Synthesis of cobalt A2B triaryl corroles bearing aldehyde and amide pyridyl groups and their performance in electrocatalytic hydrogen evolution

Article abstract of DOI:10.1039/d0nj04953k

Four A₂B type cobalt triaryl corrole complexes (1-4) bearing aldehyde and pyridyl substituents at the 10-meso-phenyl group with different spatial configurations have been synthesized and thoroughly characterized using high-resolution mass spectrometry, nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy, and single-crystal X-ray diffractometry. These complexes all exhibited good activity in the electrocatalytic hydrogen evolution reaction (HER) when acetic acid, trifluoroacetic acid (TFA) orp-toluenesulfonic acid (TsOH) was used as the proton source. The Tafel catalytic plot revealed the intrinsic characteristics of the catalytic activity. The HER followed the EECEC pathway when acetic acid was used as a proton source, while it proceeded through the EECC or EECEC pathway when using TFA or TsOH as the proton source, depending on the acid concentration. The presence of Co^III-H was detected by¹H NMR, which provided evidence for the catalytic mechanism. The amide pyridyl may function as the proton relay group since themeta-substituted cobalt corroles3and4exhibited significantly higher turnover frequency (TOF)_maxvalues than corroles1and2in an organic medium.

Full text of DOI:10.1039/d0nj04953k

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Synthesis of cobalt A2B triaryl corroles bearing
aldehyde and amide pyridyl groups and their
performance in electrocatalytic hydrogen
Cite this: New J. Chem., 2021,
45, 5127
evolution†
a
Jun-Jia Fang,a Jian Lan,a Gang Yang,a Gao-Qing Yuan,
Hai-Yang Liu *a and
Li-Ping Si*b
Four A2B type cobalt triaryl corrole complexes (1–4) bearing aldehyde and pyridyl substituents at the
10-meso-phenyl group with different spatial configurations have been synthesized and thoroughly
characterized using high-resolution mass spectrometry, nuclear magnetic resonance spectroscopy
(NMR), X-ray photoelectron spectroscopy, and single-crystal X-ray diffractometry. These complexes all
exhibited good activity in the electrocatalytic hydrogen evolution reaction (HER) when acetic acid,
trifluoroacetic acid (TFA) or p-toluenesulfonic acid (TsOH) was used as the proton source. The Tafel
catalytic plot revealed the intrinsic characteristics of the catalytic activity. The HER followed the EECEC
pathway when acetic acid was used as a proton source, while it proceeded through the EECC or EECEC
pathway when using TFA or TsOH as the proton source, depending on the acid concentration. The
presence of CoIII–H was detected by 1H NMR, which provided evidence for the catalytic mechanism. The
amide pyridyl may function as the proton relay group since the meta-substituted cobalt corroles 3 and 4
exhibited significantly higher turnover frequency (TOF)max values than corroles 1 and 2 in an organic medium.
Received 8th October 2020,
Accepted 12th February 2021
DOI: 10.1039/d0nj04953k
rsc.li/njc
cobalt6 and nickel7 complexes with pendant amine groups
exhibit more effective HER catalytic activity. The pendant
Introduction
Due to the ever-increasing consumption of fossil fuels, the amines may accelerate proton transfer between the solution
global need for sustainable and environmentally friendly and the active metal site by minimizing the energy barrier of
energy has become an enormously concerning topic.1 Hydrogen, proton transfer, which is helpful for the HER.8
the cleanest fuel, can be obtained by catalytic water splitting.2
It is well known that metal porphyrin complexes have good
Platinum is one of the most efficient catalysts for the hydrogen activity in electrocatalytic hydrogen evolution.9–11 Corrole is an
evolution reaction (HER). However, its application is seriously analogue of porphyrin and can stabilize metal centers with
limited due to its earth-scarcity.3 Therefore earth-abundant higher oxidation states. As early as 2013, Gross used cobalt tris-
metals such as cobalt, iron and nickel are in great demand for pentafluorophenyl corrole as a HER electrochemical catalyst.12
molecular HER catalysts.4
It was found that electro-withdrawing substituents were
In nature, the reduction of protons can be efficiently beneficial for the HER. The more electron-deficient fluorinated
achieved by hydrogenase, which is typically found in anaerobic corrole Co–F8 was found to have better activity than chlorinated
organisms and can convert the protons of water into hydrogen. or brominated cobalt corroles.13 Cobalt corroles with phosphonic
The amine group positioned near the [Fe–Fe] hydrogenase iron and carboxyl acid pendants also showed improved activity
center has been recognized to facilitate the formation of because of the proton-donating properties of the attached
hydrogen via proton relay.5 It has been demonstrated that groups.14 Although cobalt corrole has demonstrated promising
electrocatalytic HER activity,15–17 the question of how to design a
a Department of Chemistry, The Key Laboratory of Fuel Cell Technology of
Guangdong Province, South China University of Technology, Guangzhou 510640,
P. R. China. E-mail: chhyliu@scut.edu.cn
molecular structure to facilitate efficient HER is still not fully
resolved. Previously, we have tested the electrocatalytic HER
activity of cobalt tris-ethoxycarbonyl corrole18 and triaryl corroles
with pendent pentafluorophenyl and amino-,19 nitro-,20 cyano-,
and 4-N,N0-dimethyl-substituted21 phenyl groups in aqueous
solution, and found that electron-withdrawing corroles have
higher turnover frequencies (TOFs). In addition to controlling
b School of Materials Science and Energy Engineering, Foshan University,
Foshan 528000, P. R. China
† Electronic supplementary information (ESI) available. CCDC 1975880, 1975881,
1951203 and 1975879. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0nj04953k
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a 250 mL dry round-bottom flask along with trifluoroacetic acid
(60 mL). The solution was then stirred at room temperature for
an hour, after which triethylamine (120 mL) was added to
neutralize the reaction. The excess pyrrole was removed
through vacuum distillation, the product was purified using a
column (dichloromethane (DCM)/hexane (HEX) 1/1) and dipyrro-
methane (DPM) was obtained as a yellow powdered solid
(3.6 g, 80.7%). DPM (1.56 g, 5 mmol) and terephthalaldehyde
(0.34 g, 2.5 mmol) were dissolved in methanol (250 mL), and
then hydrochloric acid (250 mL, 0.6 mol Lꢀ1) was added. After
being stirred for 1 h, the solution was extracted with DCM.
