A unique proton coupled electron transfer pathway for electrochemical reduction of acetophenone in the ionic liquid [BMIM][BF4] under a carbon dioxide atmosphere

Article abstract of DOI:10.1039/c1gc15929a

The mechanism of electrochemical reduction of acetophenone in 1-butyl-3-methylimidazolium tetrafluroborate ([BMIM][BF₄]) under nitrogen (N₂) and carbon dioxide (CO₂) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. Under a N₂ atmosphere, acetophenone undergoes a one-electron reduction to the radical anion followed by rapid dimerization reactions with an apparent rate constant of 1.0 × 10⁶ M^-1 s^-1. In contrast, under a CO ₂ atmosphere, the electrochemical reduction of acetophenone is an overall two-electron transfer chemically irreversible process with the final electrolysis product being 1-phenylethanol, instead of the anticipated 2-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton coupled electron transfer pathway leading to the formation of 1-phenylethanol requires the presence of a sufficiently strong proton donor which is not available in neat [BMIM][BF₄]. However, the presence of CO₂ enhances the C-2 hydrogen donating ability of [BMIM]⁺ due to strong complex formation between the deprotonated form of [BMIM] ⁺, N-heterocyclic carbene, and CO₂, resulting in a thermodynamically favorable proton coupled electron transfer pathway.

Full text of DOI:10.1039/c1gc15929a

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PAPER
A unique proton coupled electron transfer pathway for electrochemical
reduction of acetophenone in the ionic liquid [BMIM][BF ] under a carbon
4
dioxide atmosphere
Shu-Feng Zhao,a,b La-Xia Wu, Huan Wang, Jia-Xing Lu,* Alan M. Bond* and Jie Zhang*
a
a
a
b
b
Received 30th July 2011, Accepted 16th September 2011
DOI: 10.1039/c1gc15929a
The mechanism of electrochemical reduction of acetophenone in 1-butyl-3-methylimidazolium
tetrafluroborate ([BMIM][BF
been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and
numerical simulation. Under a N atmosphere, acetophenone undergoes a one-electron reduction
to the radical anion followed by rapid dimerization reactions with an apparent rate constant of
4
]) under nitrogen (N
2
) and carbon dioxide (CO
2
) atmospheres have
2
6
-1 -1
1
.0 ¥ 10 M s . In contrast, under a CO atmosphere, the electrochemical reduction of
2
acetophenone is an overall two-electron transfer chemically irreversible process with
the final electrolysis product being 1-phenylethanol, instead of the anticipated
2
-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton
coupled electron transfer pathway leading to the formation of 1-phenylethanol requires the
presence of a sufficiently strong proton donor which is not available in neat [BMIM][BF
4
].
+
However, the presence of CO
strong complex formation between the deprotonated form of [BMIM] , N-heterocyclic carbene,
and CO , resulting in a thermodynamically favorable proton coupled electron transfer pathway.
2
enhances the C-2 hydrogen donating ability of [BMIM] due to
+
2
8
Introduction
pathway, with 1-phenylethanol as the main product. Under a
CO atmosphere, the intermediate complex of the one-electron
reduced form of acetophenone radical anion and CO can accept
2
Electrochemical studies of ketones have received considerable
attention in recent years due to their applications in the
pharmaceutical industry as precursors for the formation of high
value chemicals using an environmentally friendly electrocar-
boxylation reaction with carbon dioxide.
Acetophenone, which represents one of the most widely
2
an additional electron to form 2-hydroxy-2-phenylpropionic
acid. However, due to the competitive co-existence of the
9
dimerization route, the reduction reaction also generates dimers.
The ratio between 2-hydroxy-2-phenylpropionic acid and dimers
depends on the experimental conditions. In particular, the sol-
vent significantly affects both the kinetics and thermodynamics
of dimerization and electrocarboxylation reactions.
Several groups, including us, have reported the preparative
scale electrocarboxylation reaction of acetophenone in con-
ventional organic solvents using an undivided electrolysis cell
1–3
4–6
studied ketones, has the ability of accepting one or two
electrons. In conventional solvents, such as dimethylformamide
(
DMF), acetonitrile (MeCN), N-methyl-2-pyrrolidone (NMP),
methanol water mixtures etc., and under a N atmosphere, ace-
tophenone undergoes a one-electron reduction reaction followed
by a homogeneous dimerization reaction leading to the for-
2
9–12
equipped with an appropriate cathode and a sacrificial anode.
