Efficient electrocatalytic hydrogen production from H+ ions using specially designed boron-capped cobalt clathrochelates

Article abstract of DOI:10.1039/c1cc12239h

Specially designed hexachlorine-containing cobalt(ii) tris-dioximate clathrochelates were found to efficiently electrocatalyze the production of molecular hydrogen from H⁺ ions without the overpotential of this process.

Full text of DOI:10.1039/c1cc12239h

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Efficient electrocatalytic hydrogen production from H+ ions using
specially designed boron-capped cobalt clathrochelatesw
Yan Z. Voloshin,*a Alexander V. Dolganov,a Oleg A. Varzatskiib and Yurii N. Bubnova
Received 18th April 2011, Accepted 23rd May 2011
DOI: 10.1039/c1cc12239h
Specially designed hexachlorine-containing cobalt(II) tris-dioximate
clathrochelates were found to efficiently electrocatalyze the
production of molecular hydrogen from H+ ions without the
overpotential of this process.
design clathrochelate complexes that electrocatalyze the proton
reduction at the potential E close to 0 V vs. RHE in this
producing system.
Using the plot of E versus spara, we calculated the potentials
of the redox couples Co2+/+ for the clathrochelate hexachlorine-
containing cobalt(II) tris-dioximates (in particular, for the
complexes Co(Cl2Gm)2(BC6H5)2 and Co(Cl2Gm)2(Bn-C4H9)2
(Scheme 1)) to be close to that for the redox couple 2H+/H2
for HClO4 in acetonitrile solution. These clathrochelates were
prepared as described earlier [9f] and additionally purified
by column chromatography and characterized using 1H
and 13C NMR, UV-vis and IR spectra. The complexes
Co(Cl2Gm)2(BC6H5)2 and Co(Cl2Gm)2(Bn-C4H9)2 are chemically
stable in neutral and acidic media and do not hydrolyse over a
long period of time. In general, most of the clathrochelates
demonstrate unusually high thermodynamic and kinetic
stability and may easily be functionalized using the apical
and ribbed substituents.10
The molecular hydrogen derived from non-carbon sources has
emerged as a potential fuel for sustainable energy cycles that
minimize carbon dioxide emissions.1 The search for the
catalysts that would allow facilitating the proton reduction
to molecular hydrogen using cheap and earth-abundant metals
is now an area of great interest.2 Despite the recent advances in
this field, the design of catalysts with high activity and stability
in aqueous media at minimal overpotentials is a real challenge.3
Most of the previously described hydrogen-producing small
molecular systems are biomimetic metal–sulfur clusters that
mimic the hydrogenase enzymes,4 cobalt oximes, imines and
clathrochelates5 as well as cobalt and nickel metallocenes
and phosphines.6 Recently, the electrocatalytic activity of
boron-capped clathrochelate cobalt aliphatic and aromatic
tris-dioximates in hydrogen-producing systems has been
reported.5,7 The mechanistic and quantum chemical studies
suggested that the proton reduction in these systems involves
the electron transfer (ET) from the electrochemically generated
reactive cobalt(I) species. The overpotential that is caused by
the unfavorable thermodynamics of this ET is responsible for
the low overall rate of the catalytic process.8 One of the
possible ways to decrease the overpotential of the proton
reduction and, hence, to make this process more thermo-
dynamically favorable is a variation of the substituents at
the ribbed a-dioximate fragments of the cage ligands. This
allows tuning of the potentials of the metal-centered redox-
processes;9 these potentials can be deduced using the values of
their electromeric (first of all, the Hammett spara) constants.9
As a result, the potential of the electrocatalytic hydrogen
production with the ribbed-functionalized clathrochelate used
as a catalyst may be controlled. Therefore, it is possible to
The CV data for these complexes are summarized in
Table 1. All the redox waves in their CVs (Fig. 1) are
characteristic of the diffusion-controlled current processes
(as it follows from the linear plot of ip versus v1/2, where v is
a scan rate). These CVs have similar shapes and contain both
the anodic and cathodic one-electron waves. The DE values for
the anodic oxidation processes assigned to the metal-centered
redox couple Co2+/3+ are considerably higher than the
theoretical value for the reversible one-electron redox process
(56 mV).11 The reversible cathodic reduction waves with the
current ratio ic/ia for the direct and backward redox processes
equal to 1 were observed for both these cobalt complexes
(Table 1). This indicates that the anionic clathrochelate
species that resulted from the metal-centered Co2+/+ reductions
a Nesmeyanov Institute of Organoelement Compounds of the Russian
Academy of Sciences, 119991 Moscow, Russia.
