Stability and organocatalytic efficiency of N-heterocyclic carbenes electrogenerated in organic solvents from imidazolium ionic liquids

Article abstract of DOI:10.1016/j.electacta.2014.11.135

The kinetic of degradation of 1-butyl-3-methylimidazole-2-ylidene (selected as model N-heterocyclic carbene - NHC), generated in organic solvents by cathodic reduction of the parent 1-butyl-3-methylimidazolium salts BMIm-X, was studied by a simple voltammetric analysis. The effect of NHC degradation rate on the efficiency of an organocatalyzed reaction (the synthesis of γ-butyrolactone from cinnamaldehyde and trifluoromethylacetophenone) was investigated. The nature of the solvent and of the anion X^- have a remarkable effect on the stability of the NHC, the bis(trifluoromethylsulfonyl) imide anion being the best for a long lasting carbene (while acetonitrile seems to be the worst solvent). The role of X^- has been related to a competition between NHC and X^-, in the hydrogen bonding interaction with BMIm⁺. The higher the stabilization of NHC by hydrogen bond, the lower its degradation rate. These hydrogen bonding interactions, previously reported in pure BMIm-X, seem to be operative even in organic solvents containing BMIm-X at low concentrations (c < 0.1 mol L^-1). The effect of the nitrogen alkyl substituents on the degradation of NHC (and thus on its efficiency as organocatalyst) is also pointed out.

Full text of DOI:10.1016/j.electacta.2014.11.135

Electrochimica Acta xxx (2015) 122–129
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
STABILITY AND ORGANOCATALYTIC EFFICIENCY OF N-HETEROCYCLIC
CARBENES ELECTROGENERATED IN ORGANIC SOLVENTS FROM
IMIDAZOLIUM IONIC LIQUIDS
a
b
a
Marta Feroci a, , Isabella Chiarotto , Francesca D’Anna , Gianpiero Forte ,
*
c,**
Renato Noto b, 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
via Antelao, 9 00141 Rome, Italy
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
The kinetic of degradation of 1-butyl-3-methylimidazole-2-ylidene (selected as model N-heterocyclic
carbene - NHC), generated in organic solvents by cathodic reduction of the parent 1-butyl-3-
methylimidazolium salts BMIm-X, was studied by a simple voltammetric analysis. The effect of NHC
Received 2 October 2014
Received in revised form 19 November 2014
Accepted 21 November 2014
Available online xxx
degradation rate on the efficiency of an organocatalyzed reaction (the synthesis of g-butyrolactone from
cinnamaldehyde and trifluoromethylacetophenone) was investigated. The nature of the solvent and of the
anion Xꢀ have a remarkable effect on the stability of the NHC, the bis(trifluoromethylsulfonyl) imide anion
being the best for a long lasting carbene (while acetonitrile seems to be the worst solvent). The role of Xꢀ
has been related to a competition between NHC and Xꢀ, in the hydrogen bonding interaction with BMIm+.
The higher the stabilization of NHC by hydrogen bond, the lower its degradation rate. These hydrogen
bonding interactions, previously reported in pure BMIm-X, seem to be operative even in organic solvents
containing BMIm-X at lowconcentrations (c < 0.1 mol Lꢀ1). The effect of the nitrogen alkyl substituents on
the degradation of NHC (and thus on its efficiency as organocatalyst) is also pointed out.
ã 2014 Elsevier Ltd. All rights reserved.
Keywords:
cyclic voltammetry
N-heterocyclic carbene
organocatalysis
ionic liquid
anion effect
1. Introduction
conditions, several processes involving C—C, C—O and C—N bond
formation: benzoin condensation, Stetter reaction, NHC-redox
The urging need for removing or reducing any waste in
chemical processes forces modern organic synthesis to seek for
catalysts able to increase both the efficiency and the sustainability
of the procedures [1–5].
acylation, synthesis of oxygen and nitrogen containing hetero-
cycles, etc. The ability of NHCs to activate aldehydes, via inversion
of their polarity (umpolung), is an intriguing reaction extensively
investigated.
