Modulation of Self-Assembly Enhances the Catalytic Activity of Iron Porphyrin for CO2 Reduction

Article abstract of DOI:10.1002/smll.202006150

Electrochemical reduction of CO₂ in aqueous media is an important reaction to produce value-added carbon products in an environmentally and economically friendly manner. Various molecule-based catalytic systems for the reaction have been reported thus far. The key features of state-of-the-art catalytic systems in this field can be summarized as follows: 1) an iron-porphyrin-based scaffold as a catalytic center, 2) a dinuclear active center for the efficient activation of a CO₂ molecule, and 3) a hydrophobic channel for the accumulation of CO₂. This article reports a novel approach to construct a catalytic system for CO₂ reduction with the aforementioned three key substructures. The self-assembly of a newly designed iron-porphyrin complex bearing bulky substituents with noncovalent interaction ability forms a highly ordered crystalline solid with adjacent catalytically active sites and hydrophobic pores. The obtained crystalline solid serves as an electrocatalyst for CO₂ reduction in aqueous media. Note that a relevant iron-porphyrin complex without bulky substituents cannot form a porous structure with adjacent active sites, and the catalytic performance of the crystals of this relevant iron-porphyrin complex is substantially lower than that of the newly developed catalytic system. The present study provides a novel strategy for constructing porous crystalline solids for small-molecule conversions.

Full text of DOI:10.1002/smll.202006150

FULL PAPER
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Modulation of Self-Assembly Enhances the Catalytic
Activity of Iron Porphyrin for CO2 Reduction
Masahiro Tasaki, Yuki Okabe, Hikaru Iwami, Chiharu Akatsuka, Kento Kosugi,
Kohei Negita, Sinpei Kusaka, Ryotaro Matsuda, Mio Kondo,* and Shigeyuki Masaoka*
Dedicated to the memory of Prof. Dr. Jean-Michel Savéant, who made a great contribution to this field
CO2 reduction reaction underpins broad
Electrochemical reduction of CO2 in aqueous media is an important reaction to
produce value-added carbon products in an environmentally and economically
friendly manner. Various molecule-based catalytic systems for the reaction have
been reported thus far. The key features of state-of-the-art catalytic systems in
this field can be summarized as follows: 1) an iron-porphyrin-based scaffold
as a catalytic center, 2) a dinuclear active center for the efficient activation of a
CO2 molecule, and 3) a hydrophobic channel for the accumulation of CO2. This
article reports a novel approach to construct a catalytic system for CO2 reduc-
tion with the aforementioned three key substructures. The self-assembly of a
newly designed iron-porphyrin complex bearing bulky substituents with nonco-
valent interaction ability forms a highly ordered crystalline solid with adjacent
catalytically active sites and hydrophobic pores. The obtained crystalline solid
serves as an electrocatalyst for CO2 reduction in aqueous media. Note that a rel-
evant iron-porphyrin complex without bulky substituents cannot form a porous
structure with adjacent active sites, and the catalytic performance of the crystals
of this relevant iron-porphyrin complex is substantially lower than that of the
newly developed catalytic system. The present study provides a novel strategy
for constructing porous crystalline solids for small-molecule conversions.
interest because this technology can pro-
duce value-added carbon products and
counteract CO2 emissions.[2] In particular,
CO2 reduction in aqueous media is an
advantageous approach because water is
an inexpensive, abundant, and environ-
mentally benign solvent that facilitates
proton and electron transfer, which is
required to drive catalysis. Based on this
background, vast efforts have been devoted
to the development of molecule-based
catalysts for CO2 reduction.[3,4] The most
significant advantage of molecule-based
catalysts is their high tunability for catalytic
activity based on the precise modification
of molecular structures. From the excellent
examples of state-of-the-art molecule-based
catalysts for CO2 reduction, we can extract
the following important features to achieve
efficient catalysis (Scheme 1a). First, iron-
porphyrin complexes[4c,d,h,5] are a prom-
ising catalytic platform for CO2 reduction
because the complexes are based on earth-
abundant metal ions and show extremely high catalytic activity.
