A new bipyridyl cobalt complex for reductive dechlorination of pesticides

Article abstract of DOI:10.1016/j.electacta.2016.04.170

Reductive dechlorinations are especially promising reactions for improving the biodegradability and hence decreasing the environmental impact of chlorinated organic pollutants. In this context, the catalytic activity of a bipyridyl cobalt complex containing two bipyridine ligands was examined for achieving clean electrochemical dehalogenation in aqueous media. The prepared [Co(bpy(CH₂OH)₂)₂]²⁺ complex was found to exhibit high catalytic activity toward the dechlorination of chloroacetanilide herbicides. Based on preparative electrolyses, 2-electron reduction of the substrate was obtained with a dechlorination yield of 85%, underlining the selectivity and efficiency of the dehalogenation process. Cyclic voltammetry analyses highlighted the catalytic activity of the complex toward chloroacetanilide herbicides, as alachlor, metolachlor and metazachlor. An high apparent chemical rate constant k₁ of 5000 mol^-1 L s^-1 is calculated for the first step of the cathodic reduction of alachlor, based on simulated cyclic voltammetry experiments.

Full text of DOI:10.1016/j.electacta.2016.04.170

Accepted Manuscript
Title: A new bipyridyl cobalt complex for reductive
dechlorination of pesticides
Author: W.Y. He J.-M. Fontmorin P. Hapiot I. Soutrel D.
Floner F. Fourcade A. Amrane F. Geneste
PII:
S0013-4686(16)31022-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.04.170
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30-3-2016
28-4-2016
28-4-2016
Please cite this article as: W.Y.He, J.-M.Fontmorin, P.Hapiot, I.Soutrel,
D.Floner, F.Fourcade, A.Amrane, F.Geneste, new bipyridyl cobalt
A
complex for reductive dechlorination of pesticides, Electrochimica Acta
http://dx.doi.org/10.1016/j.electacta.2016.04.170
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A new bipyridyl cobalt complex for reductive dechlorination of pesticides
W. Y. Hea,b, J.-M. Fontmorina,b, P. Hapiotb, I. Soutrela, D. Flonerb, F. Fourcadea, A. Amranea,
F. Geneste*,b
a
Ecole Nationale Supérieure de Chimie de Rennes/Université de Rennes 1, CNRS, UMR 6226, 11
allée de Beaulieu, CS 50837, 35708 Rennes cedex 7, France.
b Université de Rennes 1, CNRS, UMR 6226, Equipe Matière Condensée et Systèmes Electroactifs,
Campus de Beaulieu, 35042 Rennes cedex, France.
ABSTRACT
Reductive dechlorinations are especially promising reactions for improving the
biodegradability and hence decreasing the environmental impact of chlorinated organic
pollutants. In this context, the catalytic activity of a bipyridyl cobalt complex containing two
bipyridine ligands was examined for achieving clean electrochemical dehalogenation in
aqueous media. The prepared [Co(bpy(CH2OH)2)2]2+ complex was found to exhibit high
catalytic activity toward the dechlorination of chloroacetanilide herbicides. Based on
preparative electrolyses, 2-electron reduction of the substrate was obtained with a
dechlorination yield of 85%, underlining the selectivity and efficiency of the dehalogenation
process. Cyclic voltammetry analyses highlighted the catalytic activity of the complex toward
chloroacetanilide herbicides, as alachlor, metolachlor and metazachlor. An high apparent
chemical rate constant k1 of 5000 mol-1 L s-1 is calculated for the first step of the cathodic
reduction of alachlor, based on simulated cyclic voltammetry experiments.
Keywords: dehalogenation, chloroacetanilide, alachlor, Co complex, depollution
1. Introduction
Chlorinated organic compounds are widely used in synthetic chemistry and are
prominent environmental contaminants as many of them are biorecalcitrant. Several physico-
chemical methods have been proposed for their degradation [1-6]. Among them, the catalytic
reduction of chlorinated compounds has been the subject of numerous investigations because
of its high importance both for synthetic chemistry and environment [6-11]. Transition metal
complexes such as Co(I) complexes have been identified as effective catalysts for this
Corresponding author. E-mail address: Florence.Geneste@univ-rennes1.fr
1

