Hexachloroethane reduction catalyzed by cobaloximes. Effect of the substituents on the equatorial ligands

Article abstract of DOI:10.1016/j.poly.2017.11.005

The degradation of hexachloroethane using chloridobis(dimethylglyoximato)pyridinecobalt(III), chloridobis(methylphenylglyoximato)pyridinecobalt(III) and chloridobis(diphenylglyoximato)pyridinecobalt(III) is described. To achieve the degradation, the cobal

Full text of DOI:10.1016/j.poly.2017.11.005

Accepted Manuscript
Hexachloroethane reduction catalyzed by cobaloximes. Effect of the substitu-
ents on the equatorial ligands
Sebastián Pizarro, Michael Araya, Alvaro Delgadillo
PII:
S0277-5387(17)30705-2
https://doi.org/10.1016/j.poly.2017.11.005
POLY 12907
DOI:
Reference:
Polyhedron
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3 August 2017
3 November 2017
Please cite this article as: S. Pizarro, M. Araya, A. Delgadillo, Hexachloroethane reduction catalyzed by
cobaloximes. Effect of the substituents on the equatorial ligands, Polyhedron (2017), doi: https://doi.org/10.1016/
j.poly.2017.11.005
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Hexachloroethane reduction catalyzed by cobaloximes. Effect of the substituents on the equatorial
ligands
Sebastián Pizarro1*, Michael Araya3, and Alvaro Delgadillo2
1Instituto de Investigación Multidisciplinario en Ciencias y Tecnologías, Universidad de La Serena, Casilla
599, Benavente 980; La Serena, Chile.
2Departamento de Química, Facultad de Ciencias, Universidad de La Serena, Casilla 599, Benavente 980; La
Serena, Chile.
3Centro de Investigación y desarrollo tecnológico en Algas (CIDTA), Facultad de Ciencias del Mar,
Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile.
*tel: +56 51 2204420; fax: +56 51 2204399; email: adelgadillo@userena.cl
1

Abstract
The degradation of hexachloroethane using chloridobis(dimethylglyoximato)pyridinecobalt(III),
chloridobis(methylphenylglyoximato)pyridinecobalt(III)
and
chloridobis(diphenylglyoximato)pyridinecobalt(III) is described. To achieve the degradation, the cobalt
complexes were reduced to the Co(I) oxidation state by electrochemical means. The chemical catalysis was
studied using cyclic voltammetry by monitoring the current increase near the Co(II)/Co(I) reduction potential
as the concentration of organohalogen was increased in solution. Tetrachloroethene was found as the
degradation product. The apparent rate constant for the dehalogenation of hexachloroethane was estimated for
all cobaloximes, obtaining 2.54 0.08 x105 for [Co(dmgH)2Cl(py)]; 2.38 0.06 x105 for [Co(mpgH)2Cl(py)]
and 2.79 0.15x105 for [Co(dpgH)2Cl(py)].
Keywords: cobaloxime, cyclic voltammetry, electrochemistry, organohalides, catalysis
2

