Electrochemical and homogeneous proton-coupled electron transfers: Concerted pathways in the one-electron oxidation of a phenol coupled with an intramolecular amine-driven proton transfer

Article abstract of DOI:10.1021/ja060527x

Proton-coupled electron transfers currently attract considerable attention in view of their likely involvement in many natural processes. Electrochemistry, through techniques such as cyclic voltammetry, is an efficient way of investigating the reaction mechanism of these reactions, and deciding whether proton and electron transfers are concerted or occur in a stepwise manner. The oxidation of an ortho-substituted 4,6-di (tert-butyl)-phenol in which the phenolic hydrogen atom is transferred during the reaction to the nitrogen atom of a nearby amine is taken as illustrative example. A careful analysis of the cyclic voltammetric responses obtained with this compound and its OD derivative allows, after estimation of the various thermodynamic parameters, ruling out the occurrence of the square scheme mechanism involving the proton-electron and electron-proton sequences. Simulation and comparison of the rate constant and H/D kinetic isotope effect with theoretical predictions show that the experimental value of the preexponential factor is ca. 1 order of magnitude larger than the theoretical value. Detailed calculations suggest that an electric field effect is responsible for this discrepancy. Copyright

Full text of DOI:10.1021/ja060527x

Published on Web 03/21/2006
Electrochemical and Homogeneous Proton-Coupled Electron Transfers:
Concerted Pathways in the One-Electron Oxidation of a Phenol Coupled with
an Intramolecular Amine-Driven Proton Transfer
Cyrille Costentin, Marc Robert, and Jean-Michel Save´ant*
Laboratoire d’Electrochimie Mole´culaire, UniVersite´ de Paris 7 - Denis Diderot, Case Courrier 7107,
2 place Jussieu, 75251 Paris Cedex 05, France
Received January 30, 2006; E-mail: saveant@paris7.jussieu.fr
Proton-coupled electron transfers (CPET) currently attract con-
siderable attention in view of their likely involvement in many
natural processes.1 In this connection, ortho-substituted 4,6-di(tert-
butyl)phenols in which the phenolic hydrogen atom is H-bonded
to the nitrogen atom of a nearby amine are particularly interesting
mimics, such as the molecules shown in Scheme 1 and other similar
compounds.2
Scheme 1
Figure 1. CV of 2 in acetonitrile + 0.1 M n-NBu4PF6. Temp: 20 °C.
Evidence has been presented that the oxidation of 1 by a series
Scan rate: 0.2 (a, a′, c), 5 V/s (b). a′: after addition of 0.5 M pyridine. Red
of triarylamine cation radicals follows a concerted proton electron
lines in a, b, and c: simulation of the CPE pathway (see text). Blue and
transfer (CPE) pathway (Scheme 2).2d One would expect that
green lines in c: simulation of the square scheme mechanism (see text),
with, respectively: λ ) 0.7 eV, E0AB ) 1.59, 1.39, EC0 D ) 0.23, 0.43 V vs
SCE, KAC ) 3.5 × 10-7, 9.8 × 10-4, KBD ) 9 × 1016, 3.2 × 1013.
Scheme 2
At low scan rates, the CV of 2 in acetonitrile (Figure 1) shows
a wave that is not completely chemically reversible, indicating that
•
the reaction product, OArHN+<, is not completely stable within
the time-scale of the experiment (Figure 1a). That this instability
results from deprotonation of the cation radical is confirmed by
the fact that the addition of a base such as pyridine renders the
wave completely irreversible (Figure 1a′). At 0.2 V/s, the HOArN<
wave corresponds to a fast electron transfer, with a standard
electrochemical oxidation of these compounds would equally
follow the same mechanism. Although reversible cyclic voltam-
metry (CV) has been reported in several cases,2 only one mechanis-
tic study has appeared, concluding to the occurrence of a square
scheme mechanism that would involve both the EP and PE branches
shown in Scheme 2.2c Although this proposal has been considered
as unlikely,3 no definite mechanistic conclusion has been drawn
so far.
We have found that a careful analysis of the CV responses
obtained with 2 and of its OD derivative allows, after estimation
of the various thermodynamic parameters, a definite answer to the
question. The next step was to apply the same approach to the
homogeneous oxidation of 1 and check the validity of the CPE
mechanism. Finally, the values of the reorganization energy and
preexponential factor were compared to the theoretical estimations,
leading to the proposal that the CPE reactivity is boosted by an
electric field effect in the electrochemical and homogeneous
reactions.
potential E0AD ) 0.605 V vs SCE, followed by a slow deprotona-
tion reaction (rate constant: 0.25 s-1). At higher scan rate, chemical
reversibility is restored, and the kinetics of electron transfer starts
to interfere (Figure 1b). Repeating the experiment shown in Figure
1b in the presence of 2% methanol and 2% CD3OD indicated the
existence of a small but definite kinetic isotope effect (ZH/ZD
)
1.8).4 Based on these data and on simulation of the CV response
in Figure 1c, using the previously established relationships char-
acterizing electrochemical CPE reactions,5 the preexponential factor,
Z, and the reorganization energy, λ, are estimated to be 1300 cm
s-1 and 0.8 eV, respectively.4
Although the observed kinetic isotope effect is a good clue to
the occurrence of a CPE reaction, it is interesting to see whether
the CV data could be concurrently interpreted in terms of the square
scheme mechanism involving the EP and or PE pathways shown