The organic phase was washed with water several times and
then dried with Na2SO4. Subsequently, 2,3-dicyano-5,6-
dichlorobenzoquinone (DDQ) (1.475 g, 6.5 mmol) was added
and the solution was stirred for another 20 min. The solvent
was then evaporated and the residue was purified using a silica
column (DCM/HEX 2/1). The solid was recrystallized by DCM/
HEX to obtain the pure violet freebase corrole (0.25 g, 13.9%).
MS: (ESI-HRMS) m/z 735.1321 [M + H]+, calculated: m/z
735.1251 [M + H]+. 1H NMR (CDCl3, 400 MHz, d, ppm):
d 10.34 (s, 1H), 9.14 (d, J = 12 Hz, 2H), 8.74 (d, J = 4 Hz, 2H),
8.66 (d, J = 4 Hz, 2H), 8.60 (d, J = 4 Hz, 2H), 8.38 (d, J = 8 Hz, 2H),
8.27 (d, J = 8 Hz, 2H). 19F NMR (CDCl3, 376 MHz, d, ppm):
d ꢀ137.82 (4F), ꢀ152.50 (2F), ꢀ161.58 (4F). 13C NMR (CDCl3,
101 MHz, d, ppm): d 192.33, 147.83, 147.30, 144.87, 142.28,
139.06, 136.65, 135.37, 126.06, 121.58, 117.65, 113.86, 111.85.
Fig. 1 Molecular structures of the prepared cobalt corrole complexes (1–4).
the push–pull electron density of cobalt corroles, an alternative
molecular design idea involves the spatial configuration of the
proton relay groups. Bearing this in mind, we herein wish to
report the synthesis and electrocatalytic HER activity of a new type
of A2B cobalt corroles containing pentafluorophenyl and amide
pyridyl groups with different spatial arrangements (Fig. 1). When
various acids are used as the proton source, corroles 3 and 4 have
higher TOFmax values than corroles 1 and 2 in pure DMF solution,
indicating improved catalytic activity after changing the spatial
structures.
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-
(3-formylphenyl)corrole (3-BPFC)
The synthetic route was similar to that of 4-BPFC. MS: (ESI-HRMS)
m/z 735.1249 [M + H]+, calculated: m/z 735.1237 [M + H]+. 1H NMR
(CDCl3, 500 MHz, d, ppm): d 10.27 (s, 1H), 9.14 (d, J = 4 Hz, 2H),
8.74 (d, J = 4 Hz, 2H), 8.66 (s, 1H), 8.61 (m, 4H), 8.46 (d, J = 8 Hz,
1H), 8.30 (d, J = 8 Hz, 1H), 7.97 (t, J = 8 Hz, 1H). 19F NMR (CDCl3,
Experimental
Materials and methods
All reagents were purchased from commercial suppliers and 471 MHz, d, ppm): d ꢀ137.72 (4F), ꢀ152.58 (2F), ꢀ161.52 (4F). 13
C
were used without further purification unless otherwise NMR (CDCl3, 126 MHz, d, ppm): d 192.47, 147.11, 145.10, 142.44,
pointed out. UV-Visible spectra were recorded using a Hitachi 140.05, 138.93, 136.99, 135.50, 135.33, 132.73, 128.80, 128.10,
U-3010 spectrophotometer at room temperature. Nuclear 127.41, 125.97, 117.66, 113.93, 111.56.
magnetic resonance (NMR) spectra were obtained using a Bruker
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-(4-Schiff-phenyl)
Avance III 400 MHz or 500 MHz using CDCl3, DMSO-d6 or DMF-d7
corrole (4-BPSC)
as the solvent. High-resolution mass spectra (HRMS) were mea-
sured using a Bruker Maxis impact mass spectrometer with an 4-BPSC (50 mg, 68 mmol) and isonicotinohydrazide (47 mg,
electrospray ionization (ESI) source. Cyclic voltammograms (CVs) 340 mmol) were dissolved in methanol (20 mL) and heated to
were performed using a three-electrode system including a Pt wire reflux for 24 h. TLC was used to monitor the reaction process,
as the counter electrode, glassy carbon as the working electrode, and then the solvent was evaporated and the residue was
and Ag/AgNO3 (in DMF) and Ag/AgCl (in acetonitrile/water) as the passed through a silica column (DCM/MeOH 100/1). The solid
reference electrodes with a CHI-660E electrochemical analyzer. was recrystallized in DCM and HEX, and a dark violet
Ferrocene was added as an internal standard in DMF. The compound precipitated (35 mg, 60.3%). MS: (ESI-HRMS) m/z
1
amount of hydrogen after electrolysis was monitored using an 854.1745 [M + H]+, calculated: m/z 854.1721 [M + H]+. H NMR
Agilent Technologies 7890A GC chromatograph.