N-Heterocyclic carbenes (NHCs), a new generation of organo-
Owed to their possible instability, NHCs are frequently generated
in situ by deprotonation or reduction of suitable precursors (e.g.,
imidazolium, pyrazolium, thiazolium based salts) [18].
The efficiency of a catalyst is certainly affected by its affinity
with the substrate and by its ability to trigger the substrate
structure, but also by its lifetime if it is not a stable molecule. A
significant decrease of the catalyst concentration during the
reaction could indeed negatively affect its outcome.
Therefore the lifetime of a NHC in the reaction conditions
should be regarded as a key-feature of organocatalysis and
quantitative investigations on the kinetic of NHC degradation
are of interest.
catalysts, possess both these requirements and represent
a
significant alternative to organometallic catalysts as regards cost
and environmental impact [6,7].
NHCs are cyclic carbenes bearing at least one
a-amino
substituent, frequently utilized as versatile ligands for transition
metals [8–10] and more recently as catalysts in metal-free polymer
synthesis [11]. In the last two decades NHCs have attracted
considerable interest as powerful nucleophilic organocatalysts in
molecular chemistry [12–17]. This attention is due to the versatility
and high activity of NHCs to catalyze, under mild reaction
In a previous communication we reported an electrochemical
methodology for the generation of NHCs and for the evaluation of
their stability in the parent ionic liquid as solvent, emphasizing the
possible influence of the anion on their lifetime [19].
* Corresponding author. Tel.: +39 06 49766563.
** Corresponding author.
E-mail addresses: marta.feroci@uniroma1.it (M. Feroci),
achille.inesi@uniroma1.it (A. Inesi).
0013-4686/$ – see front matter ã 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2014.11.135

M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
123
elaboration software was
a
CorrView for windows version
1
a
b
c
d
e
f
X
BF4
PF6
Bu
N
2.8d1 Scribner. A 492/GC/3 Amel microelectrode was employed,
using a Pt counter electrode and modified saturated calomel
electrode as reference electrode. Electrolyses under galvanostatic
control were carried out with an Amel 552 potentiostat equipped
with an Amel 721 integrator. A two-compartment cell was used.
The electrode apparent surface areas (flat Pt spirals, 99.9%) were ca
1.0 cm2 for the cathode and 0.8 cm2 for the anode. 1H and 13C NMR
spectra were recorded using a Bruker AC 200 spectrometer using
CDCl3 as internal standard.
H
N
Me
X
N(CF3SO2)2 (NTf2)
Cl
I
1a-h
CF3SO3 (OTf)
MeOSO3
CF3CO2
g
h
MeCN was distilled twice from P2O5 and CaH2; N,N-
dimethylformamide (DMF) was distilled from activated alumina
under reduced pressure; dimethylsulfoxide (DMSO) and 1,2-
dimethoxyethane (DME) were used without any further purifi-
cation. 1-Methyl-3-octylimidazolium bis-(trifluormethylsulfonil)
imide was obtained by anion metathesis from the corresponding
chloride according to a previously reported procedure [21]. 1-
Benzyl-3-methylimidazolium bis-(trifluoromethylsulfonil) imide
and 1-benzyl-3-butylimidazolium bis-(trifluoromethylsulfonil)
imide were synthesized according to the previously reported
procedure [22]. All other ionic liquids were purchased from
Iolitec and used after being kept at reduced pressure at 70 ꢁC for
24 h. All other reagents were purchased from Aldrich and used
as received.
Fig. 1. 1-Butyl-3-methylimidazolium salts (BMIm-X) used in this investigation.
Bu
Bu
N
N
1
_
(1)
(2)
H
+ e
+
H2
2
N
Me
N
Me
3
2
Bu
N
Bu
N
Me
N
1
_
2
- e
N
N
N
2.2. Cyclic voltammetries of ionic liquids 1a–h in organic solvents
Me
Me
Bu
3
Catholyte (0.10 mol Lꢀ1 of ionic liquid 1a–h in 5.0 mL of organic
solvent) and anolyte (2.0 mL same solvent/electrolyte) were
separated through a porous glass frit filled with methylcellulose
in DMF-Et4N-BF4. The electrolysis was carried out, under N2
Scheme 1. Electrochemical reduction of BMIm+ 2 and oxidation of NHC 3.
