Synthesis and Characterization of meso-Substituted Cobalt Tetradehydrocorrin and Evaluation of Its Electrocatalytic Behavior Toward CO2 Reduction and H2 Evolution

Article abstract of DOI:10.1021/acs.inorgchem.8b02333

A meso-aryl substituted cobalt(II) tetradehydrocorrin complex (Co(II)TDHC) has been synthesized and investigated. The corrin framework, determined by X-ray crystallographic analysis, is found to be relatively planar except at the C1 and C19 positions. Cyclic voltammetry (CV) measurements indicate two positively shifted reversible redox couples at -0.53 and -1.70 V vs Fc/Fc⁺ for [Co^II]⁺/[Co^I] and [Co^I]/([Co^I]^?- and/or [Co^II]^-) ([Co^II] = Co(II)TDHC), respectively, compared with the previously reported cobalt porphyrin complex, because the tetradehydrocorrin ligand efficiently promotes the formation of low-valent metal species due to its monoanionic character. Furthermore, it is found that the current in the CV measurement is significantly enhanced upon addition of H₂O under a CO₂ atmosphere, indicating the progression of electroreductive catalysis by Co(II)TDHC. However, controlled-potential electrolysis (CPE) using Co(II)TDHC under the same conditions shows generation of H₂ as a major product and only a small amount of CO as a CO₂ reduction product; Faradaic efficiencies are calculated to be 66.8 and 4.5%, respectively. The CPE with a buffer solution under an N₂ atmosphere reveals that the selective H₂ generation is promoted by the moderate acidification of the solution under CO₂ saturation conditions. The present study demonstrates that the significantly stabilized Co(I) species with the monoanionic ligand framework preferentially catalyzes the thermodynamically favored H₂ evolution rather than CO₂ reduction.

Full text of DOI:10.1021/acs.inorgchem.8b02333

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Characterization of meso-Substituted Cobalt
Tetradehydrocorrin and Evaluation of Its Electrocatalytic Behavior
Toward CO2 Reduction and H2 Evolution
Ayumu Ogawa,† Koji Oohora,*,†,‡ and Takashi Hayashi*,†
†Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
‡PRESTO, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: A meso-aryl substituted cobalt(II) tetradehydrocorrin complex (Co(II)TDHC) has been synthesized and
investigated. The corrin framework, determined by X-ray crystallographic analysis, is found to be relatively planar except at the
C1 and C19 positions. Cyclic voltammetry (CV) measurements indicate two positively shifted reversible redox couples at −0.53
and −1.70 V vs Fc/Fc+ for [CoII]+/[CoI] and [CoI]/([CoI]•− and/or [CoII]−) ([CoII] = Co(II)TDHC), respectively,
compared with the previously reported cobalt porphyrin complex, because the tetradehydrocorrin ligand efficiently promotes
the formation of low-valent metal species due to its monoanionic character. Furthermore, it is found that the current in the CV
measurement is significantly enhanced upon addition of H2O under a CO2 atmosphere, indicating the progression of
electroreductive catalysis by Co(II)TDHC. However, controlled-potential electrolysis (CPE) using Co(II)TDHC under the
same conditions shows generation of H2 as a major product and only a small amount of CO as a CO2 reduction product;
Faradaic efficiencies are calculated to be 66.8 and 4.5%, respectively. The CPE with a buffer solution under an N2 atmosphere
reveals that the selective H2 generation is promoted by the moderate acidification of the solution under CO2 saturation
conditions. The present study demonstrates that the significantly stabilized Co(I) species with the monoanionic ligand
framework preferentially catalyzes the thermodynamically favored H2 evolution rather than CO2 reduction.
Chart 1. Deprotonated Structures of Porphyrin, Corrin, and
Tetradehydrocorrin
INTRODUCTION
■
Cobalamin plays important roles in biological systems as a
cofactor of metalloenzymes which catalyze essential reactions
such as methyl group transfer, 1,2-rearrangement and
dehalogenation.1−5 A low-valent cobalt species, a common
key intermediate in the enzymatic reactions with cobalamin, is
stabilized by a monoanionic macrocyclic corrin framework,
which lacks one of the meso-position carbons as seen in the
dianionic porphyrin framework (Chart 1). Tuning the valency
of a series of porphyrinoid frameworks is an essential strategy
used in regulation of the reactivity of the metal center, resulting
in catalysis of a broad range of reactions in biological systems.
Thus, it is of particular interest to synthesize a porphyrinoid-
type monoanionic ligand and to study the reactivity of its
cobalt complex. Previously, Dolphin demonstrated the syn-
thesis of 1,19-disubstituted tetradehydrocorrin (Chart 1) as an
analog of cobalamin,6 and several additional studies have been
reported focusing on the synthesis and physicochemical
properties of cobalt tetradehydrocorrin derivatives.7 However,
investigations of the reactivity of cobalt tetradehydrocorrin
derivatives have been quite limited, except for the artificial
Received: August 21, 2018
© XXXX American Chemical Society
A
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enzyme models reported by our group.8 In this context, we
attempted to evaluate the reactivity of the low-valent cobalt
species using a structurally modified cobalt tetradehydrocorrin
complex.
