Electrocatalytic Hydrogen Evolution of Cobalt and Free-base Triaryl Corrole Bearing Hydroxyethyl Amino Groups

Article abstract of DOI:10.1002/ejic.201900996

Four free-base corroles (F₁₅C, F₁₂C, F₁₀C, F₈C) and their cobalt(III) complexes were used as homogeneous electrocatalysts for hydrogen evolution using acetic acid, trifluoroacetic acid and water as proton source

Full text of DOI:10.1002/ejic.201900996

A Journal of
Accepted Article
Title: Electrocatalytic Hydrogen Evolution of Cobalt and Free Base
Triaryl Corrole Bearing Hydroxyethyl Amino Groups
Authors: Ying Chen, Qi-Hang Fan, Md Sahadat Hossain, Shu-Zhong
Zhan, Hai-Yang Liu, and Li-Ping Si
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900996
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
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Electrocatalytic Hydrogen Evolution of Cobalt and Free-base
Triaryl Corrole Bearing Hydroxyethyl Amino Groups
Ying Chen,[a] Qi-Hang Fan,[a] Md Sahadat Hossain,[a] Shu-Zhong Zhan,[a] Hai-Yang Liu,*[a]
and Li-Ping Si*[b]
Abstract: Four free-base corroles (F15C, F12C, F10C, F8C) and their
cobalt(III) complexes were used as homogeneous electrocatalysts for
hydrogen evolution using acetic acid, trifluoroacetic acid and water as
proton sources. Free-base corrole was firstly reported as
electrocatalyst for H2 evolution. The results showed free-base corrole
was active in electrocatalytic hydrogen evolution reaction (HER) in
trifluoroacetic acid. And cobalt(III) corroles bearing hydroxyethyl
amino groups were more efficient in HER as compared to their
benchmark corroles. The catalytic active species of four tested
cobalt(III) corroles were CoII-H in acetic acid/dimethylformamide
solvent. In water solution, both CoIII-H and CoII-H may be involved in
HER. Interestingly, when trifluoroacetic acid was used as proton
source, CoII-H was the mainly active intermediate for F15CCo and
F10CCo, and CoIII-H for F12CCo and F8CCo, respectively. As for free-
base corrle, (•Cor)H4 and [(•Cor)H3]- were the catalytic active species.
The best electrocatalytic activity for HER is realized via anchoring
strong electron-deficient substituents in all meso-position of
corrole ligand.[11] It is also observed that other metal corroles such
as cobalt 5,10,15-tris-(nitrophenyl)corrole,[12] cobalt tris-(ethoxy-
carbonyl)corrole,[13] cobalt 5,15-bis-(4-aminophenyl)-10-(penta-
fluorophenyl)corrole,[14] as well as the metal complexes of other
corroles bearing -NO2,[15] -N3,[16] -NH2,[17] pyrenyl[18] are also
efficient electrocatalysts for HER. As compared to substitution on
meso-phenyl of corrole, β-substitution has more substantial
influence on electrochemical property of metal corrole. The most
electron-deficient cobalt perfluorinated corrole exhibits excellent
performance with TOF value of 1140 s-1 for electrocatalytic H2
evolution in the solution of 0.5 M H2SO4 at -0.8 V (vs. NHE).[19]
The electrocatalytic activity of metal corrole in HER may also be
tuned through appending of acid groups or axial ligand.[18, 20]
Recently, free-base porphyrins were reported as efficient
electrocatalysts in HER.[21] Corrole is an analogous of porphyrin,
which has more electrochemical active species including CorH3,
[(Cor)H4]+ and [(Cor)H2]-.[22] To the best of our knowledge, free-
base corrole has not been reported as HER electrocatalyst so far.
In this work, we wish to report the electrocatalytic HER by free-
base corroles bearing hydroxyethyl amino groups and their
cobalt(III) complexes (Figure 1). 5,10,15-tris(pentafluoro-
phenyl)corrole (F15C) and 5,15-bis(pentafluorophenyl)-10-
(phenyl)corrole (F10C) were used as starting corroles for
modification. The substitution of the para-fluorine atom of meso-
pentafluorophenyl group by ethanolamine afforded target corroles
in good yield. Our results showed that the electrocatalytic HER
activity of these modified corroles were improved significantly as
compared to their starting counterparts in the homogeneous
system.
Introduction
Hydrogen is a typical clean energy source. Water electrolysis is
one of the traditional and yet superior methods of hydrogen
production.[1] However, the energy of forming hydrogen is very
high in the absence of catalyst. Thus a lot of transition metal
complexes containing different ligands such as corrole,[2]
porphyrins,[3] tetraaza macrocycle,[4] pentapyridine macrocycle,[5]
polypyridyl,[6] glyoxime and tetraimine macrocycle[7] have been
developed as the electrocatalytic hydrogen evolution catalysts.