Subsequently, the very extensive utilization of NHCs in
catalyzed syntheses carried out in organic solvents spurred us
to investigate the stability of NHCs in organic solvents, according to
the assumption that the NHC lifetime might be affected also by the
nature of the medium. 1-Butyl-3-methylimidazole-2-ylidene was
chosen as reference NHC, owing to the availability of its BMIm-X
salts (1-butyl-3-methylimidazolium salts, 1a–h; Fig. 1).
NHC was generated by cathodic reduction of BMIm-X 1a–h in
organic solvent (DMF, MeCN, DME, DMSO). The concentration of
free NHC was evaluated by simple voltammetric analysis,
measuring the height of the NHC oxidation peak, as reported in
our previous papers [19,20].
atmosphere at 25 ꢁC, at a constant current (J = 15 mA cmꢀ2). After
ꢂ
the consumption of 31 C, the current was switched off, the anodic
compartment removed and the catholyte analyzed by cyclic
voltammetry at different time intervals from the end of the
electrolysis.
2.3. Electrolyses of 1a in organic solvents followed by addition of
cinnamaldehyde 4 and trifluoromethylacetophenone 5
Catholyte (1.0 mmol of BMIm-BF4 1a in 3.0 mL of organic
solvent) and anolyte (2.0 mL same solvent/electrolyte) were
separated through a porous plug filled with methylcellulose in
DMF-Et4N-BF4. The electrolysis was carried out, under N2
Aim of the present investigation was:
i)
atmosphere at 25 ꢁC, at a constant current (J = 15 mA
ꢂ
cmꢀ2).
to verify and quantify the dependence of the stability of 1-
butyl-3-methylimidazole-2-ylidene 3 (Scheme 1) on the nature
of the solvent and of the anion of the parent imidazolium salt;
After the consumption of 31 C, the current was switched off.
Cinnamaldehyde 4 (0.5 mmol) and trifluoromethylacetophenone 5
(1.5 mmol) were added to the catholyte immediately or after
40 minutes (in which the catholyte was kept under nitrogen at
room temperature) and the mixture was stirred at 60 ꢁC for 2 h and
at r.t. for 12 h.
ii)
to ascertain the correlation between the NHC lifetime and its
efficacy in NHC catalyzed reaction (the synthesis of
-butyrolactone, via NHC-catalyzed umpolung of cinnamalde-
a
g
hyde, was taken as reference reaction);
iii)
The solvent was evaporated under reduced pressure and the
residue extracted with Et2O (3 ꢃ 20 mL). The ethereal layers were
combined, the solvent was removed and the crude reaction
mixture analyzed by 1H NMR. The mixture was purifed by flash
chromatography, using n-hexane/ethyl acetate 95/5 as eluent to
to obtain indications for a substantial choice of the proper
solvent and anion for a NHC catalyzed reaction, in order to
increase its efficiency.
afford the pure 4,5-diphenyl-5-trifluoromethyl-g-butyrolactone 6
2. Experimental
in the yields reported in Table 1. -Butyrolactone 6 is a known
g
compound and gave spectral data in accordance with those
reported in the literature [23].
2.1. General Remarks
Voltammetric measurements were performed using an Amel
552 potentiostat equipped with an Amel 566 function generator
and an Amel 563 multipurpose unit in a three-electrode cell; the
curves were displayed on an Amel 863 recorder; acquisition
software was a CorrWare for windows version 2.8d1 Scribner,
3. Results and Discussion
The voltammograms recorded for solutions of BMIm-X 1a–h in
organic solvents (DMF, MeCN, DME, DMSO) show a behavior

124
M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
Table 1
Yields of
g-butyrolactone 6 from the reaction of electrogenerated NHC,
cinnamaldehyde and trifluoromethylacetophenone in different solvents. Reaction
carried out after 0 minutes (t = 0 min) or 40 minutes (t = 40 min) from the NHC
generation.a
Entry
Solvent
6, yield (t = 0 min)
6, yield (t = 40 min)
1
2
3
4
DMF
DME
DMSO
MeCN
75%
59%
58%
15%
63%
21%
5%
–
a1 mmol of BMIm-BF4 in 3.0 mL of solvent was reduced on a Pt cathode (divided cell,
r.t., N2 atmosphere, J = 15 mA
cm2, 31 C). At the end of the electrolysis or after
ꢂ
40 minutes from the end, 0.5 mmol of cinnamaldehyde and 1.5 mmol of
trifluoromethylacetophenone were added to the catholyte and the solution kept
at 60 ꢁC for 2 h and then at r.t. for 12 h. Usual workup gave the corresponding
g
-butyrolactone 6 (isolated yields) [23].
