Electrochemical CO2 reduction by a cobalt bipyricorrole complex: Decrease of an overpotential value derived from monoanionic ligand character of the porphyrinoid species

Article abstract of DOI:10.1039/c8cc08876d

A newly synthesized Co(ii) complex with a monoanionic bipyricorrole ligand is found to catalytically promote a selective CO₂ electroreduction to CO with Faradaic efficiency of 75%. Catalytic Tafel plots show that the overpotential of Co(ii) bipyricorrole is 0.35 V lower than that of a Co(ii) complex with the dianionic tetraphenylporphyrin ligand.

Full text of DOI:10.1039/c8cc08876d

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Electrochemical CO2 reduction by a cobalt
bipyricorrole complex: decrease of an
overpotential value derived from monoanionic
ligand character of the porphyrinoid species†
Cite this: Chem. Commun., 2019,
55, 493
Received 7th November 2018,
Accepted 7th December 2018
ab
a
Ayumu Ogawa,a Koji Oohora,
*
Wenting Gua and Takashi Hayashi
*
DOI: 10.1039/c8cc08876d
rsc.li/chemcomm
A newly synthesized Co(II) complex with a monoanionic bipyricorrole significantly negative electrochemical potential to generate an
ligand is found to catalytically promote a selective CO2 electroreduction active low-valent intermediate is required. In this context, it is
to CO with Faradaic efficiency of 75%. Catalytic Tafel plots show important to promote the generation of the low-valent species
that the overpotential of Co(II) bipyricorrole is 0.35 V lower than that by tuning the ligand structure. Corrin (Chart 1b), a mono-
of a Co(II) complex with the dianionic tetraphenylporphyrin ligand.
anionic porphyrinoid ligand, is the natural cofactor of cobalamin.
This ligand stabilizes the low-valent species to induce formation of
Provision of effective conversion of carbon dioxide (CO2) into the Co(I) species, which serves as a key intermediate for a broad
various compounds including fuels and chemical materials by range of reactions in biological systems.10 The tetradehydrocorrin
electrocatalysis has been recognized as one of the most important derivatives shown in Chart 1c have been synthesized as simple
objectives for realization of an environmentally sustainable society.1 analogs of cobalamin and physicochemical properties of their
Over the last three decades, a wide variety of homogenous metal cobalt complexes have been previously investigated.11 Our recent
complexes based on first-row transition metals such as Mn,2 study, however, has demonstrated that a stabilized Co(I) species
Fe,3 Co4 and Ni5 have been reported as useful and practical CO2 with a tetradehydrocorrin framework as a ligand promotes selective
reduction electrocatalysts because they are relatively inexpensive H2 evolution rather than CO2 reduction.12 Thus, we hypothesize that
and abundant. Porphyrin (Chart 1a) is a promising catalyst ligand suitable stabilization of the Co(I) species will be required to promote
because it has intrinsically high durability and excellent photo- a selective CO2 reduction reaction with a low overpotential value.
chemical and electrochemical properties. Iron porphyrin complexes Here, we employ a bipyricorrole framework (Chart 1d),13 because
have been extensively studied and are highly effective in converting the stronger Lewis basicity of the nitrogen atoms in a bipyridine
CO2 into carbon monoxide (CO).6 In contrast, investigations of moiety of bipyricorrole relative to the corresponding imine-like
cobalt porphyrin complexes have been quite limited despite their nitrogen atoms in the bipyrrolic moiety of tetradehydrocorrin
potential as CO2 reduction catalysts.7 Although cobalt complexes promises a more reactive Co(I) species in this metal complex.
formed by other porphyrinoids such as phthalocyanine8 and Therefore, we have designed and investigated a Co(II) bipyricorrole
corrole9 have been investigated as CO2 reduction catalysts, a (Co(II)BIPC) functionalized with a 2,6-dimethoxyphenyl group at a
meso-position of the bipyricorrole framework (Scheme 1).14
Chart 1 Deprotonated structures of (a) porphyrin, (b) corrin, (c) tetra-
dehydrocorrin and (d) bipyricorrole frameworks.
a Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, 565-0871, Japan.