reaction [12-15], the reactivity of the catalyst depending on the ligands structure. [Co(salen)]-
(salen=bis(salicyliden)ethylenediamine) has shown good catalytic activity toward alkyl
iodide, bromide and chloride such as tert-butyl bromide and chloride [16], n-butyl bromide
and iodide, 1,2-dibromobutane [17, 18], iodoethane [19], some of them containing a carbon in
 position of a carbonyl (3-chloro-2,4-pentanedione) or a phenyl group (PhCCl3 [14], benzal
chloride [20, 21], and benzyl chlorides [22]). Whereas [Co(salen)]- catalytic activity toward
the reduction of bromobenzene has been reported once [23], it is not reported as a good
catalyst for the reduction of aryl halides. Cobalt chelates of biological interest such as cobalt
porphyrins, and cobalamin have shown good catalytic activity toward reduction of alkyl
bromide and chlorine [24-26], ethylenes [27-29], and arenes [30-32]. Two different general
mechanisms have been proposed for the catalysis depending on the ligand L, the halogenated
compound and the electrolyte. These schemes differ by the nature of electron transfer step that
.
involved the formation of the intermediate radical R (outer-sphere) or an oxidative addition
(inner-sphere) (Scheme 1) [18, 27, 33-35].
Concerning the possible uses for an environmental purpose, some of these complexes
have been reported to keep their catalytic activity in aqueous media, which is a requirement
for such application [12, 24, 32, 36]. Among the possible catalysts, polypyridyl complexes of
cobalt are promising candidates as they display good catalytic activity for indirect
electroreduction of allyl chloride in aqueous medium [15, 37, 38]. However, the number of
bipyridyl units coordinated to the metal strongly affects the catalytic behavior of the Co(I)
polypyridyl complexes, such effect depending on the medium, electrolyte, substrates,… It was
2+
previously established that the catalytic behavior of Co(bpy)3 toward the reduction of allyl
chloride in acetonitrile involves the reduction of Co(II) to Co(I) followed by a rapid
+
decomposition into Co(bpy)2 (Scheme 2), the ligand dissociation step (Eq. 2) being the rate-
+
determining step and the Co(bpy)2 being the active catalytic species [39, 40].
This conclusion was supported by the observation of a higher catalytic efficiency with
+
+
Co(bpy)2 than with Co(bpy)3 . More recently, it was reported that CoBr2 in the presence of a
single equivalent of 2,2’-bipyridine or pyridine exhibits a high catalytic activity toward the
reduction of aromatic halides [41, 42]. In an aqueous solution containing a surfactant, the
reduced tris- complex was stabilized in the micelle and a mild kinetic enhancement was
+
reported in the catalytic reduction of allyl halides by the Co(bpy)3 complex. However, the
exact nature of the active catalyst remains difficult to establish as little is known about the
+
activity of the corresponding Co(bpy)2 complex in such aqueous conditions.
2

In this context, we have considered the catalytic activity toward dechlorination
reaction of a new polypyridyl complex that contains only two bipyridyl units:
[Co(bpy(CH2OH)2)2]2+ where bpy(CH2OH)2 is 4,4’-Bis (hydroxymethyl)-2,2’-bipyridine. In
view of potential environmental applications, its catalytic behavior was investigated in
aqueous medium by cyclic voltammetry toward the reduction of several different common
chlorinated pesticides. In complement to the cyclic voltammetry investigations, bulk
electrolysis were also performed to precise the dependence of the catalytic efficiency on the
structure of the chlorinated compound. The considered pesticides: alachlor 1, metazachlor 2,
metolachlor 3, 2,4-D 4 and the dechlorinated form of alachlor 5, are shown in Scheme 3.
2. Experimental section
2.1. Chemicals
Alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide), metazachlor (2-
chloro-N-(2,6-dimethylphenyl)-N-(1H-pyrazol-1-ylmethyl)acetamide), metolachlor (2-chloro-
N-(2-ethyl-6-methylphenyl)-N-(1-methoxypropan-2-yl)acetamide),
[2,2'-bipyridine]-4,4'-
dicarboxylic acid, sodium borohydride and Cobalt(II) tetrafluoroborate hexahydrate were
purchased from Sigma-Aldrich. 2,4-D (2-(2,4-dichlorophenoxy)acetic acid) and hexadecyl
trimethyl ammonium bromide (HDTAB) were supplied by Alfa Aesar. Deschloroalachlor (N-
(2,6-diethylphenyl)-N-(methoxymethyl)acetamide) was obtained from Alpha Chimica
(France). Graphite felt (Recycled Vein Graphite RVG 4000) was supplied by Mersen
(France). Its specific area measured by the BET (Brunauer, Emett and Teller) method, its
volume density and its carbon content were 0.7 m2 g−1, 0.088 g cm−3 and 99.9%, respectively.
2.2. Synthetic procedures
The following procedure for the preparation of [2,2'-bipyridine]-4,4'-diyldimethanol 8 was
modified from literature conditions [43].
Synthesis of diethyl [2,2'-bipyridine]-4,4'-dicarboxylate 7. [2,2'-bipyridine]-4,4'-
dicarboxylic acid 6 (500 mg, 2 mmol) was added to absolute ethanol (40 mL). Concentrated
sulfuric acid (95%) (0.5 mL) was added to the suspension and the mixture was refluxed for 4
days. The solution was cooled to room temperature, distilled water was added (40 mL) and
ethanol was then evaporated. The pH was adjusted to 7 using a 5% sodium hydroxide
solution. The formed precipitate was filtrated and washed with water, leading to a white
1
powder. Yield; 92%, H NMR (CDCl3, 300 MHz): δ (ppm) 1,38 (t, 6H, J = 7 Hz); 4.40 (q,
4H, J = 7 Hz); 7.85 (d, 2H, J = 6 Hz); 8.80 (d, 2H, J = 6 Hz); 8.88 (s, 2H).
3

