ja054167+_si_003.cif (23.69 kB)
Concerted Proton−Electron Transfer in the Oxidation of Hydrogen-Bonded Phenols
dataset
posted on 2006-05-10, 00:00 authored by Ian J. Rhile, Todd F. Markle, Hirotaka Nagao, Antonio G. DiPasquale, Oanh P. Lam, Mark A. Lockwood, Katrina Rotter, James M. MayerThree phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN
with various one-electron oxidants. The bases are a primary amine (−CPh2NH2), an imidazole, and a
pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the
phenoxyl radical in which the phenolic proton has transferred to the base, •OAr-BH+, a proton-coupled
electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols,
predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs
by a concerted proton−electron transfer (CPET) mechanism, based on thermochemical arguments, isotope
effects, and ΔΔG⧧/ΔΔG°. The data rule out stepwise paths involving initial electron transfer to form the
phenol radical cations [•+HOAr-B] or initial proton transfer to give the zwitterions [-OAr-BH+]. The rate
constant for heterogeneous electron transfer from HOAr-NH2 to a platinum electrode has been derived
from electrochemical measurements. For oxidations of HOAr-NH2, the dependence of the solution rate
constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence
between the homogeneous and heterogeneous rate constants, are all consistent with the application of
adiabatic Marcus theory. The CPET reorganization energies, λ = 23−56 kcal mol-1, are large in comparison
with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic,
based on minimum values of Hrp derived from the temperature dependence of the rate constants. These
are among the first detailed analyses of CPET reactions where the proton and electron move to different
sites.