(DMSO-d6, 500 MHz, d, ppm): d 12.32 (s, 1H), 9.22 (s, 2H), 8.97
(s, 2H), 8.85 (t, J = 5.5 Hz, 3H), 8.73 (s, 2H), 8.65 (d, J = 3 Hz, 2H),
8.29 (d, J = 6.5 Hz, 2H), 8.22 (d, J = 7 Hz, 2H), 7.95 (d, J = 5.5 Hz,
2H). 19F NMR (DMSO-d6, 471 MHz, d, ppm): d ꢀ139.57 (4F),
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-
(4-formylphenyl)corrole (4-BPFC)
The freebase corrole was synthesized according to a previous ꢀ154.78 (2F), ꢀ162.88 (4F). 13C NMR (CDCl3, 126 MHz, d, ppm):
report in the literature.22 Distilled pyrrole (160 mL, 2.30 mol) d 167.46, 162.22, 150.81, 149.97, 149.59, 147.21, 145.32, 143.99,
and pentafluorobenzaldehyde (2.8 g, 14.3 mmol) were added to 142.53, 140.64, 139.01, 137.07, 135.57, 133.71, 132.19, 131.95,
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129.15, 126.76, 123.78, 122.14, 117.63, 114.28, 65.49,
60.23, 55.32.
l
max; nm, e; Mꢀ1 cmꢀ1); 618 (1.11 ꢂ 104), 590 (8.93 ꢂ 103),
568 (7.27 ꢂ 103), 431 (4.35 ꢂ 104).
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-(3-Schiff-phenyl)
corrole (3-BPSC)
Synthesis of cobalt meso-5,15-bis(pentafluorophenyl)-10-(4-
Schiff-phenyl) corrole (3)
The synthetic method was similar to that of 4-BPSC. MS: (ESI-
HRMS) m/z 854.1725 [M + H]+, calculated: m/z 854.1721 [M + H]+.
1H NMR (DMSO-d6, 500 MHz, d, ppm): d 12.24 (s, 1H), 9.22
(s, 2H), 8.97 (s, 2H), 8.79 (d, J = 5.5 Hz, 3H), 8.74 (s, 2H), 8.61
(d, J = 33.5 Hz, 3H), 8.24 (br, 2H), 7.91 (s, 1H), 7.85 (d, J = 4 Hz,
2H). 19F NMR (DMSO-d6, 471 MHz, d, ppm): d ꢀ139.54 (4F),
ꢀ154.80 (2F), ꢀ162.86 (4F). 13C NMR (CDCl3, 126 MHz, d, ppm):
d 168.39, 162.20, 150.76, 140.71, 139.03, 136.58, 133.70, 128.40,
123.66, 117.71, 114.42, 112.05, 94.49, 72.68, 71.02, 60.71, 60.23,
56.55, 44.09.
MS: (ESI-HRMS) m/z 1172.1765 [M + H]+, calculated: m/z
1172.1760 [M + H]+. 1H NMR (CDCl3, 400 MHz): 1H NMR
(CDCl3, 400 MHz) d: 8.71 (s, 4H), 8.00 (br, 7H), 7.65 (m, 2H),
7.55 (m, 1H), 7.45 (m, 2H), 7.05 (br, 3H), 6.69 (br, 6H), 4.61
(br, 6H), 3.76 (m, 2H). 19F NMR (CDCl3, 376 MHz, d, ppm):
d ꢀ137.82 (4F), ꢀ152.50 (2F), ꢀ161.58 (4F). 13C NMR (CDCl3,
101 MHz, d, ppm): d 132.19, 131.62, 129.66, 127.30, 123.58,
121.01, 90.00, 31.93, 31.51, 30.11, 29.67, 29.34, 29.15, 27.18,
25.53, 24.86, 22.68, 14.09. 31P NMR (CDCl3, 162 MHz, d, ppm):
d 26.77 (s, 1P). UV-Vis (N,N-dimethylformamide; c; 3.2 ꢂ 10ꢀ5 M,
l
max; nm, e; Mꢀ1 cmꢀ1); 615 (7.69 ꢂ 103), 587 (6.43 ꢂ 103),
Synthesis of cobalt meso-5,15-bis(pentafluorophenyl)-10-(4-
formylphenyl)corrole (1)
422 (3.06 ꢂ 104).
The cobalt corrole complexes were synthesized according to a
previous report in the literature.23 A solution of 4-BPFC
(100 mg, 136 mmol) and Co(OAc)2ꢁ4H2O (169 mg, 680 mmol)
in methanol (20 mL) was stirred at room temperature, and then
triphenylphosphine (178 mg, 680 mmol) was added and allowed
to react for 2 h. Next, dichloromethane and water were used to
extract the organic phase and then passed through a silica
column to obtain the pure cobalt meso-5,15-bis(pentafluoro-
phenyl)-10-(4-formylphenyl)corrole (1) (75 mg, 52.4%). The
synthetic procedure for the other cobalt complexes was similar
to that of cobalt corrole 1. MS: (ESI-HRMS) m/z 1053.1239 [M + H]+,
calculated: m/z 1053.1246 [M + H]+. 1H NMR (CDCl3, 500 MHz, d,
ppm): d 10.29 (s, 1H), 8.77 (d, J = 4.5 Hz, 2H), 8.29 (d, J = 4.5 Hz,
2H), 8.23 (t, J = 4.5 Hz, 3H), 8.15 (t, J = 10 Hz, 2H), 8.06 (d, J =
4.5 Hz, 2H), 7.60 (d, J = 8 Hz, 1H), 7.06 (t, J = 7.5 Hz, 3H), 6.71 (t, J =
7.5 Hz, 6H), 4.64 (t, J = 8 Hz, 6H). 19F NMR (CDCl3, 471 MHz,
d, ppm): d ꢀ136.86 (4F), ꢀ153.87 (2F), ꢀ161.79 (4F). 13C NMR
(CDCl3, 101 MHz, d, ppm): d 192.42, 149.04, 146.12, 144.20, 143.72,
136.10, 135.50, 132.06, 131.45, 131.36, 129.34, 127.23, 126.56,
Synthesis of cobalt meso-5,15-bis(pentafluorophenyl)-10-
(3-Schiff-phenyl) corrole (4)
MS: (ESI-HRMS) m/z 1194.1551 [M + Na]+, calculated: m/z
1194.1549 [M + Na]+. 1H NMR (CDCl3, 400 MHz): 1H NMR
(CDCl3, 400 MHz) d: 8.71 (s, 4H), 8.20 (br, 8H), 7.54 (s, 2H), 7.41
(s, 1H), 7.34 (s, 1H), 6.98 (s, 3H), 6.78 (s, 6H), 4.56 (s, 6H), 3.69
(m, 2H). 19F NMR (CDCl3, 376 MHz, d, ppm): d ꢀ137.52 (4F),
ꢀ154.01 (2F), ꢀ162.27 (4F). 13C NMR (CDCl3, 101 MHz, d, ppm):
d 162.60, 145.50, 129.70, 128.63, 127.36, 124.55, 123.50, 119.07,
36.52, 35.01, 34.47, 31.94, 31.46, 30.17, 29.72, 27.27, 24.92,
22.72, 14.15. 31P NMR (CDCl3, 162 MHz, d, ppm): 30.00
(s, 1P). UV-Vis (N,N-dimethylformamide; c; 3.2 ꢂ 10ꢀ5 M, lmax; nm,
e; Mꢀ1 cmꢀ1); 618 (7.69 ꢂ 103), 591 (6.37 ꢂ 103), 421 (2.93 ꢂ 104).