Additionally, their ease of structural modification enables fine-
tuning of the activity.[4c,d,h,5,6] Second, dinuclear complexes with
closely positioned metal centers can accelerate catalysis.[3k,4i,j]
This fact indicates that the presence of adjacent catalytic active
1. Introduction
Due to the increased demand for a sustainable society, the pro-
duction of clean chemical fuels from abundant resources has
been highly sought after in recent years.[1] In this context, the
M. Tasaki, Dr. Y. Okabe, H. Iwami
Department of Life and Coordination-Complex Molecular Science
Institute for Molecular Science (IMS)
5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
M. Tasaki, Dr. Y. Okabe, H. Iwami
Department of Structural Molecular Science
SOKENDAI (The Graduate University for Advanced Studies)
Shonan Village, Hayama, Kanagawa 240-0193, Japan
H. Iwami, C. Akatsuka, K. Kosugi, Prof. M. Kondo, Prof. S. Masaoka
Division of Applied Chemistry
Graduate School of Engineering
Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
E-mail: mio@chem.eng.osaka-u.ac.jp; masaoka@chem.eng.osaka-u.ac.jp
K. Negita, Dr. S. Kusaka, Prof. R. Matsuda
Department of Chemistry and Biotechnology
School of Engineering
Nagoya University
Chikusa-ku, Nagoya 464-8603, Japan
Prof. M. Kondo
JST
PRESTO
4-1-8 Honcho, Kawaguchi 332-0012, Japan
Prof. M. Kondo, Prof. S. Masaoka
Innovative Catalysis Science Division
Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI)
Osaka University
Suita, Osaka 565-0871, Japan
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202006150.
DOI: 10.1002/smll.202006150
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Scheme 1. Schematic illustration of a) three key substructures for CO
reduction in state-of-the-art catalysts and b) the strategy of this work.
Scheme 2. Schematic illustration of a novel iron-porphyrin complex
developed in this study.
centers is important to efficiently activate the substrate molecule
CO2. Third, the use of metal–organic frameworks (MOFs), cova-
lent organic frameworks (COFs), and hydrogen-bonded frame-
works (HOFs) is an intriguing approach.[3g,j,4e,f,j,m,n,7] These
molecule-based crystalline porous materials can possess hydro-
phobic pores, which are suitable for the selective accumulation
of CO2 even in aqueous media. Such features of the MOFs and
COFs enable catalysis in aqueous media by preventing kineti-
cally favored side reaction, the reduction of water. Therefore, a
molecule-based catalyst that exhibits the aforementioned three
key features is expected to be an excellent candidate. However,
to our knowledge, the literature contains no reports on such
catalytic systems, and thus, the establishment of a strategy to
obtain materials with three key features is highly desired.
In this contribution, we report an iron-porphyrin-based
crystalline solid with adjacent catalytically active sites and
hydrophobic pores. Our strategy here is to introduce bulky
peripheral substituents with intermolecular interaction ability
(“space modulators”) to the iron-porphyrin unit (Scheme 1b).
Intermolecular interactions between space modulators enable
the assembly of catalyst molecules in an ordered fashion. As
a result, catalytic centers can arrange with the appropriate dis-
tance for the activation of CO2 molecules. The assembly also
provides well-ordered hydrophobic pores for CO2 accumula-
tion, which is similar to the case of conventional MOFs, COFs,
and HOFs. Another inherent feature of our strategy is that the
porous crystalline solid can easily be prepared by the simple
recrystallization technique. Notably, the obtained porous solid
can be disassembled to a discrete state by using the appropriate
solvent, which allows us to disclose the effect of the porous
crystalline structure on the catalytic activity. Described here is
the synthesis, characterization, and electrocatalytic activity for
CO2 reduction of the novel porous crystalline solid.
iron(III) chloride, Fe-BPPy). The characteristics of this module
can be summarized as follows. First, Fe-BPPy contains an iron-
porphyrin moiety that can serve as an excellent catalyst for CO2
reduction reactions and thus can be regarded as a suitable motif
for catalytic centers. Note also that the fourfold (D4h) symmetry
of the iron-porphyrin unit is expected to be advantageous for the
construction of a highly ordered framework structure. Second,
Fe-BPPy possesses four bulky pyrenylphenyl moieties as space
modulators. Pyrene derivatives are known to exhibit face-to-
edge (CH–π) interactions between molecules and easily form
the assembled structure.[8] The bulkiness of pyrene moieties are
expected to prevent the π–π stacking of the porphyrin moieties
and control the distance between iron centers. Third, tert-butyl
groups are introduced at the 7-positions of the pyrene moie-
ties. These groups are expected to enhance the solubility of the
molecule, which is advantageous to control the assembly/disas-
sembly process of the molecule.
