed by DFT calculations. 3 is also formed via H-atom abstraction from the corresponding μ-1,1-hydroperoxo dicopper(II) complex 2, while 3 itself is able to abstract H-atoms from weaker X-H bonds such as TEMPO-H to re-form 2. Kinetic and thermodynamic analyses evidence a concerted proton-electron transfer pathway for these processes. The thermodynamic square scheme reveals a bond dissociation free energy of 71.7 ± 1.1 kcal mol for the hydroperoxo OO-H bond of 2.

Structure and Reactivity of Copper Complexes Supported by a Bulky Tripodal N4 Ligand: Copper(I)/Dioxygen Reactivity and Formation of a Hydroperoxide Copper(II) Complex

“…Recently Meyer et al have reported the reaction of μ‐1,2‐superoxo dicopper(II) complex with TEMPO‐H, which resulted in formation of a μ‐1,1‐hydroperoxo dicopper(II) complex via HAT mechanism. The reaction exhibited activation parameters of Δ H ‡ = 9.03 ± 0.4 kcal · mol –1 and Δ S ‡ = –26.76 ± 1.7 cal · K –1 · mol –1 , which are close to our activation parameters . Kieber‐Emmons et al recently reported the reaction of a dicopper(II)‐μ‐oxo adduct with TEMPOH, which resulted in the formation of a dicopper(I,II)‐μ‐hydroxo adduct via a concerted HAT mechanism.…”

“…[60] Thus, the O-H bond dissociation energy of 6 is suggested to be around 80 kcal·mol -1 . Of note, the O-H bond dissociation energy of Meyer's μ-1,1-hydroperoxo dicopper(II) complex was estimated as 72 kcal·mol -1 [58] and Kieber-Emmons's dicopper(I,II)-μ-hydroxo complex as 77.2 kcal·mol -1 . [59] Based on the observed reactivity of the superoxide copper(II) complexes, 4 and 4Ј, a comparison of their reactivity can be made.…”

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

“…A great deal of information about structures and spectroscopic features of metal–oxygen centers can and have been obtained from synthetic models of enzymatic active sites by using fast‐time acquisition set‐ups, organic solvents, and low temperatures . However, redox potentials and electron‐transfer properties of such active species are scarce; that is, for products of O 2 stepwise reduction, in particular for copper–oxygen systems . Nonetheless, such information is important for the development of efficient devices in the energy research area such as fuel cells (O 2 ‐reduction), solar photoelectrochemical cells (water oxidation), and organic lithium–air batteries…”

“…As previously discussed, this derives from stronger electron‐withdrawing effects of the dicobalt(III) core assuming that charge at the HOMO is localized on the dioxygen core for both peroxo complexes. Interestingly, superoxo/peroxo dicopper complexes bearing a dinucleating pyrazolate‐bridged bis(tacn) (tacn=triazacyclononane) ligand have been very recently studied at much higher temperature (273 K) in acetonitrile by CV . A quasi‐reversible system was found at −0.59 V vs. Fc and ascribed to a ligand‐centered redox process according to UV/Vis spectroscopic experiments driven by chemical reagents.…”

Bioinspired molecular catalysts for homogenous electrochemical activation of dioxygen

“…[24] In particular, superoxo bridged diCu II have been reported to perform the abstraction of H atom from organic substrates. [25] Even though this was not investigated by the authors, this dicopper system is of potential interest for catalytic oxidation by O 2 with the assistance of electrochemistry. Indeed, it is expected that the formation of the peroxo bridged complex 3 + or of the superoxo bridged diCu II intermediate 2 2+ can be controlled upon polarization of the oxygenated solution at E < 0.02 V or E > 0.07 V, respectively.…”