11546
J. Am. Chem. Soc. 1997, 119, 11546-11547
Transition between Concerted and Stepwise
Dissociative Electron Transfers. An Example of
How a Change of Temperature May Trigger a
Change in Mechanism in Electrochemical
Experiments
Claude P. Andrieux, Jean-Michel Sav e´ ant,* and
Caroline Tardy
Laboratoire d’Electrochimie Mol e´ culaire de l’UniVersit e´
Denis Diderot, Unit e´ Associ e´ e au CNRS No. 438
2
place Jussieu, 75251 Paris Cedex 05, France
ReceiVed July 31, 1997
ReVised Manuscript ReceiVed October 8, 1997
Electron transfers involving the breaking of a bond may
follow, in polar solvents, two distinct mechanisms. One in
which the two events occur concertedly (reaction 1) and the
other where electron transfer (reaction 2) and bond breaking
Figure 1. Transition between a stepwise and a concerted reductive
cleavage mechanism as a function of the reducing power of the donor.
E is the electrode potential for electrochemical reactions and the
standard potential of the donor for homogeneous reactions. For oxidative
cleavages, RX being the donor, E would be the oxidative power of the
acceptor and the transition between stepwise and concerted mechanism
would occur upon decreasing E.
(
reaction 3) occur successively.1
-
•
(
RX - e f R + X
(1)
(2)
(3)
-
•(
RX - e f RX
or “non-existence” 5a of the intermediate, as often assumed in
5b
RX•( f R + X
•
(
sake of simplification, but is rather dictated by the lowest free
energy pathway.
An early observation of the transition between the two
mechanisms triggered by a variation of the reducing power of
the donor was made with the homogeneous reductive cleavage
The question of the distinction and of the transition between
the two mechanisms arises in electrochemical and bimolecular
homogeneous reactions and also in photoinduced reactions.
2
3
•
-
of triphenylmethylphenyl sulfide (giving Ph3C and PhS ) by a
On thermodynamic grounds, the passage from the stepwise to
the concerted pathway is triggered by a decrease of the cleavage
free energy of the ion radical, which may be segmented into
1
c
series of anion radical. Albeit small, the effect seems real
even if some ambiguity arises from the fact that the reorganiza-
tion energy may vary from one donor to the other in the series.
In principle, electrochemical experiments are devoid from this
ambiguity. The transition between the concerted and the
stepwise mechanism has been observed in the electrochemical
2
three main quantities as shown in the following equation.
∆
G° •(
RX fR +X(
•
) BDFE + E°RX/RX•( - E°
X /X
•(
•
+
(BDFE is the RX homolytic bond dissociation free energy, and
reductive cleavage of two sulfonium cations, Ph(CH3)S CH2-
+
the E° are the standard potentials of the subscript couples.) How
the molecular structure governs the occurrence of one or the
other mechanisms through the variations of these three param-
Ph and Naph(CH3)S CH2Ph (yielding the benzyl radical and
4
c,6
the corresponding sulfide), in acetonitrile.
The change in
mechanism was observed by means of the peak width of the
irreversible voltammetric wave, and, in all cases, the increase
in the thermodynamic driving force was produced by an increase
of the scan rate which shifts the voltammetric peak in the
negative direction. The variations of the peak are small also
(20 mV at maximum). It is thus worth trying to confirm the
reality of the change in mechanism by other means.
We have found that a variation of the temperature may
produce changes in the cyclic voltammetric wave that indeed
confirm the mechanism transition. The example we selected
1d,2,4
eters has been illustrated by several experimental examples.
The transition between the concerted and the stepwise
mechanism may also be triggered by a change in the reducing
(
or oxidizing) power of the outersphere electron donor (or
acceptor) opposed to the reactant, an electrode or an homoge-
neous reagent as sketched on Figure 1. The theoretical interest
of providing evidence that such transitions can be experimentally
observed resides in the demonstration that the concerted or
stepwise character of a reaction is not related to the “existence”
is the electrochemical reduction of the sulfonium cation Ph-
(
1) (a) Andrieux, C. P.; Sav e´ ant, J.-M. J. Electroanal. Chem. 1986, 205,
+
(CH3)S CH2Ph in acetonitrile. The cyclic voltammetric wave
4
1
3. (b) Andrieux, C. P.; Gallardo, I.; Sav e´ ant, J.-M. J. Electroanal. Chem.
