Dissociative electron transfer to organic chlorides: Electrocatalysis at metal cathodes

Article abstract of DOI:10.1039/b719936h

The reductive cleavage of a series of organic chlorides, including chloroaromatics, benzyl chlorides, activated chloroalkanes and polychloromethanes, was investigated at Ag, Cu, Pd and glassy carbon (GC) electrodes in CH₃CN + 0.1 M (C₂H₅) ₄NClO₄. The silver cathode was either a 2-mm diameter disc, fabricated from Ag wire, or nanoclusters of average diameter d = 304 nm, prepared by electrodeposition on GC. Ag, Cu and Pd electrodes have shown remarkable electrocatalytic properties for the reduction of several compounds. The peak potentials recorded at these electrodes, for example, at υ = 0.1 V s^-1 are positively shifted by 0.3-0.8 V with respect to the reduction potentials measured at a non catalytic electrode such as GC. Electrocatalysis is strictly related to the concerted nature of the dissociative electron transfer to the carbon-chlorine bond. No catalysis is observed when the dissociative electron transfer to RCl occurs according to a stepwise mechanism involving the intermediate formation of a radical anion. The catalytic surfaces affect the reaction scheme, offering a more favourable route possibly through the formation of strongly adsorbed activated complexes. the Owner Societies.

Full text of DOI:10.1039/b719936h

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Dissociative electron transfer to organic chlorides: Electrocatalysis at
metal cathodes
Abdirisak A. Isse, Silvia Gottardello, Christian Durante and Armando Gennaro*
Received 2nd January 2008, Accepted 31st January 2008
First published as an Advance Article on the web 7th March 2008
DOI: 10.1039/b719936h
The reductive cleavage of a series of organic chlorides, including chloroaromatics, benzyl
chlorides, activated chloroalkanes and polychloromethanes, was investigated at Ag, Cu, Pd and
glassy carbon (GC) electrodes in CH3CN + 0.1 M (C2H5)4NClO4. The silver cathode was either
a 2-mm diameter disc, fabricated from Ag wire, or nanoclusters of average diameter d = 304 nm,
prepared by electrodeposition on GC. Ag, Cu and Pd electrodes have shown remarkable
electrocatalytic properties for the reduction of several compounds. The peak potentials recorded
at these electrodes, for example, at u = 0.1 V sꢀ1 are positively shifted by 0.3–0.8 V with respect
to the reduction potentials measured at a non catalytic electrode such as GC. Electrocatalysis is
strictly related to the concerted nature of the dissociative electron transfer to the carbon–chlorine
bond. No catalysis is observed when the dissociative electron transfer to RCl occurs according to
a stepwise mechanism involving the intermediate formation of a radical anion. The catalytic
surfaces affect the reaction scheme, offering a more favourable route possibly through the
formation of strongly adsorbed activated complexes.
to the scission of the carbon–halogen bond. There are two
1. Introduction
possible mechanisms for this DET process. Electron transfer
The electrochemical activation of carbon–halogen bonds is a
and bond rupture may occur either in a stepwise manner with
widely explored field in organic electrochemistry. The great
the formation of an intermediate radical anion (eqn (1) and
interest for this process is due to the important roles it plays in
(2)) or in a concerted way yielding directly a radical and an ion
synthetic1,2 and environmental applications,3,4 especially the
(eqn (3)).5 The radical Rꢁ is often more easily reducible than
abatement of volatile polychlorinated organic compounds.
the parent RX molecule, especially when X = Cl or Br, so
The reductive cleavage of the carbon–halogen bond has also
that an overall two-electron reduction wave is observed
attracted much interest in the investigation of dissociative
(eqn (1)–(4)).
electron transfer processes.5–7
ꢀ
RX + eꢀ $ RXꢁ
(1)
(2)
(3)
(4)
A major drawback to the use of organic halides in electro-
synthesis is that electrochemical activation of carbon–halogen
bonds, especially chlorides and bromides, requires application of
quite negative potentials. Several approaches based on homo-
geneous electrocatalysis, mainly with transition metal com-
plexes, have been investigated to circumvent this difficulty.2,8
Also electroreduction at catalytic surfaces has been investi-
gated.9 Most of the early studies were carried out at Hg on
which adsorption phenomena and intermediate formation of
organomercurials were often found to play an important role in
the process.1 However, in the recent past, this metal has lost
interest in electroorganic chemistry because of environmental
concerns. Recently, silver has been found to possess extraordin-
ary electrocatalytic properties for a large variety of organic
halides.10–13 The electrocatalytic properties of this metal have
been widely investigated and successfully utilised in several
synthetic14 and environmental applications.4,15 Also Pd16 and
Cu17,18 have been found to possess good electrocatalytic proper-
ties for the reduction of a few alkyl bromides and iodides.