(Fig. 2) similar to the one of pure BMIm-X (previously reported
[19,24]):
i)
an increase of the cathodic current at E < -2.0 V (vs SCE) due to
the monoelectronic reduction of the imidazolium cation 2 to
NHC 3 and hydrogen (Scheme 1, reaction 1);
ii)
an irreversible anodic peak, at E > -0.6 V, in the reversed anodic
scanning due to the monoelectronic oxidation of NHC (gener-
ated during the cathodic scanning at E < -2.0 V) (Scheme 1,
reaction 2) [25].
The structure of the cation and the anion of BMIm-X, as well as
the presence and the nature of the organic solvent, might affect the
values of the electrochemical parameters of the specific salt (the
peak potential Ep and the peak current ip).
Electrolyses of BMIm-X 1a–h, under galvanostatic conditions in
a divided cell, yield the corresponding NHC 3 as product of the
cathodic reduction of the imidazolium cation 2 (Scheme 1, reaction
1). Voltammograms recorded on the catholyte at the end of the
electrolysis, show an anodic peak (scanning potential -1.0 to +0.1 V
vs SCE) related to the oxidation of electrogenerated NHC. The value
of the oxidation peak current increases on increasing the number n
of Faradays consumed during the electrolysis, i.e. on increasing the
concentration of 3 in the catholyte. Due to the proportionality
between the peak current and the NHC concentration, it is possible
to follow the decrease of the NHC concentration as a decrease in
the corresponding oxidation peak current.
Fig. 2. Cyclic voltammetric curves of 1a (0.10 mol Lꢀ1) in DME (curve a), MeCN
(curve b), DMF (curve c) and DMSO (curve d). Vitreous carbon cathode, n ,
= 0.2 V sꢀ1
T = 25 ꢁC. Scanning potential +0.6 to -3.0 V (where the current reaches the upper
limit of the instrument).
electrolysis by measuring the NHC oxidation peak current ipn
(Fig. 3; apex n: increasing times from the end of electrolysis).
ox
The behaviour of NHC 3 in different conditions (solvent, IL
anion) can be compared by plotting ipn vs tn. For a correct
ox
Since the lifetime of the NHC in solution could be affected by its
initial concentration1, the lifetime of the NHC generated from the
ionic liquids 1a–h in different solvents, DMF, MeCN, DMSO, and
comparison, a plot of the normalized value ipnox/ip0 vs tn is
ox
reported, as the voltammetric peak current is dependent on the
dielectric constant and the viscosity of the solvent (besides other
factors) and therefore it is different in different solvents for the
same NHC concentration.
DME, was determined at an identical concentration of 0.10 mol Lꢀ1
,
the ionic liquid playing also the role of supporting electrolyte.
Therefore, these solutions (5.0 mL) were electrolyzed under
galvanostatic conditions in a divided cell (Pt catode and anode,
3.1. Stability of 1-butyl-3-methylimidazole-2-ylidene
3 electrogenerated from BMIm-BF4 1a. The effect of the solvent
J = 15 mA
the current was switched off
ꢂ
cmꢀ2, T = 25 ꢁC). After the consumption of 31 coulombs,
2
.
We evaluated the kinetic of degradation of 3 in the different
organic solvents recording the voltammetric curves on the
catholyte at prefixed intervals of time tn from the end of the
Taking 3 as model carbene, we studied the NHC degradation
process in four classical solvents: acetonitrile (MeCN), N,N-
dimethylformamide (DMF), dimethylsulfoxide (DMSO) and 1,2-
dimethoxyethane (DME). Plots of ipn and ipnox/ip0 vs tn are
ox
ox
reported in Fig. 4.
