Azolium and acetate ions in DMF: Formation of free N-heterocyclic carbene. A voltammetric analysis

Article abstract of DOI:10.1016/j.elecom.2016.03.012

In order to reveal the possible formation of free N-heterocyclic carbene (NHC) in DMF-azolium and acetate solutions, the proton exchange equilibrium between azolium cations and CH₃COO^- was investigated (by cyclic voltammetry) by addi

Full text of DOI:10.1016/j.elecom.2016.03.012

Electrochemistry Communications 67 (2016) 55–58
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Azolium and acetate ions in DMF: Formation of free N-heterocyclic
carbene. A voltammetric analysis
a
b
a
b
a,c,1
Marta Feroci a, , Isabella Chiarotto , Francesca D'Anna , Luigi Ornano , Carla Rizzo , Achille Inesi
⁎
a
Dept. SBAI, Sapienza University of Rome, via Castro Laurenziano, 7, 00161 Rome, Italy
Dept. STEBICEF, Università degli Studi di Palermo, viale delle Scienze, ed. 17, 90128 Palermo, Italy
b
c
via Antelao, 9 00141 Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to reveal the possible formation of free N-heterocyclic carbene (NHC) in DMF-azolium and acetate solutions,
the proton exchange equilibrium between azolium cations and CH3COO− was investigated (by cyclic voltammetry)
by adding CH3COOH or tetrabutylammonium acetate to DMF solutions of imidazolium or thiazolium salts of
different acidity.
Received 5 February 2016
Received in revised form 3 March 2016
Accepted 15 March 2016
Available online 24 March 2016
The voltammetric analysis confirms that the deprotonation of the azolium cation by CH3COO− (with the
formation of free NHC) is significant in the case of the more acidic thiazolium cations, while it is not effective
with the less acidic imidazolium ones.
Keywords:
Cyclic voltammetry
NHC
Azolium salts
Tetrabutylammonium acetate
Acetic acid
Accordingly, the NHC-catalyzed benzoin condensation was carried out in DMF solutions of azolium salts,
tetrabutylammonium acetate, and benzaldehyde. Benzoin was isolated only starting from the more acidic
thiazolium salts.
© 2016 Elsevier B.V. All rights reserved.
Benzoin condensation
1. Introduction
solution of dialkylimidazolium acetate in the absence of an exogenous
base.
During the last decades, N-heterocyclic carbenes (NHCs) have been
frequently utilized as cheap and environmental friendly organocatalysts
in a plethora of organic syntheses [1–6].
Recently, we reported a simple electrochemical procedure to ascertain
the presence of free NHC in organic solvents [17]. Cyclic voltammetries of
solutions containing an NHC (in a detectable amount) showed the
carbene presence by its oxidation peak. The absence of NHC in 1-butyl-
3-methylimidazolium acetate (BMIm+CH3COO−)–DMF solution without
an exogenous base was emphasized. Nevertheless, it cannot be excluded
that DMF solutions of more acidic azolium acetates could contain a small
amount of free NHC.
In the present communication, we wish to report that
cyclovoltammetric measures are able to reveal the possible presence
of NHC 1a-e (also in small amount) in DMF solutions of azolium and
CH3COO− and to investigate the different behavior of imidazolium or
thiazolium and CH3COO− in the proton exchange equilibrium reaction.
Owing to their possible instability, NHCs are often generated in situ
by deprotonation of the parent azolium salts with suitable bases.
Although the addition of a base to the reaction mixture could be
regarded as necessary, some authors reported NHC-catalyzed syntheses
using imidazolium acetate in the absence of any exogenous base [7–11].
The possible presence of free NHC in dialkylimidazolium acetate
(due to the ability of CH3COO− to deprotonate the imidazolium cation,
which can be seen as the protonated carbene, NHCH+) was extensively
discussed [12].
Although the formation of NHC from dialkylimidazolium acetate
was proven in the gas phase, no NHC was directly detected in pure IL
or in solution, but its presence was inferred by its catalytic activity in
neat IL [13,14]. In fact, the values of pKa of dialkylimidazolium cation
and of acetic acid in DMSO (pKa: 22 and 12 respectively [15,16]) make
questionable the presence of a detectable amount of free NHC in a
2. Experimental
Ionic liquids 2a+ BF4−, 2b+ Cl−, 2c+ Cl−, and Bu4N+ AcO− were
purchased from Iolitec, 2d+BF4− and 2e+BF−4 were synthesized
according to the literature [18,19]. ILs were used after being kept at re-
duced pressure at 70 °C for 24 h. The cyclic voltammetry instrument
was previously described [17]. A 492/GC/3 Amel microelectrode was
employed, using a Pt wire counter electrode and modified saturated calo-
⁎
Corresponding author.
E-mail addresses: marta.feroci@uniroma1.it (M. Feroci), achille.inesi@uniroma1.it
(A. Inesi).
Tel: +39 06 49766563.
mel electrode as reference electrode [20]. The scan rate was ν = 0.2 V s−1
.
1
http://dx.doi.org/10.1016/j.elecom.2016.03.012
1388-2481/© 2016 Elsevier B.V. All rights reserved.