engineering of a hydrogen bond between metallocarboxylic
acid (M−CO2H) and the methoxy group of the peripheral
ligand substituent.30 Based on this work, our group has
recently designed a cobalt(II) tetradehydrocorrin (Co(II)-
TDHC) complex modified with 1,3-dimethoxybenzene moi-
eties at the meso-positions (Chart 2). The synthesis of
Co(II)TDHC is depicted in Scheme 1. The target molecule
was achieved via 6 steps. First, commercially available 1,3-
dimethoxybenzene was formylated, and the subsequent
reaction with pyrrole in the presence of trifluoroacetic acid
afforded dipyrromethane 2. A Vilsmeier−Haack reaction of
compound 2 gave monoformylated compound 3. Compound 4
was then obtained via a Wolf−Kishner reaction of compound
3, and subsequent acid-catalyzed coupling with compound 1
afforded tetrapyrrole 5. Finally, cobalt-templated oxidative
cyclization of tetrapyrrole 5 produced Co(II)TDHC in an 18%
yield. Typically, meso-unsubstituted or meso-methyl substituted
tetradehydrocorrin derivatives have been synthesized through
acid-catalyzed intermolecular coupling of the corresponding
2,2′-dicarboxybipyrromethane and 2-formylpyrrole followed by
metal-templated cyclization.6,7a In the case of meso-aryl
substituted tetradehydrocorrin, however, this method could
not be applied to prepare the corresponding biladiene because
of severe steric hindrance around the carbonyl group in the
pyrrole precursor.31 We are unaware of any reports of synthesis
of meso-aryl substituted tetradehydrocorrin derivatives despite
the broad versatility available for functionalization of the
tetradehydrocorrin framework. To the best of our knowledge,
this is the first example of synthesis of a meso-aryl substituted
tetradehydrocorrin. We also attempted to prepare meso-phenyl
substituted tetradehydrocorrin and meso-mesityl substituted
tetradehydrocorrin as reference compounds but was not
successful because of their low stability to promote the
spontaneous demetalation. Thus, the effect of proximal
methoxy groups toward electrochemical reaction is not
evaluated in this paper. However, 1,3-dimethoxybenzene
moiety at meso-positions seems to be important to maintain
the macrocyclic structure of meso-aryl substituted cobalt
tetradehydrocorrins.
In order to evaluate the reactivity of the low-valent cobalt
species, we have focused on reductive catalytic conversions of
small molecules such as carbon dioxide (CO2) reduction and
hydrogen (H2) evolution. A variety of investigations into H2
evolution have been carried out using cobalt macrocyclic
complexes consisting of cobalt cyclams, cobaloxymes, and
cobalt porphyrins, among others.9−13 It is known that cobalt
complexes can function as useful and practical catalysts for
CO2 reduction because they are relatively inexpensive.14−19 In
particular, cobalt porphyrin complexes have been reported to
function as CO2 reduction catalysts.20−22 Although cobalt
complexes of other porphyrinoids including phthalocyanine23
and corrole24 have also been evaluated as CO2 reduction
catalysts, significantly negative potentials to generate a low-
valent metal species are required, and low product selectivities
were observed for these catalysts due to competition between
CO2 and proton reductions. In contrast, a strategy to easily
stabilize the low-valent metal species should provide important
insights into achieving high catalytic activity and product
selectivity. On this point, investigations of cobalamin and its
derivatives as a homogeneous catalyst for CO2 photoreduction
have been demonstrated.25 However, the catalytic reaction
provides a mixture of H2, CO, and formic acid, and the
dependency of catalytic activity and the product selectivity on
the ligand valency has not yet been defined. Therefore, we have
focused on a monoanionic tetradehydrocorrin framework,
which is able to further promote efficient formation of the low-
valent cobalt species for small-molecule activation, compared
to cobalt complexes with highly saturated corrin frameworks or
well-known porphyrins providing a typical dianionic ligand
framework. In the present study, we have designed,
synthesized, and characterized the meso-aryl substituted cobalt
tetradehydrocorrin complex shown in Chart 2 to evaluate the
electrochemical behavior under a CO2 atmosphere.
Chart 2. Molecular structure of Co(II)TDHC
X-ray Crystal Structure. A single crystal of Co(II)TDHC
was successfully obtained from a solution of hexane/CH2Cl2
(v/v = 1/1) by vapor diffusion. The structure of Co(II)TDHC
was unambiguously revealed by single crystal X-ray analysis as
depicted in Figure 1. The cobalt center has a pentacoordinated
structure ligated by an oxygen atom of ethyl acetate as a
neutral axial ligand and four nitrogen atoms from the
tetradehydrocorrin ligand. Since Co(II)TDHC possesses two
chiral sp3 carbon atoms at the 1 and 19 positions,
Co(II)TDHC is a mixture of (1R, 19R) and (1S, 19S)
enantiomers with a relative population of approximately 0.8
and 0.2, respectively, in the crystal. This ratio is similar to
previously reported ratio of a cobalt tetradehydrocorrin
derivative.7a Furthermore, the aryl groups at the meso-positions
are nearly perpendicular to the tetradehydrocorrin macrocycle,
indicating that the methoxy substituents in the aryl groups are
capable of interacting with an external substrate bound to the
cobalt atom in the framework.