Results and discussion
Synthesis and characterization
123.75, 123.26, 122.79, 121.68, 121.08, 115.87, 105.65. 31P NMR Four cobalt corrole complexes were synthesized and character-
(CDCl3, 162 MHz, d, ppm): d 26.64 (s, 1P). UV-Vis (N,N- ized using NMR spectroscopy and HR-MS (see the ESI,† S1–S8).
dimethylformamide; c; 2.8 ꢂ 10ꢀ5 M, lmax; nm, e; Mꢀ1 cmꢀ1); 4-BPFC was synthesized through the reaction of DPM and
618 (7.74 ꢂ 103), 590 (6.16 ꢂ 103), 568 (5.76 ꢂ 103), 431 (3.38 ꢂ terephthalaldehyde in methanol and hydrochloric acid
104).
aqueous solution following the synthetic procedure reported
by Gross. Based on this synthetic route, we first synthetized
3-BPFC using DPM and isophthalaldehyde under the same
solution conditions with a yield of 34%. We also attempted to
Synthesis of cobalt meso-5,15-bis(pentafluorophenyl)-10-
(3-formylphenyl)corrole (2)
MS: (ESI-HRMS) m/z 1053.1224 [M + H]+, calculated: m/z synthesize the corrole with the aldehyde group at the ortho
1053.1246 [M + H]+. 1H NMR (CDCl3, 500 MHz, d, ppm): position of the benzene ring. However, despite trying several
d 10.21 (s, 1H), 8.80 (dd, J = 5.5 Hz, 18.5 Hz, 2H), 8.29 (m, synthetic routes, the product was not obtained. We then
6H), 8.07 (d, J = 4.5 Hz, 2H), 7.86 (m, 2H), 7.07 (m, 3H), 6.71 modified 4-BPFC and 3-BPFC through amidation coupling with
(t, J = 9.5 Hz, 6H), 4.63 (t, J = 12 Hz, 6H). 19F NMR (CDCl3, isonicotinohydrazide in methanol solution. In their 1H NMR
471 MHz, d, ppm): d ꢀ136.81 (4F), ꢀ153.91 (2F), ꢀ161.79 (4F). spectra, the peaks of the protons of the triphenylphosphine
13C NMR (CDCl3, 101 MHz, d, ppm): d 192.43, 146.13, 144.31, unit linked to the central cobalt atom moved in the up-field
144.02, 143.46, 139.03, 136.94, 136.16, 135.36, 133.05, 131.47, direction (d = 7.06, 3H; 6.71, 6H; and 4.64 ppm, 6H for corrole
129.27, 128.27, 127.14, 126.62, 123.73, 123.25, 122.86, 120.99, 1). For cobalt corroles 1 and 2, the aldehyde protons appeared
115.94, 105.59. 31P NMR (CDCl3, 162 MHz, d, ppm): d 27.04 in the downfield region of 10.21–10.29 ppm. After substitution
(s, 1P). UV-Vis (N,N-dimethylformamide; c; 2.8 ꢂ 10ꢀ5 M, with the amide pyridyl group, the characteristic proton peak
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Fig. 3 (a) XPS survey scan spectrum of corrole 1; (b) XPS Co 2p spectra
and (c) P 2p and (d) F 1s binding energy regions of corroles 1, 2, 3 and 4.
plane by 17.1921, 15.2661 and 10.4701, while for 3-BPSC, only
one pyrrole ring deviated from the corrole ring plane by
22.9051. The amide and pyridyl groups of 3-BPSC were relatively
closer to the cobalt center than those of 4-BPSC. Specific crystal
data are shown in Tables S1–S4 (ESI†). The structures of
complexes 1 and 2 and 4-BPSC and 3-BPSC have been deposited
with the Cambridge Crystallographic Data Center, and their
CCDC numbers are 1975880, 1975881, 1951203 and 1975879,
respectively.†
X-Ray photoelectron spectroscopy (XPS) revealed signals
corresponding to Co, F, and P for these cobalt corrole complexes.
The XPS survey scan of corrole 1 and the Co 2p, P 2p and F 1s
spectra of corroles 1–4 are depicted in Fig. 3. In Fig. 3(b), the peaks
of Co 2p2/3 and Co 2p1/2 are observed at approximately 780.0 and
795.1 eV, which is consistent with a CoIII oxidation state.25 The
binding energy of P 2p at 131.6 eV for corrole 1 confirmed the
presence of the PPh3 axial ligand shown in Fig. 3(c). XPS survey
scans of corroles 2, 3 and 4 are provided in Fig. S9 (ESI†).