These ligands are capable of tuning the redox properties of
central metals to facilitate the hydrogen evolution reactions (HER).
Metal corrole has a high reactivity towards the reduction of
proton.[8] The electrocatalytic HER activity of metal corroles may
be modulated by changing the corrole structure with proper
substituents. It has been found that metal corroles bearing
electron-withdrawing substituents exhibit significant activity for
electrocatalytic H2 evolution.[9] The cobalt tris-(pentafluoro-
phenyl)corrole can catalyze proton reduction in highly acidic
sulfuric acid solution with a TOF value of 1010 s-1 at -1.0 V (vs.
Ag/AgCl).[10] A series of similar studies of A2B type metal corrole
complexes bearing different substituents have been carried out.
[a] Prof. Hai-Yang Liu
Department of Chemistry, The Key Laboratory of Fuel Cell Technology
of Guangdong Province, South China University of Technology,
Guangzhou 510640, China
E-mail: chhyliu@scut.edu.cn
[b] Dr. Li-Ping Si
School of Materials Science and Energy Engineering, Foshan
University, Foshan 528000, China
Supporting information for this article are available on the WWW under
https:// XXXXXX
Figure 1. Molecular structures of cobalt(III) corrole.
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
Results and Discussion
was attributed to the dissociation of axial ligand PPh3 upon
reduction.[25] The two redox potential of F12CCo shifted negatively
by 120 mV and 110 mV versus F15CCo, while two redox potential
of F8CCo shifted to cathodic direction by 70 mV and 110 mV
versus F10CCo.These results reveal that there are substantial
electronic communication between corrole ligands and cobalt
center. The differences in potential may be due to the changes in
average electron density of the cobalt(III) corrole π-conjugation
system caused by the substitution of the para-fluorine atom on
meso-pentafluorophenyl group by ethanolamine. CVs of four free-
base corrole ligands F15C, F12C, F10C and F8C in DMF were
shown in Figure S2. It has been reported that free-base corroles
were in their deprotonized form [(Cor)H2]- in DMF solution.[26] Four
free-base corroles were reduced at the potentials of E1/2 = -1.60
V (F15C), -1.72 V (F12C), -1.57 V (F10C) and -1.70 V (F8C) (vs.
Ag/AgNO3), respectively.
Synthesis and Characterization. F15C and F10C were
synthesized by using the procedure described in literature.[23]
Then they were used as starting corroles for the preparation of
F12C and F8C by reacting with ethanolamine in DMF under
refluxing. The desired cobalt corroles were obtained by reaction
of Co(OAc)2 • 4H2O and free-base corroles in ethyl alcohol in the
presence of sodium acetate and triphenylphosphine (PPh3). All
synthesized corroles were well characterized by 1H-NMR, 19F-
NMR (Figure S19-30), HRMS (Figure S31-38) and UV-visible
spectra. The single crystals of F10C and F8C were obtained in
dichloromethane:n-hexane (1:8) solvent and the structure were
determined by X-ray diffraction. All characterization data are listed
in experimental section.
In order to confirm these redox couples are belong to
cobalt(III) corroles. CVs of cobalt salt, corresponding corrole
ligands, and a mixture of starting materials were examined in
degassed TBAP/DMF solution, respectively. As shown in Figure
S2 and S3, the redox events of cobalt(III) corroles were
recognized by their unique characteristics. In addition, the peak
currents displayed a linear relationship with the square root of the
scan rate (Figure S4), revealing that the current were associated
with a diffusion controlled redox reaction.
Structure Analysis. Single crystal structures of F10C and F8C
were illustrated in Figure 2. The main crystal data including the
salient bond lengths were summarized in Table S1 and Table S2.
From the side view of ORTEP diagram (Figure 2), it can be
observed that the two corrole rings were nonplanar with two
pyrrole rings tilted up and down alternately and the other two
pyrrole rings were almost coplanar. Which is similar to other
reported free-base corrole.[24] In F10C, two pyrrole rings were tilted
out of the average plane by 25.98° and 5.87°, respectively.
However in F8C, two pyrrole rings tilted out of the average plane
by 19.96° and 14.53°. Interestingly, the position of the NH protons
of F10C was different from F8C, and there was n-hexane molecule
in the unit cells of F8C crystal, while there is no any solvent
molecules in the unit cells of F10C crystal.