-Hydroxy-2-phenylpropionic acid was obtained in good yield.
7
mation of electro-inactive 2,3-diphenyl-butane-2,3-diol. Upon
2
addition of a sufficiently strong acid, this one-electron reduction
process can be converted into an overall two-electron reduction
process through a proton coupled electron transfer reaction
However, with the growing demand for environmentally friendly
processes, efforts need to be devoted to eliminate the use of
volatile and toxic solvents, such as DMF, NMP and MeCN. In
recent years, room-temperature ionic liquids (RTILs) have been
advocated in green chemical applications in organic synthesis
because they may exhibit desirable properties such as negligible
volatility, thermal and chemical stability, low toxicity and non-
flammability. In addition, their high conductivity, wide electro-
chemical potential windows and the ability to dissolve large
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, Shanghai,
2
00062, China. E-mail: jxlu@chem.ecnu.edu.cn; Fax: +86 21 6223
491; Tel: +86 21 6223 3491
3
b
School of Chemistry, Monash University, Clayton, Victoria, 3800,
Australia. E-mail: alan.bond@monash.edu, jie.zhang@monash.edu;
Fax: +61 3 99054597; Tel: +61 3 99051338, +61 3 99056289
13–17
amounts of CO
2
and organic substrates
make RTILs very
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1
8–23
useful media for electrocarboxylation reactions.
Lagrost and
Simulations of the voltammograms were undertaken with the
ꢀ
R
co-workers have reported the electrochemistry of acetophenone
in several imidazonium and ammonium based ionic liquids
containing bis(trifluromethylsulfonyl)imide anion under a N
atmosphere. They found that electroreduction of acetophe-
none in these ionic liquids follows an EC (heterogeneous electron
transfer followed by a chemical reaction) mechanism with the
C step being a rapid dimerization reaction. However, detailed
mechanistic studies of electrochemistry of acetophenone in ionic
DigiElch software package. A two-dimensional mass transport
model was used to take into account the contribution from radial
diffusion at slow scan rates. Butler–Volmer kinetics was used to
describe the potential-dependent kinetics of the heterogeneous
electron transfer.
2
24
Bulk electrolysis
◦
Potentiostatic bulk electrolysis was undertaken at 25 C under
liquids in the presence of CO
In this paper, we report detailed electrochemical studies of ace-
tophenone in the ionic liquid [BMIM][BF ] under both N and
CO atmospheres using transient cyclic voltammetry, steady-
2
have not been reported.
both N
2
and CO atmospheres in an undivided cell equipped
2
with a glassy carbon beaker cathode, a magnesium sacrificial
anode and a silver wire quasi-reference electrode immersed into
4
2
2
[
BMIM][BF
of CO (to provide a constant supply of the reactant) or
(to prevent O from entering the electrolysis cell) was
applied during the electrolysis. 5 mL of [BMIM][BF ] containing
0 mM acetophenone was used for electrolysis. The duration of
4
] and separated by a glass frit. A slow stream
state voltammetry, bulk electrolysis and numerical simulation.
2
N
2
2
Experimental
4
5
Chemicals
electrolysis, defined as the time required to reach theoretical
value of electricity consumption for a completed electrolysis, is
typically about 10 h due to the low mass transport rate associated
[
BMIM][BF
4
] was purchased from IOLITEC (Germany). The
ionic liquid was dried under vacuum at 80 C for 24 h prior to
◦
use. The water content in [BMIM][BF
4
] after such a pretreatment
with highly viscous [BMIM][BF ]. After electrolysis under a
4
step is 60 ± 10 ppm as measured by Karl Fischer titration.
N atmosphere, the products were extracted three times with
2
ꢀ
R
Acetophenone (ReagentPlus , 99%), 4-methoxy acetophenone
diethyl ether. After the electrolysis under a CO atmosphere,
2
ꢀ
R
(
99%), MeI (ReagentPlus , 99.5%), diethyl ether (Analytical
reagent, >99%) and ferrocene (≥98%) were purchased from
Sigma Aldrich and used as received. [Bu N][PF ] (GFS Chem-
the reaction mixture was esterified in [BMIM][BF ] by adding
4
anhydrous K CO (1 mmol) and MeI (3 mmol) with stirring the
2
3
◦
4
6
mixture at 50–60 C for 5 h. After this step, the reaction mixture
was extracted three times with diethyl ether. Finally, the organic
icals, Inc, 98%) was recrystallised twice with ethanol prior to
use.
layer was dried with anhydrous MgSO , filtered, concentrated
4
and then analyzed by GC–MS (HP 6890A gas chromatograph
equipped with a 5973 N mass selective detector, Agilent, USA).