The variation of ipnox or ipnox/ip0ox as function of time suggests
that:
1
The assumption that the initial NHC concentration is the same in all the
electrolyses is not strictly correct, as the rate of NHC disappearance depends on the
solvent and the anion. This means that there could be a different NHC consumption
during the electrolysis time, which would lead to a slightly different initial NHC
i)
the nature of the organic solvent considerably affects the NHC
degradation rate3 [28]. The values of the half-life time (t1/2) for
3 in the various solvents indicate that the degradation of NHC in
concentration.
2
Assuming a current yield r = 1 to the editorial staff: in this equation r must be the
greek letter r, which I cannot find here the concentration of the electrogenerated
NHC and of the BMIm+ cation in the cathodic solutions (not considering the NHC
degradation during the electrolysis time) at the end of the electrolysis can be
evaluated as: ½NHCꢄ ¼
103
3
0
0
¼ 6 :4
ꢂ
10ꢀ2molLꢀ1; ½BMImþꢄ ¼ 1 :0
ꢂ
10ꢀ1 ꢀ 6 :4 :10ꢀ2
31
ꢂ
The high viscosity of DMSO and DME (respect to MeCN and DMF) could account
for the low initial oxidation peak current in these solvents.
96500 5
ꢂ
¼3 :6
ꢂ
10ꢀ2molLꢀ1

M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
125
Fig. 3. Voltammetric curves of 1a (0.10 mol Lꢀ1) in DMF recorded on a vitreous carbon cathode (
n
= 0.2 V sꢀ1, T = 25 ꢁC), scanning potential -1.0 to +1.0 V. Curve a*: before the
electrolysis; curve aꢁ: immediately at the end of the electrolysis (31 C); curves a1-a3: after 30, 50 and 73 minutes from the end of electrolysis.
MeCN (t1/2 = 280) and in DMSO (t1/2 = 240) is more significant
than in DME (t1/2 = 930) or in DMF (t1/2 = 690). The half-life of 3
was evaluated as the time required to halve the initial NHC
The results are reported in Table 1. The yields of lactone 6 using
3 immediately after its generation are, as expected, higher than
those obtained using 3 after 40 minutes, confirming the validity of
the voltammetric results. Moreover, 1–3 hours (depending on the
solvent) after the end of the electrolysis the catholytes are no more
oxidation peak current ip0 (recorded immediately after the
ox
end of the electrolyses, i.e. from ip0ox to ipnox = 0.5 ip0ox). Among
the properties of the solvents (dielectric constant, viscosity,
polarity, etc.), polarity rather than viscosity seems to have a
able to significantly support the catalytic synthesis of
g-butyr-
olactone from cinnamaldehyde (too low NHC concentration).
noteworthy effect on the lifetime of NHC; in fact, an increase in
4
polarity leads to a decrease of t1/2
.
3.2. Stability of 1-butyl-3-methylimidazole-2-ylidene
ii)
degradation of NHC occurs irrespective of the nature of the
solvent and, 200 minutes after the end of the electrolyses, the
NHC concentration is negligible in the cathodic solutions.
3 electrogenerated from BMIm-X 1a–h. The effect of the anion
In order to assess the influence of the BMIm-X anion on the
stability of NHC 3 in organic solvents, we studied the NHC
degradation process in DMF, electrogenerating the carbene from
different BMIm-X salts. Plots of ipn and ipnox/ip0 vs tn are
In order to verify the results obtained from the voltammetric
analysis, we carried out a classical NHC-catalyzed reaction,
comparing the yields obtained using electrogenerated NHC
immediately after its generation with those obtained using NHC
after 40 minutes from its generation.
ox
ox
reported in Fig. 5.
The data show that the NHC degradation rate is strongly
dependent on the nature of the anion which surrounds the carbene
in solution (being the cation identical for all salts).
The curves allow to determine the values of NHC half-life time
t1/2 in DMF in the presence of different anions Xꢀ and of the cation
BMIm+ (Table 2).