Reduction of organic halides at inert electrodes such as
glassy carbon is a dissociative electron transfer (DET) leading
RXꢁ - Rꢁ + Xꢀ
RX + eꢀ - Rꢁ + Xꢀ
Rꢁ + eꢀ $ Rꢀ
ꢀ
The mechanism of the electrocatalytic process at Ag has been
investigated from different viewpoints. It has been shown that
the molecular structure of the organic halide,10 the nature of
the halogen atom,11,19 the surface morphology of the elec-
trode20 and adsorption of the reagents and products, especially
halide ions,11,20,21 all play an important role in the electro-
catalytic process. Also the mechanism of the dissociative
electron transfer was found to have a crucial role in the
electrocatalytic process. The catalytic surface modifies the
reaction scheme possibly through the formation of a more
favourable activated complex and adsorption of halide
anions.11,13
The electrocatalytic properties of Pd and Cu have been less
investigated. Questions on the role of various factors such as
molecular structure, solvent, adsorption phenomena and DET
mechanism have not been fully addressed. In addition, the
existence of the electrocatalytic activity as a general property,
which applies to organic halides as a class, is not yet well
`
Dipartimento di Scienze Chimiche, Universita di Padova, Via Marzolo
1, 35131 Padova, Italy
ꢂc
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counter-electrode and the reference electrode were a Pt ring
and Ag7AgI7Iꢀ, respectively. All potentials reported in this
paper are quoted versus the saturated calomel electrode (SCE).
This has been achieved by calibrating the Ag7AgI7Iꢀ reference
system at the end of each experiment against the ferricenium/
ferrocene couple (EF0
= 0.391 V vs. SCE in CH3CN) and
þ=Fc
converting the potentials to the SCE scale.
c
The working electrodes were 2-mm or 3-mm diameter discs
embedded in glass or Teflon. Glassy carbon, Ag and Cu were
fabricated from a 3-mm diameter GC rod (Tokai GC-20), a
2-mm diameter Ag wire (Alfa Aesar, 99.999%) and a 2-mm
diameter Cu wire (Alfa Aesar, 99.999%) and were polished to
a mirror finish with silicon carbide papers of decreasing grain
size (Struers, grit: 500, 1000, 2400, 4000) followed by diamond
paste (3-, 1-, 0.25-mm particle size). They were then cleaned in
ethanol in an ultrasonic bath for about 5 minutes. The Pd
electrode was a highly polished disc (3-mm diameter) from
Bioanalytical Systems. Prior to each experiment, the Pd, Cu,
Ag and unmodified GC electrodes were refreshed by polishing
with a 0.25-mm diamond paste followed by ultrasonic rinsing.
In several cases a slight passivation of the Pd electrode,
resulting in a poor reproducibility of the data, was observed
during the experiment. When this happened, the electrode was
activated by cycling the potential from ꢀ0.3 to 1.2 V vs. SCE,
before each cyclic voltammogram. This simple activation
procedure guaranteed fair reproducibility of the data.
Scheme 1
established for these two metals. So far, Pd has been reported
to have good catalytic properties for the reduction of simple
alkyl halides (RX, where R varies from CH3 to C14H29 and
X = Br, I),16 whereas data on Cu are limited to few methyl
and methylene bromides and iodides17 and haloethanols
(XCH2CH2OH, where X = Br, I).18 Herein, we report the
electrocatalytic reduction of a series of organic chlorides at Pd
and Cu electrodes. The aim of the study was to show whether
the electrocatalytic activities of these metals is limited to some
bromides and iodides or is of a more general applicability. The
molecular structures of the investigated compounds, which
were selected so as different types of organic chlorides are
represented, are shown in Scheme 1. The selected series of
compounds allows examination of the effect of the DET
mechanism on the electrocatalytic properties.
Besides Pd and Cu, silver, either in the form of nanoclusters
or bulk metal, and glassy carbon were also investigated. The
latter was used as a reference system for a non catalytic
electrode acting simply as an outer sphere electron donor.