1. Introduction
Chlorinated organic compounds are listed among the most widely used pollutants on the planet mainly due to
their versatility in various applications, such as solvents, plasticizers, and pesticides[1]. Some chlorinated
polyethylenes such as tetrachloroethene[2] and hexachloroethane[3] are used as raw materials in the
preparation of fungicides, insecticides, lubricants or plastics, and are often detected as contaminants in soils
and wastewater[4]. The United States Environmental Protection Agency has classified these substances as
possible carcinogens; consequently, their presence in the environment represents a risk to human health[5].
During the last years several technologies have been developed to degrade these type of substances. These
processes comprehend bio-degradation[6], photocatalytic degradation[7], as well the use of molecular
catalysts as cobalt porphyrins[8], cobinamides[9] and cobaloximes[10].
Gantzer and Wackett[11] reported the dehalogenation of polychlorinated ethenes using cobalamin as catalyst
and titanium(III) citrate as a sacrifice reductant. The products observed by Gantzar and Wackett correspond to
the hydrogenolysis of PCE (TCE, cis-1,2-dichloroethylene, vinyl chloride and ethene). These observations
were complemented by the experiments of Schanke and Wackett[12] which demonstrated that catalytic
reduction of chlorinated alkanes is possible by giving some alkenes as products. The main disadvantage of
these cobalt-based systems is that a reducing agent must be added to the reaction medium in order to reach the
Co (I) state[13].
In an alternative way to the addition of reducing agents McCauley et al. have electrochemically generated the
reduced forms of cobalamin and cobinamide, and demonstrated that these compounds are efficient catalysts
for the hydrogenolysis of aliphatic chloro-compounds[14].
Cobaloximes are a set of pseudo-macrocyclic bis(diglyoximato)cobalt complexes that show a pseudo-
octahedral structure where the equatorial positions are occupied by two vic-dioximes forming a pseudo-
macrocycle, and the axial positions are occupied by an anionic ligand and a nitrogenous base[15]. These
structural characteristics make them good models of vitamin B12 and related compounds since they share
many of their chemical properties, i.e. the nucleophilicity. Advantageously, it is easier to perform systematic
structural changes on cobaloximes than in vitamin B12 related compounds[16].
3

It has been observed that the electron donor/withdrawal characteristics of substituent functional groups of the
cobaloxime ligands have a variable influence on the reduction potential of cobalt, and therefore in the
catalytic properties of those compounds[17–19]. Substitution on the nitrogenous axial base has a little impact
on the electrochemical properties, while the groups on the dioxime backbone can exert a big influence on the
Co (II/I) reduction potential.
Cobaloximes have been widely studied because are good catalysts for the reduction of protons at low
overpotentials in organic solvents[20]. However, we are interested in expanding the study of these compounds
to the dehalogenation of organohalide compounds. In this context, we report the catalytic degradation of
hexachloroethane, a compound environmentally persistent, bioaccumulative and toxic, by reaction with
electrochemically generated Co(I) cobaloximes. The electrochemical potentials of the Co center have been
systematically tuned by selecting the substituents of the equatorial in order to evaluate their influence on the
catalytic process.
2. Experimental section
2.1. Materials and measurements
All the chemicals were commercially available and used as received. Complexes [Co(dmgH)2Cl(py)][21] and
[Co(dpgH)2Cl(py)][22] were synthesized according to procedures reported in literature. The description of the
synthetic procedures as well the instrumentation and characterization methods are described in the supporting
information.
2.2. Electrochemistry
Cyclic voltammetry experiments were conducted on
a Princeton Applied Research PG 580
potentiostat/galvanostat using a three-electrode single compartment cell including a glassy carbon working
electrode, a Pt wire auxiliary electrode, and an Ag/AgCl reference electrode. Tetrabutylammonium
hexafluorophosphate [NBu4][PF6] was used as the supporting electrolyte (0.1M). For all measurements,
samples were degassed by purging with nitrogen.
4

