Cobalt-based particles formed upon electrocatalytic hydrogen production by a cobalt pyridine oxime complex

Article abstract of DOI:10.1002/cssc.201300564

An open-coordination-sphere cobalt(III) oximato-based complex was designed as a putative catalyst for the hydrogen evolution reaction (HER). Electrochemical alteration in the presence of acid occurs, leading to the formation of cobalt-based particles that

Full text of DOI:10.1002/cssc.201300564

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DOI: 10.1002/cssc.201300564
Cobalt-Based Particles Formed upon Electrocatalytic
Hydrogen Production by a Cobalt Pyridine Oxime Complex
[a]
[a]
[a]
[a, b]
Sanae El Ghachtouli,* Regis Guillot, Francois Brisset, and Ally Aukauloo*
Abstract: An open-coordination-sphere cobalt(III) oximato-
based complex was designed as a putative catalyst for the hy-
drogen evolution reaction (HER). Electrochemical alteration in
the presence of acid occurs, leading to the formation of
cobalt-based particles that act as an efficient catalyst for HER
at pH 7. The exact chemical nature of these particles is yet to
be determined. This study thus raises interesting issues regard-
ing the fate of molecular-based complexes designed for the
HER, and points to the challenging task of identifying the real
catalytic species. Moreover, understanding and rationalizing
the alteration pathways can be seen as a new route to reach
catalytic particulates.
reasoned that a ligand set bearing both an oxime and a pyri-
dine functionality may offer attractive properties in the electro-
catalytic reduction of protons to H2.
We report herein on the synthesis of bis pyridine methylox-
ime (hereby abbreviated as PyMOH) cobalt complexes. Their
electrochemical properties and their activity towards the re-
duction of protons are discussed. We find that upon electroca-
talysis, there is an alteration of the initial cobalt complex to
form cobalt-based particles that are at the origin of the elec-
trocatalytic hydrogen production.
The ligand was synthesized by reacting 2-acetylpyridine with
NH OH.HCl in ethanol, and isolated as a white powder in good
2
yield. Complex 1 was obtained by reacting 2 equiv of the
ligand PyMOH with CoBr in ethanol. Upon treatment of com-
2
Research towards molecular catalysts for hydrogen production
is a contemporary scientific issue, with the development of
plex 1 with an excess of BF .Et O in dry ether under argon, the
3
2
BF -capped complex 2 was isolated as a green solid. We suc-
2
[
1]
III
cost-effective catalytic materials as its defining target. This re-
search falls in two main categories: biomimetic models and co-
ordination metal complexes for electrocatalysis. The first ap-
proach seeks primarily to understand the intricacies of the rela-
tionship between structure and function of the active sites of
the hydrogenases, the family of enzymes involved in the me-
cessfully obtained crystals of both [Co (PyMO)(PyMOH)Br ]
2
III
1 and [Co (PyMO) (BF )Br ] 2 of sufficient quality to perform X-
2
2
2
ray analysis. (see Experimental Section).
The crystal structure of 1 depicts an octahedral environment
around the cobalt(III) ion with two trans-positioned bromide li-
gands (Figure 1a). The deprotonated form of one oxime func-
tion satisfies the electroneutrality of complex 1. We note that
the proton bridges the two oxygen atoms O1 and O2. The co-
ordination scheme of the cobalt(III) ion for complex 2 is similar
to that of complex 1, with four N atoms in the equatorial belt
and two bromide ligands at trans positions. The structure of 2
[
2]
tabolism of hydrogen in bacteria and algae. To their detri-
ment, functional systems in this area are still scarce. However,
the development of coordination metal complexes based on
first-row transition metals for the hydrogen evolution reaction
(HER) has been more successful, and several molecular-based
catalysts functioning at modest overpotential have been re-
reveals that the BF bridges the oxime groups, and a contrac-
2
[
3]
ported. Among those, cobalt-based glyoxime complexes
figure as one of the most prominent molecular systems for H2
generation. Artero, Peters, and others have re-examined the
chemistry of cobalt glyoximato complexes for their electrocata-
tion of the metric distances in the N4 equatorial plane (Fig-
ure 1b). This shrinking leads at the same time to a pronounced
tilt of the pyridine groups, out of the median plane.