As to the silver electrode, the extraordinary electrocatalytic
properties of the bulk metal have widely been demonstrated.
Little is known about the possibility of using Ag nanoclusters
in electrocatalytic reduction of organic halides. A preliminary
report from our laboratory has shown interesting results, even
though the study was limited to a single compound.22 We wish
to show here a comparison of the electrocatalytic properties of
Ag nanoparticles with those of the bulk metal for the reduc-
tion of an extended series of organic chlorides.
The silver clusters were prepared by electrodeposition of Ag
on a 3-mm diameter GC disc from a 1 mM AgClO4 solution in
CH3CN + 0.1 M LiClO4. The details of the method of
preparation and characterisation of the silver-modified elec-
trode were previously published.22
A freshly prepared
electrode was used in each experiment.
3. Results and discussion
Cyclic voltammetry of compounds 1–10 was recorded in
CH3CN + 0.1 M (C2H5)4NClO4 at different scan rates in
the range from 0.04 V sꢀ1 to 4 V sꢀ1. All compounds exhibit a
single irreversible reduction peak corresponding to a 2eꢀ
reduction of RCl to RH (eqn (5)):
2. Experimental
RCl + 2eꢀ + BH - RH + Clꢀ + Bꢀ
(5)
2.1 Chemicals
Acetonitrile (Prolabo, HPLC grade) was distilled over CaH2
and stored under an argon atmosphere. Tetraethylammonium
perchlorate was prepared by mixing equimolar amounts of
tetraethylammonium bromide (Fluka, 498%) and NaClO4
(Acros, 99+%) in warm ethanol, followed by filtration of the
precipitate. The salt was recrystallised twice from H2O and
dried in a vacuum oven at 70 1C. All other reagents were
commercially available with good purity and were used with-
out further purification.
where BH is any proton donor in solution. When RH is
reducible in the accessible potential window, a further reduc-
tion peak (or peaks) attributable to the newly formed com-
pound is observed. Fig.
1 shows an example of the
voltammetric pattern of the aromatic and heteroaromatic
compounds 1–4. For these compounds the first irreversible
peak is followed by at least one reversible peak couple
corresponding to the 1eꢀ reduction of RH to a stable radical
anion RHꢁꢀ. Reductive cleavage of the chlorides 5–8 results in
the formation of hydrogenated compounds lacking easily
reducible groups. An example of the voltammetric behaviour
of this group of compounds is shown in Fig. 2, where a single
irreversible reduction peak is observed. Reduction of the two
polychlorinated compounds 9–10 occurs by successive re-
moval of chlorine atoms. Thus, several reduction peaks attri-
butable to compounds with decreasing number of chlorine
atoms are observed (Fig. 3).
2.2 Instrumentation
Electrochemical measurements were carried out on a computer-
controlled Autolab PGSTAT30 (Eco-Chimie, Utrecht, Nether-
lands) potentiostat. All experiments were carried out at 25 1C in
a three-electrode cell system using glassy carbon (GC), Ag,
either in the form of clusters deposited on glassy carbon (GC/
Ag) or bulk metal, Cu or Pd as a working electrode. The
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Fig. 1 Cyclic voltammetry of 3 mM 9-chloroanthracene in CH3CN
+ 0.1 M (C2H5)4NClO4 at u = 0.1 V sꢀ1
Fig. 2 Cyclic voltammetry of 3 mM benzyl chloride in CH3CN +
0.1 M (C2H5)4NClO4 at u = 0.1 V sꢀ1
.
.
The mechanism of the dissociative electron transfer to the
compounds under investigation here has been previously re-
ported. Both homogeneous and heterogeneous DET to com-
pounds 1–4 follow
a
stepwise mechanism.23–26 These
compounds have low lying p* orbitals in which the incoming
electron is iꢀnitially accommodated. The existence of the radical
anion RClꢁ has been experimentally observed and the kinetics
of its decomposition measured. The rate constant of the cleavage
reaction (eqn (2)) of 1ꢁꢀ is 1.4 ꢃ 102 s
,
ꢀ1 23 whereas compounds
2, 3 and 4 give short-lived radical anions, with cleavage rate
constants of 1.6 ꢃ 107 s
,
ꢀ1 24 1.6 ꢃ 108 sꢀ1 and 3.6 ꢃ 109 sꢀ1 26
,
respectively. In contrast, electron transfer to compounds 5–10
involves a s* orbital and, hence, rupture of the carbon–chlorine
bond concomitantly with the insertion of 1eꢀ is energetically
more favourable than the stepwise mechanism.7,27
The DET mechanism can conveniently be examined by
voltammetric analysis of the process. In the case of a concerted
DET, the heterogeneous electron transfer (ET) is very slow
because a bond is to be broken in concomitance with the ET.