2.3. Diffusion coefficient
The diffusion coefficients were determined using the Randles-Sevcik equation which assumes that mass
transport occurs only by a diffusion process[23]:
ꢀꢁ = (2,69 × 10ꢂ)ꢃꢄ/ꢅꢆꢇꢈ/ꢅꢉꢊꢈ/ꢅ
(1)
Where ip is the peak current (A), n the number of electron equivalents exchanged during the oxidation/redox
reversible process, A the active surface area of the working electrode (cm2), D the diffusion coefficient
(cm2·s–1), C the bulk concentration of the diffusing species (mol·cm–3) and ν the voltage scan rate (V·s–1).
3. Results and discussion
3.1 Synthesis
The cobaloximes used in this study are: [Co(dmgH)2Cl(py)] (where dmgH is dimethylglyoximato and py is
pyridine), [Co(mpgH)2Cl(py)] (where mpgH is methylphenylglyoximato) and [Co(dpgH)2Cl(py)] (where
dpgH is diphenylglyoximato). The structures of these complexes are presented in Fig. 1.
Cobaloximes (a) and (c) were previously reported. To obtain cobaloxime (b) we prepare anti-
methylphenylglyoxime by condensation between 1-phenyl-1,2-propanedione and hydroxylamine
hydrochloride in acidic ethanol solution. The cobaloxime [Co(mpgH)2Cl(py)] was prepared by coordination
of the ligand mpgH2 and pyridine to cobalt(II) chloride hexahydrate in a single step. The Fig. 2 shows the
synthesis scheme followed to obtain this compound.
3.2. Electrochemistry in solution
3.2.1. Cobaloximes
The Fig. 3 shows the cyclic voltammograms of all the cobalt(III) complexes used in this study. The three of
them show the same pattern which was described in the literature[24,25]. It consists of an irreversible
cathodic wave attributed to the Co(III) to Co(II) reduction process, which is also associated to an equilibrium
involving the loss of the chloride axial ligand[26]. To more negative potentials a quasi-reversible process is
observed and assigned to the reduction process Co(II) to Co(I). Finally, at more positive potentials is observed
5

an anodic irreversible wave which is assigned to the Co(II) to Co(III) oxidation process. The previously
described cathodic, anodic and half wave potentials, as well the diffusion coefficients for each complex are
summarized in Table 1.
There is a clear influence of the structure of the glyoxime equatorial ligands on the electrochemical behavior
of the cobalt center. The substitution of a methyl group of the dimethylglyoxime ligand by a phenyl group
results in a displacement of the Co(II) to Co(I) half-wave potential of 180 mV towards more positive
potentials. In a similar way, the substitution of all the methyl groups by phenyl groups leads to a 320 mV
positive displacement. It has been reported that substitution on the pyridine axial ligand exerts none or small
influence on the Co(II/I) half-wave potential[27].
As our objective was to use these complexes to dehalogenate reductively hexachloroethane, we initially
studied the electrochemical behavior of this compound in a cobaloxime free solution.
3.2.2. Hexachloroethane (HCA).
The Fig. 4 shows the cyclic voltammetry of HCA on DMF 0.1 M [NBu4][PF6] between +0.5 V and –1.8 V on
a carbon glass electrode working electrode. This voltammogram shows only one irreversible reduction wave
at -1.58 V vs Ag/AgCl. According to literature, this wave is assigned to a concerted reductive cleavage which
consists of the sum of two redox processes that results in the elimination of chloride [28]. The first one is a
one electron reduction which leads to the break of a carbon chloride bond generating a radical. The second
process consists of the reduction of the generated radical and the elimination of a second chlorine atom. The
process may be written as follows:
C2Cl6 + e- ꢀ ·C2Cl5 + Cl-
·C2Cl5 + e- ꢀ C2Cl4 + Cl-
(2)
(3)
Subsequent reductions process leading to dichloroethane are reported to occur at more negative potentials (> -
1,8 V). It is important to note that the potential of the cathodic wave assigned to the hexachloroethane
reduction is observed at a potential, at least, 500 mV more negative than the Co (II/I) half-wave potential of
the cobaloximes used in this study.
6