Synthesis of [2,2'-bipyridine]-4,4'-diyldimethanol 8. Sodium borohydride (1.87 g, 49
mmol) was added to a suspension of diethyl [2,2'-bipyridine]-4,4'-dicarboxylate 7 (567 mg,
1.89 mmol) dissolved in absolute ethanol (34 mL). The solution was refluxed for 3h. After
cooling to room temperature, a saturated ammonium chloride solution (24 mL/g of NH4Cl)
was added to decompose the excess of sodium borohydride. Ethanol was evaporated under
vacuum and the resulting solid was dissolved in a minimum of water using ultrasonic bath.
The solution was extracted with ethyl acetate (5 × 34 mL), dried on sodium sulfate and
1
concentrated under vacuum to give a white powder. Yield: 91%, H NMR (CD3OD, 300
MHz): δ (ppm) 4.66 (s, 4H); 7.35 (d, 2H, J = 5 Hz); 8.18 (s, 2H); 8.50 (d, 2H, J = 5 Hz).
Synthesis of [Co(bpy(CH2OH)2)2](BF4)2 9. Cobalt(II) tetrafluoroborate hexahydrate
(156 mg, 0.46 mmol) was added to 4,4’-Bis(hydroxymethyl)-2,2’-bipyridine 8 (408 mg, 1.89
mmol) dissolved in absolute ethanol (37 mL). The solution was refluxed under stirring for
24h, concentrated and dried under vacuum. The solid was dissolved in a minimum of
methanol and dichloromethane was added slowly. The formed precipitate was filtrated and
washed with dichloromethane, leading to a yellow solid. Yield: 95%, HRMS (ESI): m/z calcd
for C24H24N4O4Co [M-2BF4]++: 245.5559; found, m/z 245.5559; Found, C, 43.68; H, 4.06; N,
8.27 Calc. for C24H24BrCoF8N4O4: C, 43.35; H, 3.64; N, 8.42%.
2.3. Dechlorination conditions.
The dechlorination reaction was performed in a home-made flow cell [44]. To ensure a good
homogeneity of the potential distribution in the three dimensional working electrode, the felt
was located between two interconnected DSA counter-electrodes (dimentionally stable
anodes, AC-2004, supplied by ECS International Electro Chemical Services, France). The
compartments were separated by cationic exchange membranes (Ionac 3470 – Lanxess SAS,
Courbevoie, France). The reference electrode (Mercury-mercurous sulfate – MSE) was
positioned in the middle of the graphite felt (10 mm diameter and 12 mm width) and a
potential of -1.8 V/MSE was applied using a VersaSTAT 3 potentiostat from
Ametek/Princeton Applied (Elancourt, France). The solution (100 mg L−1 pesticide in 0.1 mol
L-1 Na2SO4) percolated the porous electrode at a flow rate of 1 mL min-1 monitored by a
Gilson minipuls 3 peristaltic pump (Middleton, WI, USA).
2.4. Analytical procedures
Electrochemical measurements. Voltammetric experiments were carried out using a
VersaSTAT3 AMETEK Model (Princeton Applied Research) potentiostat/galvanostat. A
glassy carbon electrode (7 mm2), a platinum plate auxiliary electrode, and a saturated calomel
reference electrode were used in a standard three-electrode configuration. Cyclic voltammetry
4