The peak potential, Ep, varies linearly with log u with a slope
qEp/qlog u = (ꢀ29.6/a) mV at 25 1C (eqn (6)), whereas the
peak width is given by Ep/2 ꢀ Ep = (47.7/a) mV at the same
temperature (eqn (7)).28
@Ep
1:151 RT
aF
¼ ꢀ
ð6Þ
ð7Þ
@ logu
1:857 RT
aF
Fig. 3 Cyclic voltammetry of 3 mM CCl4 in CH3CN + 0.1 M
(C2H5)4NClO4 at u = 0.1 V sꢀ1
Ep=2 ꢀ Ep ¼
.
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These equations can be used to calculate the electron transfer
coefficient, a, which is typically much smaller than 0.5.5 In the
stepwise mechanism, the rate determining step may be either
the heterogeneous ET (eqn (1)) or the chemical reaction (eqn
(2)). If reaction (2) is the rate determining step while reaction
(1) remains at equilibrium, Ep varies linearly with log u with a
slope qEp/qlog u = ꢀ29.6 mV at 25 1C, whereas the peak
width Ep/2 ꢀ Ep = 47.7 mV at the same temperature. Con-
versely, if the ET (eqn (1)) is the rate determining step, qEp/
qlog u = (ꢀ29.6/a) mV and Ep/2 ꢀ Ep = (47.7/a) mV, at
25 1C. In situations between these two limiting cases a mixed
kinetic control prevails and both qEp/qlogu and Ep/2 ꢀ Ep
assume intermediate values. For example, if a = 0.5, qEp/qlog
u lies between ꢀ29.6 and ꢀ59.1 mV at 25 1C. When the DET
process is under mixed kinetic control, application of eqn (6)
and (7) gives an apparent value of aapp 40.5. Therefore, a or
aapp is often used as a diagnostic criterion for distinguishing
the two mechanisms: aapp 40.5 indicates a stepwise mechan-
ism, whereas a { 0.5 implies a concerted mechanism.
compounds 1–4 is little affected by the nature of the electrode
material; all four compounds give essentially the same voltam-
metric pattern at all electrodes (see, for instance, Fig. 1).
Application of the above described diagnostic criteria for
mechanistic analysis clearly points out a stepwise dissociative
electron transfer for these aromatic or heteroaromatic chlor-
ides. A comparison of the peak potentials obtained at the
different electrodes (see Fig. 4 and Tables 2–4, last column)
shows no catalytic effect at GC/Ag, Cu and Pd electrodes. In
some cases the reduction potentials at these electrodes are
slightly more negative than the Ep values obtained at GC.
Overall Ep (M) ꢀ Ep (GC) values are spanned in a range of
0.1 V (from ꢀ0.076 V to 0.027 V). Similar Ep differences are
observed if one compares data obtained at any one of the
cathodes with those pertaining to the other electrodes. There-
fore, the observed Ep differences are not related to electro-
catalytic properties of the electrodes. It is more likely that such
small differences arise from slight variations of the hetero-
geneous ET rate constant at the different electrode surfaces.