986, 205, 43. (c) Severin, M. G.; Farnia, G.; Vianello, E.; Ar e´ valo, M. C.
obtained at a glassy carbon electrode at 273 K is irreversible
whatever the scan rate (from 0.1 to 100 V/s) as it is at 293 K.
Figure 2a summarizes the variations of the peak width (i.e., the
difference between the half-peak potential, Ep/2, and the peak
potential, Ep) with the scan rate at the two temperatures. At
J. Electroanal. Chem. 1988, 251, 369. (d) Sav e´ ant, J.-M. Single Electron
Transfer and SN2 Substitution. In AdVances in Physical Organic Chemistry;
Bethel, D., Ed.; Academic Press: London, 1990; Vol. 26, pp 1-130.
(
2) (a) Sav e´ ant, J.-M. Acc. Chem. Res. 1993, 26, 455. (b) Sav e´ ant, J.-M.
Dissociative Electron transfer. In AdVances in Electron Transfer Chemistry;
Mariano, P. S., Ed.; JAI Press: New York, 1994; Vol. 4, pp 53-116.
(
2
93 K, the transition between the concerted and stepwise
3) (a) Saeva, F. D. Topics Curr. Chem. 1990, 156, 61. (b) Saeva, F. D.
Intramolecular Photochemical Electron Transfer (PET) - Induced Bond
Cleavage Reactions in some Sulfonium Salts Derivatives. In AdVances in
Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: New York,
(5) (a) Meaning that the lifetime of the intermediate is larger or smaller
respectively than one vibration of the cleaving bond. (b) See for example
ref 5c. (c) Eldin, S.; Jencks, W. P. J. Am. Chem. Soc. 1995, 117, 9415.
(6) (a) Some evidence has been provided that a similar phenomenon
occurs in the reduction of p-EtOCOCHCHBrCH2Br in acetonitrile, although
the reduction mechanism is complicated by follow-up reactions. (b) Another
clear-cut example has been found recently involving the reduction of tert-
butyl-p-cyanoperbenzoate in N,N′-dimethylformamide where the convolution
technique was employed.6c A better precision than with peak-width
measurements is thus reached owing to the use of a larger part of the
information contained in each single voltammogram.6d (b) Antonello, S.;
Maran, F. J. Am. Chem. Soc. In press. (d) Sa v´ eant, J.-M.; Tessier, D. J.
Electroanal. Chem. 1975, 65, 57.
1
994; Vol. 4, pp 1-25. Arnold, B. R.; Scaiano, J. C.; McGimpsey, W. G.
J. Am. Chem. Soc. 1992, 114, 9978. (d) Chen, L.; Farahat, M. S.; Gan, H.;
Farid, S.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 6399. (e) Chen, L.;
Farahat, M. S.; Gaillard, E. R.; Farid, S.; Whitten, D. G. J. Photochem.
Photobiol. A: Chem. 1996, 95, 21.
(4) (a) Andrieux, C. P.; Le Gorande, A.; Sav e´ ant, J.-M. J. Am. Chem.
Soc. 1992, 114, 6892. (b) Andrieux, C. P.; Differding, E.; Robert, M.;
Sav e´ ant, J.-M. J. Am. Chem. Soc. 1993, 115, 6592. (c) Andrieux, C. P.;
Robert, M.; Saeva, F. D.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1994, 116,
7
864. (d) Andrieux, C. P.; Tallec, A.; Tardivel, R.; Sav e´ ant, J.-M.; Tardy,
C. J. Am. Chem. Soc. 1997, 119, 2420.
S0002-7863(97)02606-1 CCC: $14.00 © 1997 American Chemical Society