Figure 3. CVs of 0.5 mM F15CCo, F12CCo, F10CCo and F8CCo in 0.1 M
TBAP/DMF. Data was collected by using GC as working electrode, Pt as
counter electrode and Ag/AgNO3 as reference electrode at 0.1 V s-1 scan rate;
Ferrocene (*) was used as internal standard.
Figure 2. The ORTEP of F10C and F8C with 50% thermal ellipsoids. Up: top
view; Bottom: side view (the benzene ring are omitted for clarity).
Electrocatalytic Behavior of Cobalt(III) Corroles with Acetic
Acid (AcOH). The possible electrocatalytic activity of four
cobalt(III) complexes were detected in the presence of AcOH.
Figure 4 illustrates the CVs of four cobalt(III) corroles at various
AcOH concentration. With the increasing amount of AcOH, the
first reduction peak CoIII/II of four cobalt(III) corroles were almost
unchanged. The second redox couples CoII/I become irreversible
and the reverse oxidations disappeared gradually. And the
observable reduction peak potentials shifted positively by 50-110
mV. This implies that CoI is involved in HER via its proton coupling
to form CoIII-H (reaction III in Scheme 1). Catalytic current of the
Electrochemical Characterization in dimethylformamide
(DMF). Cyclic voltammograms (CVs) of corroles were recorded in
DMF using glass carbon working electrode, Ag/AgNO3 reference
electrode and platinum counter electrode. 0.1 M [n-Bu4N]ClO4
was used as supporting electrolyte. Ferrocene was added as
internal standard. All potentials were reported versus Ag/AgNO3
without calibration. Four cobalt(III) corroles displayed two redox
events on their CVs in DMF solution, which were assigned as
reversible CoII/I and irreversible CoIII/II couple, respectively (Figure
3, Table S3 and Figure S1). The irreversibility of CoIII/II redox peak
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
system were triggered near -1.74 V (F15CCo), -1.77 V (F12CCo), -
buildup. The TOF values follows an order of F12CCo > F15CCo >
F8CCo ~ F10CCo. Interestingly, F12CCo and F8CCo exhibited
higher TOF than their counterpart F15CCo and F10CCo complexes.
It may be caused by the ethanolamine substitution of the para-
fluorine atom of meso-C6F5 group, which results in the more
electron-rich of cobalt center to faciliate the hydrogen generation.
1.78
V (F10CCo) and -1.82 V (F8CCo) (vs. Ag/AgNO3),
respectively. The catalytic current increased significantly and
were proportional to the concentration of AcOH (Figure S5). This
suggests that these cobalt(III) corroles have a promising activity
for hydrogen evolution. With the AcOH concentration varying from
0.00 mM to 5.33 mM, the reduction peaks of CoII/I (around -1.4 V)
increased in a small extent. However a remarkable catalytic wave
appeared at around -1.75 V (vs. Ag/AgNO3). This indicates that
the reduction of CoIII-H to CoII-H (reaction IV) was involved in the
catalysis (Scheme 1). The CoII-H intermediate coupled with
protons to furnish the hydrogen production (reaction V). Similar
catalytic behavior has also been reported for metal porphyrin
catalytic HER systems.[27] For four cobalt complexes, the
hydrogen evolution is mainly go through: I→II→III→IV→V or
EECEC (E: electron; C: charge) pathway as shown in Scheme 1.
Scheme1. Possible pathways of electrocatalytic hydrogen evolution by
cobalt(III) corroles.
Table 1. Catalytic activity of cobalt corroles in DMF.
Catalysts
F15CCo
F12CCo
F10CCo
F8CCo
E0[a]/V
-1.43
-1.49
-1.48
-1.56
Epc[b]/V
-1.38
-1.40
-1.40
-1.46
Ecat[c]/V
-1.80
-1.80
-1.80
-1.90
TOF[d]/h-1
202
Charge[e]/mC
116
143
80
276
127
Figure 4. CVs of 0.5 mM (a) F15CCo, (b) F12CCo, (c) F10CCo and (d) F8CCo at
different concentration of AcOH in 0.1 M TBAP/DMF. Data was collected by
using GC as working electrode, Pt as counter electrode and Ag/AgNO3 as
reference electrode at 0.1 V s-1 scan rate; Ferrocene (*) was used as internal
standard.
140
82
[a] The reduction potential of CoII/I without AcOH. [b] The reduction potential
of CoII/I under the condition of 5.33 mM AcOH. [c] The peak potential of
catalytic wave. [d] The TOF value of 2 min CPE. [e] Charge accumulated
during the 2 min CPE. Condition: Under the -1.45 V vs. Ag/AgNO3, 2 min
electrolysis of DMF solution were conducted in the present of 1.62 μM
complexes.