3.3. Dehalogenation experiments
In the cyclic voltammetry experiments an increase of the cathodic current is observed after the addition of
hexachloroethane to a cobaloxime solution, which is consistent with a chemical catalysis process. This
behavior is depicted in Fig. 5, 6 and 7 which shows the cyclic voltammograms of the cobaloximes after the
addition of 2 to 18 equivalents of the HCA. All these voltamograms were recorded at a scan rate of 100 mV s-
1 using a concentration of 0.125 mM in 0.1M DMF [NBu4][PF6] for all the complexes.
The catalytic currents were observed at a slightly more negative potential than the observed for the couple
Co(II)/Co(I), but 500 mV to 900 mV more positive than the reduction potential observed for HCA. This
behavior can be attributed to competing side processes such as substrate consumption (when the substrate
concentration at the electrode surface is depleted relative to the bulk concentration) or catalyst
deactivation[29].
The apparent rate constant for the degradation of HCA was estimated from the dependence between the
current increase and the square root of the substrate concentration using the model of chemical catalysis
described in the literature[30]. Hence, for a reaction of first order for both the catalyst and the organohalogen,
the cathodic current is related to the concentration of the catalyst (mol cm-3) and the square root of the
concentration of the organohalogen (mol L-1). The expression for this relation is represented by equation 1:
ꢈ/ꢅꢍꢘꢙꢕꢈ/ꢅ
(4)
ꢈ/ꢅ
ꢍ
ꢕ
ꢀꢋ = ꢃꢌꢆ ꢉꢎꢏꢐꢑꢎꢒꢀꢓꢔ ꢇ
ꢖ
ꢗꢁꢁ
Where n is the number of electrons for the reaction (n=2), F is Faraday constant (96485 C mol-1), A is the area
of the electrode surface (0.072 cm2), D is the diffusion coefficient (cm2s-1) and k is the apparent rate constant
for the reaction (M-1s-1). The figures 5b, 6b and 7b shows the fitting of the cathodic current to this model. The
apparent rate constants, as well the electrochemical parameters are resumed on table 1.
Head space gas-chromatography experiments were carried out after 60 minutes of electrolysis of cobaloxime
hexachloroethane solutions of a 1:8 molar ratio. In each case, the potential employed for the electrolysis was
slightly more negative than the potential observed for the catalytic waves. The only product found in these
experiments was tetrachloroethylene (see Support Information).
7

In the literature, the processes of dehalogenation catalyzed by structurally similar cobalt complexes are
described starting with a nucleophilic attack of the reduced cobalt complex to the organohalogen, eliminating
a halogen anion and forming an organometallic bond[28].
This mechanism is plausible just if the halogenated derivative does not possess a great steric hindrance, since
the reaction proceeds in a similar way as the SN2 type. In the case of HCA a reaction of this type is very
unlikely to proceed due to the steric hindrance of the chlorine atoms present in the molecule.
We propose the reductive dehalogenation reaction for HCA to occur by an electron transfer associated with
the formation of an intermolecular halide bridge between the substrate and the catalyst as is schematized in
Fig. 8. The first step begins with a one electron reduction of the cobaloxime followed by the exchange of the
chloride ligand with a solvent molecule. Subsequently, a second one electron reduction generates a Co (I)
metallic center. The substrate interacts with the reduced Co through a chloride bridge causing a heterolytic
cleavage of the C-Cl bond; the pentachloroethane radical can undergo a subsequent reductive elimination
reaction generating tetrachloroethylene.
A similar mechanism was proposed by Payne for the reductive degradation of 2,6-dibromophenol by vitamin
B12 as a catalyst[32].
Fontecave, Artero, and others have studied extensively these systems for proton reduction from acid sources
in organic solvents[33–36]. They have reported a marked dependence of the reduction activity, expressed on
the apparent rate constant, with the electron-withdrawal characteristics of the glyoxime substituents, whereas
the half-wave potential of the Co(II)/Co(I) process turns more positive, the hydrogen evolution decreases[37].
In a clear difference with those observations, the apparent rate constants for the hexachloroethane catalytic
reduction found in this study (Table 1) do not show extended differences. In an opposite direction, the higher
magnitude rate constant was observed for the cobaloxime displaying the better electron-withdrawal
substituent. This behavior could be explained since the latter reaction should be independent on the basicity of
the cobalt center, and probably driven by conformational halogen effects[38,39].
8