was carried out in 0.1 mol L-1 Na2SO4, containing 0.1 mol L-1 hexadecyl trimethyl ammonium
bromide (HDTAB) as cationic surfactant, under a dinitrogen atmosphere. For comparison
with numerical simulations, the background was subtracted.
Numerical simulations of the voltammograms were performed with the KISSA 1D program10
using the default numerical options with the assumption of planar diffusion. Butler−Volmer
law was considered for the electron transfer kinetics [45-47]. The transfer coefficient, α, was
taken as 0.5, and the diffusion coefficients were estimated from voltammetric signals at
different scan rates based on the Randles-Sevčik equation ([Co(bpy(CH2OH)2)2](BF4)2: 7.8 ×
10-7 cm2 s-1; Alachlor: 3.2 × 10-7 cm2 s-1).
High Performance Liquid Chromatography (HPLC). Alachlor concentration was
determined by HPLC using a Waters 996 system equipped with waters 996 PDA (Photodiode
Array Detector) and Waters 600 LCD Pump. The separation was achieved on a Waters C-18
(5 µm; 4.6 × 250 mm) reversed-phase and the mobile phase consisted of a mixture of
acetonitrile/ultra-pure water (70/30, v/v). The flow rate was set at 1 mL min-1 and 50 µL
injections were used. Detection was carried out at 230 nm and the retention time was
approximately 8.3 min.
The same apparatus with the same column were used to quantify 2,4-D. The mobile phase
consisted of a mixture of acetonitrile/ultra-pure water (60/40, v/v) and 0.1% Formic acid.
Detection was carried out at 284 nm and the retention time was approximately 4.6 min.
Before HPLC analysis of alachlor and 2,4-D, the pH of the electrolyzed solutions should be
raised at 2.
Ultra-pressure liquid chromatography-MS/MS. The devices used are detailed in a
previous work [4]. The analytes were separated by a Waters Acquity UPLC system (Waters
corporation, Milford, MA, USA), consisting of an Acquity UPLC binary solvent manager, an
Acquity UPLC sample manager and an Acquity UPLC column heater equipped with a Waters
Acquity UPLC BEH C18 column (2.1 mm × 100 mm, 1.7 μm particle size) thermostated at 45
°C (Milford, MA, USA). Gradient LC elution was performed with 0.1% formic acid in 10%
acetonitrile in ultrapure water as mobile phase A and acetonitrile added 0.1% (v/v) of formic
acid as mobile phase B. Separation of the analytes on the column was performed with a
gradient of phase A/phase B at 0.4 mL min−1 flow rate. The starting eluent composition
consisted of 100% phase A which was then linearly increased to reach 100% of phase B at 5
min. At 6.5 min, the composition was changed again to 100% of phase A. The separated
compounds were detected with a Waters Micromass Quattro Premier (Waters Corporation,
Manchester, UK) triple quadripole mass spectrometer. It was operated with an electrospray
5

source in positive ionization mode with a cone potential of 20V. The ionization source
conditions were: capillary voltage of 3 kV, source temperature 120 °C and desolvation
temperature 350 °C. The cone and desolvation gas flows (N2) were 50 and 750 L h−1,
respectively. Analyses were performed in full scan mode. Spectra were acquired between 60
and 300 m/z and the data were treated with Micromass MassLynx 4.1 software.
Ion chromatography. The concentration of chloride ions (diluted five times) before
and after electrolysis were determined by DIONEX DX120 ion chromatography equipped
with a conductivity detector and a DIONEX AS19 (4 × 250 mm) ion-exclusion column.
Potassium hydroxide was used as eluant and the flow rate was 1 mL min-1. The detection was
carried out by conductivity with a Self Regenerating Suppressor (SRS).
3. Results and discussion
3.1. Synthesis and cyclic voltammetry analyses
The ligand [2,2'-bipyridine]-4,4'-diyldimethanol 8 was synthesized from [2,2'-bipyridine]-4,4'-
dicarboxylic acid 6 by esterification and then reduction with NaBH4 (Scheme 4) [48].
The [Co(bpy(CH2OH)2)2]2+ complex 9 was then prepared by refluxing 2 to 4 equivalents of
[2,2'-bipyridine]-4,4'-diyldimethanol 8 with Co(BF4)2 in ethanol.
The redox behavior of the complex was first examined in aqueous medium in the
presence of a surfactant to stabilize the [Co(bpy(CH2OH)2)2]+ intermediate [37]. As seen in
Figure 1, the voltammogram exhibits two well-defined reversible waves at 0.0 and -1.2 VSCE
corresponding respectively to the CoIII/II and CoII/I systems (Fig. 1). It indicates that the
different redox oxidation states of the complex are stable on the time scale of the cyclic
voltammetry (some tens of seconds) and that the electron transfers kinetics are relatively fast
in this medium. The complex synthesized in the presence of 4 equivalents of 8 exhibited the
same two reversible systems, showing that the same complex is obtained even in the presence
of an excess of ligand.
The redox behavior of the chlorinated pesticides, alachlor, metolachlor and metazachlor was
examined in the same conditions. A broad and irreversible wave at a negative potential
(around -1.4 V/SCE) close to the solvent discharge was visible as highlighted in Figure 2.
6

