Electrocarboxylation of chloroacetonitrile mediated by electrogenerated cobalt(I) phenanthroline

Article abstract of DOI:10.1016/j.elecom.2010.07.020

The electrocarboxylation of chloroacetonitrile mediated by [Co(II)(phen)₃]²⁺ has been investigated. Cyclic voltammetry studies of [Co(II)(phen)₃]²⁺ have shown that [Co(I)(phen)₃]⁺, an 18 electron complex, activates chloroacetonitrile by an oxidative addition through the loss of a phenanthroline ligand to give [RCo(III)(phen)₂Cl]⁺. The unstable one-electron-reduced complex underwent Co-C bond cleavage. In carbon dioxide saturated solution, CO₂ insertion proceeds after reduction of the alkylcobalt complex. A catalytic current is observed which corresponds to the electrocarboxylation of chloroacetonitrile into cyanoacetic acid. Electrolyses confirmed the process and gave faradic yield of 62% in cyanoacetic acid at potentials that are about 0.3 V less cathodic than the one required for Ni(salen).

Full text of DOI:10.1016/j.elecom.2010.07.020

Electrochemistry Communications 12 (2010) 1360–1362
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Electrocarboxylation of chloroacetonitrile mediated by electrogenerated
cobalt(I) phenanthroline
⁎
P.-L. Fabre, O. Reynes
Université de Toulouse, Université Paul Sabatier, Laboratoire de Génie Chimique, Bat. 2R1, 118 route de Narbonne, F-31062 Toulouse cedex 09, France
CNRS, Laboratoire de Génie Chimique UMR 5503, F-31062 Toulouse cedex 09, France
a r t i c l e i n f o
a b s t r a c t
The electrocarboxylation of chloroacetonitrile mediated by [Co(II)(phen)3]2+ has been investigated. Cyclic
voltammetry studies of [Co(II)(phen)3]2+ have shown that [Co(I)(phen)3]+, an 18 electron complex,
activates chloroacetonitrile by an oxidative addition through the loss of a phenanthroline ligand to give [RCo
(III)(phen)2Cl]+. The unstable one-electron-reduced complex underwent Co–C bond cleavage. In carbon
dioxide saturated solution, CO2 insertion proceeds after reduction of the alkylcobalt complex. A catalytic
current is observed which corresponds to the electrocarboxylation of chloroacetonitrile into cyanoacetic
acid. Electrolyses confirmed the process and gave faradic yield of 62% in cyanoacetic acid at potentials that
are about 0.3 V less cathodic than the one required for Ni(salen).
Article history:
Received 16 June 2010
Received in revised form 15 July 2010
Accepted 16 July 2010
Available online 23 July 2010
Keywords:
Electrocarboxylation
Cobalt tris (phenanthroline)
Electrocatalysis
© 2010 Elsevier B.V. All rights reserved.
Chloroacetonitrile
Cyanoacetic acid
1. Introduction
high overpotential, a yield of 71% was reported [21]. Electrogenerated
cobalt(I) complexes have also been used as catalytic systems for
Electrocarboxylation of organic halides has been extensively studied
as a synthetic method towards carboxylic acid [1–5]. Catalytic systems
based on transition-metal complexes have proved to be an efficient way.
Several nickel [6–8], palladium [9] and cobalt [10–12] complexes have
been reported. Many works were devoted more specially on the
electrosynthesis of cyanoacetic acid [13–16] whose derivatives are
precious starting materials in pharmaceutical [17] and agrochemical[18]
synthesis. Actually, the industrial manufacture of cyanoacetic acid is
based on the reaction between chloroacetic acid and alkali metal
cyanides [19]. This process is dangerous andenvironmentally unfriendly.
For these reasons, the development of new synthetic ways has been
appraised. Recently, Gennaro et al. have shown the electrocatalytic
properties of a silver electrode for the synthesis of cyanoacetic acid
through the reduction of chloroacetonitrile under CO2.
electrocarboxylation of organic halides [10,12]. The reported cobalt
complexes present a square planar structure allowing the oxidative
addition of alkyl halides on cobalt which seems to be the key step in
the carboxylation process. Hexacoordinated cobalt complexes such as
[Co(phen)3]2+ have never been used as potential catalysts due to the
hexacoordination of Co(I) which would prevent the oxidative addition of
alkyl halides [22,23]. However, it has been shown that the phenanthro-
line ligands are labile in the [Co(II)(phen)3]2+ complex [24,25].
