Ligand Identity Induced Generation of Enhanced Oxidative HAT Reactivity for a CuII2(O2•¬-) Complex Driven by Formation of a CuII2(-OOH) Compound with a Strong O-H Bond.

A superoxide bridged dicopper(II) complex, [CuII2(XYLO)(O2•-)]2+ (1) (XYLO ≡ binucleating m-xylyl derivative with a bridging phe-nolate ligand donor and two bis[(2-{2-pyridyl)ethyl)amine arms), was generated from chemical oxidation of the peroxide bridged di-copper(II) complex [CuII2(XYLO)(O22-)]+ (2), using ferrocenium (Fc+) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me10Fc+, a 1  2 equilibrium was established, allowing for calculation of the reduction potential of 1 as -0.525 ± 0.01 V vs. Fc+/0. Addition of 1 equiv. of strong acids to 2 afforded the hydroperoxide dicopper(II) species [CuII2(XYLO)(OOH)]2+ (3). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The pKa of 3 was thus deter-mined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex (3) O¬-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [CuII2(XYLO)(O2•-)]2+ (1), as demonstrated in H-atom abstraction reactions with certain phenolic ArO¬-H and hydro-carbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reac-tion of TEMPO-H with 1 in MeTHF; k (-100 °C) = 5.6 M-1 s-1. DFT calculations reveal how the structure of 1 may minimize stabiliza-tion of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation of copper (or other metal ion) properties of bound O2-derived reduced species, such as pKa, reduction potential, and BDFE; these may be relevant to the properties (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.