Voltammetric measurements
Voltammetric measurements were undertaken with a CHI760
electrochemical analyzer (CH Instruments, Texas, USA) using a
standard three-electrode cell. For transient cyclic voltammetric
experiments, glassy carbon (GC (1.8 and 3 mm diameters)) or
other metallic macrodisc electrodes (Ag (2 mm diameter), Cu
Results and discussion
Transient cyclic voltammograms of acetophenone reduction at a
macrodisc electrode
(
2 mm diameter), Ni (2 mm diameter), Ti (2 mm diameter) and
Electroreduction of acetophenone occurs in the very negative
Pt (2 mm diameter)) or a carbon fiber microdisc electrode (33 mm
diameter) was used as the working electrode. Ag wire was used as
the quasi-reference electrode and Pt wire was used as the counter
electrode. This quasi reference potential scale was then converted
potential region where direct electroreduction of CO may also
occur. Therefore, cyclic voltammetric experiments were first
2
conducted to identify suitable electrodes for the investigation
of acetophenone reduction in [BMIM][BF ] under a CO
4
2
+
to the Fc/Fc (Fc = ferrocene) reference potential scale. Working
atmosphere without suffering any interference from the direct
electrodes were polished using 0.3 mm or 0.05 mm alumina slurry
on a polishing cloth (BAS), sonicated in deionised water, rinsed
with water and acetone and then dried with nitrogen before
use. For the steady-state voltammetric measurements, a carbon
fiber microdisc electrode (7 mm diameter) was used together
with the aforementioned counter and reference electrodes. All
voltammetric experiments were undertaken at room temperature
of 25 ± 2 C in a dry box. The exact radii (r) of the carbon
fiber microdisc electrodes used for quantitative studies were
determined to be 3.6 and 18 mm respectively, based on the steady-
electroreduction of CO . Cyclic voltammograms of 10 mM
2
acetophenone (under a N atmosphere) and saturated CO were
2
2
obtained at several commonly used electrodes, including Ag
(2 mm diameter), Cu (2 mm diameter), Ni (2 mm diameter),
Ti (2 mm diameter), Pt (2 mm diameter) and GC (3 mm
-
1
diameter), at a scan rate of 0.05 V s (Fig. 1). In all cases, no
companion oxidation process was observed to accompany the
electroreduction of acetophenone within the potential region
of investigation when the potential direction was reversed and
scanned in the positive potential direction. This suggests that the
reduction of acetophenone is chemically irreversible on the time
scale of the measurement due to the presence of a homogeneous
reaction involving the electrogenerated acetophenone radical
anion. The homogeneous reaction coupled to electron transfer is
most likely the dimerization reaction according to the previous
◦
state diffusion limiting current (I
MeCN/0.1 M [Bu N][PF ], using the following equation and
a known diffusion coefficient (D) value of 2.4 ¥ 10 cm s .
L
) for the oxidation of Fc in
25
4
6
-
5
2
-1 26
I
L
= 4nFrDC
(1)
7,8
24,27
where n is the number of electrons transferred (n = 1 in this case);
F is Faraday’s constant and C is the concentration of Fc.
studies in both conventional solvents and ionic liquids.
The
electroreduction of acetophenone remained fully irreversible
3
462 | Green Chem., 2011, 13, 3461–3468
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Fig. 1 Cyclic voltammograms recorded at a scan rate of 0.05 V s- using designated working electrodes (Ag, Cu, Ni, Ti and Pt (2 mm diameter),
1
4 2 2
and GC (3 mm diameter)) in [BMIM][BF ] containing 10 mM acetophenone under a N atmosphere ( ) and saturated CO at 1 atm (---).
+
Table 1 Peak potentials (V vs. Fc /Fc) obtained for the reduction of
CO
2
and acetophenone in [BMIM][BF ] at designated electrodes under
4
a
voltammetric conditions
Electrodes
Ag
Cu
Ni
Pt
Ti
GC
E
E
P,CO2
-2.23
-2.17
-2.34
-2.20
-2.36
-2.19
—
-2.27
—
-2.49
—
-2.18
P,acetophenone
a
Experimental conditions are summarized in Fig. 1.
even when a scan rate of 10 V s- was used. The peak potential
for acetophenone reduction at the Ti electrode (-2.49 V) is
much more negative than those recorded at Cu (-2.20 V), Ni
1
(
-2.19 V), glass carbon (GC) (-2.18 V) and Ag (-2.17 V). The
electroreduction of acetophenone at the Pt electrode (-2.27 V),
although not as negative as at Ti, is much closer to the
solvent negative potential limit, which is more positive at the
Pt electrode.