RESULTS AND DISCUSSION
■
Molecular Design and Synthesis of Co(II)TDHC. For
achieving effective CO2 reduction, it is necessary to design a
finely tuned catalyst considering access of substrate,26
facilitation of proton transfer,27,28 and inhibition of catalyst
dimerization.29 To address these structural requirements, we
took advantage of the properties of the proximal methoxy
group, employing it as a pendant Lewis base, because a similar
pendant Lewis base is known to promote cleavage of a C−OH
bond of protonated CO2 in a reaction intermediate via
Electrochemical/Spectroscopic Characterization. The
redox behaviors of Co(II)TDHC under an N2 atmosphere
were evaluated by cyclic voltammetry (CV) using a glassy
carbon electrode as the working electrode. All potentials in this
paper are reported relative to the Fe(III)/Fe(II) couple at 0.00
V using ferrocene (Fc). In a DMF solution of Co(II)TDHC,
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a
Scheme 1. Synthesis of Co(II)TDHC
a
n
(a) TMEDA, BuLi, DMF, THF, 88%; (b) pyrrole, TFA, 71%; (c) POCl3, DMF, 69%; (d) H2NNH2·H2O, KOH, ethylene glycol, 84%; (e) 1,
HCl, CH3OH, H2O, 47%; (f) Co(OAc)2·4H2O, p-chloranil, CH2Cl2, CH3OH; (g) HClO4, H2O, CH2Cl2, 18% in 2 steps. TMEDA = N,N,N′,N′-
tetramethylethylenediamine, TFA = trifluoroacetic acid.
Figure 1. X-ray crystal structure of Co(II)TDHC with 50% thermal ellipsoid probability. (a) Top view and (b) side view. Hydrogen atoms and the
−
nonbonding counteranion (ClO4 ) are omitted for clarity, and only the (1R,19R) enantiomer is shown as the major configuration of the disordered
groups. Ethyl acetate as a neutral axial ligand in the top view and an aryl group at the 10-position in the side view are also omitted.
two reversible peaks were found with E1/2 of −0.53 and −1.70
V (Figure 2). The redox couples are confirmed to be diffusion-
controlled process by the CV measurements under the various
scan rate conditions (Figures S9 and S10). The first redox
potential has a slightly positive shift compared to that of meso-
unsubstituted cobalt tetradehydrocorrin (E1/2 ∼ −0.65 V).7b
This result indicates that introduction of an aryl group at each
meso-position stabilizes the π-electrons of the tetradehydro-
corrin macrocycle, resulting in lower electron density, as
previously reported for meso-tetraphenylporphyrin deriva-
tives.32−34
In order to characterize the species generated by one-
electron reduction at −0.53 V, the titration experiment with
cobaltocene was performed under anaerobic conditions.
Cobaltocene is a one-electron reductant with a reduction
potential of −1.3 V, which is between the first and second
redox potentials of Co(II)TDHC.35 It is known that cobalt
tetradehydrocorrin derivatives display significant absorption
changes depending on the valency of the cobalt ion.7a The
UV−vis spectrum of Co(II)TDHC shows characteristic
absorption peaks at 510 and 615 nm (Figure 3a, blue line),
whereas a new absorption peak at 573 nm appears upon
addition of cobaltocene with a concomitant decrease of
absorption peaks at 510 and 615 nm via several isosbestic
points. The titration curve of the absorption reached a plateau
when a small excess of cobaltocene (∼0.8 mM) was added
(Figure 3b). These changes are similar to those observed in the
reduction of the previously reported Co(II) tetradehydrocorrin
to Co(I) tetradehydrocorrin.7a The reduction of Co(II)TDHC
using NaBH4 as a reductant was also monitored with the same
spectral changes (Figure S12).
Figure 2. Cyclic voltammogram of Co(II)TDHC (0.5 mM) in
anhydrous DMF with 0.1 M TBAPF6 at a scan rate of 100 mV·s−1
under an N2 atmosphere.
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Figure 3. (a) Changes in UV−vis absorption spectra of Co(II)TDHC (50 μM in CH2Cl2) upon addition of cobaltocene ranging from 0 mM
(blue) to 0.84 mM (red). Asterisk (*) shows an absorption peak of cobaltocene and cobaltocenium. The absorption peak of cobaltocene at 365 nm
was negligible because of the strong absorptions of Co(II)TDHC and Co(I)TDHC (Figure S11).35 (b) Absorbance changes of Co(II)TDHC
upon addition of cobaltocene. (c) EPR spectrum of Co(II)TDHC in THF at 100 K. (d) EPR spectrum of Co(I)TDHC in THF at 100 K with an
excess amount of NaBH4.
EPR spectra of Co(II)TDHC were measured at 100 K.
Eight hyperfine lines were observed due to the interaction with
the Co nucleus (I = 7/2) with g// = 1.99, supporting the
presence of a Co(II) species with the unpaired electron of the
Co atom in a low-spin d7 configuration (Figure 3c).7c,d Upon
addition of excess NaBH4, the signals derived from the Co(II)
species completely disappear, indicating formation of the
Co(I) species (Figure 3d).7d These results suggest that NaBH4
is capable of reducing Co(II)TDHC to Co(I)TDHC. The
chemical reductions of Co(II)TDHC monitored by UV−vis
and EPR spectra indicate that the reduction event observed at
−0.53 V is attributed to the [CoII]+/[CoI] couple ([CoII] =
Co(II)TDHC). Compared to the cobalt complex formed by
dianionic tetraphenylporphyrin (Co(II)TPP), the redox
potential of Co(II)TDHC is positively shifted by 0.75 V
Figure 4. Cyclic voltammograms of Co(II)TDHC (0.5 mM) in dry
(E1/2(CoII/I) of Co(II)TPP is −1.28 V) due to the
monoanionic ligand. Co(I)TDHC is highly stable and can
be maintained for a few days even under aerobic conditions.