The UV/Vis spectra of 1 and 2 (see Supporting Information,
[
4]
ꢀ1
ꢀ1
lytic properties in the HER. Long and co-workers have report-
ed on robust cobalt polypyridine complexes showing remark-
Figure S1) show an intense band at 280 nm (16075m cm )
ꢀ1
ꢀ1
and 310 nm (16070m cm ) respectively, that we assign to
ligand p!p* transitions. A band of weaker intensity observed
[
5]
able catalytic properties for H production, while Gray and
2
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
collaborators have shown in a recent paper that the cobalt (bi-
siminopyridine) complex in aqueous solution is a highly active
at 342 nm (3205m cm ) and 400 nm (2355m cm ), respec-
tively, probably corresponds to ligand to metal charge-transfer
[
6]
[6]
catalyst for the reduction of water. Based on these results, we
(LMCT) transitions.
III
The electrochemical behavior of [Co (PyMO)(PyMOH)Br ]
2
III
[
a] Dr. S. E. Ghachtouli, Dr. R. Guillot, Dr. F. Brisset, Prof. A. Aukauloo
Institut de Chimie Molꢀculaire et des Matꢀriaux d’Orsay
UMR-CNRS 8182, Universitꢀ de Paris-Sud XI
1
and [Co (PyMO) (BF )(Br) ] 2 complexes were studied in ace-
2
2
2
tonitrile (0.1m NaClO ). Cyclic voltrammetry (CV) measurements
4
of 1 display one quasi-reversible wave at 0.45 V vs. SCE, attrib-
9
1405 Orsay (France)
III/II
Fax: (+33)0-169-154-756
E-mail: ally.aukauloo@u-psud.fr
uted to the Co
couple, and an irreversible wave on the
II/I
cathodic side at around ꢀ1 V vs. SCE, for the putative Co
couple (Figure 2a). In the case of 2, we observe a shift of the
Co couple towards less-positive potential by ca. 200 mV (Fig-
[b] Prof. A. Aukauloo
CEA, iBiTec-S
III/II
Service de Bioꢀnergꢀtique Biologie Structurale et Mꢀcanismes (SB2SM)
ure 2b). This is in agreement with findings of a stronger inter-
action between the metal ion and the ligands, leading to a sta-
bilization of the metal d orbitals. Intriguingly, the second re-
9
1191 Gif-sur-Yvette (France)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300564.
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duction process is also displaced to a less-negative potential
(
from ꢀ1 V to ꢀ0.74 V vs. SCE), together with a gain in the
electrochemical reversiblity. We attribute this shift towards less
cathodic values to the electron-withdrawing nature of the di-
fluoroboryl capping group, which confines uptake of the
second electron mainly to the ligand skeleton. For comparison,
II/I
under similar experimental conditions the Co couple of [Co-
(
DMGBF ) ] (DMG: dimethyl glyoxime) was detected at ꢀ0.55 V
2
7]
2
[
vs. SCE, supporting the assertion that the pyridine groups are
less electron-withdrawing than the boryl-oximato functions. In-
II/I
terestingly, the potential of the Co couple falls within the
same potential window (ꢀ0.71 V vs. SCE) as in a case where
the fluoroboryl-oximato groups were replaced by two iminic
[8]
functions.
The CV scans of both 1 and 2 in acetonitrile change strongly
upon addition of increasing amounts of trifluroacetic acid (TFA,
[9]
pK 12.7 in acetonitrile). The absence of a genuine S-shaped
a
electrocatalytic wave, as expected for molecular-based cataly-
sis, and the different reduction processes taking place upon
addition of protons prompted us to investigate the effect of
proton addition on the cobalt complexes in more detail. For
stability reasons (see Figures S2 and S3) we could only study
the electrochemical behavior of compound 2. The addition of
TFA leads to several marked changes in the shape of the CV
curve. Importantly, the electrochemical behavior does not
change in the presence of the weaker acids triethylammonium
or acetic acid. With TFA, we noticed the increased intensity of
III
Figure 1. Crystal structure of a) [Co (PyMO)(PyMOH)Br
2
] 1; Co-N1
(
1.9891(19) ꢂ) Co-N2 (1.978(2 ꢂ), Co-N3 (1.9092(19 ꢂ) Co-N4 (1.916(2 ꢂ); and
III
b) [Co (PyMO)
1.900(2 ꢂ) Co-N4 (1.902(2 ꢂ). Hydrogen atoms and solvent molecules are
omitted for clarity.