Based on the general reactivity of halogenated alkyl-compounds, these different
electrochemical peaks could be ascribed to the global 2-electron reduction of the C-Cl bond
leading to the corresponding dehalogenated form (Eq. 3) [49].
R-X + 2e- + H+→ RH + X-
(3)
The catalytic activity of the [Co(bpy(CH2OH)2)2]2+ complex toward the reduction of
chloroacetanilide pesticides of Scheme 3, was then examined by cyclic voltammetry. Addition
of the chloroacetanilide pesticides to a solution of [Co(bpy(CH2OH)2)2]2+ led to a
considerable increase of the cathodic peak current corresponding to the reduction of CoII in
CoI, while the reverse anodic peak decreased (Fig. 3).
The observed modifications (increase of the current and decrease of the reversibility at the
level of the reduction) are indicative of a catalytic activity by the CoI complex toward the
reduction of the chloroacetanilide herbicides.
Similar experiments were performed with an aromatic chlorine pesticide, 2,4-D (4)
(Scheme 3). In the presence of 2,4-D, the cathodic wave of the CoII/I system was enhanced
while the anodic wave was totally suppressed (data not shown). Since direct reduction of 2,4-
D has been previously observed around -1.5 VSCE and attributed to a one-electron reduction of
carboxylic acid into carboxylate anion [50], it is difficult to conclude from these sole
experiments on the catalytic activity of the CoI complex toward the dechlorination of 2,4-D.
3.2. Exhaustive electrolysis
To confirm the cyclic voltammetry analyses, the cathodic reduction of alachlor (3.7 × 10-4
mol L-1; 100 ppm) was performed at -1.65 V/MSE (i.e. -1.25 V/SCE) in aqueous medium in
the presence of the [Co(bpy(CH2OH)2)2]2+ complex. Notice that a Mercury Sulfate Electrode
(MSE) was used as reference to avoid any pollution by chlorides during the ions solution
analysis. The reaction was carried out in a flow electrochemical cell with recycling of the
solution and with substrate to catalyst ratios of 3 and 4. The working electrode is a graphite
felt of 1 cm3. Interestingly, this material is available in larger scale as it is suitable for higher
volumes of effluent to treat. The evolution of the solution composition was followed by
regular sampling and HPLC and UPLC analyses. The variations of the normalized
concentrations of alachlor and the corresponding dehalogenated product are presented in
Figure 4, displayed for the most efficient substrate to catalyst ratio 3, as discussed below. A
7