The syntheses of the molecular module Fe-BPPy were per-
formed according to the procedure shown in Scheme S1 in
the Supporting Information. Initially, the Suzuki–Miyaura
coupling of 2-bromo-7-tert-butylpyrene was reacted with 4-for-
mylphenylboronic acid to afford 4-(7-(tert-butyl)pyrene-2-yl)
benzaldehyde in 67% yield. The obtained aldehyde derivative
was then reacted with pyrrole in the presence of trifluoro-
acetic acid (TFA) in CH2Cl2 at room temperature to provide a
macrocyclic structure. Subsequently, 2,3-dichloro-5,6-dicyano-
p-benzoquinone (DDQ) was added to the reaction mixture
to oxidize the porphyrinogen, and the free base porphyrin
(5,10,15,20-tetrakis(4-(7-tert-butyl)pyren-2-yl)phenyl)porphyrin,
H-BPPy) was obtained in 24% yield. The H-BPPy was charac-
1
terized by elemental analysis and H NMR spectroscopy. The
metal insertion reaction to the free base porphyrin, H-BPPy,
was performed by heating the mixture of H-BPPy and iron(II)
chloride tetrahydrate as a metal source under reflux condi-
tions in o-dichlorobenzene (o-DCB) for 1.5 h. After the reaction,
methanol was added to the reaction mixture to form a precipi-
tate. The precipitate was collected by filtration, and the targeted
compound, Fe-BPPy, was obtained with 80% yield. Fe-BPPy
was characterized by elemental analysis and UV–vis absorption
spectroscopy.
2. Results and Discussion
A key to success for obtaining the porous crystalline catalyst is to
use a molecular module that has a catalytic center moiety with
peripheral intermolecular interaction sites. Scheme 2 illustrates
the design of the molecular module developed in this study
(5,10,15,20-tetrakis(4-(7-tert-butyl)pyren-2-yl)phenyl)porphyrinato
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UV–vis absorption spectra of H-BPPy and Fe-BPPy
were measured in o-DCB, and the results are shown in
Figures S3 and S4 in the Supporting Information. The spec-
trum of the iron-porphyrin complex without pyrene moieties,
5,10,15,20-tetrakis(phenyl)porphyrinato
iron(III)
chloride
(Fe-TPP), is also displayed in Figure S5 in the Supporting
Information. In the spectrum of H-BPPy, sharp bands that are
assignable to pyrene moieties[9] were observed in the UV region
(λmax = 314, 328, and 344 nm; Table S1, Supporting Information).
In addition, a strong absorption band, the so-called Soret band,
and four weaker bands, the so-called Q-bands, were observed at
430 nm and at 521, 558, 596, and 652 nm, respectively. These
absorption bands in the visible region are characteristic of por-
phyrin derivatives.[10] The Fe-BPPy exhibited bands attributed to
the pyrene moieties at 314, 329, and 344 nm and a Soret band
associated with the porphyrin moiety at 428 nm. The position of
these bands is similar to that of H-BPPy. In contrast, Fe-BPPy
exhibited a smaller number of Q-bands (λmax = 513, 557, and
695 nm) than H-BPPy. This is due to the degeneration of the
highest occupied molecular orbital (HOMO) and the HOMO-1
upon the generation of more symmetric structures via metala-
tion.[11] This observation supports that an iron ion is inserted
into the porphyrin ring of Fe-BPPy. Notably, the positions of
the Soret band and Q-bands are almost identical between Fe-
BPPy and Fe-TPP (Soret band: λmax = 420 nm, Q-bands: λmax
=
510, 556, and 691 nm; Table S1, Supporting Information), which
suggests that the electronic structure of the porphyrin unit in
Fe-BPPy is analogous to that of Fe-TPP.