E-mail: voloshin@ineos.ac.ru; Fax: +7 (449)135-5085
b Vernadskii Institute of General and Inorganic Chemistry of the
National Academy of Sciences of Ukraine, 03680 Kiev, Ukraine
w Electronic supplementary information (ESI) available: The materials
used as well as the details of the electrochemical data collections and
gas chromatography experiments. See DOI: 10.1039/c1cc12239h
Scheme 1
c
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Table 1 The redox parameters (mV) and characteristics of the redox
processes for the cobalt(II) clathrochelates Co(Cl2Gm)2(BC6H5)2 and
Co(Cl2Gm)2(Bn-C4H9)2
negative potentials when an ET is fast. For the reactions of the
first order with respect to a catalyst’s concentration and of the
second order with respect to an acid’s concentration, the
plateau current (ic) may be described by the following
Parameter
Co(Cl2Gm)3(BC6H5)2
Co(Cl2Gm)3(Bn-C4H9)2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
equation:13 ic ¼ nFA½catꢁ Dk½Hþꢁ , and the linear correlation
Oxidation
Epa
Epc
1170
1060
110
1130
1120
100
between a plateau current and the concentration of an acid
should be observed as it is in the case of both the clathrochelate
complexes (see SI, Fig. SI1). This suggests that the electro-
chemical reaction studied is of the second order with respect to
HClO4 concentration.
DE
ic/ia
Reduction
Epa
Epc
0.85
0.90
ꢀ260
ꢀ200
60
ꢀ270
ꢀ210
60
According to the following equation, a plot of ic/ip versus
HClO4 concentration (Fig. 2) should be linear for different
DE
ic/ia
1
1
scan rates (where ip is the peak current in the absence of an
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
RTk½Hþꢁ
acid):14
¼
. The rate constants for hydrogen
ic
ip
n
0:4463
Fn
production from the H+ ions, which were calculated from the
slope values (Fig. 3), for the complexes Co(Cl2Gm)2(BC6H5)2
and Co(Cl2Gm)2(Bn-C4H9)2 are equal to 2.00 ꢂ 103 and
1.46 ꢂ 103 molꢀ2 l2 sꢀ1, respectively. It should be noted that
these electrocatalytic reactions occur without an overpotential.
As the potentials for the hydrogen production in these systems
are close to that of the couples Co2+/+ for the clathrochelates
Co(Cl2Gm)2(BC6H5)2 and Co(Cl2Gm)2(Bn-C4H9)2, we suggested
that the reduced cobalt(I)-containing clathrochelate species are
involved in the catalytic cycle; hence, we performed the UV-vis
spectroelectrochemical study of this process. The gradual
electrochemical reduction of these macrobicycles on a platinum
grid electrode caused two intensive broad bands to appear at
approximately 550 and 680 nm (see SI, Fig. SI2); the latter
values are similar to those for the preparatively isolated
cobalt(I) complexes ((n-C4H9)4N)[Co(Cl2Gm)3(BC6H5)2] and
(((CH3)2N)4P)[Co(Cl2Gm)3(Bn-C4H9)2].9
Fig. 1 CVs for 1.0 mM acetonitrile solutions of the complexes
Co(Cl2Gm)3(BC6H5)2 (a) and Co(Cl2Gm)2(Bn-C4H9)2 (b) in 0.1 M
((n-C4H9)4N)BF4 in the absence (1) and in the presence of HClO4:
5 (2), 10 (3), 15 (4), and 25 mM (5). Scan rate is 100 mV sꢀ1
,
GC electrode relative to Ag/AgCl/KClaq
.