E-mail: oohora@chem.eng.osaka-u.ac.jp, thayashi@chem.eng.osaka-u.ac.jp
b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho,
Scheme 1 Synthesis of Co(II)BIPC. (a) 2,6-Dimethoxybenzaldehyde, HCl,
H2O, ethanol, 51%; (b) NaOH, ethanol, 81%; (c) nBuLi, CuCl2, diethyl ether, 24%;
Kawaguchi 332-0012, Japan
(d) nBuLi, DMF, THF, 41%; (e) trifluoroacetic acid, THF (f) Co(OAc)2ꢀ4H2O,
† Electronic supplementary information (ESI) available. CCDC 1876629. For ESI and
CH3OH, CH2Cl2, then NaClO4ꢀH2O, H2O, CH3OH, 25% in 2 steps.
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc08876d
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Co(II)BIPC was successfully synthesized according to Scheme 1
(see Scheme S1, ESI,† for details). The target complex was achieved
via acid-catalyzed intermolecular coupling of corresponding 5,50-
dicarboxydipyrromethane and 6,60-diformyl-2,20-bipyridine followed
by cobalt insertion in a 25% yield. The zinc analog (Zn(II)BIPC) was
also synthesized as a reference compound according to Scheme S1,
ESI.† Zn(II)BIPC was fully characterized by NMR spectroscopy
(Fig. S9 and S10, ESI†), in contrast to Co(II)BIPC which shows
paramagnetic Co(II) behavior (vide infra).
The structure of Co(II)BIPC was determined by X-ray crystallo-
graphic analysis. In the crystal, the macrocycle of bipyricorrole is
nearly planar and the coordination site of the cobalt ion is open
to allow entry of external substrates as shown in Fig. 1. An aryl
moiety linked to the C11 atom is close to being perpendicular to
the macrocycle, indicating that the methoxy substituents in the
aryl group are positioned to interact with the external substrate
bound to the cobalt ion in the framework, in an arrangement
which is similar to a Mn-based complex described in a previous
report by Rochford.15
Cyclic voltammetry (CV) of Co(II)BIPC in a DMF solution
under an N2 atmosphere displays two reversible peaks with E1/2
of ꢁ0.87 V and ꢁ1.75 V (Fig. 2a).16 The first reduction event at
ꢁ0.87 V was confirmed by EPR experiments to represent cobalt-
based reduction of Co(II)BIPC to form Co(I)BIPC (Fig. S13, ESI†).
Eight hyperfine peaks can be seen in Fig. S13a (ESI†) due to the
interaction with the Co nucleus (I = 7/2) with g8 = 1.99,
supporting the presence of a Co(II) species with the unpaired
electron of the Co atom in a low-spin d7 configuration. Upon
addition of an excess amount of NaBH4 as a one-electron
reductant, the signals derived from the Co(II) species completely
disappear (Fig. S13b, ESI†), suggesting the formation of the Co(I)
species. The absorption change corresponding to [Co(II)BIPC]/
[Co(I)BIPC] was also monitored using cobaltocene as a one-
electron reductant, which has a reduction potential of ꢁ1.3 V
(Fig. S14, ESI†). Furthermore, the absence of the redox peak at
ꢁ0.87 V in the CV measurement of Zn(II)BIPC rules out a ligand-
based reduction at the first reduction of Co(II)BIPC, whereas the
presence of the redox peak at ꢁ1.75 V of Zn(II)BIPC indicates
that the second reduction of Co(II)BIPC is attributed to the
ligand-based reduction (Fig. S15, ESI†). Compared to a Co(II)
Fig. 2 (a) CV of Co(II)BIPC (0.5 mM) in dry DMF with 0.1 M TBAPF6 under
an N2 atmosphere. (b) CVs of Co(II)BIPC (0.5 mM) in dry DMF containing
0.1 M TBAPF6 under N2 (black) and CO2 with various concentrations of H2O
(from blue to purple): [H2O] = 0, 1, 2, 3, 4, and 5 M. Scan rate: 100 mV sꢁ1
.
complex of dianionic tetraphenylporphyrin (Co(II)TPP, E1/2(CoII/I) =
ꢁ1.28 V, Fig. S25, ESI†), the first redox potential of Co(II)BIPC is
positively shifted by 0.41 V because of the significant stabilization of
the Co(I) species by the monoanionic bipyricorrole ligand.