56
M. Feroci et al. / Electrochemistry Communications 67 (2016) 55–58
All cyclic voltammetries were recorded on 5 mL of DMF-0.1 M azolium
salt at 25 °C.
Electrolyses were carried out in the same conditions as the
voltammetries, under galvanostatic conditions (I = 20 mA cm−1) in a
divided cell, and stopped after 31 C. Then the catholyte was analyzed
by voltammetry.
2.1. Benzoin condensation
0.5 mmol of azolium salt, 1.0 mmol of Bu4N+AcO− (or DBU), and
1.0 mL of DMF were stirred at rt for 5 min, then benzaldehyde
(2.0 mmol) was added and the solution was kept at 65 °C for 2 h [21].
Usual workup gave the results reported in Table 1.
3. Results and discussion
Cyclic voltammetry is a technique able to evaluate in which cases the
acetate ion is a base strong enough to deprotonate an azolium cation.
For this purpose, imidazolium (less acidic) and thiazolium (more acidic)
salts were considered. The structures of carbenes 1a-e and of the corre-
sponding azolium salts are reported in Fig. 1.
The extent of the deprotonation of azolium cations (2a–e+) by ace-
tate ion to yield the corresponding NHCs 1a–e and acetic acid (in the
equilibrium Reaction 1, see below) can be at first studied evaluating
the effect on the cyclic voltammetry of the addition of acetic acid to a
azolium salt–DMF solution.
NHCHþ þ CH3COO−⇄NHC þ CH3COOH
ðReaction 1Þ
In fact, the voltammetric curves of azolium salts in DMF (Fig. 2, black
curves) are characterized by
i. a reduction peak related to the monoelectronic reduction of the
azolium cation to NHC and H2 (Ep (red) b −2.5 V vs SCE);
ii. an oxidation peak, in the reverse scan, related to the monoelectronic
oxidation of NHC electrogenerated in the direct scan (Ep (ox) N 0.0 V
vs SCE) [22,23].
Fig. 1. Structures of NHCs and azolium salts. (NHCH+X−) considered in this work
(Bn: CH2Ph; Mes: 2,4,6-trimethylphenyl).
3.1. Addition of acetic acid
3.1.1. Cathodic scan
As regards 2b+Cl−, 2c+Cl−, 2d+BF4−, and 2e+BF−4 DMF solutions,
the voltammetric curves show a pre-peak in addition to the major
oxidation peak. The current of these pre-peaks is dependent on the
structure of the azolium salt. The presence of the two oxidation peaks
required a further investigation to correlate them to the oxidation of
the electrogenerated NHC.
The addition of acetic acid to the azolium salt–DMF solution has a
remarkable effect on the peak current relative to the electrogeneration
of NHC (cathodic scan). In fact, the reduction peak current in the
presence of CH3COOH (ipred, red and blue curves) is higher than that
recorded in its absence (ip0red, black curve, Fig. 2).
These higher values of peak current could be due to
Azolium salts–DMF solutions were electrolyzed in a divided cell
under galvanostatic conditions. The voltammetric curves recorded on
the catholyte (starting potential −1.0 V) show, in the anodic scan,
only one oxidation peak (no pre-peak, Fig. 2, inserts).
Therefore, the presence of a pre-peak could be related to the history
of the electrodic surface during the cathodic scan. We suppose that the
pre-peak and the major peak could be related to the oxidation of NHC
both at the electrodic surface and in the bulk of the solution.
- the direct reduction of CH3COOH to H2 and CH3COO−;
- a proton exchange between CH3COOH and electrogenerated NHC,
yielding again the azolium cation, the increase of the current being
caused by the reduction of this re-formed azolium cation (reverse
of Reaction 1).
Although the peak potential for the reduction of acetic acid in our
experimental conditions is lower than −3.5 V, we cannot exclude a
minimal2 cathodic reduction at higher potentials, especially at high
concentrations. Moreover, according to the values of pKa in DMSO (acetic
acid: 12; imidazolium salts: 20–22; thiazolium salts: 16–19 [24]), the
ipred/ip0red ratio of imidazolium cations 2a+ and 2b+ is higher than that
of thiazolium cations 2c+, 2d+ and 2e+ (i.e., a higher protonation extent
for the more basic NHCs), and this ratio increases on increasing the
Table 1
Benzoin condensation.
⁎
Entry
Azolium salt
Benzoin, yield
1
2
3
4
5
2a+BF−4
2b+Cl−
2c+Cl−
2d+BF−4
2e+BF−4
Traces
2%
46%
12%
13%
2
A substantial co-reduction of acetic acid during the cathodic scan of the cyclic volt-
0.5 mmol azolium salt, 1.0 mmol Bu4N+ AcO−, 1 mL DMF, 5 min rt. Then 2.0
mmol benzaldehyde, 65 °C, 2 h.
ammetry must be excluded, as the increase in the cathodic current is very different for
the different salts in the presence of the same concentration of acetic acid.
⁎