Next, the redox peak at −1.70 V is considered to be attributed
not to reduction of the Co(I) species to form the Co(0)
species but to reduction of the tetradehydrocorrin ligand to
generate [CoI]•− and/or [CoII]− after the one-electron transfer
from Co(I) to the ligand, according to a previous report.7d
Electrochemistry under a CO2 Atmosphere. To
investigate the activity of Co(II)TDHC for CO2 reduction,
CV measurements were conducted in dry and wet CO2-
saturated DMF ([CO2] ∼ 0.23 M) containing 0.1 M TBAPF6
under a CO2 atmosphere. As shown in Figure 4, only a small
increase of current was observed in an electrochemical solution
saturated by CO2. Under a CO2 atmosphere, the addition of
DMF containing 0.1 M TBAPF6 at a scan rate of 100 mV·s−1 under an
N2 atmosphere (black). Addition of H2O under a CO2 atmosphere
(from blue to red): [H2O] = 0, 1, 2, 3, 4, and 5 M.
H2O to the solution clearly enhances the current after the
second reduction process. This result indicates that reductive
catalysis occurs in the presence of CO2 and H2O after the two-
electron reduction of Co(II)TDHC. Further addition of H2O,
however, does not cause an increase in current, indicating that
the reaction rate of reductive electrolysis does not depend on
the concentration of H2O in the presence of >5 M H2O
(Figure S13). As a proton source, 2,2,2-trifluoroethanol (TFE)
or CH3OH was also employed. In the case of TFE, a large
enhancement of the current was observed in the CV
experiment even under an N2 atmosphere, indicating the H2
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evolution (Figure S15). In contrast, addition of CH3OH under
a CO2 atmosphere also enhanced the current in the CV
experiment, whereas a negligible increase in the current was
observed upon addition of CH3OH under an N2 atmosphere
(Figure S17).36
To evaluate the product(s) generated by the electrocatalysis,
controlled-potential electrolysis (CPE) experiments were
carried out at a potential of −2.11 V for 1 h in a CO2-
saturated DMF solution in the presence of various amounts of
H2O, and the Faradaic efficiencies were determined from the
quantification of gaseous and liquid products by gas
chromatography and ion chromatography analyses, respec-
tively (Figure 5). In the CO2-saturated dry DMF solution,
with the Faradaic efficiency of 0.5%, and no CO generation
was detected. Thus, the electrolysis potential of −2.11 V is
found to be appropriate in our system.
Electrochemistry under an N2 Atmosphere in the
Presence of Buffer Solution. To determine whether the H2
evolution reaction is caused by acidification of DMF/H2O
cosolvent under CO2-saturated conditions, a buffer solution
was used to regulate a pseudo-pH value of the electrochemical
solution. The buffer solution is a mixture containing 15 mM
each of 2-(N-morpholino)ethanesulfonic acid (MES), N-
tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid
(TAPS), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
(HEPES), and N-cyclohexyl-2-aminoethanesulfonic acid
(CHES).37 We have employed the buffer solution for our
experiments, and the pseudo-pH values of N2-saturated DMF/
buffer solutions were found to be similar to those of CO2-
saturated DMF/H2O solutions.38 Figure 6 shows the effect of
Figure 5. Faradaic efficiencies of CO (red) and H2 (blue) generations
in the presence of various concentrations of H2O after 1 h of
electrolysis at −2.11 V under CO2-saturated conditions.
Figure 6. Cyclic voltammograms of Co(II)TDHC (0.5 mM) in DMF
with 0.1 M TBAPF6 at a scan rate of 100 mV·s−1 under an N2
atmosphere. Addition of buffer solution (from blue to red): 0, 1.8, 3.6,
5.4, 7.2, and 9.0% v/v.
small amounts of CO and H2 were detected in the gas phase,
and the formation of formic acid was not observed in the liquid
phase. Upon addition of H2O, H2 was dominantly generated
and only a small amount of CO was produced. Under each set
of conditions, no formic acid was detected in the liquid phase.
In particular, in the presence of 2 M H2O, the Faradaic
efficiency of H2 generation reached 66.8%, while the Faradaic
efficiency of CO production as a CO2 reduction product was
only 4.5%. The CPE results suggest that enhancement of
current in CV measurements is caused by H2 generation
catalyzed by two-electron reduced species of Co(II)TDHC
rather than CO2 reduction.
Further addition of H2O results in decomposition of
Co(II)TDHC during electrolysis. To ensure that the catalytic
activity was not derived from electrodeposition on the
electrode surface, a rinse test was performed after the CPE
experiments. In the presence of less than 2 M H2O, a negligible
increase in current was observed, suggesting that the catalyst
remains homogeneous during the electrolysis (Figure S19a). In
contrast, the largely increased current in the presence of 3 M
H2O compared to a fresh working electrode suggests the
formation of active and adsorbable decomposed materials
(Figure S19b). CPE experiments were also carried out at more
negative (−2.34 V) and at more positive (−2.00 V) potentials
in the presence of 2 M H2O. Electrolysis at −2.34 V resulted in
H2 and CO generation with the Faradaic efficiencies of 39.2
and 7.8%, respectively. However, CPE under this condition led
to the formation of decomposed materials of the catalyst to
adsorb on the working electrode as confirmed by the rinse test.