are stable in the CV time scale. To the best of our
knowledge, the reduction potentials Co2+/+ for the complexes
Co(Cl2Gm)2(BC6H5)2 and Co(Cl2Gm)2(Bn-C4H9)2 (Eap are
equal to ꢀ260 and ꢀ270 mV, respectively) are the lowest of
those known for the cobalt clathrochelates,5,9,11 and these
values are very close to the standard one for the proton
reduction to produce molecular hydrogen from HClO4 in
We also studied the electrocatalytic reduction of the H+
ions in the presence of the clathrochelate Co(Cl2Gm)3(Bn-C4H9)2
in mixed water–acetonitrile 1 : 1 solution: in this mixture, the
complex is soluble enough to obtain the reproducible electro-
chemical results. The CV for this mixed solution contains a
reversible Co2+/+ reduction wave in the absence of an acid
(Fig. 4). The addition of HClO4 resulted in the electrocatalytic
current at the potential close to that for this redox couple in
water–acetonitrile mixture. This fact also suggests that in the
mixed solution as well as in pure acetonitrile the macrobicyclic
complex with an encapsulated cobalt(I) ion is the catalytically
active form in the hydrogen-producing reaction.
acetonitrile solution (EH0 ClO ¼ ꢀ260 mV versus Ag/AgCl/KClaq
4
in acetonitrile as a solvent).12 Thus, these complexes seem to
be very promising catalysts of hydrogen production in this
system. Indeed, the addition of HClO4 to the acetonitrile
solutions of the clathrochelates Co(Cl2Gm)3(Bn-C4H9)2 and
Co(Cl2Gm)3(BC6H5)2 resulted in the formation of the same
catalytic cathodic reduction Co2+/+ waves (Fig. 1), confirming
that the clathrochelate cobalt(I)-containing species are the
catalytically active forms in these systems as well as in the
case of the aliphatic and aromatic cobalt clathrochelates.5,7 An
increase in HClO4 concentration causes a slight negative shift
of the catalytic wave, together with an increase in its intensity;
the latter then reaching a plateau. The controlled-potential
electrolysis of the complexes Co(Cl2Gm)3(Bn-C4H9)2 and
Co(Cl2Gm)3(BC6H5)2 confirmed the production of the molecular
hydrogen in the Faraday yields of approximately 99%. This
suggests that the observed enhancements of a current are
caused by the electrocatalytic proton reduction; the molecular
hydrogen formation was confirmed by gas chromatography
analysis of the gaseous reaction products (see Supporting
information, SI).
Fig. 2 Plot of ic/ip versus HClO4 concentration for 1.0 mM aceto-
nitrile solutions of the clathrochelates Co(Cl2Gm)3(Bn-C4H9)2 (a) and
Co(Cl2Gm)3(BC6H5)2 (b) in 0.1 M ((n-C4H9)4N)BF4 at the scan rates
400 (’), 200 (m), 100 (K) and 25 (.) mV sꢀ1 (GC electrode).
To determine the reaction orders of this electrocatalytic
process, we performed a series of the CV experiments under
electrocatalytic conditions with a large excess of an acid at
c
7738 Chem. Commun., 2011, 47, 7737–7739
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Notes and references
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electrocatalytic reduction of H+ to molecular hydrogen with
minimal overpotentials. Such complexes (or their analogs)
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mercapto-groups for
a
gold electrode).9 The potentials
at which the clathrochelates Co(Cl2Gm)3(BC6H5)2 and
Co(Cl2Gm)3(Bn-C4H9)2 catalyze the production of hydrogen
from the H+ ions are significantly more positive than those of
other dioximate macrocyclic and macrobicyclic systems that
are known to produce gaseous H2.5,15 The synthetic versatility
of this family of metal complexes provides new prospects for
fine tuning their electrochemical properties and reactivity
together with an elaboration of the modified electrodes and
photoelectrodes: their apical substituents with hydrophilic
groups (in particular, protono- and ionogenic ones) and those
with terminal reactive groups may be used to obtain the water-
soluble clathrochelate electrocatalysts able to be immobilized
on an electrode surface. Moreover, based on the preliminary
calculations of the electrochemical characteristics of the metal
complexes, this approach may be used for the design of efficient
electrocatalysts for a wide range of hydrogen-producing systems.
The authors gratefully acknowledge the support of RFBR
(grants 09-03-00540, 09-03-90454, 09-03-12231, and 10-03-00613)
and RAS (programs 6, 7 and P7).
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7737–7739 7739