M. Feroci et al. / Electrochemistry Communications 67 (2016) 55–58
57
Fig. 2. Voltammetries of DMF solutions of azolium salts (0.1 M) in the absence (curve a, black) and in the presence of acetic acid (curve b, red, 0.1 M acetic acid; curve c, blue, 0.2 M acetic
acid). Vitreous carbon cathode, ν = 0.2 V s−1, T = 25 °C. Scan potential: 0.0 to −3.6 to +1.5 to 0.0 V vs SCE. Inserts: voltammetries after cathodic reduction of azolium cation. Scan
potential: −1.0 to +1.0 to −1.0 V vs SCE.
CH3COOH concentration. These results emphasize that at the electrodic
surface (during the cathodic scan on DMF solutions containing the
azolium salts and acetic acid) CH3COOH is converted into acetate anion
and H2.
the pre-peak (Fig. 2). Further addition of acetic acid (0.2 mol L−1
)
does not change the overall anodic current. Accordingly, NHC present
at the electrodic surface could be “killed” by the presence of acetic acid.
3.2. Addition of acetate anion
3.1.2. Anodic scan
It is interesting to consider the effect of the presence of acetic acid on
the anodic scan of the voltammetries of azolium salts in DMF. The
comparison between the oxidation peak current of the corresponding
NHC (electrogenerated during the cathodic scan) in the absence
(ip0ox) and in the presence (ipox) of acetic acid shows a different trend
with the different azolium salts.
In order to evaluate the effective capability of the acetate anion to
deprotonate the azolium cations, we added tetrabutylammonium
acetate (Bu4NOAc) to azolium salt–DMF solutions and recorded the
voltammetric curves in the range −1.0 to 2.0 V, thus avoiding the
electrogeneration of NHC.
The addition of Bu4NOAc to DMF solutions of imidazolium salts
2a+BF−4 and 2b+Cl− did not modify the curves, according to the inabil-
ity of CH3COO− to deprotonate NHCH+ 2a+ and 2b+ (Fig. 3, red and
blue curves vs the black one).
In the case of imidazolium 2a+BF4−, the initial oxidation peak splits
into two very close oxidation peaks. The values of the overall anodic cur-
rent in the presence of acetic acid are slightly lower than in its absence
(according to the protonation of NHC by CH3COOH).
On the contrary, the addition of Bu4NOAc to DMF solutions of
thiazolium salts (2c+Cl−, 2d+BF4−, and 2e+BF4−) brought about the ap-
pearance of an anodic current corresponding to the oxidation of NHCs
Surprisingly, the addition of acetic acid (0.1 mol L−1) to 2b+Cl−
,
2c+Cl−, 2d+BF4−, 2e+BF−4 DMF solutions causes the disappearance of
Fig. 3. Voltammetries of DMF solutions of azolium salts (0.1 M) in the absence (curve a, black) and in the presence of tetrabutylammonium acetate or DBU (curve b, red, 0.1 M Bu4NOAc or
DBU; curve c, blue, 0.2 M Bu4NOAc or DBU). Vitreous carbon cathode, ν = 0.2 V s−1, T = 25 °C. Scan potential: −1.0 to +2.0 to −1.0 V vs SCE.