In contrast, CPE at −2.00 V gave only a trace amount of H2
addition of the buffer solution on the CV profiles under an N2
atmosphere. Using the buffer solution, a comparable catalytic
current to the CO2-saturated solution containing H2O was
observed after generation of the two-electron reduced species.
Further addition of the buffer solution did not show any effect
on the enhancement of current (Figure S20), which is
consistent with the results obtained from addition of H2O
under a CO2 atmosphere (vide supra). As a control experiment,
the electrochemical behavior upon addition of H2O under an
N2 atmosphere was also evaluated, and no enhancement of
current was observed, suggesting that moderate acidification of
the electrochemical solution is essential for the reductive
catalysis (Figure S22).
To obtain further evidence, the CPE experiments with the
buffer solution under an N2 atmosphere were conducted. H2
generation was observed and the Faradaic efficiencies were
found to be generally consistent with the Faradaic efficiencies
measured in CO2-saturated DMF solution containing H2O
(Figure 7). These results suggest that acidification of the bulk
solution by the saturation of CO2 generates a sufficient amount
of protons to promote electrocatalytic H2 generation by
Co(II)TDHC.
A plausible mechanism for electrocatalysis promoted by
Co(II)TDHC toward H2 evolution and CO2 reduction is as
follows: The one-electron reduction species of Co(II)TDHC,
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acidification of the electrochemical solution. In the present
study, we have demonstrated that the highly stabilized Co(I)
species is inert for the reactions, whereas one more reduced
species [Co(I)]•− and/or [Co(II)]− promotes thermodynami-
cally favored H2 evolution, since Co(II)TDHC works as a
catalyst under relatively mild conditions (pH ∼ 8) with nearly
100% Faradaic efficiency. Moreover, we have succeeded in
demonstrating that the monoanionic ligand is useful for
generating an electrocatalytically active low-valent metal
species at much more positive potentials than dianionic
porphyrinoid ligands. Efforts to design new catalysts to achieve
CO2-selective reduction with low overpotential are now
underway in our laboratory.
Figure 7. Faradaic efficiencies of CO (red) and H2 (blue) generations
with 1.8 and 3.6% v/v of buffer solution after 1 h of electrolysis at
−2.11 V under an N2 saturation conditions. CO generated in the
presence of 3.6% v/v buffer solution is derived from the
decomposition of the catalyst or degradation of DMF/buffer
components.
EXPERIMENTAL SECTION
■
Materials and Methods. NMR spectra were recorded on a
Bruker DPX 400 (400 MHz) and Bruker Avance III HD (400 MHz)
spectrometers at 298 K. Chemical shifts are reported in ppm relative
to the residual solvent resonances. ESI-TOF MS analyses were
performed on a Bruker micrOTOF-II mass spectrometer. UV−vis
spectral measurements were carried out with a Shimadzu UV-3600
Plus double-beam spectrophotometer with a thermostated cell holder.
EPR spectroscopic measurements were performed with a Bruker
EMXmicro spectrometer at the X-band (9.66 GHz) microwave
frequency with 151.7 mW microwave power and 5.00 G of
modulation amplitude. During EPR measurements, the sample
temperature was maintained at 100 K using liquid N2 vapor.
Tetrabutylammonium hexafluorophosphate (TBAPF6, >98%) was
purchased from Tokyo Chemical Industry Co., Ltd., and recrystallized
from heated ethanol and dried under vacuum before use. Distilled
water was demineralized using a Millipore Integral 3 apparatus. All
other reagents of the highest guaranteed grade available were obtained
from commercial sources and were used as received unless otherwise
indicated.
Synthesis of 2,6-Dimethoxybenzaldehyde (1). To a flame-
dried flask were added 2,6-dimethoxybenzene (5.1 mL, 39.0 mmol),
N,N,N′,N′-tetramethylethylenediamine (6.6 mL, 44.3 mmol), and
THF (60 mL). The solution was cooled to 0 °C, and then 1.6 M
nBuLi hexane solution (29.3 mL, 46.9 mmol) was added dropwise.