Subsequently, the electrochemical properties of the porphy-
rins were investigated. Figure 1a shows cyclic voltammograms
(CVs) of the porphyrins in 0.1 m tetra-n-butyl ammonium
perchlorate (TBAP)/o-DCB under Ar atmosphere. H-BPPy
exhibited two reversible redox waves at −1.72 and −2.00 V
(vs ferrocene/ferrocenium (Fc/Fc+); Figure 1a, black line),
and these waves are assigned to the reduction processes of
the porphyrin moiety.[12] In the CV of Fe-BPPy, three revers-
ible redox waves were observed at −0.83, −1.58, and −2.17 V
(Figure 1a, red line; Table S2, Supporting Information), and
these peaks are putatively assigned to Fe(III)/Fe(II), Fe(II)/
Fe(I), and Fe(I)/Fe(0) redox couples according to a previous
report.[13] For the CV of Fe-TPP, three redox peaks assign-
able to the reduction processes of the iron ion were observed
at −0.84, −1.61, and −2.23 V (Figure 1a, blue; Table S2, Sup-
porting Information), and the redox potentials of Fe-TPP are
quite similar to those of Fe-BPPy. It should be also noted that
the redox potentials Fe-TPP in 0.1 m TBAP/o-DCB are almost
identical to the reported values.[6c] The results demonstrate
that the effect of pyrene moieties on the electronic structure
of the porphyrin moiety is almost negligible and is consistent
with the results of UV–vis absorption spectroscopy (vide
supra). We also performed electrochemical measurements of
Fe-BPPy under a CO2 atmosphere to investigate its electrocat-
alytic activity (Figure 1b). The CVs were measured in a 0.1 m
TBAP/N-methylpyrrolidone (NMP) solution. In the measure-
ments under CO2, NMP was used as a solvent instead of o-DCB
because of the low solubility of proton source, H2O,[14] which is
essential for the catalysis,[4c] in o-DCB. Note that N,N-dimethyl-
formamide (DMF), a common solvent for iron-porphyrin
systems, could not be used because of the low solubility of
Figure 1. a) CVs of H-BPPy, Fe-BPPy, and Fe-TPP in 0.1
m
TBAP/o-DCB under Ar atmosphere (WE: GC; CE: Pt wire; RE:
Ag/Ag+; scan rate: 100 mV s−1). b) CVs of Fe-BPPy (0.3 × 10−3 m)
in a 0.1 m TBAP/NMP solution under Ar atmosphere (blue line)
and CO2 atmosphere with 1 m TFE (red line) (WE: GC; CE: Pt wire;
RE: Ag/Ag+; scan rate: 20 mV s−1). c) CVs of [Fe-BPPy]cryst in 0.5 m
NaHCO3 aqueous solution under Ar atmosphere (black line) and CO2
atmosphere (red line) (WE: carbon cloth with predeposited crys-
tals of [Fe-BPPy]cryst; CE: Pt wire; RE: Ag/AgCl (sat. KCl); scan rate:
20 mV s−1).
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Fe-BPPy in DMF. In the CVs of Fe-BPPy under Ar atmosphere,
three reversible reduction peaks, which are assigned to the
Fe(III)/Fe(II), Fe(II)/Fe(I), and Fe(I)/Fe(0) redox couples, were
observed and are similar to the results obtained in a 0.1 m
TBAP/o-DCB solution. When the measurement was performed
under a CO2 atmosphere with added trifluoroethanol (TFE)
as a proton source, a large increase in the irreversible current
was observed at the potential close to the third redox wave.
The results indicate that Fe-BPPy can catalyze electrochemical
CO2 reduction. The similar experiments were also performed
for Fe-TPP, and a catalytic current at similar potential and with
similar intensity was observed (Figure S6, Supporting Informa-
tion). This observation implies that the electrocatalytic activity
two iron porphyrin complexes, Fe-BPPy and Fe-TPP, are almost
identical in homogeneous state.
To further confirm the origin of the irreversible current, con-
trolled potential electrolysis (CPE) of Fe-BPPy was conducted. A
0.1 m TBAP/NMP solution containing 0.3 × 10−3 m Fe-BPPy and
1 m TFE was electrolyzed at −2.14 V (vs Fc/Fc+) under a CO2
atmosphere for 1 h (for details of the CPE experiment; see also
Pages S20 and S21 in the Supporting Information). After elec-
trolysis, both gas and liquid phases were analyzed by gas chro-
matography and liquid phase chromatography, respectively, and
CO as a major product and a small amount of formic acid as
a minor product were detected (Table S3, Supporting Informa-
tion). The formation of dihydrogen (H2) was hardly detected.