Here, we report for the first time, the electrocatalytic properties of
[Co(I)(phen)3]+ for the electrocarboxylation of chloroacetonitrile.
Cyclic voltammetry studies reveal the activity of [Co(I)(phen)3]+
towards chloroacetonitrile and CO2. Preparative electrolyses were
performed in order to calculate to the faradic yield of cyanoacetic acid.
2. Experimental
NCCH2Cl þ CO2 þ 2e−
Cl− þ NCCH2CO−2
ð1Þ
2.1. Reagents and equipments
The reduction of chloroacetonitrile occurs at a less cathodic potential
(around 0.7 V) than those required on glassy carbon and the faradic
yield (around 90%) is excellent [20]. Among the electrocatalysts based
on transition-metal complexes, the best results were obtained with Ni
(salen). Under an applied potential around −1.7 V which represents a
1,10-phenanthroline, cobalt(II) chloride hexahydrated, chloroaceto-
nitrile, and potassium hexafluorophosphate (KPF6) were used as
received. Tetrabutylammonium hexafluorophosphate (TBAPF6) was
recrystallised from ethylacetate and dried at 80 °C under vacuum.
Acetonitrile (HPLC grade) was stored on molecular sieves (3 Å).
Cyanoacetic acid was analysed by HPLC using a HP 1100 HPLC equipped
with a UV–Vis detector operating at 200 nm and a Sugar SH1011 column
eluted by 5.10−3 mol L−1 H2SO4. The[Co(phen)3](PF6)2 was synthesized
as described in the literature [24].
⁎
Corresponding author. Université de Toulouse, Université Paul Sabatier, Laboratoire
de Génie Chimique, Bat. 2R1, 118 route de Narbonne, F-31062 Toulouse cedex 09,
France. Tel.: +33 5 61 55 61 21; fax: + 33 5 61 55 61 39.
E-mail address: reynes@chimie.ups-tlse.fr (O. Reynes).
1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2010.07.020

P.-L. Fabre, O. Reynes / Electrochemistry Communications 12 (2010) 1360–1362
1361
2.2. Cyclic voltammetry and electrolyses
gram (curve C) is severely modified. The electrochemical system
appears irreversible and the cathodic peak is slightly increased. The
reverse peak height increases when the potential scan speed is
increased and decreases when chloroacetonitrile is added. This is
typically an EC system in which the following reaction would be the
oxidative addition of chloroacetonitrile (RCl) on the metal. This
reaction implies the release of a phenanthroline ligand which has
been observed for [Co(II)(phen)3]2+ [24,25]:
The cyclic voltammetry experiments were conducted in a classical
three electrode cell under argon or carbon dioxide atmospheres. A
saturated calomel electrode (SCE), a platinum wire and a glassy
carbon (GC) disk (diameter: 3 mm) were used as reference, counter
and working electrodes respectively. All experiments were carried out
in dry acetonitrile+TBAPF6 0.1 mol L−1. Simulated voltammograms
(ESP) have been computed with Nervi's program in order to evaluate
the kinetic parameters [26].
The electrolyses were performed in an undivided cell equipped with
a magnesium bar used as a sacrificial anode, a saturated calomel
electrode (SCE) as the reference electrode and carbon fibres as the
working electrode. Acetonitrile+TBAPF6 0.1 mol L−1 solution (volume
25 mL) containing chloroacetonitrile and [Co(II)(phen)3](PF6)2 was
saturated with carbon dioxide by bubbling during 30 min ([CO2]=
0.28 mol L−1 [27]). The electrolysis potential was −1.4 V/SCE. The
electrolyses were stopped when the current dropped to 10% of its initial
value. The cyanoacetic acid was extracted using the procedure
previously reported by Gennaro et al. [21].