The catalytic electrodes GC/Ag, Cu and Pd all show re-
markable electrocatalytic properties for the reduction of com-
pounds 5–10. Fig. 2 and 3 show examples of the voltammetric
pattern of these compounds, which exhibit irreversible reduc-
tion peaks at all electrodes. The general features of the
voltammograms at the catalytic electrodes are similar to those
obtained at bare GC, except for the values of the peak
potentials which are considerably more positive at GC/Ag,
Cu and Pd electrodes than at bare GC electrode. In addition,
the peaks obtained at the catalytic electrodes are quite sharp,
suggesting some adsorption phenomena, especially in the case
of chloroform and carbon tetrachloride. The results of the
voltammetric analysis are collected in Tables 2–4. It can be
seen that the slopes qEp/qlog u are highly negative for all
compounds regardless of the nature of the electrode material,
indicating that the DET process is under kinetic control of a
very slow electron transfer. The values of the transfer coeffi-
cients calculated from these slopes are very small (often a o
The data obtained from the voltammetric analysis at GC are
collected in Table 1. The values of qEp/qlog u and Ep/2 ꢀ Ep
measured for the reduction of 9-chloroanthracene are practi-
cally identical with the theoretical values predicted for a fast
ET followed by a rate determining first-order chemical reac-
tion. The slopes qEp/qlog u obtained for compounds 2–4 show
that, in the investigated range of u, the process is under mixed
kinetic control. As shown in Table 1, application of eqn (6)
and (7) leads to aapp 40.5. These data are in good agreement
with previous reports and clearly point out that the reductive
cleavage of these four molecules occurs according to a stepwise
mechanism. The data on the aliphatic compounds 5–10 show a
different picture. In all cases the heterogeneous ET is the rate
determining step and is characterised by a quite low value of a
(generally a o 0.35). Again these results confirm literature
data showing that aliphatic halides undergo concerted DET
because formation of s* radical anions is energetically less
favourable than fragmentation to Rꢁ and Xꢀ, which in some
cases may give in-cage electrostatic interactions.5,27
0.35). Also the a values obtained from the peak width, Ep/2
ꢀ
The results of the voltammetric analyses at Ag-modified
GC, Cu and Pd electrodes are collected in Tables 2–4, whereas
some representative examples of cyclic voltammograms at
these electrodes are shown in Fig. 1–3. The voltammetric
behaviour of the compounds at the catalytic surfaces strongly
depends on the mechanism of DET. The reductive cleavage of
Ep, are very small except in the case the two polychloro-
methanes 9 and 10, which give quite sharp reduction peaks
possibly due to adsorption of the reagents and/or products.
According to the diagnostic criteria set for the mechanistic
analysis of electrode processes underlying irreversible peaks,
the reductive cleavage of the aliphatic chlorides 5–10 at all
Table 1 Voltammetric data for the reduction of RCl in CH3CN + 0.1 M (C2H5)4 NClO4 at 298.15 K at glassy carbon
ab
Ep/2 ꢀ Ep (mV)
ad
DET mechanism
a
Ep (V vs. SCE)
c
RCl
qEp/qlog u (mV)
1
2
3
4
5
6
7
8
9
10
a
ꢀ1.775
ꢀ2.161
ꢀ1.960
ꢀ2.297
ꢀ2.172
ꢀ1.909
ꢀ1.968
ꢀ2.080
ꢀ2.086
ꢀ1.578
b
ꢀ32
ꢀ35
ꢀ34
ꢀ47
ꢀ94
ꢀ73
ꢀ93
ꢀ86
ꢀ139
ꢀ125
0.92
0.84
0.87
0.63
0.32
0.40
0.32
0.34
0.21
0.24
51
59
60
64
160
119
126
133
155
125
0.93
0.81
0.80
0.74
0.30
0.40
0.38
0.36
0.31
0.38
Stepwise
Stepwise
Stepwise
Stepwise
Concerted
Concerted
Concerted
Concerted
Concerted
Concerted
u = 0.1 V sꢀ1
.
Calculated from qEp/qlog u = ꢀ1.15RT/aF. Average of the values obtained at different scan rates in the 0.04–4 V sꢀ1
c
d
range. Calculated from Ep/2 ꢀ Ep = 1.857RT/aF.
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Table 2 Voltammetric data for the reduction of RCl in CH3CN + 0.1 M (C2H5)4NClO4 at 298.15 K at silver nanoclusters deposited on GC
ab
Ep/2 ꢀ Ep (mV)
ad
DET mechanism
DEp (V)
a
Ep (V vs. SCE)
c
e
RCl
qEp/qlog u (mV)
1
2
3
4
5
6
7
8
9
10
a
ꢀ1.774
ꢀ2.156
ꢀ1.941
ꢀ2.275
ꢀ1.720
ꢀ1.586
ꢀ1.263
ꢀ1.505
ꢀ1.274
ꢀ1.020
b
ꢀ33
ꢀ37
ꢀ35
ꢀ44
ꢀ92
ꢀ93
ꢀ94
ꢀ95
ꢀ97
ꢀ62
0.90
0.80
0.84
0.67
0.32
0.32
0.32
0.31
0.30
0.48
53
58
59
72
120
143
135
131
82
0.90
0.82
0.81
0.66
0.40
0.33
0.35
0.36
0.58
0.57
Stepwise
Stepwise
Stepwise
Stepwise
Concerted
Concerted
Concerted
Concerted
Concerted
Concerted
0.001
0.005
0.019
0.022
0.452
0.323
0.705
0.575
0.812
0.558
83
u = 0.1 V sꢀ1
.