Fig. 2 Thermal ellipsoid plots (50% probability) of the X-ray structures of
(a) corrole 1, (b) corrole 2, (c) 4-BPSC, and (d) 3-BPSC.
disappeared, demonstrating the formation of cobalt corroles 3
and 4.
We obtained the single-crystal X-ray structures of cobalt
corroles 1 and 2; unfortunately, we observed only the freebase
Electrochemical studies
corrole structures of 3 and 4 in the single crystals obtained The electrochemical properties of the cobalt corrole complexes
through the slow evaporation of hexane/dichloromethane were estimated preliminarily using cyclic voltammetry (CV) in
mixed solvent, as shown in Fig. 2.
DMF with a complex concentration of 0.83 mM, glassy carbon
The single-crystal X-ray structures of corroles 1 and 2 as the working electrode, 0.1 M TBAP as the supporting
showed that the Co ion was coordinated by four N atoms with electrolyte, Ag/AgNO3 as the reference electrode and ferrocene
a PPh3 group bonded at the axial position. The Co ion deviated as an external standard. The ferrocene redox couple is shown in
from the N4 plane by 0.3563 Å and 0.4163 Å in corroles 1 and 2, Fig. S10 (ESI†). The potentials in this work are referenced to the
respectively, due to the steric repulsion from the axial PPh3 ferrocenium/ferrocene (Fc+/Fc) couple except when specifically
ligand. The distances between the Co and P atoms were noted. The CVs of these cobalt corrole complexes are shown in
2.2052(16) Å and 2.2088(9) Å, respectively. Furthermore, the Fig. 4. Cobalt corrole complexes 1, 2, 3, and 4 showed irreversible
average distance of Co–N bond 1.874(4) Å and 1.882(2) Å for oxidation peaks at 0.28, 0.27, 0.26, and 0.29 V (Epa versus
corrole 1 and 2 were consistent with a d6 CoIII electronic ferrocene), respectively. The first reduction peaks of the cobalt
configuration.24 In the non-metallized corrole structures of corroles, which were assigned to CoIII/CoII, appeared at ꢀ0.81,
4-BPSC and 3-BPSC, the N4 plane showed less planarity than ꢀ0.83, ꢀ0.79, and ꢀ0.82 V, respectively. The changes in the
those of cobalt corroles 1 and 2. For 4-BPSC, the planes of the spatial structure and electron density led to the differences in
three pyrrole rings were tilted away from the average corrole the reduction potentials. They are all irreversible due to the
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proton reduction was investigated in DMF using acetic acid
(AcOH, pKa = 13.2 in DMF29), trifluoroacetic acid (TFA, pKa = 3.5
in DMF30) or p-toluenesulfonic acid (TsOH, pKa = 2.6 in DMF29
)
as the proton source. As shown in Fig. 5(a), in the case of
corrole 1, with the addition of acetic acid the first reduction
peak was almost identical to that of the blank, while the second
reduction couple changed obviously, which indicated that the
catalytically active species was CoI. In the presence of acetic
acid, CoI combined with a proton and formed CoIII–H species.
Note that the second reduction peak was still observed, and
that the catalytic current increased at more negative potentials,
which revealed that CoIII–H needed to be further reduced to
release hydrogen.31 Similarly, the CVs of corroles 2, 3 and 4
were measured under the same solution conditions.
To confirm the existence of CoIII–H, we used 1H NMR
spectroscopy to monitor the formation of the hydride complex
(see below). Furthermore, UV-Vis kinetic spectroscopy was
applied to study the reaction process.32 The UV-vis absorbance
changed with the addition of cobaltocene and acid, as shown in
Fig. 6. An excess (25 equivalents) of cobaltocene was added to a
0.038 mM DMF solution of the complex to reduce CoIII to CoI,
and different concentrations of acetic acid were then added. In
the UV-Vis spectrum, the absorbance decreased evidently at the
moment that acetic acid was added, which indicated the
formation of CoIII–H. The pKa value of CoIII–H can be calculated
(vide infra, kinetic study). The ic for 18 equiv. of AcOH and the ip
for 0.83 mM of the corrole complex are displayed in Fig. S12
(ESI†). The ic/ip plot is shown in Fig. S13 (ESI†). The value
linearly increased with the addition of acetic acid at first and
then reached a plateau. This might be attributed to the con-
sumption of the catalyst at high acid concentration. Based on
experimental data and previous references,31,33 we suggested a
catalytic mechanism for these four complexes when acetic acid
was used as the proton source as shown in Scheme 1. (I–II–III–
V–VI or EECEC pathway; E: electron, C: charge.)
Fig. 4 CVs obtained for complexes 1, 2, 3 and 4 (0.83 mM) in DMF
containing 0.1 M TBAP as the electrolyte with GC as the working electrode
and ferrocene as a reference electrode.
dissociation of the –PPh3 ligand.26 The second reversible redox
couples, which were attributed to CoII/CoI, were observed at
ꢀ1.96, ꢀ1.96, ꢀ1.98, and ꢀ1.98 V, respectively. All electro-
chemical values are summarized in Table 1.
The redox peak current of these corrole complexes had a
linear relationship with v1/2 from 100 to 400 mV sꢀ1, as shown
in Fig. S11 (ESI†), which is consistent with the diffusion-limited
electron transfer process.27 The diffusion coefficient D for the
second reduction peak was calculated to be 1.35 ꢂ 10ꢀ5, 1.41 ꢂ
10ꢀ5, 1.55 ꢂ 10ꢀ5 and 8.84 ꢂ 10ꢀ6 cm2 sꢀ1, respectively. The
D values were determined using the Randle–Sevcik eqn (1):28
ꢀ
ꢁ
1=2
npFvD
RT
ip ¼ 0:4463npFACcat
(1)
where ip is the peak current, np is the electron stoichiometry, F
is the Faraday constant, v is the sweep rate, R is the ideal gas
constant, and T is the temperature. The diffusion coefficients
in this work were nearly the same as those of cobalt corrole
complexes reported by our group.21
Proton reduction of acetic acid in DMF
To examine the differences in their catalytic capability and
mechanisms, the performance of the complexes in catalyzing
Table 1 Redox potentials of 1–4 from CV measurements
E1/2 (V)
E1/2 (V)
E1/2 (V)
No.