Fig. 2 Cyclic voltammograms recorded at a 1.8 mm diameter GC
-1
electrode (scan rate = 0.1 V s ) in neat [BMIM][BF
b) CO (1 atm) atmospheres, and for [BMIM][BF ] containing 10 mM
acetophenone under (c) N and (d) CO (1 atm) atmospheres. Cyclic
4 2
] under (a) N or
Direct electroreduction of CO also occurs at all electrodes in
2
(
2
4
the potential range of acetophenone reduction except for the GC
electrode. GC is known to be a more inert electrode, compared
2
2
+
voltammograms obtained for the Fc/Fc process (around 0 V) with
2.7 mM Fc are included for comparison.
28
to metallic electrodes for direct CO reduction. Peak potentials
2
for the irreversible reduction of both acetophenone and CO
summarized in Table 1.
Based on the observations outlined above, GC was selected
as the electrode material of choice for detailed investigations of
2
are
fact that the oxidation peak current increases almost proportion-
ally with the increase in acetophenone concentration (Fig. 3),
suggests that this new compound is not resulting from the
the electroreduction of acetophenone under both N
atmospheres. Under a N atmosphere, the acetophenone reduc-
tion peak current is proportional to the square root of the scan
2
and CO
2
reaction between water (less than 3.0 mM in the [BMIM][BF
4
]
2
used in our studies) or other impurities in [BMIM][BF ] and
4
the electroreduction products of acetophenone. A similar ob-
-
1
-1
8
rate in the range of 0.05 V s to 1.0 V s . This suggests that both
the heterogeneous electron transfer and coupled homogeneous
servation reported by Amatore and co-workers in their studies
of electroreduction of acetophenone in DMF in the presence
of cyclodextrin was attributed to the formation of a head-to-
tail type of dimer (1-[4-(1-hydroxy-ethylidene)-cyclohexa-2,5-
dienyl]-1-phenyl-ethanol). To investigate whether this is also the
case in our study, voltammetric experiments with 4-methoxy
acetophenone were also undertaken. The formation of a
25
reaction are rapid on the time scale of these measurements.
A small oxidation process at -1.09 V was also observed in
the reverse cycle of potential after the irreversible reduction of
acetophenone (Fig. 2). This indicates the formation of a new
compound during the electroreduction of acetophenone. The
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4 2
Scheme 1 Reaction scheme proposed for reduction of acetophenone in [BMIM][BF ] under a N atmosphere.
if the observation of increased current in the presence of CO
due to the decrease in viscosity of [BMIM][BF ], Fc was added to
the solution and cyclic voltammograms of the reversible Fc /Fc
process were also recorded under both N and CO atmospheres
Fig. 2). The fact that the oxidation peak current for the Fc /Fc
process remains essentially unaltered suggests that the viscosity
of [BMIM][BF ] does not vary appreciably in the presence of
CO . Consequently, the increase of acetophenone reduction
peak current and positive shift of peak potential in the presence
of CO are attributed to the occurrence of new chemistry, likely
involving an additional electron transfer step. In the presence of
CO , a minor new oxidation process with a peak potential of
0.68 V was found in the reverse potential scan direction after
2
is
4
+
2
2
+
(
4
2
2
2
-
the potential was switched following the irreversible reduction
of acetophenone.
-1
Fig. 3 Cyclic voltammograms (scan rate = 0.1 V s ) recorded under a
atmosphere with a 1.8 mm diameter GC electrode in [BMIM][BF
containing designated concentrations of acetophenone (a: 5 mM, b:
0 mM, c: 50 mM and d: 100 mM). The inset figure shows a cyclic
Steady-state voltammograms of acetophenone reduction at a
carbon fiber microdisc electrode
N
2
4
]
1
To gain further mechanistic insights, near steady-state voltam-
metric measurements of acetophenone reduction under both
-1
voltammogram (scan rate= 0.1 V s ) of 10 mM 4-methoxy acetophenone
recorded under a N atmosphere with a 3.0 mm diameter GC electrode
in [BMIM][BF ].