blank has been carried out in the same conditions for comparison. Direct reduction of alachlor
occurred at -1.65 V/MSE, leading to a degradation of alachlor of 27% (Table 1) with
formation of deschloroalachlor 5 in 25%.
As seen in Figure 4, deschloroalachlor 5, which exact structure has been confirmed by
UPLC/MS/MS (see Supporting Information), is the only observed product during the
electrochemical catalysis and its formation followed the degradation of alachlor with a final
yield of 85%, underlining the selectivity and the efficiency of the reduction process.
Additionally, the final concentration of chloride ions was measured by ion chromatography
(Table 1) at the end of the electrolyses and confirmed a total dechlorination of alachlor for a
substrate/catalyst ratio of 3. Concerning the faradic yield, a current efficiency of 55% was
estimated from the charge passed during the electrolysis and considering a 2-electron
reduction process and a total dechlorination. This value could probably be improved by an
optimization of the cell. For comparison, a direct electrolysis without catalyst was also
performed at -1.8 V/MSE. A reduction of alachlor of 90% was obtained with a current
efficiency of 17% due to the competition with hydrogen evolution at this potential. This result
underlines the interest of the indirect catalytic process that occurs at lower potential. It is
noticeable that the reduction of alachlor into deschloroalachlor proceeds via a 2-electron
process using the [CoI(bpy(CH2OH)2)2]+ complex, whereas the reduction with the
corresponding tris-bypiridine analog has been reported as a monoelectronic process, leading
to the formation of 1,5-hexadiene from allyl chloride [37, 38].
Similar experiments were carried out with 2,4-D as a starting material. A potential of -1.8
V/MSE was applied for a concentration of 3.7 × 10-4 mol L-1 (100 ppm) of 2,4-D and
substrate to catalyst ratios of 2.5 and 2. Samples were taken from the solution during the
electrolysis, acidified to pH 2 and analyzed by HPLC for observing the evolution of 2,4-D
concentration during the electrolysis as done previously for the reduction of alachlor. As seen
in Figure 5, a decrease of the substrate concentration is observed upon electrolysis, leading to
a degradation of 22 and 38% of 2,4-D for substrate to catalyst ratios of 2 and 2.5, after 10h of
electrolysis, respectively. These results are much lower than those obtained for alachlor,
underlining a poor catalytic activity of the [CoI(bpy(CH2OH)2)2]+ catalyst toward the 2,4-D
reduction. The reductive dechlorination of 2,4-D is therefore possible at -1.4 V/SCE
corresponding to the formation of the CoI complex but with a much lower efficiently than for
the dechloration of alachlor. Our observations fall in line with literature reports about catalytic
2+
dechlorination in organic media. Indeed at the reduction potential of CoII(bpy)3 into
8

+
CoI(bpy)3 , only an aliphatic chloride of 1,1-Bis(4-chlorophenyl)-2,2,2-trichloroethane was
reduced [51], [52]. More recently, a study on CoBr2bpy catalyst underlined the poor catalytic
activity of the CoIBrbpy complex toward the reduction of aromatic chloride that was slightly
enhanced by using Co0bpy complex [42].
3.3. Mechanistic study
The reduction of chlorinated compounds has been previously reported as an outer-sphere or
an inner-sphere mechanism depending on the substrate and media [18, 27, 33, 34]. To get
some insights about the mechanism occurring in aqueous media with the studied chlorinated
pesticides, the possibility of an outer-sphere mechanism was considered to realize if this class
of complexes plays a more chemical role than to be a simple charge transfer mediator and
thus how the structure of the complex is an important point in the efficiency of the catalysis.
The voltammograms of the reduction of [CoII(bpy(CH2OH)2)2]2+ complex were recorded for
different concentrations of alachlor and at different scan rates. As general observations, the
cathodic peak current of the Co complex considerably increased with the concentration of
alachlor in the range of investigated scan rates (0.05-0.2 V s-1). For the highest scan rates ≥
0.1 V s-1, the voltammograms exhibit two reduction waves (see for example Fig. 6b for a 2 ×
10-3 mol L-1 concentration of alachlor), the first one corresponding to the reduction of the CoII
complex and the second one to the direct reduction of alachlor at the electrode. For the lowest
scan rates, it is noticeable that the anodic peak corresponding to the oxidation of CoI into CoII
remain visible even at high concentrations of alachlor (12-20 × 10-3 mol L-1). This is
indicative of a fast catalytic reaction since the catalyst was detected under its CoI form during
the anodic scan and all alachlor was consumed (the direct reduction was not visible).
Simplified numerical simulations of the voltammograms were performed to derive an
apparent charge transfer rate between the catalyst and the substrate RX (k1). The considered
outer-sphere mechanism is presented on Scheme 5. It involves two monoelectronic charge
transfers between the CoI catalyst and the substrate. The electrochemical apparent charge
transfer standard rate constant ks was first estimated as 0.01 cm s-1 from the cyclic
voltammetry of the reduction of [CoII(bpy(CH2OH)2)2]2+ complex, assuming a Buttler-Volmer
law and considering the peak-to-peak potential separation between the cathodic and anodic
9