4. Conclusions
In cyclic voltammetry experiments, the addition of HCA to electrochemically generated Co(I) cobaloximes
produces catalytic waves which are consistent with a chemical catalysis process.
Although the Co complexes employed show substantial differences on the Co (II/I) reduction potential, the
observed rate constant do not show significant differences. Under the experimental conditions employed in
this study, only tetrachloroethene was observed as a degradation product.
5. Acknowledgments
The authors acknowledge: DIULS PI 17161; Plan de Mejoramiento Institucional en Eficiencia Energética y
Sustentabilidad Ambiental ULS-1401.
6. References
[1]
B. Huang, C. Lei, C. Wei, G. Zeng, Chlorinated volatile organic compounds (Cl-VOCs) in
environment - sources, potential human health impacts, and current remediation technologies,
Environ. Int. 71 (2014) 118–138. doi:10.1016/j.envint.2014.06.013.
[2]
[3]
[4]
C. Barton, Tetrachloroethylene, Encycl. Toxicol. 4 (2014) 498–502. doi:10.1016/B978-0-12-386454-
3.00436-X.
S.B. Jazayeri, R. Bazl, M. Biglar, M. Amanlou, Hexachloroethane, in: Encycl. Toxicol., Elsevier,
2014: pp. 882–889. doi:10.1016/B978-0-12-386454-3.01129-5.
F. Aulenta, M. Majone, P. Verbo, V. Tandoi, Complete dechlorination of tetrachloroethene to ethene
in presence of methanogenesis and acetogenesis by an anaerobic sediment microcosm,
Biodegradation. 13 (2002) 411–424. doi:10.1023/A:1022868712613.
[5]
(EPA) United States Environmental Protection Agency, Toxicological review of Hexachloroethane
(CAS No. 67-72-1) in support of summary information on the Integrated Risk Information System
(IRIS), (2011). https://cfpub.epa.gov/ncea/iris/iris_documents/documents/toxreviews/0167tr.pdf.
9

[6]
[7]
[8]
[9]
M.E. Walsh, P. Kyritsis, N.A.J. Eady, H.A.O. Hill, L.L. Wong, Catalytic reductive dehalogenation of
hexachloroethane by molecular variants of cytochrome P450(cam) (CYP101), Eur. J. Biochem. 267
(2000) 5815–5820. doi:10.1046/j.1432-1327.2000.01666.x.
E. Jho, N. Singhal, S. Turner, Tetrachloroethylene and hexachloroethane degradation in Fe(III) and
Fe(III)-citrate catalyzed Fenton systems, J. Chem. Technol. Biotechnol. 87 (2012) 1179–1186.
doi:10.1002/jctb.3746.
S.O. Obare, T. Ito, G.J. Meyer, Controlling Reduction Potentials of Semiconductor-Supported
Molecular Catalysts for Environmental Remediation of Organohalide Pollutants, Environ. Sci.
Technol. 39 (2005) 6266–6272. doi:10.1021/es048058r.
J.E. Argüello, C. Costentin, S. Griveau, J.M. Savéant, Role of protonation and of axial ligands in the
reductive dechlorination of alkyl chlorides by vitamin B12 complexes. Reductive cleavage of
chloroacetonitrile by Co(I) cobalamins and cobinamides, J. Am. Chem. Soc. 127 (2005) 5049–5055.
doi:10.1021/ja042940f.
[10] S. Kliegman, K. McNeill, Dechlorination of chloroethylenes by cob(I)alamin and cobalamin model
complexes., Dalt. Trans. 9226 (2008) 4191–4201. doi:10.1039/b801679h.
[11] C.J. Gantzer, L.P. Wackett, Reductive dechlorination catalyzed by bacterial transition-metal
coenzymes, Environ. Sci. Technol. 25 (1991) 715–722. doi:10.1021/es00016a017.
[12] C. a. Schanke, L.P. Wackett, Environmental reductive elimination reactions of polychlorinated
ethanes mimicked by transition-metal coenzymes, Environ. Sci. Technol. 26 (1992) 830–833.
doi:10.1021/es00028a025.
[13] A.D. Follett, K. McNeill, Evidence for the Formation of a cis-Dichlorovinyl Anion upon Reduction of
cis-1,2-Dichlorovinyl(pyridine)cobaloxime, Inorg. Chem. 45 (2006) 2727–2732.
doi:10.1021/ic052020d.
[14] K.M. McCauley, S.R. Wilson, W.A. van der Donk, Synthesis and Characterization of Chlorinated
Alkenylcobaloximes To Probe the Mechanism of Vitamin B12-Catalyzed Dechlorination of Priority
10