The synthesis of g-butyrolactone 6 from cinnamaldehyde 4 and
trifluoromethylacetophenone 5 (Scheme 2) was chosen as model
reaction [23].
As it can be derived from Fig. 5 and Table 2, the stabilizing
action of the anion N(SO2CF3)2ꢀ (as previously reported by others
[29,30]) with respect to other anions is considerable and the same
effect is obtained when the solvent is the parent ionic liquid
(Table 2, entries 9–12).
4
Values of viscosity η and polarity ET(30) of the solvents are reported, together
with the halflife time τ1/2 of NHC 3 (this paper).
Solvent
hꢅ(mPaꢅs)
t1/2 (min)
ET(30) (kcal/mol)a
DMSO
DMF
ACN
1.948[26]
0.796[26]
0.361[26]
0.4089[27]
24
69
28
93
45.1
43.8
45.6
35.8
As an example, the value of the half-life time of NHC in DMF is
ꢀ
eight times higher in the presence of N(SO2CF3)2 than in the
presence of Clꢀ.
DME
The remarkable reactivity of NHC depends on its nucleophilicity
and its strong basicity (pKa = 22–24 in DMSO) [11] and its possible
a
The ET(30) value for a solvent is simply defined as the transition energy of a
dissolved betaine dye measured in kcal/mol [28].

126
M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
ii)
iii)
a dime
the rea
(reactio
the rea
1
50
40
30
20
10
0
BMIm-BF4
BMIm-BF4
iv)
DMF-N
DMF
0,8
MeCN
MeCN-N
DMSO-N
DME-N
DMSO
DME
0,6
0,4
0,2
0
0
50
100
150
200
0
50
100
150
200
t /min
t /min
Fig. 4. NHC generated by cathodic reduction (31 C) of 1a in different solvents. t = 0: end of the electrolysis. Left: NHC oxidation peak current ipnox vs time. Right: ratio between
NHC oxidation peak current and initial NHC oxidation peak current ipnox/ip0 vs time.
ox
As the voltammetric analyses were carried out in dry solvents,
under continuous N2 bubbling, reactions (3) and (4) can be ruled
out. In any case, the reactivity of NHC is due to the availability of its
lone pair, so the more this lone pair is free to react, the shorter the
half-life time of the carbene.
Recently, the stability and the reactivity of 1,3-dialkylimida-
zole-2-ylidenes, in the presence of the parent ionic liquid, were
extensively investigated [29] and the formation of a hydrogen-
bonded adduct I was hypothesized (Scheme 4, reaction 1).
As the reactivity of NHC depends mainly on the availability of
the lone pair localised in an sp2-orbital of the C2 atom, the
formation of adduct I should decrease the NHC reactivity while
O
O
O
electrogenerated 3
O
+
H
Ph
CF3
Ph
CF3
Ph
4
6
5
Ph
Scheme 2. NHC-catalyzed synthesis of g-butyrolactone 6.
instability in organic solvent containing an imidazolium salt can be
possibly related to (Scheme 3):
i)
an exchange of proton with the solvent (reaction 1),
1
50
BMIm-BF4
DMF
DMF
BMIm-BF4
BMIm-NTf2
BMIm-Cl
BMIm-PF6
BMIm-I
BMIm-OTf
BMIm-MeOSO3
BMIm-CF3CO2
BMIm-NTf2
BMIm-Cl
BMIm-I
0,8
BMIm-OTf
40
30
20
10
0
BMIm-PF6
BMIm-MeOSO3
BMIm-CF3CO2
0,6
0,4
0,2
0
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
t /min
t /min
Fig. 5. NHC generated by cathodic reduction (31 C) of different BMIm-X in DMF. t = 0: end of the electrolysis. Left: NHC oxidation peak current ipn vs time. Right: ratio
ox
between NHC oxidation peak current and initial NHC oxidation peak current ipnox/ip0 vs time.
ox

M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
127
Table 2
Bu
Bu
Bu
Bu
Stability of 1-butyl-3-methyl-2-ylidene 3. Effect of the counterion on NHC half-life
time t1/2. Data from Fig. 5.