scans [53]. The charge transfer coefficient was taken α = 0.5 and the diffusion coefficient of
[CoII(bpy(CH2OH)2)2]2+ measured as 7.8 × 10-7 cm2 s-1.
In a first series of simulation, the first reduction step was taken as the rate-determining step,
all other steps being considered as fast (k2>>k1, Fig. 7a). Within this simplification, it was not
possible to get a proper fit between the simulated voltammograms and the experimental data
(Fig. 7a), where the disappearance of the anodic wave already coincides with the increase of
the cathodic peak current (for example, the reversibility character is almost lost for a 8 × 10-3
mol L-1 concentration of alachlor, see Fig. 7a).
In a second set of simulations (Fig. 7b), the second reduction step, which is normally
faster in outer-sphere mechanisms as the neutral radical is generally easier to reduce than the
starting RX [54], was considered as a slow step. Only very low values of k2 (< 50 mol-1 L s-1
i.e. k2<< k1) allow maintaining the presence of the anodic wave of the Co complex as the
cathodic peak increases upon addition of alachlor. Under these conditions, the simulated
voltammograms display a reasonable agreement with the experimental ones (Fig. 7b, see the
supplementary material for an easier comparison). From these results, an apparent chemical
rate constant k1 of 5000 mol-1 L s-1 was estimated. It is worth noting that this value is 6 orders
of magnitude higher than the apparent chemical rate constant estimated for the reduction of
+
allyl chloride by the CoI(bpy)3 complex [15, 37, 38, 55]. It clearly underlines the high
catalytic activity of the [CoI(bpy(CH2OH)2)2]+ complex containing two bipyridyl ligands,
toward dehalogenation of alachlor. The difficulty to fit the experimental voltammograms and
the necessity for considering a slower second charge transfer suggest a more “chemical
character” of the catalysis that was not considered owning to the complexity of the aqueous
medium with surfactant.
4. Conclusions
This example of cobalt complex containing only two bipyridine ligands confirms all
the interest of this class of molecules for achieving clean electrochemical dehalogenation of
chlorinated pesticides. The prepared [Co(bpy(CH2OH)2)2]2+ complex exhibits high catalytic
activity toward the dechlorination of chloroacetanilide herbicides. Based on preparative
electrolyses performed on alachlor, 2-electron reduction of the substrate to deschloroalachlor
was obtained with 85% of yield, underlining the high selectivity and efficiency of this
dehalogenation process. We could highlight the high apparent chemical rate constant k1 of
10

5000 mol-1 L s-1 followed by an apparent slower second reduction step. This observation
suggests that the catalytic process is not a simple succession of electron transfer steps as the
condition required on the second electronic transfer is not reasonable for a purely redox
mediator. A more detailed study on the mechanism involving the Co complex will be
envisaged in a simplified medium, with comparison with other cobalt complexes. The high
catalytic activity of the Co complex containing two bipyridine ligands to achieve
dechlorination of chloroacetanilide herbicides is particularly encouraging for applications in
depollution processes. To still improve the performances of this dehalogenation process, work
is currently in progress to investigate the catalytic activity of the [Co(bpy(CH2OH)2)2]2+
complex in heterogeneous conditions, avoiding by this way its presence in solution before a
further biological treatment.
Supporting information
1H-NMR of intermediate products, mass spectrometry of the electrolyzed solution and of
deschloroalachlor and comparison of the experimental results and simulations obtained in cyclic
voltammetry.
Acknowledgments
Profs. I Svir and C. Amatore are warmly thanked for providing us the KISSA 1D
software package, which was used for the mechanistic simulations of the voltammograms. W.
Y. He thanks the China Scholarship Council for a Ph.D grant.
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15

Figure 1. Cyclic voltammograms on a 3 mm diameter glassy carbon electrode in water + 0.1 mol L-1 Na2SO4 +
0.1 mol L-1 HDTAB of 1.9 × 10-3 mol L-1 of [Co(bpy(CH2OH)2)2]2+ prepared from 2 (----) and 4 (____) eq. of
[2,2'-bipyridine]-4,4'-diyldimethanol 8. Scan rate 0.02 V s-1.
Figure 2. Cyclic voltammograms in 0.1 mol L-1 Na2SO4 + 0.1 mol L-1 HDTAB without (----) and with 4 × 10-3
mol L-1 alachlor (_____), 4 × 10-3 mol L-1 metazachlor (_____), 4 × 10-3 mol L-1 metolachlor (……) on a 3 mm
diameter glassy carbon electrode. Scan rate 0.02 V s-1
16

