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

Abstract

Direct experimental determination of redox properties of superoxo (O ) and peroxo (O ) embedded in dicopper complexes bearing an unsymmetrical binucleating ligand was achieved using cryo-electrochemistry and cryo-spectroelectrochemistry in dichloromethane. Cyclic voltammetry for dicopper(I) (1 ) oxidation to a Cu Cu mixed-valent species (1 ) under inert atmosphere at 193 K reveals slow heterogeneous electron-transfer kinetics, indicative of a large reorganization energy. Oxygenation of the dicuprous complex 1 gives the bridged peroxo dicopper(II) species 3 , which is reversibly oxidized to the superoxo complex 2 at E =0.11 V (vs. SCE) with a small inner sphere electron-transfer reorganization energy, λ =0.54 eV, determined from variable temperature electrochemical impedance spectroscopy. The data suggest that the O /O redox process occurs directly on the O -derived core.

Structurally Characterized μ-1,2-Peroxo/Superoxo Dicopper(II) Pair

“…In subsequent work, it was demonstrated that C can be reversibly oxidized at low potential (−0.59 V vs Fc/Fc + ), giving the spectroscopically characterized μ-1,2-superoxo dicopper­(II) complex D (Scheme ) whose H atom abstraction (HAA) thermochemistry could be established . Using a ligand scaffold with a central phenolate bridge, Karlin et al studied a related Cu II 2 (O 2 •– ) complex, which was obtained by reacting the mixed-valent Cu I Cu II precursor with O 2 and was shown to consist of a mixture of μ-1,2 and μ-1,1 superoxo isomers in rapid equilibrium. , A kinetically facile superoxo/peroxo interconversion with low reorganization energy (at E = −0.24 V vs Fc/Fc + ) was revealed, reflecting that the redox process occurs at the O 2 -derived unit. , Most recent work focused on correlations between properties of the supporting ligands and superoxo HAA capabilities, in view of the very different OO-H bond dissociation free energies (BDFEs) of the hydroperoxo products derived from D and one of Karlin’s phenolate-based Cu II 2 (O 2 •– ) complexes (69.4 kcal/mol vs 81.8 kcal/mol). , However, crucial structural characterization of any dicopper superoxo species is lacking so far.…”

“…Experimental and computational evidence reveal an S = 1/2 spin ground state for the Cu II –O 2 •– –Cu II core and a negligible energy penalty associated with moderate changes of the Cu–O–O–Cu dihedral angle in both the peroxo and superoxo species. This work provides a structural basis for the previously observed , low reorganization energy of the kinetically facile 1e – interconversion of such superoxo/peroxo dicopper­(II) couples.…”

“…17,18 A kinetically facile superoxo/peroxo interconversion with low reorganization energy (at E = −0.24 V vs Fc/Fc + ) was revealed, reflecting that the redox process occurs at the O 2 -derived unit. 18,19 Most recent work focused on correlations between properties of the supporting ligands and superoxo HAA capabilities, 20 in view of the very different OO-H bond dissociation free energies (BDFEs) of the hydroperoxo products derived from D 21 and one of Karlin's phenolate-based Cu II 2 (O 2 •− ) complexes (69.4 kcal/mol 22 vs 81.8 kcal/mol). 16,20 However, crucial structural characterization of any dicopper superoxo species is lacking so far.…”

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

“…Here, the irreversible 1e reduction of 3 R is comparable to the monoelectronic and reversible electron-exchange reactions detected for the end-on superoxo/peroxo pyrazolate-and xylO-based complexes (entries 4 and 5). 74,75 Interestingly, the reduction potential of 3 NO 2 (entry 8) is close to that of Kodera's side-on peroxodicopper(II) species (entry 7), although the latter is a 2e process. 76 Overall, using such comparisons to make educated assignments of the electrochemical processes remains tentative given the large variety of ligands, charge, nuclearity, and bonding topology of the reported complexes.…”

“…395second system (process D) at E D (−1 75. V < E D < −1.21 V),396 which is quasi-reversible or irreversible, depending on R 397 (Figures 4 and S31).…”