To further investigate the catalytic activity, controlled-
potential electrolysis experiments (CPE) were performed in
AcOH/DMF solution and the results were shown in Figure S6 and
Table 1. The onset overpotential was observed 242 mV for
Electrocatalytic Behavior of Cobalt(III) Corroles with
Trifluoroacetic Acid (TFA). The pathways of electrocatalytic H2
evolution are highly depended on the nature of proton source.[28]
Hence, different catalytic mechanisms may be observed when
using weak acid and strong acid. In this study, we also performed
the catalytic experiment by using TFA as proton source. Figure 5
shows the CVs of F15CCo and F12CCo complexes at different TFA
concentrations. For F15CCo, with the increasing of TFA
concentration, a catalytic current peak might be observed near
CoII/I couple at -1.38 V (vs. Ag/AgNO3). This implies that the CoI
species is involved in the HER reaction (Scheme 1, reaction III).
Interestingly, at higher TFA concentration, the catalytic peak near
F15CCo, F12CCo and F8CCo. While
a little higher onset
overpotential was observed 292 mV for F10CCo. This indicates
F10CCo exhibits lower activity than other cobalt(III) corroles. At
different applied overpotential from 342 mV to 942 mV, significant
charge accumulation could be observed for all tested cobalt(III)
corrole catalysts. The more negative potential were applied, the
more amounts of charge buildup were observed. With the addition
of AcOH, the reduction potential of CoII/I of four cobalt complexes
shifted positively, eg. from -1.43 V to -1.38 V (vs. Ag/AgNO3) for
F15CCo when AcOH concentration increased from 0.00 mM to
5.33 mM (Table 1). The catalytic peak appeared around the -1.80
V (vs. Ag/AgNO3). F12CCo exhibited the highest TOF and charge
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
-1.38 V vanishes and a remarkable new catalytic peak appeared
at -1.56 V (vs. Ag/AgNO3). This is interpreted as CoIII-H/CoII-H
peak caused by the accumulation of CoIII-H species at higher TFA
concentrations. This HER catalytic behavior was also observed
for other systems.[29] F10CCo complex exhibited similar catalytic
behavior to F15CCo (Figure S7). So HER for F15CCo and F10CCo
are mainly going through I→II→III→IV→V or EECEC pathway
(Scheme 1).
are reported versus Ag/AgCl in MeCN/H2O. From Figure 6a, it can
be seen that there was nearly no catalytic current for the control
system even if the potential is reached -1.5 V (vs. Ag/AgCl). In
the presence of cobalt corrole catalysts, the onset of catalytic
current was observed near -1.22 V (vs. Ag/AgCl). This suggests
that the four synthesized cobalt corroles can catalyze the
decomposition of water to produce H2. Furthermore, a strong
catalytic peak could be observed around -1.71 V vs. Ag/AgCl
(Table S4). The catalytic current increased with the increasing
amount of the catalysts and showed a linear relationship with the
catalysts concentration (Figure 7 and Figure S8). We also found
that the catalytic current increased remarkably with the lowering
of the pH value of buffers (Figure S9). The irreversible waves
around -1.7 V were catalytic wave for hydrogen evolution pathway
IV as shown in Scheme 1, which is a proton coupled electron
transfer process.[31] The weak irreversible wave around -0.47 V
(Figure 6b) is due to the CoIII-PPh3 to CoII, which follows hydrogen
evolution by pathway VI (Scheme 1) and is an integral part of
regenerating the CoI species. This indicates axial ligand PPh3 is
involved in the catalytic reaction. So the hydrogen evolution in
neutral MeCN/H2O may go through: I→II→III→IV→V (EECEC)
or I→II→III→VI (EECC) pathway as shown in Scheme 1.
Figure 5. CVs of 0.5 mM catalysts at different concentration of TFA in 0.1 M
TBAP/DMF. (a) and simplified (b) is for F15CCo, (c) and simplified (d) is for
F12CCo. Data was collected by using GC as working electrode, Pt as counter
electrode and Ag/AgNO3 as reference electrode at 0.1 V s-1 scan rate;
Ferrocene (*) was used as internal standard.
For F12CCo complex, the catalytic peak were triggered
Figure 6. CVs of 1.62 μM (a) F15CCo, F12CCo, F10CCo and F8CCo in MeCN/H2O
(v/v = 2/3, pH = 7.0) containing in 0.1 M KCl. (b) is the amplified CVs of catalysts
in orange box in (a). Data was collected by using GC as working electrode, Pt
as counter electrode and Ag/AgCl as reference electrode at 0.1 V s-1 scan rate.
around the -1.39
V (vs. Ag/AgNO3). With the increasing
concentration of TFA, the catalytic current increased and peak
potential shifted negatively. Which implies that the CoI species is
involved for proton reduction (reaction III). Interestingly, the
reversible CoII/I redox peak could be observed clearly at lower
TFA concentration. This is due to the fast total consumption of
substrate in the reaction layer under the low TFA concentration.