2þ
½CoðIIÞðphenÞ3ꢀ þ e−
½CoðIÞðphenÞ3ꢀþ
½RCoðIIIÞðphenÞ2Clꢀþ þ phen:
ð2Þ
ð3Þ
½CoðIÞðphenÞ3ꢀþ þ RCl
Assuming a simple EC scheme [30], the oxidative addition was
evaluated through the RIp analysis (k around 60 M−1s−1 [29] and
simulated [26]). The kinetic constant (k) is around 4 order of magnitude
lower than those reported for planar Co(I)salen in the presence of n-butyl
halides [30]. This suggests that the formation of the 16 electron complex,
[Co(I)(phen)2]+, is the kinetic control step in the oxidative addition
pathway. When free phenanthroline is added, the backward peak is
increased which is in favour of the [Co(I)(phen)2]+ formation
assumption.
3. Results and discussion
Fig. 2 shows cyclic voltammograms of [Co(II)(phen)3]2+ in aceto-
nitrile under an argon atmosphere when the potential scan is reversed
at −1.5 V before the reduction of Co(I) into Co(-I). Curve A is related to
the reduction of [Co(II)(phen)3]2+ alone and curves B and C are the
voltammograms when increasing amounts of chloroacetonitrile are
added to the solution. As above mentioned, the signal of the Co(II)/Co(I)
couple traduces the EC scheme ((2) and (3)). Moreover, a new cathodic
peak appears at Epc=−1.4 V. The peak current increases with the
chloroacetonitrile concentration but is not linearly related to the square
root of the potential scan speed. A backward peak appears when the
potential scan speed is higher than 9 V/s. The associated electrochemical
system is the monoelectronic reduction [10] of the product of the
oxidative oxidation (3), according to an ECE scheme (2, 3, 4):
In the cathodic domain, the cyclic voltammograms of [Co(II)
(phen)3]2+ in non-aqueous solvent under argon atmosphere exhibit
three electrochemical reductions at −0.95 V, −1.65 V and −2.12 V
corresponding to the Co(II)/Co(I), Co(I)/Co(-I) and free phenanthro-
line reductions respectively [22]. In a CO2-saturated solution, the
catalytic reduction of CO2 was demonstrated through the Co(I)/Co(-I)
couple but it requires high overpotential [23].
On the contrary, the Co(II)/Co(I) couple presents a nice reduction
potential, around −1 V, but the unreactivity of Co(I) was attributed to
the weak nucleophilicity of Co(I) and to a complete coordination
sphere around the metal (three phenanthroline ligands) [22,23].
Nevertheless, the reactivity of Co(I) can be demonstrated.
Fig. 1 shows cyclic voltammograms of [Co(II)(phen)3]2+ in acetoni-
trile. As already described, curve A, the reduction of [Co(II)(phen)3]2+
into [Co(I)(phen)3]+ follows a reversible monoelectronic process
(E°= 0.96 V, k°=0.045 cm s−1 , evaluated by the ΔEp method [28]
and simulated [26]). As expected, in a CO2-saturated solution, the
voltammogram (curve B) is unchanged. No specific reaction between
[Co(I)(phen)3]+ and CO2 takes place. On the contrary, under an argon
atmosphere and after addition of chloroacetonitrile, the voltammo-
½RCoðIIIÞðphenÞ2Clꢀþ þ e−
½RCoðIIÞðphenÞ2Clꢀ:
ð4Þ
The resulting Co(II)-alkyl complex is known to undergo a rapid
homolytic or heterolytic Co–R bond cleavage [10].
½RCoIIðphenÞ2ꢀþ
½RCoIIðphenÞ2ꢀþ
R• þ ½CoIðphenÞ2ꢀþ Homolytic
ð5Þ
ð6Þ
R
− þ ½CoIIðphenÞ2ꢀ2þ Heterolytic
Fig. 1. Cyclic voltammograms in CH3CN +TBAPF6. v =50 mV/s. (—) 2 mmol L−1
[Co(II)(phen)3]2+ under argon, (———) 2 mmol L−1 [Co(II)(phen)3]2+ under carbon
dioxide, and (-·-·-·)2 mmol L−1 [Co(II)(phen)3]2+ and 8 mmol L−1 chloroacetonitrile
under argon.
Fig. 2. Cyclic voltammograms in CH3CN+ TBAPF6 under argon. v=100 mV/s.