Pollutants., Inorg. Chem. 41 (2002) 393–404. doi:10.1021/ic011023x.
[15] G.N. Schrauzer, R.J. Holland, Hydridocobaloximes, J. Am. Chem. Soc. 93 (1971) 1505–1506.
doi:10.1021/ja00735a040.
[16] G.. Schrauzer, Organocobalt Chemistry of Vitamin B12 Model Compouns (Cobaloximes), Acc.
Chem. Res. 1 (1968) 97–103. doi:10.1021/ar50004a001.
[17] X. Hu, B.S. Brunschwig, J.C. Peters, Electrocatalytic hydrogen evolution at low overpotentials by
cobalt macrocyclic glyoxime and tetraimine complexes, J. Am. Chem. Soc. 129 (2007) 8988–8998.
doi:10.1021/ja067876b.
[18] S. Pizarro, P. Astudillo, F. Gajardo, I. Brito, A. Delgadillo, Synthesis and Characterization of a New
Cobaloxime-Terpyridine Compound, Crystals. 7 (2017) 175. doi:10.3390/cryst7060175.
[19] S. Pizarro, G. Saá, F. Gajardo, A. Delgadillo, Synthesis and Characterization of
Chloridobis(dimethylglyoximato)4-(2-ferrocenylvinyl)pyridinecobalt(III): A New Heterobinuclear
Cobaloxime, J. Chem. 2016 (2016) 1–5. doi:10.1155/2016/1385236.
[20] P. Du, K. Knowles, R. Eisenberg, A Homogeneous System for the Photogeneration of Hydrogen from
Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst,
J. Am. Chem. Soc. 130 (2008) 12576–12577. doi:10.1021/ja804650g.
[21] W.Y. Fan, Z. Bin Tan, J.I. Koh, Proton reduction using cobalt glyoximes with isothiocyanate and
aniline axial ligands, Polyhedron. 96 (2015) 38–43. doi:10.1016/j.poly.2015.04.030.
[22] S. Concepción, M. Aguiló, X. Solans, M. Font-Altaba, Synthesis and Structure of Chloro ( ligand )
bis ( diphenylglyoximato ) cobalt ( Complexes, Inorganica Chim. Acta. 127 (1987) 153–159.
doi:10.1016/S0020-1693(00)82115-5.
[23] C. Voltammetry, Voltammetric techniques, in: Inorg. Electrochem., Royal Society of Chemistry,
Cambridge, n.d.: pp. 49–136. doi:10.1039/9781847551146-00049.
[24] M. Razavet, V. Artero, M. Fontecave, Proton electroreduction catalyzed by cobaloximes: functional
11