After stirring for 1 h at 0 °C, DMF (4.5 mL, 58.0 mmol) was added,
and the mixture was further stirred for 1 h at 0 °C. The solution was
allowed to warm up to room temperature and the reaction was
quenched by addition of 4 N HCl (aqueous) (12 mL). The reaction
mixture was extracted with ethyl acetate, washed with saturated NaCl
(aqueous), dried over Na2SO4, and the solvent was evaporated. The
target compound 1 was obtained by reprecipitation in a hexane/
Co(I)TDHC, is inert toward both CO2 reduction and H2
evolution, which was confirmed by UV−vis absorption
spectroscopic measurements where exposure of a THF
solution of Co(I)TDHC to CO2 and H2O did not show any
change in absorption spectra (Figure S23). This behavior is
distinctly different from behavior exhibited by a series of Co(I)
species of normal porphyrin which promote H2 evolution via a
Co(III)−H species.10,11 In fact, no CO and H2 evolution was
observed in the CPE experiment conducted at −1.3 V. These
findings suggest that the Co(I)TDHC seems to be stabilized
by the monoanionic tetradehydrocorrin framework to the
extent that it cannot overcome the activation energy of the
reaction with CO2. The E1/2(Co(II)/Co(I)) of Co(II)TDHC
is −0.53 V, which is 0.75 V more positive than that of
Co(II)TPP. In contrast, under the two-electron reduction
conditions carried out at −2.11 V, two-electron reduced
species of Co(II)TDHC works as a catalyst toward H2
evolution and CO2 reduction. In particular, the plausible
intermediate [Co(I)]•− and/or [Co(II)]− then may easily
form thermodynamically favored Co−H species resulting in H2
evolution. Even though Co(II)TDHC has poor activity toward
CO2, we have found that Co(II)TDHC catalyzes H2
generation under relatively mild conditions (pH ∼ 8)
compared to the previously reported porphyrinoid-type H2
generating catalysts which require the addition of a strong
Brønsted acid.9−11
1
CH2Cl2 solution as a white powder (5.69 g, 88%). H NMR (400
MHz, CDCl3) δ: 3.87 (s, 6H, OCH3); 6.55 (d, J = 8.4 Hz, 2H, Ar−
H); 7.43 (t, J = 8.4 Hz, 1H, Ar−H); 10.49 (s, 1H, CHO). 13C{1H}
NMR (100 MHz, CDCl3) δ:56.14, 103.92, 114.34, 136.05, 162.27,
189.53. ESI-TOF MS: m/z = 189.0521 [M + Na]+, calculated for
C9H10O3Na 189.0522.
CONCLUSIONS
■
This work is the first example of introduction of an aryl group
into the meso-positions of the tetradehydrocorrin framework to
functionalize the corresponding cobalt complex. In contrast to
unstable meso-phenyl and meso-mesityl substituted cobalt
tetradehydrocorrins, it was figured out that 1,3-dimethox-
ybenzene moiety at meso-positions is essential to maintain the
structure of meso-aryl substituted cobalt tetradehydrocorrins.
Co(II)TDHC shows an enhancement of current in CV
measurements under a CO2 atmosphere upon addition of H2O
as a proton source. In CPE experiments conducted at −2.11 V
for Co(II)TDHC, however, we found that Co(II)TDHC
dominantly catalyzes H2 evolution rather than CO2 reduction.
CPE with buffer solution under an N2 atmosphere offers
evidence that H2 evolution is promoted by moderate
Synthesis of 2,2′-((Dimethoxyphenyl)methylene)bis(1H-
pyrrole) (2). The mixture of compound 1 (1.00 g, 6.02 mmol) and
pyrrole (10.5 mL, 0.151 mol) was treated with trifluoroacetic acid (50
μL, 0.65 mmol) and stirred for 5 min at room temperature. After
stirring, the mixture was quenched by 0.1 M NaOH (aqueous) (100
mL), and the organic layer was separated with ethyl acetate, washed
with saturated NaCl (aqueous), dried over Na2SO4, and the solvent
was evaporated. Purification was carried out by SiO2 column
chromatography (hexane/ethyl acetate = 4/1) followed by recrystal-
lization with hexane to yield compound 2 as a white powder (1.21 g,
71%). 1H NMR (400 MHz, CDCl3) δ: 3.76 (s, 6 H, OCH3); 5.96 (m,
2H, pyrrole βH); 6.14 (m, 2H, pyrrole βH); 6.22 (s, 1H, meso-H);
6.65 (m, 2H, pyrrole αH); 6.66 (d, J = 8.4 Hz, 2H, Ar−H); 7.23 (t, J
= 8.4 Hz, 1H, Ar−H); 8.54 (s, 2H, NH). 13C{1H} NMR (100 MHz,
CDCl3) δ: 32.66, 56.51, 105.91, 107.78, 116.19, 119.64, 128.24,
F
DOI: 10.1021/acs.inorgchem.8b02333
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
133.06, 158.26. ESI-TOF MS: m/z = 305.1250 [M + Na]+, calculated
for C17H18N2O2Na 305.1260.
C45H41CoN4O6 792.2353. Elemental analysis: C 60.51, H 4.85, N
5.30. Calcd for C49H49ClCoN4O12 C 60.03, H 5.04, N 5.72.
X-ray Crystallography. The single crystal X-ray diffraction
studies were carried out with Rigaku XtaLAB synergy. The structure
was refined on F2 by full-matrix least-squares method, using SHELXL-
2016/6. Hydrogen atoms were included in the refinement on
calculated positions riding on their carrier atoms.
Synthesis of 5-((2,6-Dimethoxyphenyl)(1H-pyrrol-2-yl)-
methyl)-1H-pyrrole-2-carbaldehyde (3). DMF (4.5 mL, 58.1
mmol) was treated with POCl3 (0.67 mL, 7.38 mmol) at 0 °C, and
the resulting solution was stirred for 1 h to give the Vilsmeier reagent.