(A) 2 mmol L−1 [Co(II)(phen)3]2+, (B) 2 mmol L−1 [Co(II)(phen)3]2+ and 4 mmol L−1
chloroacetonitrile, and (C) 2 mmol L−1 [Co(II)(phen)3]2+ and 8 mmol L−1 chloroacetonitrile.

1362
P.-L. Fabre, O. Reynes / Electrochemistry Communications 12 (2010) 1360–1362
fold excess of chloroacetonitrile 10−2 mol L−1), the electrolyses were
performed under potentiostatic conditions, Eapplied =−1.4 V. The
faradic yield calculated in cyanoacetic acid was 62 1% (the electric
charge Qpassed was around 47 C for a theoric one Qtheoric of 52 C) [32].
This good faradic yield was obtained at potentials that are about 0.3 V
less cathodic that one required for Ni(salen) [21].
Whatever is the cleavage, the global electron exchange is bielec-
tronic: reduction (4) and reduction of the radical R• or reduction of
CoII(phen)22+. Indeed, the R•/R− potential is around −0.7 V [20] and
according to the complexation coefficient α, the E°(7) must be higher
than E°(2).
½CoðIIÞðphenÞ2ꢀ2þ þ e−
½CoðIÞðphenÞ2ꢀþ
ð7Þ
4. Conclusion
The resulting CoI(phen)2+ reacts with chloroacetonitrile through an
oxidative addition; the kinetic constant k(8) must be higher than k(3)
because of the under-coordination of CoI(phen)2+:
We have shown that the Co(I)phenanthroline complex is suitable
for electrocarboxylation processes. The carboxylation follows the
oxidative addition of chloroacetonitrile. No direct reduction of carbon
dioxide was observed which enhances the selectivity of this
electrocatalytic process. The preparative electrolyses show that
cyanoacetic acid can be obtained with a good faradic yield under
less cathodic working potentials compared to nickel(salen). Work is
in progress for the development of the electrosynthesis procedure.
½CoðIÞðphenÞ2ꢀþ þ RCl
½RCoðIIIÞðphenÞ2Cꢀlþ:
ð8Þ
The catalytic cycle (4, 5/6, 7, 8) explains the catalytic peak current at
Epc=−1.4 V which increases with the chloroacetonitrile concentration.
Note that the resulting carbanion can react with electrophilic com-
pounds, acetonitrile or itself to form different compounds [31].
Fig. 3 gives the evidence of the CO2 activation by the [RCo(II)
(phen)2Cl] complex. As shown in Fig. 1, [Co(I)(phen)3]+ does not
react with CO2. When [Co(II)(phen)3]2+ and chloroacetonitrile are
present in a CO2-satured solution (curve C), the intensity of the
cathodic peak at Ep,c =−1.4 V corresponding to the reduction (4) is
greatly enhanced according to a catalytic process. The increase of the
reduction current of [RCo(III)(phen)2Cl]+ implies that CO2 enters in
the catalytic cycle (4, 5/6, 7, 8) and interferes in the reduction of
chloroacetonitrile. The mechanism could go through the addition of
CO2 followed by the coupling of CO2 and the R substrate as reported
for Ni or Pd complexes [6,9].
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½RCoðIIÞðphenÞ2Clꢀ þ CO2
½RCoðIIÞðphenÞ CO2ꢀþ þ Cl−
ð9Þ
ð10Þ
ð11Þ
2
½RCoðIIÞðphenÞ CO2ꢀþ
½CoðIIÞðphenÞ RCO2ꢀþ
2
2
½CoðIIÞðphenÞ RCO2ꢀþ
RCO−2 þ ½CoIIðphenÞ2ꢀ
2þ
2
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[32] Calculated as (Q RCOOH/Q passed). Where Q RcooH =2 F NRcooH (NRcooH is the number
of moles of cyanoacetic produced by electrolysis) and Q consumed is the measured
electric charge.
Fig. 3. Cyclic voltammograms in CH3CN + TBAPF6. v =100 mV/s. (A) 2 mmol L−1
[Co(II)(phen)3]2+ under argon, (B) 2 mmol L−1 [Co(II)(phen)3]2+ and 4 mmol L−1
chloroacetonitrile under argon, and (C) 2 mmol L−1 [Co(II)(phen)3]2+ and 4 mmol L−1
chloroacetonitrile in a CO2-satured solution.