Ox I
Red I
Red II
1
2
3
4
0.23
0.23
0.15
0.16
ꢀ0.63
ꢀ0.64
ꢀ0.64
ꢀ0.66
ꢀ1.92
ꢀ1.93
ꢀ1.95
ꢀ1.95
Fig. 5 CVs of corrole complexes (a) 1, (b) 2, (c) 3 and (d) 4 (0.83 mM) with
the addition of different amounts (0, 2, 4, 10, 18 equiv.) of acetic acid.
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Fig. 6 UV-Vis absorbance of the band at 566 nm after the addition of cobaltocene to corrole 1 in DMF under a nitrogen atmosphere, followed by the
addition of (a) 25 equiv., (b) 37.5 equiv. or (c) 50 equiv. acetic acid.
Scheme 1 Possible catalytic cycles of electrocatalytic hydrogen evolu-
tion by the cobalt corrole complexes.
Fig. 7 CVs of complexes of corrole (a) 1, (b) 2, (c) 3 and (d) 4 (0.83 mM) in
the presence of different amounts (0, 2, 4, 6, 10, 14, 18, 22 equiv.) of TFA.
Proton reduction of trifluoroacetic acid (TFA) in DMF
groups may act as active centers for the HER. A similar
phenomenon has also been observed for a catalyst bearing a
pendent amine group.5 The values of ic/ip are presented in
Fig. S14 and S15 (ESI†).
We also examined the catalytic proton reduction performance
of the complexes using TFA as the proton source. As shown in
Fig. 7, for corroles 1 and 2, with the addition of TFA, the second
reduction peaks became irreversible and the onset potential
moved towards the anodic position by approximately 300 and
200 mV. The degree of movement was much greater than that
observed for acetic acid. At a TFA concentration of 4.98 mM,
two plateaus appeared (magenta line), while at a high concentration
of 18.26 mM, the two plateaus merged into one. That indicated
that two catalytic mechanisms were involved in the proton
reduction process.31 As can be seen from Fig. 7(a), at the first
plateau at ꢀ1.98 V, CoIII–H could combine with one proton and
release hydrogen (I–II–III–IV or EECC), and the second plateau
represented further reduction (I–II–III–V–VI or EECEC). The
catalytic mechanisms were the same for the other three corrole
complexes. It is noteworthy that cobalt corroles 3 and 4
exhibited two additional waves between ꢀ1.3 V and ꢀ1.7 V in
the presence of TFA and TsOH. These two peaks might be
assigned to the reduction of the protonated pyridyl amide
group in the stronger acid. In this case, the pyridyl and amide
Proton reduction of TsOH in DMF
Another strong acid, p-toluenesulfonic acid (TsOH), was also
used as the proton source. As illustrated in Fig. S16(c) (ESI†),
after 2 equiv. of acid were added, the first reduction at ꢀ0.97 V
was almost unchanged, but the second reduction wave became
irreversible, implying a proton reduction process. Two peaks
appeared at approximately ꢀ2.2 V, corresponding to further
reduction after the formation of CoIII–H. Similarly to in the case
of TFA, the further reduction waves merged into a single peak
at ꢀ2.11 V at higher acid concentration. Hence, two catalytic
pathways proceeded simultaneously. Corroles 1, 2 and 4 had
similar catalytic behavior to corrole 3. The cobalt-centered and
ligand-centered proton reduction processes occurred together.
Note that the cobalt-centered catalytic mechanism was
dominant, and the catalytic proton reduction route was similar
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to that of TFA.34 The plots of ic (in the presence of 22 equiv.
TsOH) and ip (0.83 mM concentration of cobalt corrole
complexes) are shown in Fig. S17 and S18 (ESI†). These results
showed that the HER activity of corrole complexes 1–4 was
strongly dependent on the acid proton source. Corrole 4
showed the highest ic/ip value among all the tested acids,
suggesting that the amide and pyridyl substituents of the
corrole may function as proton relay groups to facilitate proton
reduction. The values of ic/ip in acetic acid were significantly
lower than those in TFA or TsOH. When TFA was used as the
proton source, the ic/ip value reached as high as 15, and the
maximum TOF of corrole complex 3 was 202.05 sꢀ1. In contrast,
in TsOH, the maximum TOF of 187.61 sꢀ1 was achieved by
corrole complex 4.
Fig. 8 Catalytic Tafel plots of corrole complexes 1–4 in the presence of
TFA and TsOH.
were EECC pathways, eqn (3) was applied,
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
RTk2
Fv
Cs0ubst
Corrole 1 and 2 and their corresponding freebase forms
bear aldehyde groups. To clarify the possibility of reducing
the aldehyde to alcohol at the applied negative potentials,
we carried out cyclic voltammetry scans for 8 h at a potential
i
ip0
0:4463
ꢄ
ꢅ
¼
(3)
F
1 þ exp
ðE ꢀ EP0=Q
Þ
RT
of ꢀ1.1 V to ꢀ1.9 V vs. ferrocene. The FT-IR spectra of where E0P/Q is the half wave of the catalytic wave obtained from
these four complexes revealed that the stretching vibration of the CV in the absence of acid. In this work, E0P/Q represented
CQO at approximately 1700 cmꢀ1 was still present after the half waves of the second reduction peaks of the cobalt
electrolysis (Fig. S19, ESI†), and no sharp hydroxyl absorption complexes. The slope of i/i0p vs. 1/1 + exp[F/RT(E ꢀE0P/Q)] could
peak corresponding to
a primary alcohol was observed be used to calculate TOFmax (Fig. S20 and S21, ESI†), and the
near 3640 cmꢀ1. This indicated that the aldehyde groups catalytic Tafel plot is shown in Fig. 8. The TOFmax values of these
could not be reduced to alcohol at the applied potential. cobalt complexes using TFA and TsOH are summarized in
Due to the addition of the electrolyte TBAP to DMF for the Table 2. The maximum TOFs of corroles 3 and 4 reached
electrolysis, strong amide C–H and C–N vibration bands 202.05 and 187.61 sꢀ1 in TFA and TsOH, respectively.
appeared at approximately 3000 cmꢀ1 and 1090 cmꢀ1
respectively.