2 2 2
(BF )Br ] 2; Co-N1 (1.953(2 ꢂ) Co-N2 (1.957(2 ꢂ), Co-N3
(
II/I
the reduction wave in the region of the Co couple, shifted to
more positive values by ca. 60 mV (Figure 3, Wa). This wave
reaches a maximum upon the addition of two protons and
two electrons. A second wave at around ꢀ0.9 V vs. SCE (Wb) is
ꢀ
+
also detected, which peaks at the addition of 4e and 4H
(
Figure S4). These electrochemical–chemical processes can be
related to, firstly, the hydrogenation of two imine bonds of oxi-
mepyridine ligand and, secondly, the hydrogenolysis of two
NꢀO bonds, according to previous studies on the electrochem-
ical behavior of cobalt complexes containing oxime ligands in
[10]
the presence of acid. Furthermore, such behavior has recent-
ly been reported by Savꢁant et al. in the electrocatalytic con-
4
Figure 3. Cyclic voltammetry in acetonitrile (0.1m of NaClO ) of 0.5 mm of 2
Figure 2. Cyclic voltammetry in acetonitrile (0.1m of NaClO
and b) 2. Glassy carbon: working electrode, V=100 mVs , T=293 K.
4
) of 1 mm of a) 1,
in the presence of increasing trifluoroacetic acid [TFA], 0–11 mm. Glassy
carbon: working electrode, V=100 mVs , T=293 K.
ꢀ1
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version of three glyoximato ligands around a caged cobalt(II)
[
11]
ion in the presence of acid. Our results evidence that the
same electrochemical processes occur for a cobalt complex
with an open coordination sphere bearing oximato groups
and pyridine groups without any direct activation of the
[
12]
proton at the reduced metallic center.
A more detailed
mechanistic study is beyond the scope of this report and will
be addressed in the future.
Upon scanning to more-cathodic potentials, we noticed
a more intense wave (Wc). The formation of this wave is fol-
lowed by the appearance of a desorption wave at around
0.07 V vs. SCE. We therefore assign these waves to the catalytic
activity of a chemical deposit on the surface of the graphite
electrode. Polarizing the electrode at ꢀ1.1 V vs. SCE for com-
plex 2 for about 60 s leads to the formation of a new wave at
ca. ꢀ1 V vs. SCE (Figure 4, blue CV trace). We propose that this
Figure 5. SEM image of cubes deposited after 18 h of electrolysis at ꢀ1.1 V
2
vs. SCE onto glassy carbon (1 cm ) of 2 mm of 2 in an acetonitrile solution
(
4
0.1m NaClO ) in the presence of 44 mm of TFA.
average size of 1 to 2 mm. Energy dispersive X-ray spectrosco-
py (EDS) analysis of a sample of the cubic particles revealed
that they are constituted mainly of cobalt and oxygen. (see
Figure S7). However, the exact nature of these particles is yet
to be determind more precisely.
A chemically modified glassy carbon (GC) electrode with the
cobalt-based particles using complex 2 as starting material
was prepared, rinsed under inert conditions and used as
a working electrode for catalytic hydrogen production in an
aqueous solution containing 0.1m phosphate buffered at pH 7.