Calculated from qEp/qlog u = ꢀ1.15RT/aF. Average of the values obtained at different scan rates in the 0.04–4 V sꢀ1
c
d
e
range. Calculated from Ep/2 ꢀ Ep = 1.857RT/aF. DEp = Ep(GC/Ag) ꢀ Ep(GC).
Table 3 Voltammetric data for the reduction of RCl in CH3CN + 0.1 M (C2H5)4NClO4 at 298.15 K at Cu
ab
Ep/2 ꢀ Ep (mV)
ad
DET mechanism
DEp (V)
a
Ep (V vs. SCE)
c
e
RCl
qEp/qlog u (mV)
1
2
3
4
5
6
7
8
9
10
a
ꢀ1.776
ꢀ2.159
ꢀ1.949
ꢀ2.270
ꢀ1.616
ꢀ1.564
ꢀ1.416
ꢀ1.486
ꢀ1.273
ꢀ1.009
b
ꢀ36
ꢀ32
ꢀ33
ꢀ44
ꢀ92
ꢀ85
ꢀ84
ꢀ70
ꢀ97
ꢀ70
0.82
0.92
0.90
0.67
0.32
0.35
0.35
0.42
0.30
0.42
51
54
63
72
118
135
127
120
69
0.93
0.88
0.76
0.66
0.40
0.35
0.38
0.40
0.69
0.66
Stepwise
Stepwise
Stepwise
Stepwise
Concerted
Concerted
Concerted
Concerted
Concerted
Concerted
ꢀ0.001
0.002
0.011
0.027
0.556
0.345
0.552
0.594
0.813
0.569
72
u = 0.1 V sꢀ1
.
Calculated from qEp/qlog u = ꢀ1.15RT/aF. Average of the values obtained at different scan rates in the 0.04–4 V sꢀ1
c
d
e
range. Calculated from Ep/2 ꢀ Ep = 1.857RT/aF. DEp = Ep(Cu) ꢀ Ep(GC).
investigated electrodes, catalytic and non catalytic surfaces
alike, follows a concerted mechanism.
values of Ep. This effect can be observed if the ET is the rate
determining step, or at least partially contributes to the kinetic
control, which is the case of compounds 2–4.
The results of this study show a clear and crucial link
between DET mechanism and electrocatalysis at metal cath-
odes. Regardless of the nature of the electrode material,
reduction of compounds 1–4 occurs in two steps, i.e., hetero-
geneous ET followed by dehalogenation of the ensuing radical
anion in solution. GC/Ag, Cu and Pd electrodes show no
catalytic effect for this process, which means that the role of
the electrode is that of an outer sphere electron donor without
any specific interaction with the reagent. However, the double
layer structure may vary slightly with the electrode material
and this will have some effect on the heterogeneous electron
transfer rate constant and, as a consequence, also on the
The reductive cleavage of compounds 5–10 occurs accord-
ing to a concerted DET mechanism. As shown by the data in
Tables 2–4, GC/Ag, Cu and Pd electrodes have remarkable
electrocatalytic activities for the reduction process. In this case
the role of the electrode is not limited to that of an outer
sphere electron donor. It is deeply involved in the reduction
process through surface interactions, which modify consider-
ably the kinetics and/or thermodynamics of the dissociative
electron transfer. We propose the reaction mechanism de-
picted in Scheme 2. There is no clear evidence that RCl and
its reduction products Rꢁ and Xꢀ are strongly adsorbed on the
Table 4 Voltammetric data for the reduction of RCl in CH3CN + 0.1 M (C2H5)4NClO4 at 298.15 K at Pd
ab
Ep/2 ꢀ Ep (mV)
ad
DET mechanism
DEp (V)
a
Ep (V vs. SCE)
c
e
RCl
qEp/qlog u (mV)
1
2
3
4
5
6
7
8
9
10
a
ꢀ1.774
ꢀ2.218
ꢀ2.036
ꢀ2.291
ꢀ1.720
ꢀ1.626
ꢀ1.420
ꢀ1.747
ꢀ1.293
ꢀ0.950
b
ꢀ46
ꢀ52
0.64
0.57
0.66
0.46
0.29
0.23
0.30
0.21
0.29
0.38
58
82
93
84
115
170
150
321
85
0.82
0.58
0.51
0.57
0.41
0.28
0.32
0.15
0.56
0.36
Stepwise
Stepwise
Stepwise
Stepwise
Concerted
Concerted
Concerted
Concerted
Concerted
Concerted
0.001
ꢀ0.057
ꢀ0.076
0.006
0.452
0.283
0.548
0.333
0.790
0.628
ꢀ45
ꢀ64
ꢀ101
ꢀ130
ꢀ98
ꢀ143
ꢀ101
ꢀ78
134
u = 0.1 V sꢀ1
.