2
N
2
and CO
2
atmospheres were undertaken in [BMIM][BF ]
4
4
with a 7 mm diameter carbon fiber microdisc electrode. The
results obtained with three different acetophenone concen-
trations (1.0, 10.0 and 50.0 mM) are shown in Fig. 4. The
steady-state diffusion limiting currents are proportional to
head-to-tail type of dimer is prevented during this electrore-
duction process due to the presence of a methoxy group at the
para position on the aromatic ring. The fact that no oxidation
process was observed (see inset of Fig. 3) suggests that the
small oxidation peak at -1.09 V (Fig. 3) could be assigned to
the oxidation of the head-to-tail type of dimer. Based on these
the concentration of acetophenone under both CO
atmospheres. Importantly, the steady-state diffusion limiting
currents obtained under a CO atmosphere are twice those
obtained under N . Since acetophenone is believed to undergo a
one-electron reduction reaction under a N atmosphere (Scheme
1), an overall two-electron transfer reaction must occur under a
CO atmosphere. The doubling of the steady-state current in the
presence of CO implies that the follow-up chemistry is not rate
2
and N
2
2
2
8,24
experimental observations and the results from the literature,
the following reaction scheme is proposed. The important roles
2
24
played by the ionic liquid during the formation of dimers are
omitted in Scheme 1 for simplicity.
2
2
Upon saturation of [BMIM][BF
4
] with CO
2
, the acetophe-
limiting and is faster than the dimerization reaction (Scheme 1).
Under our experimental conditions, acetophenone is likely to
undergo a two-electron reduction pathway via a proton coupled
electron transfer reaction described in Scheme 2 which is known
to occur in the presence of a sufficiently strong acid or via the
electrocarboxylation reaction described in Scheme 3 which has
none reduction peak current magnitude increased significantly
and the peak potential shifted to a more positive value.
Compton and co-workers found that the viscosity of 1-
ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide
8
29
decreased by nearly 20% when saturated with CO
2
. To ascertain
3
464 | Green Chem., 2011, 13, 3461–3468
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glassy carbon beaker cathode and a Mg sacrificial anode in an
undivided cell.
After electrolysis under a N
the electrolysis were separated from [BMIM][BF
2
atmosphere, the products of
] by extraction
4
with diethyl ether. Products obtained after electrolysis and
identified by GC–MS along with their yields are summarized
in Table 2. No 1-phenylethanol could be detected by GC–MS
analysis. These results confirm that the formation of dimers
(100%) after electroreduction of acetophenone (Scheme 1) is
the major reaction pathway. However, the GC–MS results do
not provide information on the proportion of each dimer since
they are essentially indistinguishable by GC–MS under our
experimental conditions.
Under a CO
2
atmosphere, one possible product is 2-hydroxy-
2-phenylpropionic acid resulting from the electrocarboxylation
reaction. To identify this product by GC–MS, it is necessary
to convert the acid to an ester through an esterification
reaction with MeI. After esterification, products were separated
from [BMIM][BF ] by extraction with diethyl ether. Products
4
obtained after electrolysis were identified by GC–MS and their
yields determined; all results are summarized in Table 2. The
data obtained reveal that the formation of 1-phenylethanol
after electroreduction of acetophenone (Scheme 2) is the major
reaction pathway, an outcome which is totally different to that
found in molecular solvents (DMF, MeCN and NMP). In
these conventional organic solvents, carboxylation and dimer-
ization reactions are the major chemical reactions that follow
radical anion formation. This pathway is also highly unexpected
Fig. 4 Near steady-state voltammograms for the reduction of acetophe-
none (1 mM, 10 mM and 50 mM) in [BMIM][BF ] recorded at a carbon
fiber microdisc electrode (7.2 mm diameter) at a scan rate of 10 mV s
under N ) or CO (---) atmospheres.
9,12
4
-1
2
(
2
in the [BMIM][BF ] medium, since a proton coupled electron
4
transfer reaction pathway (Scheme 2) requires the presence
8
Scheme 2 Reaction scheme for reduction of acetophenone in acidic
media.
of a moderately strong acid. The acidity of [BMIM][BF
4
]
31
is very weak (pKa about 21–23. ). Furthermore, previous
studies by Fredlake et al. suggest that the addition of CO
32
2
to ionic liquids does not modify the strength of hydrogen bonds
between ionic liquids and the solvatochromic dye, Reichardt’s
dye 33. Consequently, the enhanced proton donating ability of
Scheme 3 Reaction scheme for reduction of acetophenone in conven-
[
BMIM][BF
4
] in the presence of CO is assumed to be induced
2
tional solvent media under a CO
2
atmosphere.
by another thermodynamically favorable processes.