We found that the overpotential for proton reduction is
ꢀ
2
4
00 mV for a geometric current density of 2 mAcm at pH 7,
Figure 4. Cyclic voltammograms of 2 (0.5 mm) in acetonitrile (0.1m of
NaClO
4
): in the presence of 11 mm of TFA (red line), after electrolysis for 60 s
and the shift in the overpotential when compared to the blank
electrode is around 750 mV towards more positive value at the
same pH for a similar current density (Figure 6). As a compari-
son, the overpotential value we found is lower than those re-
at ꢀ0.8 V vs. SCE (green line), after electrolysis for 60 s at ꢀ0.98 V vs. SCE
(
black line) and after electrolysis for 60 s at ꢀ1.1 V vs. SCE (blue line);
ꢀ
1
2
V=100 mVs at a glassy carbon electrode (0.07 cm ).
process leads to the electrosynthesis of a new species, catalytic
towards the HER, at the surface of the electrode. Of note, the
initial CV trace is regained after polishing the electrode surface.
More importantly, the shape of the CV trace did not change
upon polarization of the electrode after Wa and Wb (Figure 4,
CV traces in green and black), and the shape of the CV trace
changes only upon polarization of the electrode at a potential
more negative than Wb. Furthermore, control experiments per-
formed under the same conditions but with only acid or with
a solution of the cobalt salt Co(NO ) containing the same acid
3
2
did not reveal any electrocatalytic activity in the same poten-
tial window (see Figure S5), supporting the assertion that the
electrochemical route to the formation of the active species
emanating from the molecular complex 2 is different from the
free cobalt(II) ions source.
Of relevance to this finding, we performed electrolysis of 2
2
Figure 6. Linear sweep voltammetry in 0.1m phosphate buffer at pH 7. in
the absence of catalyst (black line); of working electrode washed with water
and placed into a pure phosphate electrolyte after 60 s of electrolysis at
at ꢀ1.1 V vs. SCE at a glassy carbon electrode (1 cm ) in pres-
ence of acid. A scanning electron microscopy image of the
electrode after electrolysis is shown in Figure 5. The modified
electrode with 2, is decorated with cubic-like particles with an
ꢀ
1.1 V vs. SCE in acetonitrile in the presence of 2 (0.5 mm) and 11 mm of
TFA (red line). Glassy carbon: working electrode, V=100 mVs , T=293 K.
ꢀ
1
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ported for the majority of cobalt molecular complexes and
The modified electrode exhibits good catalytic activity towards
the hydrogen evolution reaction (HER) at pH 7. This study thus
raises interesting issues regarding the fate of molecular-based
complexes designed for the HER, and points to the challenging
task of identifying the real catalytic species given the primary
importance of the outcome of this research area in the devel-
opment of new catalytic materials. Moreover, understanding
and rationalizing the alteration pathways can be seen as a new
route to reach catalytic particulates.
slightly higher than that of the film of H -CoCat deposited on
2
[
13]
FTO electrode reported recently. However, any further com-
parison would be risky given the fact that we do not know the
actual amount of catalytically active cobalt sites at the surface
of the electrode, and because the morphologies of the depos-
ited materials are different.
Bulk electrolysis of particles deposited in acetonitrile at the
2
surface of the glassy carbon electrode (1 cm ) was performed
in an aqueous solution containing 0.1m phosphate buffered at
pH 7 at a controlled potential (ꢀ1.1 V vs. SCE). A quasilinear
dependence of the charge with time was observed during ca.
Experimental Section
4
h with 80% Faradaic yield of H production (see Figure S6).
Pyridine methyloxime (PyMOH): To a solution of 2-acetyl pyridine
2
(2 g, 14.7 mmol) in ethanol (15 mL) was added one equivalent of
As mentioned earlier, the challenge behind developing mo-
hydroxylamine hydrochloride in ethanol (15 mL). The solution was
stirred under reflux for 24 h, the mixture was concentrated, and
cold water (120 mL) was added to the reaction mixture at room
temperature. The white precipitate was filtered, washed with cold
lecular-based catalyst is to provide a more cost-efficient cata-
lytic material for hydrogen production than platinum. In
proton exchange membrane technology, hydrogen is pro-
duced in acidic medium with platinized electrodes. With this in
1
water, and dried by suction (1.36 mg, 60% yield). H NMR
mind we prepared
a
modified glassy carbon electrode
(
300 MHz, CDCl ): d=8.668 (d, 1H), 7.868 (d, 1H), 7.706 (t, 1H),
3
2
(0.07 cm ) by depositing a drop of a solution of 2 dissolved in
7.296 (t, 1H), 2.429 ppm (s, 3H). Elemental analysis for: C H N O
7 8 2
acetonitrile (2 mm) and leaving the sample to dry in air. The
disc was then smeared with a perfluorosulfonate ion-exchange
polymer (Nafion117) to prevent leaching of deposit at the sur-
face of the electrode. Cyclic voltammograms obtained after
continual cycling between 0 and ꢀ1 V vs. SCE in a 0.5m aque-
calcd (%): C 61.75, H 5.92, N 20.58; found (%): C 61.69, H 6.03, N
2
0.58.