The faradaic efficiency (F.E.) for the conversion of CO2 to CO
was 65.9%. Note that the formation of CO was not detected in
the absence of Fe-BPPy. These results clearly indicate that the
irreversible current corresponds to electrocatalytic CO2 reduc-
tion by Fe-BPPy. We also performed CPE of Fe-TPP under
identical conditions, and the formation of CO with similar
faradaic efficiency was confirmed (Table S3, Supporting Infor-
mation). In both cases, the formation of HCOOH as a minor
product was confirmed, although no other products, including
H2 and highly reduced products of CO2 (i.e., HCHO, CH3OH,
and CH4), were detected. We also determined the TOF values
of CO production for Fe-BPPy and Fe-TPP based on the results
of the CPE experiments (see Pages S17–S19 in the Supporting
Information for details), and the TOF values were determined
to be 369 (Fe-BPPy) and 576 s−1 (Fe-TPP). As these values are
comparable, it is suggested that catalytic activity of Fe-BPPy and
Fe-TPP are similar in homogeneous state. Collectively, the elec-
tronic structure and the catalytic activity of Fe-BPPy and Fe-TPP
can be regarded as almost identical in homogeneous solution
states.
this structure, the 1D column stabilized by head-to-head CH–π
interactions between pyrenyl moieties was observed along the
a axis. In this structure, the distances between iron centers
are relatively short (5.92 and 6.21 Å; see Figure 2b for details),
which is expected to serve as adjacent active centers suitable to
activate CO2 molecules.[15] As previously reported by Naruta
and co-workers, by using the two iron centers with appropriate
distance, the holding of CO2 molecules can be facilitated.[15]
Furthermore, such iron centers can provide a local push–pull
mechanism where one iron ion acts as a Lewis base to push
an electron pair to the coordinated CO2 molecule and another

iron ion acts as a Lewis acid to promote C O bond cleavage
leading to the formation of CO.[15] These 1D columns were fur-
ther assembled via head-to-tail CH–π interactions (Figure 2c),
which resulted in the formation of a porous framework structure
([Fe-BPPy]cryst; Figure 2d). In this structure, t-butyl groups assist
the pyrene moieties to interact with each other by filling the
space close to the porphyrin ring. The size of the pore entrance in
[Fe-BPPy]cryst was estimated to be 9.60 × 10.39 Å2 by considering
the van der Waals radii of the constituent atoms. The pores in
[Fe-BPPy]cryst are surrounded by hydrophobic aromatic rings
of pyrenylphenyl moieties and are large enough to accumu-
late small gaseous molecules. Therefore, it is expected that the
pores in this porous crystalline solid can function as space to
accumulate CO2 molecules. We also examined the self-assembly
of Fe-TPP and determined its crystal structure ([Fe-TPP]cryst).
The crystallographic data for [Fe-TPP]cryst are summarized in
Figure S12 and Table S7 in the Supporting Information, and the
packing structures are shown in Figure S13 in the Supporting
Information. Unlike [Fe-BPPy]cryst, [Fe-TPP]cryst has a closely
packed structure without adjacent catalytic centers and pores
due to the small peripheral substituents installed at the meso-
positions of the porphyrin ring. To further clarify the porosity of
[Fe-BPPy]cryst and [Fe-TPP]cryst, gas adsorption studies were then
conducted. [Fe-BPPy]cryst shows uptake for gases, N2 and CO2 at
77 and 195 K, respectively (Figure S14, Supporting Information),
indicating its porous nature and ability to accumulate CO2 gas
into its pores. In contrast, [Fe-TPP]cryst exhibited negligible uptake
for both N2 and CO2 (Figure S14, Supporting Information)
because of its nonporous structure. These results clearly demon-
strate that noncovalent intermolecular interactions between bulky
spacers of Fe-BPPy provide the framework structure, [Fe-TPP]cryst
,
which has advantageous features for CO2 reduction aqueous
media; 1) adjacent catalytic centers suitable for the activation of
CO2 molecules, and 2) hydrophobic pores suitable for the accu-
mulation of CO2 molecules.