,
The formation of CoIII–H was proved to be a key step in the
release of hydrogen.38 In the current system, the existence of a
cobalt hydride active species might be proved by 1H NMR
spectroscopy, as shown in Fig. S22 (ESI†). The signals of the
active hydrogen of CoIII–H appeared at around d = ꢀ13 ppm,
and the species was obtained by the addition of two equivalents
of cobaltocene and five equivalents of trifluoroacetic acid under
a nitrogen atmosphere. The equilibrium constant K of CoIII–H
was determined using the equation CoI + H+ $ CoIII–H, and a
kinetic study was carried out via UV-Vis spectroscopy based on
the absorbance changes versus time upon the addition of
cobaltocene and acid.32 As shown in Fig. S23–S27 (ESI†), after
the addition of cobaltocene, the absorbance at 566 nm
increased significantly, but then decreased after the addition
of acetic acid, corresponding to the formation of cobalt
hydride. Fig. S28–S30 (ESI†) show the DAꢀ1 vs. [AcOH]ꢀ1 plots
used to determine the equilibrium constants K, and the corres-
ponding pKa values of cobalt hydride in DMF were calculated
using eqn (4),
Foot of the wave analysis (FOWA) and kinetics study
Upon the addition of TFA or TsOH, the catalytic waves did not
follow an S-type plot, so foot-of-the-wave analysis (FOWA) was
applied to extract the catalytic kinetic information from the
corresponding cyclic voltammograms. FOWA considers only
the foot of the wave of the catalytic portion, and thus can
eliminate the effects of substrate consumption and deactivation
of the catalysts. The catalytic Tafel plots can be derived from the
CV and used to analyze the intrinsic properties of the
catalyst.35,36 The TOF follows eqn (2):
TOFmax
ꢂ
ꢄ
ꢅ
TOF ¼
(2)
ꢃ
F
1 þ exp
E ꢀ E1=2
RT
pKDa MF(CoIII–H) = pKaDMF(Acid) ꢀ pK
(4)
k1k2
¼
where TOFmax
Cs0ubst
. Based on the experimental
k1 þ k2
results, we assumed k1 c k2, giving us the equation TOFmax
=
k2C0subst, by introducing the overpotential, Z ¼ E0;ap ꢀ E;
H
þ=H2
Table 2 TOFmax values of corroles 1–4 in the presence of 22 equiv. of
TFA/TsOH
where C0subst is the bulk concentration of the species indicated
by the subscript and is the overpotential. EH0;ap ¼ EH0
ꢀ
þ=H2
þ=H2
Corrole 1
Corrole 2
Corrole 3
Corrole 4
0:059 pKa; where EH0
¼ ꢀ1:2 V vs. ferrocene in DMF and the
þ=H2
TOFmax(TFA)
TOFmax(TsOH)
146.60
90.43
134.56
62.17
202.05
93.15
84.31
187.61
pKa values of acids are given above.37 Because the catalytic routes
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Table 3 pKDa MF(CoIII–H) values of corroles 1–4 in three different acids
pKDa MF(CoIII–H)
Corrole 1
Corrole 2
Corrole 3
Corrole 4
AcOH
TFA
TsOH
12.176
3.024
2.333
10.511
3.18
1.242
11.716
2.828
2.476
12.165
2.978
1.826
The pKDa MF(CoIII–H) values of all the cobalt complexes in the
different acids are summarized in Table 3.
Fig. 9 (a) Charge increase for corrole 1 (2.34 mM) at different overpoten-
tials. (b) GC traces of corroles 1–4 after 1 h of electrolysis with 0.5 mL CH4
as an internal standard.
Tafel analysis and charge transfer resistance (Rct)
The charge transfer resistance in the organic phase was
evaluated using the AC impedance technique. The open circuit
potential was set as the fixed potential and the frequency range
was 1 MHz to 0.1 Hz. The Rct was tested in pure DMF solution
containing 0.83 mM of the complex and 18 equiv. AcOH or
22 equiv. TFA/TsOH. The intrinsic resistances (Rs) of all four
complexes in DMF solution with AcOH were near 1000 O, as
shown in Fig. S31 (ESI†), while in DMF with TFA (Fig. S32,
ESI†), the Rs values of corroles 1–4 were 1188, 563, 876 and
833 O, respectively. Similarly, in DMF with TsOH (Fig. S33,
ESI†), the Rs values for corroles 1–4 were 917, 605, 878 and
752 O, respectively. These results suggested that corrole 2 had a
higher charge transfer rate and catalytic proton reduction
activity than the other three corrole complexes in the presence
of TFA and TsOH. Cobalt corrole 4 had a higher charge transfer
rate than 3, which may be due to its amide pyridyl groups being
closer to the cobalt center, thus benefiting the proton transfer.
The Tafel plots for a 0.83 mM concentration of corroles 1–4
were analysed using the same solution conditions as in the
charge transfer resistance (Rct) tests. The anodic and cathodic
branches of the Tafel plots were linearly fitted, as shown in Fig.
