This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 5353–5358 5353 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 5353–5358 H-bond relays in proton-coupled electron transfers. Oxidation of a phenol concerted with proton transport to a distal base through an OH relay Cyrille Costentin, Marc Robert, Jean-Michel Save´ant* and Ce´dric Tard Received 26th October 2010, Accepted 9th December 2010 DOI: 10.1039/c0cp02275f Four molecules comprising a phenol moiety and a distal pyridine base connected by an intermediary H-bonding and an H-bonded alcohol group have been synthesized and their electrochemistry has been investigated by means of cyclic voltammetry. The molecules differ by the substituent at the alcohol functional carbon and by methyl groups on the pyridine. The reaction follows a concerted proton–electron transfer pathway as confirmed by the observation of a significant H/D kinetic isotope effect in all four cases. The standard rate constants characterizing each of the four compounds are analyzed in terms of reorganization energy and pre-exponential factor. Intramolecular and solvent reorganization energies appear as practically constant in the series, in which a previously investigated aminophenol is included, whereas significantly different pre-exponential factors are observed. That the latter, which is a measure of the efficiency of proton tunneling concerted with electron transfer, be substantially smaller with the H-bond relay molecules than with the aminophenol is related to the fact that two protons are moved in the first case instead of one in the second. Within the H-bond relay molecules, the pre- exponential factor varies with the substituent present at the alcohol functional carbon in the order CF 3 4 H 4 CH 3 , presumably as the result of a fine tuning of the balance between the H-bond accepting and H-bond donating properties of the central OH group. The kinetic H/D kinetic isotope effect increases accordingly in the same order. 1. Introduction Long-distance electron transfer 1,2 and long-distance proton transfer 3 are important issues in a number of natural systems. Processes in which electron and proton transfers are coupled and involve different sites (PCET reactions) are currently attracting intense attention with particular emphasis on the possibility that the two steps be concerted giving rise to CPET (concerted proton–electron transfer) reactions as opposed to stepwise pathways in which proton transfer precedes (PET) or follows (EPT) electron transfer (Scheme 1). 4–6 The occurrence of concerted processes requires a short distance between the group being oxidized and the proton acceptor (and vice versa for a reduction process), which usually but not necessarily involves the formation of a hydrogen-bond between the two groups as in an emblematic system such as the tyrosine– histidine couple in Photosystem II. 7 The distances over which the proton may travel as the result of a CPET reaction are therefore limited to the rather small values that correspond to the formation of an H-bond in the starting molecule. We have recently explored successfully the idea according to which this distance might be substantially increased by Scheme 1 PCET oxidation of a phenol ArOH bearing an attached proton acceptor, Nz. Concerted (horizontal) and stepwise (oblique) pathways. Laboratoire d’Electrochimie Mole ´culaire, Unite ´ Mixte de Recherche Universite ´ – CNRS No 7591, Universite ´ Paris Diderot, Ba ˆtiment Lavoisier, 15 rue Jean de Baı¨f, 75205 Paris Cedex 13, France. E-mail: [email protected]PCCP Dynamic Article Links www.rsc.org/pccp PAPER
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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 5353–5358 5353
H-bond relays in proton-coupled electron transfers. Oxidation
of a phenol concerted with proton transport to a distal base
through an OH relay
Cyrille Costentin, Marc Robert, Jean-Michel Saveant* and Cedric Tard
Received 26th October 2010, Accepted 9th December 2010
DOI: 10.1039/c0cp02275f
Four molecules comprising a phenol moiety and a distal pyridine base connected by an
intermediary H-bonding and an H-bonded alcohol group have been synthesized and their
electrochemistry has been investigated by means of cyclic voltammetry. The molecules differ by
the substituent at the alcohol functional carbon and by methyl groups on the pyridine. The
reaction follows a concerted proton–electron transfer pathway as confirmed by the observation of
a significant H/D kinetic isotope effect in all four cases. The standard rate constants
characterizing each of the four compounds are analyzed in terms of reorganization energy and
pre-exponential factor. Intramolecular and solvent reorganization energies appear as practically
constant in the series, in which a previously investigated aminophenol is included, whereas
significantly different pre-exponential factors are observed. That the latter, which is a measure of
the efficiency of proton tunneling concerted with electron transfer, be substantially smaller with
the H-bond relay molecules than with the aminophenol is related to the fact that two protons are
moved in the first case instead of one in the second. Within the H-bond relay molecules, the pre-
exponential factor varies with the substituent present at the alcohol functional carbon in the order
CF3 4 H 4 CH3, presumably as the result of a fine tuning of the balance between the H-bond
accepting and H-bond donating properties of the central OH group. The kinetic H/D kinetic
isotope effect increases accordingly in the same order.