In order to establish a possible mechanism which supports a
proton coupled electron transfer pathway, it is crucial to identify
been found to apply in conventional molecular solvents, such as
9,10,30
MeCN, DMF and NMP, in the presence of CO .
2
the source of the proton. In the ionic liquid [BMIM][BF
4
], a
proton may be derived from a trace impurity (e.g. H O) or
the C-2 hydrogen in [BMIM] . Since the water content in the
2
Preparative scale bulk electrolysis
+
31
To verify that Scheme 1 is applicable to the acetophenone
reduction under a N atmosphere and to identify which of the
reaction pathways applies under a CO atmosphere, preparative
[BMIM][BF ] used in these studies is relatively low (less than
4
+
2
3.0 mM), the C-2 hydrogen of [BMIM] is the most likely source
+
2
of the proton. In principle, [BMIM] can donate its C-2 hydrogen
scale bulk electrolysis experiments were undertaken using a large
to a strong base, such as the electrogenerated acetophenone
a
Table 2 Products obtained from preparative scale reductive electrolysis of 50 mM acetophenone in [BMIM][BF
4
]
b
Product yield (%)
Atmosphere
P = 1 atm)
Applied potential
(V vs. Fc /Fc)
1
+
c
(
Anode/Cathode
Charge/F mol-
dimer
acid
0
alcohol
Conv. (%)
N
2
Mg/GC
Mg/GC
1.0
2.0
-2.28
-2.23
100
trace
0
97
66
65
d
CO
2
1
a
Dimer = 1-[4-(1-hydroxy-ethylidene)-cyclohexa-2,5-dienyl]-1-phenyl-ethanol and 2,3-diphenyl-butane-2,3-diol; acid = 2-hydroxy-2-phenylpropionic;
b
c
d
alcohol = 1-phenylethanol. Gas chromatography yields based on the consumed substrate. Acetophenone reduced/total acetophenone. Identified
after conversion to an ester through an esterification reaction with CH I.
3
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Fig. 5 (a) Cyclic voltammogram for the reduction of 10 mM acetophenone in [BMIM][BF
4
] recorded at a carbon fiber microdisc electrode (36 mm
-1
-1
diameter) at scan rates of 5 V s or 100 V s and (b) voltammograms simulated based on the mechanism described in Scheme 5 and the parameters:
0
-1
-10
-7
2
-1
6
-1 -1
E = -2.265 V, k
s
= 0.035 cm s , a = 0.5, R = 30 000 X, C
d
= 2.3 ¥ 10 F, D = 5 ¥ 10 cm s for all species, k = 1.0 ¥10 M s .
f
33
radical anion, and form a N-heterocyclic carbene. However,
+
the acidity of [BMIM] is too weak to favour this scenario and
the dimerization reaction of the acetophenone radical anion
is very rapid. Therefore, the proton coupled electron transfer
reaction pathway (Scheme 2) leading to the formation of 1-
phenylethanol does not occur in neat [BMIM][BF
4
] under a N
2
atmosphere. However, the situation prevailing in the presence of
CO
stable complex with N-heterocyclic carbene.
process in the presence of CO
2
is different. It has been reported that CO
2
can form a highly
34–36
The follow up
2
is highly thermodynamically
+
and kinetically favorable. Plausibly, [BMIM] could donate
its C-2 hydrogen much more readily to the electrogenerated
acetophenone radical anion and the benzhydrol monoanion.
Consequently, it is proposed that a proton coupled electron
transfer reaction can occur favorably through the reaction
pathway given in Scheme 4.
Scheme 4 Reaction scheme proposed for reduction of acetophenone in
BMIM][BF ] under a CO atmosphere.
[
4
2
fiber microdisc electrode was used to obtain very fast scan rate
voltammetric data for acetophenone reduction under both N
and CO atmospheres.
Under a N atmosphere, the electroreduction of acetophenone
2
2
Kinetics of coupled homogeneous chemical reactions occurring
during the electroreduction of acetophenone
2
-
1
is completely chemically irreversible at a scan rate of 5 V s .