To a solution of compound 2 (0.96 g, 3.41 mmol) in DMF (11.3 mL)
at 0 °C was added dropwise the freshly prepared Vilsmeier reagent
(2.5 mL), and the mixture stirred for 2 h at 0 °C. Then, the reaction
mixture was treated with saturated CH3COONa (aqueous) (36 mL)
for 4 h at room temperature, and the mixture was extracted with ethyl
acetate, washed with saturated NaCl (aqueous), dried over Na2SO4,
and the solvent was evaporated. Purification was carried out by SiO2
column chromatography (hexane/ethyl acetate = 3/2) to afford
compound 3 (0.74 g, 69%). 1H NMR (400 MHz, CDCl3) δ: 3.75 (s,
6H, OCH3); 5.92 (m, 1H, pyrrole βH); 6.10−6.13 (m, 2H, pyrrole
αH, βH); 6.16 (s, 1H, meso-H); 6.62 (d, J = 8.4 Hz, 2H, Ar−H); 6.69
(m, 1H, pyrrole βH); 6.85 (m, 1H, pyrrole βH); 7.22 (t, J = 8.4 Hz,
1H, Ar−H); 8.83 (s, 1H, NH); 9.08 (s, 1H, NH); 9.33 (s, 1H, CHO).
13C{1H} NMR (100 MHz, CDCl3) δ: 33.02, 56.46, 105.77, 107.69,
Electrochemistry. Electrochemical studies were performed using
a potentiostat (CompactStat, Ivium Technologies). A single-compart-
ment cell was used for all cyclic voltammetry (CV) experiments with a
polished glassy carbon working electrode (3 mm in diameter from
ALS Co., Ltd. was used unless otherwise indicated.), a Pt wire counter
electrode, and an Ag|AgCl reference electrode (3 M NaCl (aqueous),
from ALS Co., Ltd.). All experiments were referenced relative to the
Fe3+/2+ couple at 0.0 V using ferrocene (Fc) as an internal standard.
All electrochemical experiments were performed with 0.1 M TBAPF6
as a supporting electrolyte. All solutions were purged with N2 or CO2
before CVs were measured. Pseudo-pH values were measured with an
F-72 Horiba pH meter equipped with a 9615−10D Horiba electrode.
Controlled-Potential Electrolysis. Controlled-potential electrol-
ysis (CPE) experiments were carried out in a custom-made single
compartment cell designed in our laboratory with a custom silicon top
to hold each electrode and a joint for sealing with septa and gas
sampling. This apparatus included a glassy carbon plate (5.8 cm2
surface area, from ALS Co., Ltd.) as a working electrode, a Pt wire
counter electrode separated from the bulk solution by a porous glass
frit (from EC FRONTIER Co., Ltd.), and an Ag|AgCl reference
electrode (3 M NaCl (aqueous)) separated from the solution by a
Vycor tip (from ALS Co., Ltd.). A potentiostat (CompactStat, Ivium
Technologies) was used to apply potentials and record current. The
CPE experiments were performed in DMF with various amounts of
proton source and 0.1 M TBAPF6. CPE solutions were purged with
N2 or CO2 for 20 min prior to electrolysis. The potential drift during
the electrolysis experiments was negligible, which was confirmed by
measuring the redox potential of ferrocene before and after the
electrolysis. Gaseous products after CPE experiments were analyzed
using 50 μL sample aliquots taken from the headspace of the
electrochemical cell and injected on a Shimadzu BID-2010 plus series
gas chromatograph with a 250 m × 0.5 mm I.D. micropacked column
with helium as a carrier gas at a flow rate of 3.0 mL/min. For the
liquid products, 1 mL of sample solution was diluted twice with H2O,
filtered, and analyzed with a Shimadzu ion chromatography system
equipped with a Shodex IC SI-35 4D column equilibrated with a 1.8
mM Na2CO3 aqueous solution as eluent at a flow rate of 0.3 mL/min.
Faradaic efficiencies were determined by dividing the measured
amount of H2 and CO by the expected amount of H2 and CO
calculated based on the charge passed during the CPE experiments.
The CPE experiments were performed at least twice, and the reported
Faradaic efficiencies are average values. As a rinse test, the glassy
carbon electrode was removed from the bulk solution after CPE and
washed carefully with DMF. Then the working electrode was
immersed in a catalyst-free CO2-saturated DMF/H2O cosolution
with 0.1 M TBAPF6 and CV measurements were obtained.
108.09, 109.45, 117.59, 117.78, 129.09, 130.25, 131.51, 144.66,
158.03, 178.12. ESI-TOF MS: m/z = 333.1221 [M + Na]+, calculated
for C18H18N2O3Na 333.1210.
Synthesis of 2-((2,6-Dimethoxyphenyl)(1H-pyrrol-2-yl)-
methyl)-5-methyl-1H-pyrrole (4). The mixture of compound 3
(735 mg, 2.37 mmol) and KOH (456 mg, 8.13 mmol) in ethylene
glycol (3.5 mL) was treated with hydrazine hydrate (0.48 mL, 9.71
mmol) and refluxed for 1 h. After cooling to room temperature, ethyl
acetate was added and the mixture was washed with saturated NaCl
(aqueous), dried over Na2SO4, and the solvent was evaporated.
Purification was carried out with SiO2 column chromatography
(hexane/ethyl acetate = 3/2) to yield compound 4 (590 mg, 84%).