Further increasing of the TFA concentration from 15.33 mM to
25.33 mM, CoII/I redox couple disappeared and merged with
catalytic wave. These observations are typical for “total”
catalysis.[29a, 30] in which the reaction is so fast that the intensity of
current is controlled by the diffusion of acid. Similar catalytic
behavior were also observed for F8CCo (Figure S7). Hence the
electrocatalytic hydrogen evolution for F12CCo and F8CCo may
mainly progress via: I→II→III→VI (EECC) pathway (Scheme 1).
Electrocatalytic Hydrogen Evolution in Neutral Acetonitrile
(MeCN)/water (v/v = 2/3). Electrolysis of water is one of the most
attractive method of hydrogen evolution. Inspired by the catalytic
activity of hydrogen evolution from acid/DMF, We conducted
electrochemical and electrocatalytic studies in neutral MeCN/H2O
solvent. Because the catalysts were not water-soluble, mixed
solvent MeCN/H2O (v:v = 2:3) was used as aqueous solution
system. Ag/AgCl was used as reference electrode. All potential
Figure 7. CVs of various concentration of (a) F15CCo, (b) F12CCo, (c) F10CCo
and (d) F8CCo. Data was collected in MeCN/H2O (v/v = 2/3, pH = 7.0) containing
0.1 M KCl by using GC as working electrode, Pt as counter electrode and
Ag/AgCl as reference electrode at 0.1 V s-1 scan rate.
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
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Table 2. Charge accumulation and corresponding TOF value of four cobalt(III) corroles under the different time.
2 min CPE
1 h CPE
11 h CPE
Catalyst
Charge/mC
173
TOF/h-1
299
Charge/mC
9324
TOF/h-1
538
Charge/mC
TOF/h-1
986
F15CCo
F12CCo
F10CCo
F8CCo
187870
217508
103699
154027
182
350
11721
5089
751
1268
500
191
191
271
122
210
5540
317
803
Conditions: CPE was conducted by using 1.62 μM catalysts in 0.25 M phosphate buffer (pH = 7.0) under -1.45 V vs. Ag/AgCl. Data was collected by using GC as
working electrode, Pt as counter electrode and Ag/AgCl as reference electrode at 0.1 V s-1 scan rate, 0.1 M KCl was used as supporting electrolyte. All data have
been subtracted from the blank.
Figure 8. Plot of TOF, faradaic efficiency and hydrogen volume of F15CCo
(purple), F12CCo (red), F10CCo (green) and F8CCo (black) after 1 h electrolysis
of MeCN/H2O solution. Data was collected by using GC as working electrode,
Pt as counter electrode, Ag/AgCl as reference electrode at 0.1 V s-1 scan rate,
and 0.1 M KCl was used as supporting electrolyte.
Figure 9. Cyclic voltammograms of 0.5 mM (a) F15C and (b) F12C, (c) F10C and
(d) F8C at different concentration of TFA in 0.1 M TBAP/DMF. Data was
collected by using GC as working electrode, Pt as counter electrode and
Ag/AgNO3 as reference electrode at 0.1 V s-1 scan rate.
During the measurement of CVs, bubbles evolution could be
observed, showing they are very HER active. To further
investigate their catalytic efficiency, 2 min CPE was performed in
0.25 M phosphate buffers containing 1.62 μM catalysts. Charge
buildup was recorded at several different applied potentials. In 2
min CPE, the charge buildup by electrolyzing control bulk solution
was only 180 mC at -1.45 V (vs. Ag/AgCl) (Figure S10). Upon
addition of cobalt corroles, significant charge buildup might be
observed (Figure S11). For F12CCo, the charge buildup reached
362 mC during 2 min electrolysis of MeCN/H2O solution. The net
charge buildup increment was 182 mC and the calculated TOF
turned out 350 h-1. After 1 h electrolysis at -1.45 V (vs. Ag/AgCl),
the formed hydrogen was detected and quantified via GC (Figure
S12) and the relevant results are summarized in Figure 8 and S13.
Among all tested cobalt corroles, F12CCo exhibited the best
catalytic performance as indicated its hydrogen volume, faradaic
efficiency and TOF values. When the electrolysis time extended
to 11 hours, TOF of F12CCo increased remarkably up to 1268 h-1
(Table 2). Within the time span of 11 h electrolysis at -1.45 V (vs.
Ag/AgCl), the catalytic current of four complexes remained almost
constant, and the charge increment was proportional to
electrolysis time (Figure S14 and S15). Which is attributed to the
nearly unchanged catalytic activity of catalysts during electrolysis.