The CV trace of Co(II)BIPC measured under a CO2 atmo-
sphere exhibits a significant current enhancement after the
second reduction process (Fig. 2b, blue profile). Addition of
H2O as a proton source results in a continuous increase in
current at ꢁ2.17 V and the catalytic current reaches saturation
with 5 M H2O (Fig. 2b and Fig. S16, ESI†). The catalytic current
was not observed upon addition of H2O under an N2 atmosphere
(Fig. S17, ESI†). To determine the reaction corresponding to the
current enhancement in the CV measurements, controlled-
potential electrolysis (CPE) experiments for Co(II)BIPC were
conducted at a potential of ꢁ2.17 V for 1 h in a CO2-saturated
DMF solution containing 5 M H2O. Analysis of the gas phase by
gas chromatography revealed selective CO production with
Faradaic efficiency (FE) of 75%, whereas FE of a competing H2
evolution was found to be 6% (Table 1, entry 1). Formic acid in
the liquid phase was not detected by ion chromatography.
A rinse test performed after the CPE demonstrated negligible
Table 1 Electrochemical data for Co(II)BIPC
FE (CO)/FE (H2)a icat/ip
kobs (sꢁ1
)
b
c
Entry Atm. Additive
1
2
3
CO2 5 M H2O
CO2 5 M TFE
75/6
77/6
6.3
—
3.9
7.8
—
3.0
Fig. 1 X-ray crystal structure of Co(II)BIPC with 50% thermal ellipsoid
probability. (a) Top view and (b) side view. Hydrogen atoms and the non-
bonding counter anion (ClO4ꢁ) are omitted for clarity. Only the major
configuration of the disordered groups is shown. Methanol as a neutral
axial ligand in the top view and an aryl group in the side view are also
omitted for clarity.
N2
9.0% v/v buffer 0/84
a
CPE experiments in entries 1 and 3 were performed at ꢁ2.17 V and
b
CPE in entry 2 was conducted at ꢁ2.28 V. Calculated at ꢁ2.17 V.
c
Determined as an apparent rate constant at ꢁ2.17 V at a scan rate of
100 mV sꢁ1 using the equation in ref. 3a (see ESI for details).
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enhancement of the current compared to a fresh working electrode
(Fig. S18, ESI†). This indicates that the catalytic performance is not
derived from an electrodeposition on the electrode. The UV-vis
spectra of an aliquot of the electrochemical solution before and
after the CPE experiments show that nearly 90% of the complex is
maintained after 1 h of CPE, indicating that Co(II)BIPC has high
durability in electrocatalysis (Fig. S19, ESI†). The alternative proton
source 2,2,2-trifluoroethanol (TFE) was also investigated. Upon
addition of TFE under a CO2 atmosphere, current enhancement
was observed in the CV measurement, which reached a maximum
when 5 M TFE was added (Fig. S20, ESI†). The CPE experiment
conducted at ꢁ2.28 V in CO2-saturated DMF with 5 M TFE revealed
that the enhanced current in the CV measurement corresponds to
Fig. 4 Catalytic Tafel plots of Co(II)TPP (black), Co(II)BIPC with 5 M H2O
(red) and Co(II)BIPC with 5 M TFE (blue).
a CO2-to-CO reduction with FE of 77% along with the competing Co(II)BIPC is able to function both as a CO2 reduction catalyst
H2 evolution with FE of 6% (Table 1, entry 2).17
and an H2 evolution catalyst under the same pseudo-pH condi-
To investigate the competing H2 evolution catalyzed by tions. However, the apparent catalytic rate constants (kobs) of CO2
Co(II)BIPC in the presence of H2O under a CO2 atmosphere, reduction and H2 evolution suggest that the reaction rates of CO2
pseudo-pH regulation experiments were conducted under CO2-free reduction are clearly faster than those of H2 evolution in the range
conditions using a buffer solution containing a mixture of 15 mM of pseudo-pH from 7.44 to 8.53 (Fig. 3b).20 In particular, the rate
each of MES, TAPS, HEPES and CHES.18 The pseudo-pH values of constant under CO2 in the presence of 5 M H2O is 2.6 times greater
N2-saturated DMF/buffer solutions were found to be similar to those than the rate constant measured under N2 in the presence of 9.0%
of CO2-saturated DMF/H2O solutions.19 Addition of the buffer v/v buffer solution (Table 1 and Fig. 3b). The difference in the
solution under N2 resulted in a continuous increase of current in reaction rates between CO2 reduction and H2 evolution supports
the CV measurements (Fig. 3a). The CPE experiment with a 9.0% v/v the results of the selective CO2 reduction by Co(II)BIPC.