N
N
N
N
+
(1)
(2)
H
H
N
Me
N
Me
N
Me
N
Me
Entry
Anion X
Solvent
t1/2/mina
bb
1
2
3
4
5
6
7
8
9
MeOSO3
Cl
OTf
PF6
CF3CO2
BF4
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
BMIm-BF4
BMIm-CF3CO2
BMIm-PF6
BMIm-NTf2
14
30
37
43
52
0.75
0.95
0.57
0.44
0.74
0.55
0.75
0.42
0.55
0.74
0.44
0.42
I
Bu
N
Bu
Bu
N
N
+
X
H
H
X
H
X
69
N
Me
N
N
Me
I
194
262
20c
50c
80c
220c
Me
III
II
NTf2
BF4
CF3CO2
PF6
Scheme 4. Hydrogen bond stabilization of NHC.
10
11
12
NTf2
between NHC and BMIm+ (adduct I) or between Xꢀ and BMIm+
(adduct II). Nonetheless, the hydrogen bonding effect does not
seem to be the only parameter in NHC stability (see for example
entry 7 vs entry 10, Table 2). The anion effect could be due in fact to
a
t1/2of 3 was evaluated by voltammetric analysis as the time required to halve
the initial NHC oxidation peak current ip0ox (recorded immediately after the end of
the electrolyses, i.e. from ip0 to ipox = 0.5 ip0ox).
ox
b
Kamlet-Taft b; parameter as a measure of the Hydrogen Bond Accepting ability
of anions [31].
c
a
combined influence of hydrogen bond ability and ionic
dimension (for the same value, the stabilization of larger anions
seems to be more important. As an example, PF6ꢀ and NTf2ꢀ anions
have the same value -0.44 and 0.42, respectively -, but very
Data from ref. [19].
b
b
different ionc radius -3.44 and 4.39 Å, respectively [32] -, with a t1/
increasing its stability.
of 43 and 262 minutes, respectively). Further studies will be
devoted to this subject.
2
Nevertheless, anions as Clꢀ, with high hydrogen bond accepting
ability (measured from the Kamlet-Taft
b parameter, Table 2) [31],
The voltammograms in Fig. 2 show that the values of Epox (the
peak potentials for the oxidation of 3 at the electrode surface) are
not significantly affected by the nature of the solvent (as well as by
the anion Xꢀ). This result could appear rather unexpected, if
compared with the strong influence of X and of the solvent on the
reactivity and stability of NHC in the bulk of the solution, and it
deserves a specific study.
In addition, several authors reported NHC catalyzed reactions
performed using imidazolium-X salts (X: Cl, CH3COO) in the
absence of any base purposely added to the reaction mixture
[33,34]. The results of these reactions were justified admitting the
formation of NHC by an equilibrium of proton exchange between
the imidazolium cation and Xꢀ (Xꢀ as endogenic base). Neverthe-
less, no direct experimental support of the presence of free NHC in
the ionic liquid or in organic solvent was given [29] (NHC-
containing hydrogen bonded structures were, on the contrary,
evidenced in the gas-phase by spectroscopic analysis [35]).
About this subject, we never observed the NHC oxidation peak
in the voltammetric curves recorded for BMIm-Cl or BMIm-
CH3COO (pure or in solution). These results suggest that Xꢀ
(chloride or acetate anions) is unable to generate NHC from BIMm+
to a significant concentration to be revealed by cyclic voltammetry
(a technique which is indeed quite sensitive).
could yield the more stable adduct II (Scheme 4, reaction 2),
displacing the equilibrium (1) to left by removing BMIm+.
Accordingly, the concentration of free carbene, which is more
reactive than the bonded carbene I [29], increases.