Figure 3. Cyclic voltammograms on a 3 mm diameter glassy carbon electrode of [Co(bpy(CH2OH)2)2]2+
complex (1.9 × 10-3 mol L-1) in 0.1 mol L-1 Na2SO4 + 0.1 mol L-1 HDTAB before (____) and after (----) addition
of a) 4 × 10-3 mol L-1 alachlor b) 4 × 10-3 mol L-1 metazachlor c) 3.9 × 10-3 mol L-1 metolachlor. Scan rate 0.1 V
s-1.
17

a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
Time / min
b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
Time / min
Figure 4: Exhaustive electrolysis. Evolution of the normalized concentration C/C0 of alachlor (■) and
deschloroalachlor (▲) vs electrolysis time for the cathodic reduction of alachlor (3.7 × 10-4 mol L-1; 100 ppm)
performed at -1.65 V/MSE (equivalent to -1.25 V/SCE) in 0.1 mol L-1 Na2SO4 without a) and with 1.24 × 10-4
mol L-1 [Co(bpy(CH2OH)2)2]2+ b) at 1 mL min-1. Calachlor and Cdeschloroalachlor are the concentrations of alachlor and
deschloroalachlor at a given time and C0 the initial concentration of alachlor. Error bars are based on two
reproducibility measurements.
18

Figure 5. Exhaustive electrolysis. Evolution of the normalized concentration C2,4-D/C0 vs electrolysis time for
the cathodic reduction of 2,4-D (4.5 × 10-4 mol L-1) performed with [Co(bpy(CH2OH)2)2]2+ (2.3 × 10-4 mol L-1
(■); 1.8 × 10-4 mol L-1 (●)) in 0.1 mol L-1 Na2SO4 (pH=7.2, 1 mL min-1).
19

Figure 6. Cyclic voltammograms of [Co(bpy(CH2OH)2)2]2+ complex (1.9 × 10-3 mol L-1) in 0.1 mol L-1 Na2SO4
+ 0.1 mol L-1 HDTAB before (____) and after addition of 2 × 10-3 mol L-1 (……), 4 × 10-3 mol L-1 (-----), 8 × 10-3
mol L-1 ____), 12 × 10-3 mol L-1 (…..), 20 × 10-3 mol L-1 (-----) alachlor on a 3 mm diameter glassy carbon
(
electrode at a) 0.05 V s-1 and b) Scan rate 0.2 V s-1. The background was subtracted.
20

Figure 7. Simulations of voltammograms of Figure 6a considering the second reduction step as a) fast k2>> k1
and b) low k2 << k1 = 5000 mol-1 L s-1 [Co(bpy(CH2OH)2)2]2+ complex (1.9 × 10-3 mol L-1) in 0.1 mol L-1
Na2SO4 + 0.1 mol L-1 HDTAB before (____) and after addition of 2 × 10-3 mol L-1 (……), 4 × 10-3 mol L-1 (-----),
8 × 10-3 mol L-1
(
____), 12 × 10-3 mol L-1 (…..), 20 × 10-3 mol L-1 (-----) alachlor on a 3 mm diameter glassy
carbon electrode at 0.05 V s-1.
21

Scheme 1. Inner-sphere and outer-sphere mechanisms of dehalogenation reactions catalyzed by CoI complexes.
[CoI(L)]- + RX → [CoIIIR(L)] + X- (Inner-sphere)
.
[CoI(L)]- + RX → [CoII(L)] + X- + R (Outer-sphere )
Scheme 2. Cathodic reduction of Co(bpy)32+ complex involving a loss of a bipyridyl ligand.
Scheme 3: Structure of the pesticides involved in this study and of deschloroalachlor.
Scheme 4: Synthesis of the [Co(bpy(CH2OH)2)2]2+ complex.
Scheme 5. Outer-sphere mechanism considered for the catalytic reduction of alachlor by the
[Co(bpy(CH2OH)2)2]2+ complex to simulate cyclic voltammograms.
22

Table 1. Alachlor electrolysis performed at -1.65 V/MSE in aqueous medium
Experiment Substrate/catalyst ratio Alachlor conc./mol L-1 Time/h Conversion/% Yield of Cl- a/%
Entry 1
Entry 2
Entry 3
No catalyst
3.7 × 10-4
3.7 × 10-4
3.7 × 10-4
1
1
1
27
25
3
100
99.6
100
93
4
a measured by ion chromatography
23