Moreover the pH value of solution increased with the electrolysis
time going. Electrolytic solution become alkaline which is in
consist with the splitting of H2O produced H2. This suggests that
four cobalt(III) corroles are stable catalysts.
Electrocatalytic Studies of Free-base Corroles in DMF. In this
section, TFA is used as proton source. Figure 9 shows the CVs of
four free-base corroles at different TFA concentration and the
electrochemical data are summarized in Table S5. Interestingly,
free-base F12C and F8C exhibited different TFA concentration
dependent electrochemical response from F15C and F10C. Based
26]
on the literatures,[22,
the electrochemical HER of free-base
corrole may well depicted by Scheme 2. For F12C and F8C, with
the addition of TFA, [(Cor)H2]- was protonated to form
electroactive species CorH3 and [(Cor)H4]+ successively (Scheme
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
2, I and II). The reduction peak of [(Cor)H2]- at around -1.75 V (vs. Conclusions
Ag/AgNO3) for F12C (-1.78 V vs. Ag/AgNO3 for F8C) disappeared.
At the same time, two reduction peaks corresponding to CorH3 at
around -1.5 V (vs. Ag/AgNO3) and [(Cor)H4]+ at -1.6 V (vs.
Ag/AgNO3) appeared. These peaks were enhanced remarkably
and exhibited a little negatively shifted with the increasing TFA
concentration. This indicates F12C and F8C are HER active and
two HER pathways work simultaneously (Scheme 2, IV and VI).
Free-base corrole F15C and F10C exhibited similar
electochemical behavior (Figure 9a and 9c). As exampled by F10C
(Figure 9c), the reduction peak current of [(Cor)H2]- at around -1.7
V (vs. Ag/AgNO3) gradually decreased with the increasing
concentration of TFA. When TFA concentration reached 5.33 mM,
a distinct reduction peak of CorH3 at around -1.6 V (vs. Ag/AgNO3)
appeared and its peak current increased significantly with the
incresing TFA amount from 5.33 mM to 10.67 mM. With further
increasing TFA concentration, CorH3 reduction peak current
reached its maximun and [(Cor)H4]+ reduction peak appeared.
These observations suggest that F15C and F10C catalyzed HER is
mainly via I→V→VI pathway (Scheme 2) at lower TFA
concentration, and II→III→IV pathway only works at higher TFA
concentration. The different electrochemical behavior of two sets
free-base corroles may be caused by the more difficult
protonation of CorH3 species for F15C and F10C as compared with
F12C and F8C. This is also evidenced by UV-Vis spectrascopic
titration experiment (Figure S16). Upon addition of TFA, the UV-
visible spectra of four free-base corrole were blue shifted similarly.
For F12C and F8C, a new absorption band appeared at around
675 nm and a red shift of soret band might be observed at high
TFA concentration. This indicates that CorH3 is protonated to form
[(Cor)H4]+.[22a] No such a specetroscopic features were observed
for F15C and F10C. We also found that F12C degraded after
electrolysis for 5 h. And F15C exited in the form of [(Cor)H2]- after
electrolysis for 1 h (Figure S17). 2 min CPE was shown in Figure
S18, there was significant charge accumulation after the addition
of free-base corrole, also suggesting the four free-base corrole
were catalytic activity in HER.
In conclusion, we here present the first report of the
electrocatalytic HER of free-base corrole. The hydroxyethyl amino
groups are responsible for the enhanced catalytic activity of
modified corrole as compared with the corresponding starting
corrole counterparts. Although free-base corrole ligands are
active in electrocatalytic HER, they were unstable in TFA/DMF
and degraded in a short time during the catalysis. In contrast, their
cobalt(III) corroles were durable electrocatalysts in HER. Different
catalytic species may be observed for different proton sources.
For free-base corrole, when TFA was used as proton source, the
active species were (•Cor)H4 and [(•Cor)H3]- for F12C and F8C, and
mainly [(•Cor)H3]- for F15C and F10C. For cobalt corrole complexes,
the HER current response of F15CCo and F10CCo was mainly
from CoII-H species. While the CoIII-H was mainly responsible for
HER current response of F12CCo and F8CCo. In addition, the
HER catalytic pathways of four cobalt(III) corroles in acetic acid
were similiar. CoII-H was the active species for final hydrogen
evolution in acetic acid. Interestingly, the HER in water was a
proton coupled electron transfer process. Both CoIII-H and CoII-H
might be involved in catalysis. Our work also proves that the
nucleophilic substitution of the para-fluorine atom of meso-C6F5
groups by N-nucleophiles or other nucleophiles may provide an
effective way for synthesis of new corrole-based HER
electrocatalysts.