models for hydrogenases., Inorg. Chem. 44 (2005) 4786–95. doi:10.1021/ic050167z.
[25] W.T. Eckenhoff, W.R. McNamara, P. Du, R. Eisenberg, Cobalt complexes as artificial hydrogenases
for the reductive side of water splitting, Biochim. Biophys. Acta - Bioenerg. 1827 (2013) 958–973.
doi:10.1016/j.bbabio.2013.05.003.
[26] E. Ngameni, J. Ngoune, A. Nassi, M.M. Belombe, R. Roux, Electrochemical studies and electronic
spectroscopic examination of some cobaloximatic complexes based on 2,3-butanedione dioxime or
dimethylglyoxime with identical or mixed axial ligands, Electrochim. Acta. 41 (1996) 2571–2577.
doi:10.1016/0013-4686(96)00071-0.
[27] D.W. Wakerley, E. Reisner, Development and understanding of cobaloxime activity through
electrochemical molecular catalyst screening, Phys. Chem. Chem. Phys. 16 (2014) 5739.
doi:10.1039/c4cp00453a.
[28] B. Huang, A.A. Isse, C. Durante, C. Wei, A. Gennaro, Electrocatalytic properties of transition metals
toward reductive dechlorination of polychloroethanes, Electrochim. Acta. 70 (2012) 50–61.
doi:10.1016/j.electacta.2012.03.009.
[29] K.J. Lee, N. Elgrishi, B. Kandemir, J.L. Dempsey, Electrochemical and spectroscopic methods for
evaluating molecular electrocatalysts, Nat. Rev. Chem. 1 (2017) 39. doi:10.1038/s41570-017-0039.
[30] J.M. Saveant, Molecular Catalysis of Electrochemical Reactions, in: Elem. Mol. Biomol.
Electrochem., John Wiley & Sons, Inc., Hoboken, NJ, USA, 2006: pp. 251–297.
doi:10.1002/0471758078.ch4.
[31] D.R. Burris, C. a. Delcomyn, M.H. Smith, a. Lynn Roberts, Reductive dechlorination of
tetrachloroethylene and trichloroethylene catalyzed by vitamin B12 in homogeneous and
heterogeneous systems, Environ. Sci. Technol. 30 (1996) 3047–3052. doi:10.1021/es960116o.
[32] K. a P. Payne, C.P. Quezada, K. Fisher, M.S. Dunstan, F. a Collins, H. Sjuts, C. Levy, S. Hay, S.E.J.
Rigby, D. Leys, Reductive dehalogenase structure suggests a mechanism for B12-dependent
dehalogenation., Nature. 517 (2014) 513–516. doi:10.1038/nature13901.
12

[33] C. Baffert, V. Artero, M. Fontecave, Cobaloximes as functional models for hydrogenases. 2. Proton
electroreduction catalyzed by difluoroborylbis(dimethylglyoximato)cobalt(II) complexes in organic
media, Inorg. Chem. 46 (2007) 1817–1824. doi:10.1021/ic061625m.
[34] M. Bacchi, G. Berggren, J. Niklas, E. Veinberg, M.W. Mara, M.L. Shelby, O.G. Poluektov, L.X.
Chen, D.M. Tiede, C. Cavazza, M.J. Field, M. Fontecave, V. Artero, Cobaloxime-based artificial
hydrogenases., Inorg. Chem. 53 (2014) 8071–8082. doi:10.1021/ic501014c.
[35] A. Fihri, V. Artero, M. Razavet, C. Baffert, W. Leibl, M. Fontecave, Cobaloxime-based
photocatalytic devices for hydrogen production, Angew. Chemie - Int. Ed. 47 (2008) 564–567.
doi:10.1002/anie.200702953.
[36] A. Bhattacharjee, M. Chavarot-Kerlidou, E.S. Andreiadis, M. Fontecave, M.J. Field, V. Artero,
Combined Experimental–Theoretical Characterization of the Hydrido-Cobaloxime [HCo(dmgH) 2 (P
n Bu 3 )], Inorg. Chem. 51 (2012) 7087–7093. doi:10.1021/ic2024204.
[37] B.H. Solis, S. Hammes-Schiffer, Substituent effects on cobalt diglyoxime catalysts for hydrogen
evolution., J. Am. Chem. Soc. 133 (2011) 19036–9. doi:10.1021/ja208091e.
[38] M. Bühl, V. Golubnychiy, On the intermediacy of chlorinated alkylcobalt complexes in the reductive
dehalogenation of chloroalkenes. A first-principles molecular dynamics study, Organometallics. 26
(2007) 6213–6218. doi:10.1021/om7007474.
[39] M. Bühl, I.V. Vrček, H. Kabrede, Dehalogenation of chloroalkenes at cobalt centers. A model density
functional study, Organometallics. 26 (2007) 1494–1504. doi:10.1021/om070027s.
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Table 1: Reduction potentials, diffusion coefficients and apparent rate constants of the cobaloximes used in
this study.
EPC
(V) EPA
(V) E1/2
Co(II/I)
(V) D (cm2/s)
kapp (M-1s-1)
Co(III/II)
Co(III/II)
HCA
Co(III)(dmgH)2Cl(py)
Co(III)(mpgH)2Cl(py)
Co(III)(dpgH)2Cl(py)
-0.54
-0.45
-0.37
0.13
0.17
0.23
-0.95
-0.77
-0.63
5.36x10-6
4.81x10-6
3.23x10-6
2.54 0.08 x105
2.38 0.06 x105
2.79 0.15 x105
14

