Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex

The
kinetics and mechanism(s) of the reactions of [K­(Krypt)]­[LCuO2] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]­hexacosane,
L = a bis­(arylcarboxamido)­pyridine ligand) with 2,2,6,6-tetramethylpiperdine-N-hydroxide (TEMPOH) and the para-substituted phenols XArOH (X = para substituent NO2, CF3,
Cl, H, Me, tBu, OMe,
or NMe2) at low temperatures were studied. The reaction
with TEMPOH occurs rapidly (k = 35.4 ± 0.3 M–1 s–1) by second-order kinetics to
yield TEMPO• and [LCuOOH]− on
the basis of electron paramagnetic resonance spectroscopy, the production
of H2O2 upon treatment with protic acid, and
independent preparation from reaction of [NBu4]­[LCuOH]
with H2O2 (Keq =
0.022 ± 0.007 for the reverse reaction). The reactions with XArOH also follow second-order kinetics, and analysis of the
variation of the k values as a function of phenol
properties (Hammett σ parameter, O–H bond dissociation
free energy, pKa, E1/2) revealed a change in mechanism across the series, from
proton transfer/electron transfer for X = NO2, CF3, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer)
for X = OMe, NMe2 (data for X = H, Me, tBu are intermediate between the extremes).
Thermodynamic analysis and comparisons to previous results for LCuOH,
a different copper–oxygen intermediate with the same supporting
ligand, and literature for other [CuO2]+ complexes
reveal significant differences in proton-coupled electron-transfer
mechanisms that have implications for understanding oxidation catalysis
by copper-containing enzymes and abiological catalysts.