Experimental Section
Materials and Methods. All chemicals and reagents were commercially
purchased and without further purification, except that pyrrole was freshly
distilled prior to use. NMR were recorded with a Bruker Avance III 400 MHz
spectrometer in deuterated reagent solution. The high resolution ESI mass
spectra (HRMS) were recorded on
a Bruker maxis impact mass
spectrometer. UV-visible spectra were measured with a Hitachi U-3010
spectrophotometer.
The Cyclic voltammograms (CVs) were performed by using a CHI-660E
electrochemical analyzer with a three-electrode system in degassed 0.1 M
[(n-Bu)4N]ClO4 (TBAP)/DMF and 0.1 M KCl acetonitrile/water (MeCN/H2O)
system (MeCN:H2O = 2: 3, v: v). The three-electrode cell is composed of
a 0.01 cm2 glassy carbon electrode as the working electrode, a platinum
wire as the auxiliary electrode, Ag/AgNO3 (in DMF solution) and Ag/AgCl
(in MeCN/H2O solution) as the reference electrodes. During the cyclic
voltammetry measurements, ferrocene was added to DMF solution as
internal standard. Controlled potential electrolysis (CPE) experiments
were conducted in a sealed glass double-compartment cell (working
compartment and auxiliary compartment). 40 mL 0.1 M [(n-Bu)4N]ClO4
(TBAP)/DMF were separately added to the working compartment and the
auxiliary compartment. The DMF solution contained 1.62 μM catalysts and
maximum amount of acid that corresponding complexes can be resistant.
Similarly, the two compartment cell were filled with 40 mL pH=7
MeCN/H2O solution (2: 3, v: v) using water as proton source. Both
compartments were continuously saturated by purging nitrogen for at least
60 min. The extended CPE experiments of all complexes were also
conducted under identically electrolytic condition. Assuming that every
electron is applied to the reduction of protons on electrode surface, on the
Scheme 2. Possible pathways of electrocatalytic H2 generation by four free-
base corroles using TFA as proton source.
basis of the following equation calculated the TOF value[23a]
:
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
ESI ([M+H]+): 707.1298; calcd for C37H17F10N4: 707.1288. UV-Vis (CH2Cl2)
푇푂퐹 = ∆퐶/(퐹 ∙ 푛1 ∙ 푛2 ∙ 푡)
(1)
λmax/nm (log ε): 411 (4.58), 561 (3.79), 613 (3.58).
∆퐶 is the charge build after deducting the blank during CPE. 퐹 is faraday
constant. 푛1 is the mole of electron required to produce 1 mole of hydrogen.
푛2 is the mole of catalyst in the electrolyte. 푡 is the electrolysis time. The
overpotential (vs. SHE) is calculated by the following equation. Equation
Synthesis of 5,15-bis(4-(2-hydroxyethyl)-amino-2,3,5,6-tetrafluorophenyl)-
10-phenyl)corrole (F8C). The free-base corrole was synthesized by similar
method of F12C mentioned above. A solution of 18 mg (0.025 mmol) F10
C
(2) and (3) are used for organic phase, (4) and (5) for aqueous solution[23a]
:
in 2 mL DMSO was added to a 10 mL flask with 0.2 mL ethanolamine. The
handing of the reaction mixture is the same to F12C. The final product was
obtained by column chromatography on silica gel (300-400 mesh) using
methylene chloride/methanol (97: 3, v: v) as eluent. Then afforded a purple
solid with 89% yield (17.6 mg). 1H NMR (400 MHz, CD3OD, δ, ppm): 9.68
(d, 2H), 9.33 (d, J = 76.1 Hz, 2H), 9.19 (d, J = 13.0 Hz, 4H), 8.81 (s, 2H),
8.38 (s, 3H), 4.56(m, 4H), 4.42 (s, 4H), 3.99 (m, 3H), 1.95 (d, J = 15.4 Hz,
1H). HRMS-ESI ([M+H]+): 789.2235; calcd for C41H29F8N6O2: 789.2219.
UV-Vis (CH2Cl2) λmax/nm (log ε): 416 (4.58), 567 (3.82), 614 (3.65).