Figure captions
Fig. 1: Structure of the cobaloximes used in this study: (a) [Co(dmgH)2Cl(py)]; (b) [Co(mpgH)2Cl(py)]; (c)
[Co(dpgH)2Cl(py)].
Fig. 2: Scheme of the synthesis. i) NH2OH*HCl, HCl, MeOH. ii) Cobalt(II) chloride hexahydrate, pyridine,
MeOH.
Fig. 3: Overlapping of cyclic voltammograms of the cobaloximes: [Co(dmgH)2Cl(py)] black line,
[Co(mpgH)2Cl(py)] red line and [Co(dpgH)2Cl(py)] blue line. All were recorded at a concentration of 1.5 mM
in DMF 0.1 M [NBu4][PF6].
Fig. 4: Cyclic voltammogram of a 2.3 mM HCA recorded in DMF 0.1 M [NBu4][PF6].
Fig. 5: a) Cyclic voltammogram of Co(III)(dmgH)2Cl(py) 0.125 mM in 0.1M [NBU4][PF6]. Registered at a
scanning speed of 100 mV s-1 (black) in the presence of 2, to 16 equivalents of HCA (blue). b)
Dependence of the current on the square root of HCA concentration. The data was adjusted to
IC=m[HCA]1/2 + c, m = -2.03x10-3, c = 1.68x10-5, R2 = 0.9929.
Fig. 6: a) Cyclic voltammogram of Co(III)(mpgH)2Cl(py) 0.125 mM in 0.1M [NBU4][PF6]. Registered at a
scanning speed of 100 mV s-1 (black) in the presence of additions of 2 to 16 equivalents of HCA
(blue). b) Dependence of the current increase on the HCA concentration. The data was adjusted to
IC=m[HCA]1/2 + c, m = -1.73x10-3, c = 1.31x10-5, R2 = 0.9949.
Fig. 7: a) Cyclic voltammogram of Co(III)(dpgH)2Cl(py) 0.125 mM in 0.1M [NBU4][PF6]. Registered at a
scanning speed of 100 mV s-1 (black) in the presence of additions of 2 to 16 equivalents of HCA (blue). b)
Dependence of the current increase on the HCA concentration. The data was adjusted to IC=m[HCA]1/2 + c, m
= -1.57x10-3, c = 1.33x10-5, R2 = 0.9991.
Fig. 8: General reductive dehalogenation mechanism of hexachloroethane using electrochemically generated
Co(I) cobaloximes.
15

Fig. 1
16

Fig. 2
17

Fig. 3
18

Fig. 4
19

Fig. 5
20

Fig. 6
21

Fig. 7
22

Fig. 8
23

Graphical abstract-synopsis
Three cobaloximes were used as catalysts for the reductive degradation of hexacloroethane. Tetrachloroethane
was found as the degradation product. Although the Co complexes employed show substantial differences on
the Co (II/I) reduction potential, the observed rate constant do not show significant differences
24