The partial chemical reversibility required for quantifying the
Voltammetric data obtained at a 1.8 mm diameter macrodisc
GC electrode suggest that the coupled homogeneous reaction
kinetics are very fast (Fig. 2). In order to quantify fast kinetics
using transient voltammetry, a sufficiently fast scan rate is
required so that the kinetics becomes relatively slow with respect
to the time scale of the measurement. In order to overcome the
intolerable uncompensated resistance effects occurring at very
fast scan rates when using a macroelectrode, microelectrodes are
kinetics was achieved when the scan rate was increased to 100 V
-1
s
(Fig. 5). To quantify the kinetics of dimerization reactions
using numerical simulation, a simplified reaction mechanism
was proposed in Scheme 5 taking into account the combined
contribution from two dimerization reactions.
0
In this scheme, E , a and k
s
represent the formal reversible
potential, electron transfer coefficient and formal heterogeneous
electron transfer rate constant, respectively. k stands for the ap-
parent rate constant of dimerization reactions. For quantitative
f
37–41
conventionally used.
Consequently, a 36 mm diameter carbon
4 2
Scheme 5 Simplified reaction scheme proposed for reduction of acetophenone in [BMIM][BF ] under a N atmosphere.
3
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+
numerical simulations, it is important to accurately know the
diffusion coefficients of all species involved in the reaction. The
enhanced C-2 hydrogen donating ability of [BMIM] resulting
from the strong complex formation between the deprotonated
-
7
2
-1
+
diffusion coefficient of 5.0 ¥ 10 cm s for acetophenone is
calculated from the steady-state diffusion limiting current of
acetophenone reduction (Fig. 4) using eqn (1). The diffusion
coefficients of other species involved in the reactions are assumed
to be the same as for acetophenone. Simulated voltammograms
obtained using a heterogeneous electron transfer rate constant
form of [BMIM] , N-heterocyclic carbene, and CO
2
. In this case,
the follow up chemical reactions are too fast to be measured
by cyclic voltammetry under our experimental conditions. A
lower limit of the rate constant associated with the rate limiting
chemical reaction step in this process is estimated to be 3.0 ¥
5
-1
10 s by numerical simulation based on a simplified reaction
-
1
of 0.035 cm s , taken from the previous studies by Lagrost
scheme.
24
-7
2
-1
et al., a D value of 5.0 ¥ 10 cm s and other parameters
specified in the caption of Fig. 5 agree well with the experimental
results. The major discrepancy between experimental results
and simulated data is attributed to the fact that double layer
Research of this kind should lead to new insights into the
reactivity of aromatic ketones in ionic liquids in the presence
of CO and other molecules in a green chemistry context.
Further investigations using other aromatic ketones and room
temperature ionic liquids are in progress.
2
capacitance (C ) is not constant as assumed in the simulations.
d
The second order rate constant for the dimerization reactions
6
-1
-1
is estimated to be 1.0 ¥ 10 M
s
based on the experiment
Acknowledgements
versus theory comparisons. This value falls within the range of
5
-1 -1
6
-1 -1
0.8 ¥ 10 M
s
to at least 4.0 ¥ 10 M
s
reported previously
This work was financially supported by the Project for the
National Natural Science Foundation of China (20973065),
for the same system in other imidazolium based ionic liquid
24,27
media.
Under a CO
none remained completely chemically irreversible even at a
“Chen Guang” project supported by Shanghai Municipal
2
atmosphere, the electroreduction of acetophe-
Education Commission and Shanghai Education Development
Foundation, China (10CG26), Basic Research in Natural Sci-
ence Issued by Shanghai Municipal Committee of Science,
China (08dj1400100), the Fundamental Research Funds for
the Central Universities, China, Shanghai Leading Academic
Discipline Project, China (B409) and the Australian Research
Council.
-
1
scan rate of to 5 000 V s (the upper limiting achievable
with our potentiostat). Thus, the kinetics of the follow-up
chemical reaction are now too fast to be measured under our
experimental conditions. However, this observation explains
why dimer formation is negligible under a CO
2
atmosphere. A
lower limit of the rate constant associated with the chemical
5
-1
reaction is estimated to be 3.0 ¥ 10 s . This value is obtained
using a numerical simulation based on a simplified reaction
mechanism described in Scheme 6. In Scheme 6, it is assumed
that the first process is reversible and the second protonation
of the benzhydrol monoanion is the rate limiting step. Since
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