Open AccessLigand Identity-Induced Generation of Enhanced Oxidative Hydrogen Atom Transfer Reactivity for a CuII2(O2•–) Complex Driven by Formation of a CuII2(−OOH) Compound with a Strong O–H Bond
Author(s) -
David A. Quist,
Melanie Ehudin,
A. Schaefer,
Gregory L. Schneider,
Edward I. Solomon,
Kenneth D. Karlin
Publication year - 2019
Publication title -
journal of the american chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.115
H-Index - 612
eISSN - 1520-5126
pISSN - 0002-7863
DOI - 10.1021/jacs.9b05277
Subject(s) - chemistry , ligand (biochemistry) , density functional theory , moiety , hydrogen atom abstraction , medicinal chemistry , photochemistry , stereochemistry , dissociation (chemistry) , hydrogen peroxide , crystallography , radical , computational chemistry , organic chemistry , biochemistry , receptor
A superoxide-bridged dicopper(II) complex, [Cu II 2 (XYLO)(O 2 •- )] 2+ ( 1 ) (XYLO = binucleating m -xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu II 2 (XYLO)(O 2 2- )] + ( 2 ), using ferrocenium (Fc + ) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using Me 10 Fc + , 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 acid to 2 afforded the hydroperoxide-bridged dicopper(II) species [Cu II 2 (XYLO)(OOH)] 2+ ( 3 ). An acid-base equilibrium between 3 and 2 was achieved through spectral titrations using a derivatized phosphazene base. The p K a of 3 was thus determined 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 [Cu II 2 (XYLO)(O 2 •- )] 2+ ( 1 ), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with 1 in MeTHF, with k (-100 °C) = 5.6 M -1 s -1 . Density functional theory (DFT) calculations reveal how the structure of 1 may minimize stabilization 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 and properties of copper (or other metal ion) bound O 2 -derived reduced species, such as p K a , reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.