buffer solution under an N2 atmosphere, where the pseudo-pH
The catalytic Tafel plots of Co(II)BIPC and Co(II)TPP clarify
value of the bulk solution is similar to that of a CO2-saturated DMF the electrochemical properties and give insights into the advantages
solution containing 5 M H2O, revealed selective H2 evolution with of the monoanionic bipyricorrole ligand over the dianionic
FE of 84% (Table 1, entry 3). These findings indicate that porphyrin ligand.21 Each plot was obtained by determining
the apparent catalytic rate constant values (kcat) of Co(II)BIPC
with 5 M H2O, Co(II)BIPC with 5 M TFE22 and Co(II)TPP23 from
the catalytic currents in the CV measurements, which gave kcat
of 7.8, 19.8 and 30.4 sꢁ1, respectively (Fig. S22–S26, ESI†). Fig. 4
reveals that Co(II)BIPC is capable of working as a CO2 reduction
catalyst with much smaller overpotential (Z) than Co(II)TPP
formed by the dianionic ligand, whereas the reaction rate of
Co(II)BIPC is similar to that of Co(II)TPP. In particular, the
maximum TOF (TOFmax) is achieved with Z of 0.68 V by
Co(II)BIPC with 5 M H2O, which is 0.35 V more positive than
Co(II)TPP in which the Z for TOFmax is 1.03 V. This result
unambiguously demonstrates the advantage of the monoanionic
ligand as a component of a CO2 reduction catalyst to decrease the
overpotential in the electrochemical reaction.
In summary, Co(II)BIPC, which consists of the Co(II) complex
with a monoanionic bipyricorrole ligand possessing a 2,6-dimethoxy-
phenyl group at the meso-position, catalyzes a selective CO2
reduction reaction in the presence of proton sources, as con-
firmed by CPE experiments.24 Although Co(II)BIPC was found to
function both as a CO2 reduction catalyst and an H2 evolution
catalyst under the same pseudo-pH conditions, the difference in
reaction rates between CO2 reduction and H2 evolution leads to
a selective CO2-to-CO reduction under a CO2 atmosphere.
Furthermore, we have demonstrated using catalytic Tafel plots
that the monoanionic ligand is useful for generating an electro-
catalytically active low-valent species at much more positive
Fig. 3 (a) CVs of Co(II)BIPC (0.5 mM) in DMF with 0.1 M TBAPF6 under an
N2 atmosphere at a scan rate of 100 mV sꢁ1 upon addition of buffer
solution ranging from 0% v/v (black) to 14.4% v/v (purple). (b) Reaction rate
dependence upon addition of H2O under a CO2 atmosphere (black) and
potentials than a dianionic porphyrinoid ligand. Compared to
our previous work using a Co(II) complex of tetradehydrocorrin
upon addition of buffer solution under an N2 atmosphere (blue) determined
at ꢁ2.17 V in the CV results with a scan rate of 100 mV sꢁ1
.
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(E1/2(CoII/I) = ꢁ0.53 V) that promotes the selective H2 evolution
rather than CO2 reduction, Co(II)BIPC exhibits a negatively
shifted E1/2(CoII/I) at ꢁ0.87 V and catalyzes selective CO2-to-CO
reduction, indicating that the moderate stabilization of the Co(I)
species is favorable to promote the selective CO2 reduction
reaction. We believe that the present findings will contribute
to the development of efficient CO2 reduction catalysts with low
overpotential.
This work was financially supported by Grants-in-Aid for
Scientific Research provided by JSPS KAKENHI Grant Numbers
JP15H05804, JP16K14036, JP16H06045, JP16H00837, JP16H00758
and JP18K19099. We appreciate support from JST PRESTO
(JPMJPR15S2) and SICORP. We thank Prof. M. Miura (Osaka
Univ.) for the crystal structural analysis.
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Conflicts of interest
There are no conflicts to declare.
14 We also attempted to prepare a phenyl substituted Co(II) bipyricorrole
instead of 2,6-dimethoxyphenyl substituted Co(II) bipyricorrole as a
reference complex, but the synthesis was unsuccessful due to failure
of the intermolecular coupling reaction of the corresponding bipyrro-
methane and bipyridine. Thus, the effect of the proximal methoxy group
on the electrochemical reaction is not discussed in this paper.
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496 | Chem. Commun., 2019, 55, 493--496
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