1H NMR (400 MHz, CDCl3) δ: 2.20 (s, 3H, CH3); 3.75 (s, 6H,
OCH3); 5.74 (m, 1H, pyrrole βH); 5.79 (m, 1H, pyrrole βH); 5.90
(m, 1H, pyrrole βH); 6.10 (m, 1H, pyrrole βH); 6.13 (s, 1H, meso-
H); 6.63 (d, J = 8.4 Hz, 2H, Ar−H); 6.64 (m, 1H, pyrrole αH); 7.19
(t, J = 8.4 Hz, 1H, Ar−H); 8.21 (s, 1H, NH); 8.51 (s, 1H, NH).
13C{1H} NMR (100 MHz, CDCl3) δ: 13.23, 32.71, 56.48, 105.27,
105.61, 106.11, 107.73, 115.98, 119.74, 126.18, 128.09, 131.29,
133.38, 158.30. ESI-TOF MS: m/z = 319.1418 [M + Na]+, calculated
for C18H20N2O2Na 319.1417.
Synthesis of 5,5′-((2,6-Dimethoxyphenyl)methylene)bis(2-
((2,6-dimethoxyphenyl)(5-methyl-1H-pyrrol-2-yl)methyl)-1H-
pyrrole) (5). To a mixture of compound 4 (483 mg, 1.63 mmol),
compound 1 (135 mg, 0.81 mmol) and methanol (82 mL), HCl (4.0
mL) in H2O (82 mL) were added and stirred for 12 h at room
temperature with light protection. The reaction mixture was extracted
with CH2Cl2, washed with saturated NaCl (aqueous), dried over
Na2SO4, and the solvent was evaporated. Purification was carried out
by Al2O3 (neutral, grade III) column chromatography (CH2Cl2)
followed by reprecipitation (CH2Cl2/CH3OH) to give compound 5
as a pale yellow powder (283 mg, 47%). ESI-TOF MS: m/z =
763.3465 [M + Na]+, calculated for C45H58N4O6Na 763.3466.
Synthesis of 5,10,15-Tri(2,6-dimethoxyphenyl)-1,19-dime-
thyltetradehydrocorrinato Cobalt(II) Perchlorate (Co(II)-
TDHC). A mixture of compound 5 (50.0 mg, 67.5 μmol) and
Co(OAc)2·4H2O (50.4 mg, 0.202 mmol) in CH2Cl2 (17 mL) and
methanol (17 mL) was treated with p-chloranil (8.3 mg, 33.8 μmol)
for 2 h at room temperature. After stirring, solvents were evaporated
and crude product was redissolved in CH2Cl2. To this mixture was
added 0.7% HClO4 (aqueous), and the mixture was stirred for 30 min
at room temperature. The reaction mixture was extracted with ethyl
acetate, washed with H2O, dried over Na2SO4, and the solvent was
evaporated. The crude product was purified by SiO2 column
chromatography (CH2Cl2/acetone = 4/1) followed by further
purification by Al2O3 (basic, grade IV) column chromatography
(CH2Cl2/acetone = 4/1), and the blue fraction was collected.
Evaporation of the solvent afforded a black solid of Co(II)TDHC as a
racemic mixture of (1R,19R) and (1S,19S) enantiomers (11 mg,
18%). ESI-TOF MS: m/z = 792.2357 [M]+, calculated for
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02333.
NMR spectra, cyclic voltammograms, redox peak
currents, UV−vis absorption spectra (PDF)
Accession Codes
CCDC 1862337 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
G
DOI: 10.1021/acs.inorgchem.8b02333
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Hydrogen Production from Water. Inorg. Chem. 1985, 24, 2373−
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bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(11) Maher, A. G.; Passard, G.; Dogutan, D. K.; Halbach, R. L.;
Anderson, B. L.; Gagliardi, C. J.; Taniguchi, M.; Lindsey, J. S.; Nocera,
D. G. Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt
Chlorin. ACS Catal. 2017, 7, 3597−3606.
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Evolution from Neutral Water under Aerobic Conditions Catalyzed
by Cobalt Microperoxidase-11. J. Am. Chem. Soc. 2014, 136, 4−7.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: oohora@chem.eng.osaka-u.ac.jp (K.O.).
*E-mail: thayashi@chem.eng.osaka-u.ac.jp (T.H.).
ORCID
Koji Oohora: 0000-0003-1155-6824
Takashi Hayashi: 0000-0002-2215-935X
Notes
(14) Chapovetsky, A.; Do, T. H.; Haiges, R.; Takase, M. K.;
Marinescu, S. C. Proton-Assisted Reduction of CO2 by Cobalt
Aminopyridine Macrocycles. J. Am. Chem. Soc. 2016, 138, 5765−
5768.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was 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 Dr. Y.
Nishii and Prof. M. Miura (Osaka University) for the crystal
structural analysis.
(15) Fisher, B.; Eisenberg, R. Electrocatalytic Reduction of Carbon
Dioxide by Using Macrocycles of Nickel and Cobalt. J. Am. Chem. Soc.
1980, 102, 7361−7363.
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P. D.; Derat, E.; Artero, V. Molecular Cobalt Complexes with Pendant
Amines for Selective Electrocatalytic Reduction of Carbon Dioxide to
Formic Acid. J. Am. Chem. Soc. 2017, 139, 3685−3696.
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Mobius, M. Electrocatalytic reduction of O2 and CO2 with
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I
DOI: 10.1021/acs.inorgchem.8b02333
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