S34–S36 (ESI†). The slopes of the red dashed lines ranged from
41 to 60 mV decꢀ1 and represented a Heyrovsky step. The blue
coloured lines indicated a Volmer step and had slopes between
homogeneous systems are also summarized in Table S5 (ESI†).
To further investigate the generation of hydrogen, the gas after
an hour of electrolysis was analyzed using GC with 0.5 mL
methane as an internal standard. As shown in Fig. 9(b), the
volumes of hydrogen were 0.637, 0.422, 0.172 and 0.270 mL,
which were consistent with the TOFs obtained in the 2 min
electrolysis experiment. The hydrogen evolution after electro-
lysis without any catalyst (bare glassy carbon electrode) and a
bare reused electrode under the same conditions were also
analyzed using GC (Fig. S38, ESI†). No significant difference
was observed between the hydrogen production in the systems
using the bare fresh and re-used glassy carbon electrode,
further demonstrating that the tested cobalt corrole complexes
were stable and were not obviously absorbed on the electrode.
Corrole 4 had a higher TOF than 3, which indicated that their
spatial configurations had an evident effect on their catalytic
activity in an aqueous medium. The crystal structures of
freebase corroles 3 and 4 showed that the para-substituted
amide group resulted in obvious distortion of the corrole ring
while the meta-substituted pyrrole ring had good planarity. The
distance between the amide proton relay and the cobalt centre
might be one reason for the different catalytic activity. The
amide substituents might also participate in intramolecular
interactions and the formation of hydrogen bonds due to the
arrangement of the corrole ring, which might inhibit the activity
of the catalyst.41 Interestingly, we found that corroles 1 and 2
with an aldehyde-group-substituted benzene ring have higher
TOFs than corroles 3 and 4. The detailed catalytic mechanism
and the influence of the spatial structures and proton relay
transfer on the catalytic activity require further exploration.
119 mV and 149 mV decꢀ1 39
.
Catalyzing hydrogen evolution in an aqueous medium
The three different acids were used as proton sources to
evaluate the resulting catalytic abilities of the systems, and
the experimental results indicated that this ability was related
to their spatial structures. It was obvious that all four cobalt
corrole complexes had the ability to catalyze the hydrogen
evolution reaction. To evaluate their catalytic performance in
an aqueous medium, the controlled potential electrolysis (CPE)
Stability and durability in an aqueous medium
technique was used in a mixed neutral medium (acetonitrile/ In order to evaluate the stability and durability of the corrole
water = 2/3) at different overpotentials. The four corrole com- complexes, controlled potential electrolysis (CPE) at a potential
plexes were added to the mixed solvent at a concentration of of ꢀ1.25 V versus NHE was carried out in a neutral 0.25 M
2.34 mM. To our surprise, with the addition of corrole 1, the phosphate buffer. The current density of these complexes
charge increased to 111.1 mC with a TOF of 184.3 hꢀ1 at an remained nearly unchanged at 5.986, 4.712, 2.318, and
overpotential of 838 mV, as shown in Fig. 9(a). Similarly, the 3.218 mA cmꢀ2 for 1, 2, 3, 4, respectively, during the 6 h
TOFs of corroles 2, 3, and 4 were calculated to be 171.0 hꢀ1
,
electrolysis experiment (Fig. S39, ESI†). We also examined the
113.6 hꢀ1, and 129.5 hꢀ1, respectively, using the relevant UV-vis absorption differences in DMF solution before and after
equation.40 The plots of the TOFs and overpotentials of corroles electrolysis, as shown in Fig. S40 (ESI†). The spectra were
1–4 are shown in Fig. S37 (ESI†). For comparison, the TOFs of almost the same, which indicated these four catalysts were
other reported first-row transition metal complexes in stable and did not decompose structurally.
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Conclusion
Four A2B-type cobalt corrole complexes containing pentafluoro-
phenyl and pyridyl amine groups with different spatial
structures have been prepared and thoroughly characterized.
The results indicated that the efficiency of proton reduction
was closely related to the properties of the acids used. When
acetic acid was used as the proton source, the cobalt corrole
formed a CoIII–H intermediate after two-step one-electron
reduction; the CoIII–H was further reduced to a CoII–H species
and released hydrogen after coupling with a proton (EECEC
pathway). When trifluoroacetic or benzenesulfonic acid was
used as the proton source, the catalytic process proceeded via
the EECC pathway at high acid concentration and via the
EECEC pathway at low acid concentration. The formation of
the active species CoIII–H was detected by H NMR. The values
1
of pKDa MF(CoIII–H) in different acid environments were explored
using UV-Vis kinetic studies. FOWA was used to analyze the
intrinsic characteristics of catalytic proton reduction. In the
presence of TFA, corrole 3 had the highest TOFmax of 202.05 sꢀ1
,
and the TOFmax of corrole 4 reached 187.61 sꢀ1 in TsOH. These
values indicated that the substituted amide pyridyl group was
more helpful in catalyzing proton reduction. In neutral aqueous
solution, cobalt corrole 4 has a higher TOF than cobalt corrole 3.
This implies that the amide pyridyl group may function as a
proton relay group. The meta-position amide pyridyl is closer to
the cobalt center and thus more efficient in facilitating the
proton reduction. Interestingly, all four prepared cobalt corroles
performed well in the electrocatalytic HER in neutral aqueous
solution, and the TOF of cobalt corrole 1 could even reach as
high as 184.3 hꢀ1. We suggest that the aldehyde group may also
act as an inter- or intramolecular proton relay group in aqueous
solution. Further investigations along this line are still ongoing
in our laboratory.
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Conflicts of interest
There are no conflicts to declare.
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R. Cao, Phys. Chem. Chem. Phys., 2014, 16, 1883–1893.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NNSFC) (No. 21671068 and 22005052).
26 B. Li, Z. Ou, D. Meng, J. Tang, Y. Fang, R. Liu and
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