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M. Feroci et al. / Electrochemistry Communications 67 (2016) 55–58
(1c, 1d, and 1e), according to the ability of CH3COO− to deprotonate
NHCH+ 2c+, 2d+, and 2e+ (Fig. 3, red and blue curves vs the black one).
To verify this last inference, we have studied the 2c+Cl−DBU–DMF
solution. DBU is a base strong enough to deprotonate 2c+ to NHC.
Therefore, this last solution contains free NHC. The voltammetric curves
recorded on this solution and on the 2c+Cl−acetate–DMF solution
show the same anodic peaks (Fig. 3).
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3.3. The benzoin condensation, a synthetic support
To bear out these results, we carried out a classical NHC-catalyzed
reaction, the benzoin condensation [18,21], by adding benzaldehyde
to a DMF solution of azolium salts 2a-e+-X− and Bu4NOAc, and keeping
the reaction mixture at 65 °C for 2 h. No attempt was made to optimize
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Benzoin was obtained in moderate yields (46–12%) in the presence
of thiazolium salts (Table 1, entries 3–5) while it was isolated in negligi-
ble yields (b2%) in the presence of imidazolium salts (Table 1, entries 1
and 2). In order to exclude the hypothesis that NHCs 1a-e were not
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condensation under the same experimental conditions, but using the
stronger base DBU instead of Bu4NOAc. Benzoin was obtained in 38%
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Therefore, in our experimental conditions, a strong base (DBU but not
CH3COO−) is able to deprotonate the imidazolium salt and give NHC,
thus catalyzing the benzoin condensation.
4. Conclusions
The use of N-heterocyclic carbenes as organocatalysts needs, in prin-
ciple, their generation by deprotonation of the corresponding azolium
salt with a base. Recently, it was reported that CH3COO− is a base strong
enough to deprotonate some imidazolium cations; in this case, the use
of an imidazolium acetate (in the absence of any added base) would
be sufficient to carry out organocatalyzed reactions.
The voltammetric analysis of DMF solutions of imidazolium
and thiazolium salts in the presence of CH3COOH or CH3COO− makes
it possible to determine which azolium–acetate couple gives NHC.
We can exclude a significant presence of free NHC in solutions of
imidazolium salts in the presence of an excess of CH3COO−, while we
confirm the presence of free NHC in thiazolium salts–DMF solutions in
the presence of CH3COO−, demonstrating the ability of CH3COO− to
deprotonate these thiazolium cations.
Acknowledgment
We thank Mr. Marco Di Pilato for his support to the experimental
part of this work, and Sapienza University of Rome for financial support.
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