in Scheme 2. In this purpose we need to estimate the standard
potentials and equilibrium constants defined in Scheme 2, which
obeys the following relationships:
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J. AM. CHEM. SOC. 2006, 128, 4552-4553
10.1021/ja060527x CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
E0AD ) E0AB - (RT/F) ln KBD ) EC0 D - (RT/F) ln KAC
(1)
As a first approximation, EC0 D can be equated with the standard
potential of 4. Comparison of this standard potential to that of the
tri-tert-butylphenate6 points to a strong through-space electrostatic
stabilization of the zwitterionic form. It follows from eq 1 that KAC
) 3.5 × 10-7. Similarly, EA0 B can be equated with the standard
potential of tri-tert-butylphenol (1.59 V vs SCE).7 Thus, from eq
1: KBD ) 9 × 1016. Simulation8 of the voltammogram expected at
0.2 V/s, taking for the rate constant of intramolecular proton
exchange the maximizing value of 1013 s-1 in the downhill direction9
(blue line in Figure 1c), is clearly incompatible with the experi-
mental data. These estimates of the E0’s and K’s assumed that the
electrostatic and H-bonding stabilization of C is equal to the
electrostatic stabilization of 4 and that H-bonding stabilization of
A and B are approximately the same. Such approximations may
lead to an underestimation of the contributions of the PE and EP
pathways. Adding 0.2 eV, which is an average value of H-bond
energy,10 to the preceding value of EC0 D and subtracting the same
quantity from E0AB provide an upper (and very optimistic) limit to
the contributions of the PE and EP pathways. As seen in Figure
1c, even with these very favorable values, simulation (green line
in Figure 1c) is clearly incompatible with the experimental data.
We may use the same estimated values to examine the occurrence
of the PE and EP pathways in the homogeneous oxidation of 1 by
a series of triarylamine cation radicals2d taking into account the
difference in the value of EA0 D, 0.769 V vs SCE instead of 0.605,
since we have passed from compounds 2 to 1.4 The results shown
in Figure 2 (P and Q designate the triarylamines and their cation
radicals, respectively) clearly point to the incompatibility of the
PE and EP pathways with the experimental data, in agreement with
the mechanism suggested in ref 2d.
Figure 2. Oxidation of 1 by triarylamine cation radicals. Red dots:
experimental data.2d Red line: simulation of the CPE pathway. a, b
simulation (λ ) 1 eV) of the PE [E0CD ) 0.54, KAC ) 5.4 × 10-8 (blue),
E0CD ) 0.34, KAC ) 1.5 × 10-4 (green)] and EP [E0AB ) 1.7, KBD ) 5 ×
1015 (blue), EA0 B ) 1.5, KBD ) 2 × 1012 (green)] pathways.
Figure 3. Potential energy profiles for proton transfer in the electrochemical
CPET to 2 in the absence (left) and presence (right) of an electric field. A,
B, C, D are the corresponding species at the transition state.
experimental value. A similar effect is expected in the homogeneous
case since an electric field is exerted by the positive charge borne
by the electron acceptor. Calculation of the deuterium kinetic effect
also leads to a value compatible with the experimental data.4
Investigation of other examples of electrochemical intramolecular
CPE reactions is in progress, aiming in particular at further assessing
the electric field effect that we have observed.
Having established that the reaction mechanism involves a CPE
transfer in the electrochemical and homogeneous cases, we may
now examine whether the magnitudes of the observed preexpo-
nential factor, reorganization energy, and isotope kinetic effect are
compatible with current models. In the electrochemical case, it
appears that λ is close to what is expected, whereas Z is abnormally
large. λ may be compared to the λ (0.7 eV) characterizing the outer-
sphere electron transfer to a similar molecule, namely tri-tert-butyl-
phenol,7 which undergoes the same charge variation and a similar
intramolecular reorganization upon electron transfer. Addition of
an extra solvent reorganization term related to proton transfer, of
the order of 0.1 eV,4 falls in line with the value used to fit exper-
imental data.
The value of the preexponential factor essentially reflects the
conditions under which the proton tunnels (Figure 3). It can be
predicted from the model of electrochemical CPET reactions5 and
from the attending quantum chemical estimations as equal to 120
cm s-1 4
. This value is 1 order of magnitude smaller than the
experimental value.
In the homogeneous case, the simulation of the experimental
data shown in Figure 2 was performed with λ ) 1.15 eV and Z )
1010 M-1 s-1. The preexponential value is only 1 order of magnitude
lower than the maximum value for an homogeneous bimolecular
reaction (1011 M-1 s-1). It thus seems too high for a CPE reaction
as it is in the electrochemical case. These observations may be
explained as follows.
The electrochemical reaction takes place in a strong electric field,
thus leading to the stabilization of the zwitterionic form and
decreasing the proton tunneling barrier (Figure 3). Calculations of
the electrochemical preexponential factor taking this effect into
account4 leads to a value of 1176 cm s-1, compatible with the
Supporting Information Available: Experimental and simulation
procedures; estimation of the various theoretical parameters. This ma-
terial is available free of charge via the Internet at http://pubs.acs.org.
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