Encouraged by the aforementioned results, we then explored
the development of porous crystalline solids that serve as electro-
catalysts for CO2 reduction via the self-assembly of Fe-BPPy. To
an o-DCB solution of Fe-BPPy, acetonitrile was slowly diffused
by the liquid diffusion method, and crystalline materials suit-
able for single-crystal X-ray structural analysis were obtained. An
Oak Ridge Thermal Ellipsoid Program (ORTEP) drawing and a
summary of the crystallographic data of the obtained crystal are
shown in Figure 2a and Table S6 in the Supporting Informa-
tion, respectively. The crystal contains Fe-BPPy, which has an
iron-porphyrin center with four peripheral pyrenylphenyl moi-
eties at the meso-positions of the porphyrin ring. The packing
structure of the obtained crystal is shown in Figure 2a–d. In
The catalytic activity of this porous crystalline solid,
[Fe-BPPy]cryst, for CO2 reduction under electrochemical con-
ditions in aqueous media was then investigated. The CVs of
[Fe-BPPy]cryst are shown in Figure 1c. The measurements were
performed using the carbon cloth with predeposited crystals of
[Fe-BPPy]cryst as a working electrode (for the details of the prep-
aration of the electrode; see Page S30 in the Supporting Infor-
mation). As a reference electrode, Ag/AgCl was used instead
of Ag/Ag+ because Ag/AgCl is a more suitable reference elec-
trode for electrochemical measurements in aqueous media. In
the CV under Ar atmosphere in 0.5 m NaHCO3 aqueous solu-
tion, a reversible redox peak was observed at −0.35 V (vs Ag/
AgCl; Figure 1c, black line; see Page S31 in the Supporting
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Figure 2. a) An ORTEP drawing of Fe-BPPy (50% probability ellipsoids). Hydrogen atoms and disordered atoms are omitted for clarity. C = gray,
N = blue, Cl = green, and Fe = orange. b) 1D columnar structure observed in [Fe-BPPy]cryst. c) Intercolumnar interactions observed in [Fe-BPPy]cryst
.
d) Packing structure of [Fe-BPPy]cryst along the a axis.
Information for the details of interpretation of the CV).
Under a CO2 atmosphere, an increase in the irreversible cur-
rent was observed at ≈−0.95 V (Figure 1c, red line), indicating
the electrocatalytic activity of [Fe-BPPy]cryst in aqueous media.
To quantify the product of the electrocatalysis, the CPE of
[Fe-BPPy]cryst was performed. Similar to the cyclic voltammetry
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measurements, a carbon cloth with a predeposited complex
was used as the working electrode, and electrolysis was con-
ducted under CO2 at −1.2 V (vs Ag/AgCl) for 1 h in 0.5 m
NaHCO3 aqueous solution (for details of the CPE experiment;
see Pages S33 and S34 in the Supporting Information). After
electrolysis, both the gas and liquid phases were analyzed. As a
result, CO was a major product, and small amounts of formic
acid and H2 were detected as minor products (Figure S18 and
Table S8, Supporting Information). Note that the formation
of other products was not detected, although total faradaic
efficiency for the production of CO, formic acid, and H2 was
75.9%. The selectivity for CO production reached 89%, even in
aqueous media. The F.E. for the conversion of CO2 to CO was
determined to be 67.5%, and the turnover number (TON) was
determined to be 14.5. In addition, to ascertain that CO origi-
nated from CO2 reduction, isotope labeling experiments were
performed. We carried out CPE of [Fe-BPPy]cryst both in a) a
NaH13CO3 aqueous solution under a 13CO2 atmosphere and in
b) a NaH12CO3 aqueous solution under a 12CO2 atmosphere.