Calculated from qEp/qlog u = ꢀ1.15RT/aF. Average of the values obtained at different scan rates in the 0.04–4 V sꢀ1
c
d
e
range. Calculated from Ep/2 ꢀ Ep = 1.857RT/aF. DEp = Ep(Pd) ꢀ Ep(GC).
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Scheme 2 Schematic representation of the concerted DET process at
a catalytic electrode surface.
where BDE and DS are the bond dissociation energy and
entropy of R–X. None of these contributions depends on the
electrode material.
Since the DET process follows a concerted mechanism, it is
characterised by a very high activation free energy stemming
from the bond dissociation energy.5 In fact, in the case of an
outer sphere concerted DET the intrinsic barrier DGa0 , i.e. the
value of DGa when the reaction free energy is zero, is the sum
of two contributions: DGa0 = (lo + BDE)/4, where lo is the
solvent reorganisation energy. In the case of an electrocatalytic
electrode, an activated complex characterised by strong inter-
actions between RCl and electrode surface (see Scheme 2) is
formed. Because of this strong interaction the free energy of
the activated complex is significantly lowered with respect to
the outer sphere ET path.13 Consequently, the activation free
energy of this inner sphere ET is significantly lower than that
predicted for an outer sphere concerted DET. In other words,
the effect of the catalytic surface is equivalent to a decrease of
the BDE contribution to DG0a. Also lo of the electrocatalytic
DET may decrease because of the interaction of the activated
complex with the surface, which may lower the number of
solvent molecules involved.
Fig. 4 Shift of the reduction peak of RCl at (J) GC/Ag, (n) Cu and
(&) Pd electrodes plotted versus Ep at glassy carbon. Data were
obtained in CH3CN + 0.1 M (C2H5)4NClO4 at u = 0.1 V sꢀ1
.
electrode surface. Indeed, most of the compounds show
voltammetric responses typical of electrode processes under
diffusion control. We have previously shown that, in the case
of benzyl chloride, adsorption of RCl is slow and weak, so that
both adsorbed and dissolved RCl molecules are reduced at the
same potential and only one reduction peak is observed.11
Most likely, this is a general behaviour of organic chlorides,
since interactions with silver and perhaps with the other
catalytic metals, at the negative potentials required for their
reduction, are weak. It should be recognised, however, that
electrocatalysis implies surface involvement resulting in a
modification of the outer sphere DET mechanism to a more
favourable reaction route involving an inner sphere electron
transfer.
In principle, the positive shift of the reduction peak poten-
tials may be due to thermodynamic and/or kinetic reasons. If
we consider that the adsorption of reactants is weak, as
mentioned above, the thermodynamics could be affected only
by adsorption of products, resulting in a shift of the standard
reduction potential to less negative values. Unfortunately,
there are very few data on the adsorption of halide ions at
these metals in aprotic solvents. It has recently been reported
that Brꢀ and Iꢀ are not adsorbed on Ag at potentials more
negative than ꢀ1.33 V and ꢀ1.50 V vs. SCE, respectively.29 In
addition, the order of adsorption of halide ions at this metal is
Clꢀ o Brꢀ o Iꢀ in acetonitrile.29 Therefore, it is reasonable to
assume that Clꢀ is not strongly adsorbed at the reduction
potentials of the investigated chlorides. On the other hand,
organic radicals could be adsorbed to form organometallics as
in the case of Hg electrode. However, if this were the case, one
would expect a pre-peak, corresponding to the formation of a
monolayer of adsorbed products. Typically, electrode pro-
cesses involving strong adsorption of products exhibit voltam-
metric responses presenting sharp pre-peaks.28b Since no pre-
peak was observed, at least in the investigated range of scan
rates, we are inclined to exclude adsorption contributions to
the thermodynamics of the process. Finally, the standard
potential of the concerted DET depends on the following
contributions:5
The decrease of DGa results in a decrease of the required
overpotential and hence in a positive shift of Ep with respect to
the value observed at GC.