The results of Table 2 attest that anions Xꢀ play a significant role
in the kinetic of NHC degradation, giving very different t1/2 for
different salts. The same effect of the bis(trifluorosulfonil) imide
anion on t1/2 was observed in pure BMIm-X, i.e. in the absence of
organic solvents [19]. Consequently, the role of Xꢀ was related to a
competition in the hydrogen bonding interaction (effective in
solution of organic solvent as well as in BMIm-X as solvent)
Bu
Bu
N
N
+ HS
+ S
(1)
(2)
N
Me
N
Me
Bu
Bu Bu
Bu
N
N
N
N
+
N
Me
N
N
N
Me
MeMe
3.3. Stability of electrogenerated 1,3-dialkylimidazole-2-ylidenes. The
effect of the alkyl substituents
Bu
Bu
Finally, we studied the effect, on the stability of the
corresponding NHC, of different alkyl substituents on the nitrogen
atoms of the imidazolium cation for two series of salts in DMF:
tetrafluoroborates (Fig. 6, upper plots) and bis(trifluoromethyl-
sulfonyl) imides (Fig. 6, lower plots).
N
N
+ O2
(3)
(4)
O
N
Me
N
Me
The behavior of these NHCs is quite similar (stabilizing effect of
NTf2ꢀ with respect to BF4ꢀ), with the striking exception of BMIm-
NTf2 (as previously pointed out), whose stability is much higher
than that of the other NHC. When aromatic substituents on the
nitrogen atoms are considered, the NHC behavior is not clear.
Studies are currently ongoing to understand this reactivity.
Bu
Bu
Bu
N
N
N
OH
H
+ H2O
or
N
Me
N
Me
N
Me
ring opening
Scheme 3. Possible NHC degradation reactions.

128
M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
50
40
30
20
10
0
1
BMIm-BF4
OMIm-BF4
BMIm-BF4-N
OMIm-BF4-N
BnMIm-BF4-N
BnBIm-BF4-N
BnMIm-BF4
0,8
BnBIm-BF4
0,6
0,4
0,2
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
t /min
t /min
50
40
30
20
10
0
1
0,8
0,6
0,4
0,2
0
BMIm-NTf2
OMIm-NTf2
BnMIm-NTf2
BnBIm-NTf2
BMIm-NTf2-N
OMIm-NTf2-N
BnMIm-NTf2-N
BnBIm-NTf2-N
0
50
100
150
200
250
300
0
50
100
150
200
250
300
t /min
t /min
Fig. 6. NHC generated by cathodic reduction (31 C) of imidazolium salts in DMF. t = 0: end of the electrolysis. Left: NHC oxidation peak current ipn vs time. Right: ratio
ox
between NHC oxidation peak current and initial NHC oxidation peak current ipnox/ip0 vs time.
ox
Imidazolium cations: BMIm = 1-butyl-3-methylimidazolium; OMIm = 1-methyl-3-octylimidazolium; BnMIm = 1-benzyl-3-methylimidazolium; BnBMIm = 1-benzyl-3-
butylimidazolium.
between Xꢀ and BMIm+ (adduct II). These hydrogen bonding
4. Conclusions
interactions, previously reported in pure BMIm-X, seem to be
operative also in organic solvents containing BMIm-X at low
concentration (c < 0.1 mol Lꢀ1). The efficiency of the NHC-
The degradation of NHCs in organic solvents has a remarkable
effect on their efficiency as organocatalysts. A simple voltam-
metric methodology allows to investigate the stability of NHCs
generated in organic solvents (by cathodic reduction of the
corresponding BMIm-X salts) and to determine their lifetime,
which is critical for the proper utilization of these solutions for
NHC-catalyzed syntheses. The nature of the solvent and of the
anion Xꢀ has a considerable effect on the kinetic of degradation of
NHC (due to dimerization, protonation or ring opening reactions).
The role of Xꢀ was related to a competition in the hydrogen
bonding interaction between NHC and BMIm+ (adduct I) and
catalyzed synthesis of
g-butyrolactones, used as a test for the
stability of NHCs, confirmed the results of the voltammetric
analysis.
Acknowledgements
We thank Mr Marco Di Pilato for his support to the
experimental part of this work, and Miur (Italy) and Sapienza
University of Rome for financial support.

M. Feroci et al. / Electrochimica Acta xxx (2015) 122–129
[20] I. Chiarotto, M. Feroci, G. Forte, M. Orsini, A. Inesi, Proton-Exchange
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