1. Introduction
Long-distance electron transfer1,2 and long-distance proton
transfer3 are important issues in a number of natural systems.
Processes in which electron and proton transfers are coupled
and involve different sites (PCET reactions) are currently
attracting intense attention with particular emphasis on the
possibility that the two steps be concerted giving rise to CPET
(concerted proton–electron transfer) reactions as opposed to
stepwise pathways in which proton transfer precedes (PET) or
follows (EPT) electron transfer (Scheme 1).4–6 The occurrence
of concerted processes requires a short distance between the
group being oxidized and the proton acceptor (and vice versa
for a reduction process), which usually but not necessarily
involves the formation of a hydrogen-bond between the two
groups as in an emblematic system such as the tyrosine–
histidine couple in Photosystem II.7 The distances over which
the proton may travel as the result of a CPET reaction are
therefore limited to the rather small values that correspond to
the formation of an H-bond in the starting molecule.
We have recently explored successfully the idea according
to which this distance might be substantially increased by
Scheme 1 PCET oxidation of a phenol ArOHbearing an attached proton
acceptor, Nz. Concerted (horizontal) and stepwise (oblique) pathways.
Laboratoire d’Electrochimie Moleculaire, Unite Mixte de RechercheUniversite – CNRS No 7591, Universite Paris Diderot,Batiment Lavoisier, 15 rue Jean de Baıf, 75205 Paris Cedex 13,France. E-mail: [email protected]
a Energies in eV, pre-exponential factors in cm s�1. b From ref. 18. c From the energy of the starting molecule in the geometry of the distal cation
radical (see Scheme 1); in parentheses: from the energy of the distal cation radical in the geometry of the starting molecule. d Zref = 5.4� 104 cm s�1 is
the value expected for a simple electron transfer reaction.18
Fig. 4 Calculation of the intramolecular reorganization energies, li.
5358 Phys. Chem. Chem. Phys., 2011, 13, 5353–5358 This journal is c the Owner Societies 2011
the cation radical regarding heavy atoms and the positions of
the hydrogen atoms were re-optimized so as to get the minimal
energy on the hydrogen potential energy surface. Similarly, for
compound 1 the energy of the cation radical, En,r, was
calculated in the geometry of the neutral molecule regarding
heavy atoms while the positions of the hydrogen atoms
were re-optimized. The reorganization energies are finally
calculated from: li = En,c � En,n (and also li = Ec,n � Ec,c
for compound 1 and AP).
4. Concluding remarks
In summary, electrochemical oxidation of the H-bond relay
molecules depicted in Scheme 2, investigated by means of
cyclic voltammetry, follows a concerted proton–electron
transfer mechanism in all cases as confirmed by the observa-
tion of a significant H/D kinetic isotope effect. The standard
rate constant, which characterizes the intrinsic reactivity of
each molecule and its variation with temperature and with the
replacement of the moving protons by deuterons may be
analyzed in terms of reorganization energy (intramolecular
and solvent reorganization) and pre-exponential factor. The
first of these parameters appears as practically constant in the
series, including a previously investigated aminophenol
whereas significant differences are observed concerning the
pre-exponential factor. The latter, which is a measure of the
efficiency of proton tunneling concerted with electron transfer,
is substantially smaller with the H-bond relay molecules than
with the aminophenol as expected from the fact that two
protons are moved in the first case instead of one in the
second. Within the H-bond relay molecules, the pre-exponential
factor varies with the substituent present at the alcohol func-
tional carbon in the order CF3 4 H 4 CH3, presumably as
the result of a subtle balance between the H-bond accepting
and H-bond donating properties of the central OH group. As
expected, the kinetic H/D kinetic isotope effect increases in the
same order.
Acknowledgements
Financial support from Agence Nationale de la Recherche
(Programme blanc PROTOCOLE) is gratefully acknowledged.
Notes and references
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