III
[
Co (PyMO)(PyMOH)Br ] 1: CoBr ·2H O (200 mg, 0.78 mmol) dis-
2
2
2
solved in ethanol/methanol (1:1, 4 mL) was added to 2 equiv of
the ligand PyMOH dissolved in ethanol (4 mL). The brown mixture
was stirred at room temperature overnight and concentrated by
evaporation. A brown precipitate was filtered, washed with ethanol
and diethyl ether, and the powder was dried by suction (306 mg,
ꢀ
1
ous solution of H SO under argon at a scan rate 5 mVs are
2
4
given in Figure 7. The current density steadily increases to
8
0% yield). The powder was further dissolved in acetonitrile, and
after slow diffusion of diethyl ether, brown monocrystals of 1 suita-
ble for X-ray analysis were obtained. Elemental analysis for:
CoC H Br N O calcd (%): C 34.31, H 3.09, N 11.43; found (%): C
14
15
2
4
2
3
4.62, H 3.39, N 11.39.
III
[
Co (PyMO) (BF )Br ] 2: To a solution of complex 1 (200 mg,
2
2
2
0
.41 mmol) in diethyl ether (10 mL) was added 1.2 equiv of
CH COONa, and the mixture was stirred under argon atmosphere
3
at room temperature for 30 min. BF ·Et O (1 mL) was added to the
3
2
mixture, and the brown–green suspension was stirred for 24 h at
room temperature under argon. The green precipitate was collect-
ed by filtration, washed with water and diethyl ether, and the
powder was dried by suction (154 mg, 70% yield). Green mono-
crystals of 2 suitable for X-ray analysis were obtained via diethyl
ether vapor into a acetonitrile solution of 2. Elemental analysis for
2
: CoC H Br BF N O calcd (%): C 31.26, H 2.62, N 10.42; found
14 14 2 2 4 2
(
%): C 31.72, H 2.53, N 10.54.
Figure 7. Linear sweep voltammetry obtained in aqueous solution of H
0.5m) of a glassy carbon modified electrode with Nafion (blue line); and of
a glassy carbon modified electrodes with 2 (2 mm) and Nafion. GC
2 4
SO
(
Acknowledgements
2
ꢀ1
(0.07 cm ), V=5 mVs .
This work was supported by CNRS project Commerce H , ANR
2
ꢀ
2
TechbBioPhyp and the LABEX CHARMMMAT.
reach a value of ca. 21 mAcm at around ꢀ1 V vs. SCE. This
gradual increase in the current density of the catalytic wave
for the reduction of protons can be directly inferred to the
chemical changes observed for complex 2 in solution at the
surface of the electrode.
Keywords: cobalt · electrocatalysis · homogeneous catalysis ·
hydrogen · oximes
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ences therein.
Published online on && &&, 0000
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S. E. Ghachtouli,* R. Guillot, F. Brisset,
A. Aukauloo*
Unexpected Co-operation: An open-co-
ordination-sphere cobalt(III) pyridine
oxime complex is designed as catalyst
for the hydrogen evolution reaction
(HER). However, electrochemical altera-
tion in presence of acid leads to the for-
mation of cobalt-based particles that
act as efficient HER electrocatalyst at pH
&& – &&
Cobalt-Based Particles Formed upon
Electrocatalytic Hydrogen Production
by a Cobalt Pyridine Oxime Complex
7, pointing to the challenging task of
identifying the real catalytic species in
the development of new materials.
ꢀ
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ChemSusChem 0000, 00, 1 – 5
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