It is well known that silver has extraordinary electrocataly-
tic properties towards the reduction of organic halides. Most
of the literature data pertain to bulk metal electrodes either
fabricated from a silver wire or prepared by electrodeposition
of thick films of Ag on Pt. One preliminary study on Ag
nanoparticles (average diameter d = 304 nm) deposited on
GC has shown good results for the electroreduction of benzyl
chloride. The data reported here show that Ag nanoparticles
have remarkable electrocatalytic properties for a broad range
of organic chlorides. It may be interesting to compare the
electrocatalytic activities of Ag nanoparticles with those of the
bulk metal. Since only some of the compounds under investi-
gation have been previously studied at bulk Ag electrodes, we
recorded cyclic voltammograms for the whole series of com-
pounds at a silver disc, prepared from a 2-mm diameter Ag
wire, in the same conditions in which the data at GC/Ag
(Table 2) were obtained. The comparison is illustrated in
Fig. 5, where the peak potentials measured at u = 0.1 V sꢀ1
at GC/Ag, EGp C/Ag, are plotted versus Ep values obtained at
bulk Ag, EAp g, at the same scan rate. As clearly seen, the plot is
a straight line with a slope of 0.998 and an intercept of ꢀ0.01
V. The peak potentials obtained at the two electrodes are
identical within an experimental error of ca. 11 mV, suggesting
o
o ꢁ
ꢁ
E RX/R +Xꢀ = E X /Xꢀ ꢀ BDE + TDS
(8)
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sociative electron transfer to RCl and electrocatalysis. There is
good electrocatalysis only when DET to the organic chlorides
follows a concerted mechanism. In fact, in the case of con-
certed DET at an inert electrode, electron transfer is kineti-
cally hampered by the concomitant rupture of the
carbon–chlorine bond. The role of the catalytic surface ap-
pears to be that of lowering the activation energy of the
concerted DET process possibly through the formation of
more favourable activated complexes. In principle, the cata-
lytic surface may also affect the free energy of the reaction if
the reagents or the products are strongly adsorbed. However,
the voltammetric data would be indicative of a catalytic
process without strong adsorption of neither RCl nor its
reduction products.
The catalytic surfaces Ag, Cu and Pd have similar electro-
catalytic properties for the reduction of most of the investi-
gated compounds, suggesting that Cu and Pd are potential
alternatives to Ag for the electrocatalytic reduction of organic
halides. It is to be stressed, however, that the data reported
here are based merely on voltammetric investigations. Unlike
Ag, which has been tested even in large-scale electrosynth-
eses,14,30 neither Cu nor Pd has yet been tested in preparative-
scale applications.
Fig. 5 Comparison between the reduction peak potentials of RCl in
CH3CN + 0.1 M (C2H5)4NClO4 measured at u = 0.1 V sꢀ1 at Ag
nanoclusters (GC/Ag) and bulk Ag.
that the electrocatalytic activities of Ag are preserved upon
decreasing the dimensions of the metal down to particles of the
size of ca. 300 nm.
Let us now compare the electrocatalytic activities of Ag, Cu
and Pd electrodes. Fig. 4 shows a plot of Ep(M) ꢀ Ep(GC)
versus Ep(GC), where Ep(M) and Ep(GC) stand for the peak
potentials measured at catalytic metal surfaces and at glassy
carbon, respectively. Assuming GC is an outer sphere electron
donor, we identify Ep(M) ꢀ Ep(GC) as a measure of the
electrocatalytic properties of the metal M. Examination of
the data illustrated in Fig. 4 shows that Ag, Cu and Pd
electrodes have comparable electrocatalytic activities for the
reduction of almost all investigated compounds. This is an
important result underlining the potentialities of these elec-
trode materials in electrocatalysis.
Acknowledgements
This work was financially supported by the Ministero dell’U-
`
niversita e della Ricerca (MUR).
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