Note that the use of same isotope C both for CO2 and NaHCO3
is essential because there exists equilibrium between CO2 and
structure, as evidenced by single-crystal X-ray structural
analysis and gas adsorption experiments (vide supra). In this
experiment, Fe-TPP ([Fe-TPP]cryst) was deposited on a carbon
cloth, and CPE in aqueous media under a CO2 atmosphere was
performed. It should be also noted that the surface-active con-
centrations were almost identical between [Fe-BPPy]cryst and
[Fe-TPP]cryst as evidenced by cyclic voltammetry (Figure S16,
Supporting Information). After 1 h of electrolysis at −1.2 V
(vs Ag/AgCl) in 0.5 m NaHCO3 aqueous solution, both gas-
eous and liquid products were quantified. As a result, the for-
mation of CO, formic acid, and H2 was confirmed. The F.E.
value for the conversion of CO2 to CO, TON, and selectivity
for CO production were determined to be 40.9%, 5.0, and 79%,
respectively (Table S6, Supporting Information). These values
are substantially lower than those of ([Fe-BPPy]cryst (F.E.: 67.5%,
TON: 14.5, and selectivity: 89%), which clearly demonstrates
that [Fe-BPPy]cryst has superior activity compared to [Fe-TPP]cryst
for electrocatalytic CO2 reduction in aqueous media. Similar to
[Fe-BPPy]cryst, the formation of products other than CO, formic
acid, H2 was not confirmed. As evidenced by UV–vis absorp-
tion spectroscopy and electrochemical measurements, the elec-
tronic structures of Fe-BPPy and Fe-TPP are almost identical
(vide supra). Note also that the catalytic activities of these iron
porphyrins are quite similar under homogeneous conditions
(vide supra). In contrast, the results of the CPE experiments
in a heterogeneous system revealed that the electrocatalytic
performance of [Fe-BPPy]cryst is much higher than that of
−
bicarbonate anion (HCO3 ). Under condition (a), 94% of 13CO
was produced (Figure 3a), as determined by mass spectrometry,
whereas near-quantitative formation of 12CO was confirmed
under condition (b) (Figure 3b). These results clearly indi-
cate that CO originated from CO2 in the catalysis mediated by
[Fe-BPPy]cryst. The stability of [Fe-BPPy]cryst during the elec-
trolysis was explored by scanning electron microscopy (SEM)
measurements. Figures S20 and S21 in the Supporting Infor-
mation show the SEM images of the crystals deposited on the
carbon cloth before and after the electrolysis. The morphology
of the crystals was maintained even after the electrolysis, sug-
gesting the high stability of the system.
[Fe-TPP]cryst
.
To further explore the effect of structure of [Fe-BPPy]cryst
on catalytic activity, cyclic voltammetry of [Fe-BPPy]cryst in
D2O was also performed. As shown in Figure S22 in the Sup-
porting Information, the catalytic current was largely sup-
pressed when measurement was performed in D2O, indicating
that the rate-limiting step of the reaction involves the transfer
of proton. It should be noted that kinetic isotope effect (KIE)
could not precisely be determined because electrolyte, NaHCO3
is not deuterated. However, we have roughly estimated the KIE
To clarify the effect of the porous crystalline structure of
[Fe-BPPy]cryst, we also investigated the catalytic activity of
Fe-TPP in the heterogeneous state. Note that the crystal-
line state of Fe-TPP ([Fe-TPP]cryst) did not exhibit a porous
Figure 3. Mass spectra of CO generated after the CPE of [Fe-BPPy]cryst a) under 13CO2 using a NaH13CO3 aqueous solution and b) under 12CO2 using
a NaH12CO3 aqueous solution.
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3. Conclusion
A novel highly symmetric iron-porphyrin complex with
pyrene-based peripheral noncovalent interaction sites
(Fe-BPPy) has been newly designed and synthesized. Fe-BPPy
was easily self-assembled at room temperature via nonco-
valent CH–π interactions between pyrenyl moieties, and a
porous crystalline framework, [Fe-BPPy]cryst, formed. This
[Fe-BPPy]cryst exhibited the following characteristic features:
a) adjacent catalytically active sites and b) pores surrounded
by hydrophobic aromatic rings. As [Fe-BPPy]cryst can be dis-
assembled to Fe-BPPy in organic media, the catalytic activity
of [Fe-BPPy]cryst and Fe-BPPy was investigated under electro-
chemical conditions and was compared to that of the relevant
iron-porphyrin complex ([Fe-TPP]cryst and Fe-TPP). The elec-
trochemical studies of Fe-BPPy and Fe-TPP in organic solvent
revealed that the electronic structures and catalytic activities
of these porphyrins are almost identical under homogeneous
conditions. In contrast, the investigation of the catalytic
activity of [Fe-BPPy]cryst and [Fe-TPP]cryst in aqueous media
demonstrated that [Fe-BPPy]cryst can catalyze electrochemical
CO2 reduction with higher performance than [Fe-TPP]cryst in
terms of F.E., TON, and selectivity for CO production. Given
that the catalytic activities of Fe-BPPy and Fe-TPP in organic
solvents (i.e., homogeneous conditions) are quite similar,
the differences in catalytic activity observed in [Fe-BPPy]cryst
and [Fe-TPP]cryst are concluded to arise from their difference
in the packing structure. In other words, the porous crystal-
line nature of [Fe-BPPy]cryst with adjacent catalytically active
sites is expected to be the origin of its superior activity com-
pared to that of [Fe-TPP]cryst. Collectively, the introduction of a
space modulator around a catalytic center is an advantageous
approach for developing porous crystalline catalysts for small-
molecule conversions.
Scheme 3. Schematic illustration to summarize the features of
[Fe-BPPy]cryst and [Fe-TPP]cryst
.
value assuming that the contribution of proton from NaHCO3
is negligible, and the value was estimated to be 95 (for details
of the determination of KIE value, see Pages S38 and S39 in
the Supporting Information). This large KIE value strongly
indicate that the rate-determining step involves the transfer of
proton. This observation suggests that hydrophobic cavity effi-
ciently accumulate CO2 and the adjacent metal centers facilitate
the coordination of CO2 to the metal centers, and thus these
processes are accelerated not to be the rate-determining step.
Additionally, we have analyzed the electrochemical proper-
ties of [Fe-BPPy]cryst and [Fe-TPP]cryst by Tafel plots. In general,
if a Tafel slope is close to 59 mV dec−1, fast electron transfer
to CO2 occurs and the following chemical process should be
a rate-determining step, while one electron reduction of CO2
should be a rate-determining step when a Tafel slope is close
to 118 mV dec−1.[16] In our experiment, [Fe-BPPy]cryst exhib-
ited a Tafel slope relatively close to 59 mV dec−1, whereas the
Tafel slope of [Fe-TPP]cryst were close to 118 mV dec−1 (for
details, see Page S40 in the Supporting Information). We
also performed the electrochemical impedance spectroscopy
(EIS) measurements of [Fe-BPPy]cryst and [Fe-TPP]cryst under
[CCDC 2033419 and 2033431 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.]
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
CO2 atmosphere. From the results of EIS measurements, it Acknowledgements
was revealed that [Fe-BPPy]cryst has smaller charge transfer
This work was supported by KAKENHI (19H02734 (R.M.), 17H06444,
resistance compared to [Fe-TPP]cryst, indicating that elec-
tron transfer process more easily occur in [Fe-BPPy]cryst than
[Fe-TPP]cryst (for details, see Pages S41 and S42 in the Sup-
porting Information). The results of Tafel plot and EIS measure-
ments indicate that the [Fe-BPPy]cryst and [Fe-TPP]cryst undergo
different CO2 reduction mechanism; rate-determining step of
CO2 reduction mediated by [Fe-BPPy]cryst is a chemical process,
whereas that of [Fe-TPP]cryst involves electron transfer. These
facts clarify the importance of porous crystalline structures with
adjacent catalytic centers and hydrophobic pores for achieving
electrocatalytic CO2 reduction in aqueous media (Scheme 3).
19H00903, and 20K21209 (S.M.), and 15H05480, 17K19185, 17H05391,
19H04602, and 20H02754 (M.K.)) from the Japan Society for the
Promotion of Science. This work was also supported by JST PRESTO
(Grant Number JPMJPR20A4 (M.K.)), JST CREST (Grant Number
JPMJCR20B6 (S.M.)), Research Grants in the Natural Sciences from the
Mitsubishi Foundation, and the Tokuyama Science Foundation.
Conflict of Interest
The authors declare no conflict of interest.
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Data Availability Statement
Research data are not shared.
Keywords
CO2 reduction, electrocatalysis, iron porphyrin, porous crystalline solid,
self-assembly
Received: October 2, 2020
Revised: February 5, 2021
Published online:
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