Overpotential = Applied potential − E퐻퐴
(2)
+
퐸퐻퐴 = 퐸퐻 − (2.303푅푇/퐹)푝퐾푎,퐻퐴
(3)
( )
4
Overpotential = Applied potential − E푝퐻
E푝퐻 = 0.059pH
(5)
Synthesis of meso-5,10,15-Tris(pentafluorophenyl)corrole Co(III)-PPh3
(F15CCo). The cobalt(III) corrole was prepared according to published
procedure.[34] 19.9 mg (0.025 mmol) F15C, 30 mg (0.366 mmol) sodium
acetate were added to a 25 mL flask containing 10 mL ethyl alcohol, stirred
for 5 min. Then 30 mg (0.12 mmol) Co(OAc)2•4H2O and 50 mg (0.191
mmol) triphenylphosphine (PPh3) were added. The reaction mixture was
stirred for 1.5 h and extracted with 50 mL CH2Cl2. The organic layer was
collected and washed with water three times. The crude product was
purified by silica gel column chromatography (300-400 mesh) with
dichloromethane/ n-hexane (1: 4, v: v) as eluent. A black solid product was
obtained with 90% yield (26 mg). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.69
(d, J = 4.5 Hz, 2H), 8.35 (d, J = 4.1 Hz, 2H), 8.26 (d, J = 4.7 Hz, 2H), 8.09
(d, J = 4.4Hz, 2H), 7.00 (t, J=7.1Hz, 3H), 6.71-6.60(m, 6H), 4.59 (dd,
J=11.1Hz, 6H). 19F NMR (400 MHz, CDCl3, δ, ppm): -137.23 (d, J = 19.7
Hz, 4F), -137.55 (d, J = 20.7 Hz, 2F), -153.49 (m, 2F), -153.56 (d, J = 5.3
Hz, 1F), -161.86 (d, J = 16.4 Hz, 4F), -161.95 (d, J = 7.9 Hz, 2F). HRMS-
ESI ([M+H]+): 1115.0799; calcd for C55H24CoF15N4P: 1115.0826. UV-Vis
(CH2Cl2) λmax/nm (log ε): 377 (4.85), 410 (4.86), 583 (4.72).
+
퐸퐻 is the standard potential for the solvated proton-dihydrogen couple,
푝퐾푎,퐻퐴 is the dissociation constant of acid, 퐹 is faraday constant, 푅 is the
gas constant, 푇 is the temperature, pH is 7. The gas mixture was evaluated
by Agilent Technologies 7890A gas chromatograph. CH4 was added to the
reaction mixture as an internal standard.
Crystal structure studies. Single crystals of F8C and F10C obtained by slow
recrystallization from 1: 8 dichloromethane: n-hexane. Under 100(10) K,
purple crystals were measured using a Rigaku Oxford AtlasS2 CCD
diffractometer with 0.71073 Å Mo Kα radiation. Data were consolidated
with diffractometer’s software. The structures were solved and refined with
the Olex2 software package. Non-hydrogen atoms were refined
anisotropically by a full-matrix least squares procedure based on F2
(SHELXL-2018) and all hydrogen atoms were treated with a riding model.
Additional information is listed in Table S1 and cifreport file (SI) in
Supporting Information. (CCDC: 1943607 and 1943609).
Synthesis and Characterization. Synthesis of meso-5,10,15-tris(penta-
fluorophenyl)corrole (F15C). The compound was synthesized by using
previously described procedure.[23b] 1H NMR (400 MHz, CDCl3, δ, ppm):
9.09 (d, J = 3.2 Hz, 2H), 8.79 (d, J = 3.5 Hz, 2H), 8.61 (d, J = 3.8 Hz, 2H),
8.57 (d, J = 3.6 Hz, 2H). HRMS-ESI ([M+H]+): 797.0820; calcd for
C37H12F15N4: 797.0817. UV-Vis (CH2Cl2) λmax/nm (log ε): 408 (4.68), 562
(3.90), 606 (3.66).
Synthesis of 5,10,15-tris(4-(2-hydroxyethyl)-amino-2,3,5,6-tetrafluoro-
phenyl)corrole Co(III)-PPh3 (F12CCo). Preparation procedure is the same
to F15CCo. The eluent was methylene chloride/ ethanol (91: 9, v: v). Purple
solid product was obtained with a yield of 87% (27 mg). 1H NMR (400 MHz,
CD3OD, δ, ppm): 9.11 (s, 2H), 8.91 (s, 2H), 8.84 (s, 2H), 8.65 (s, 2H), 8.08
(d, J = 57.4 Hz, 4H), 7.50 (s, 3H), 7.17 (s, 6H), 5.18 (s, 6H), 4.43 (s, 6H),
4.25 (s, 6H), 3.87 (s, 2H). 19F NMR (400 MHz, CD3OD, δ, ppm): -142.98
(d, J = 7.5 Hz,1F), -143.04 (d, J = 7.6 Hz,1F), -143.31 (d, J = 7.7 Hz, 1F),
-143.37 (d, J = 7.5 Hz, 1F), -143.73 (dd, J = 22.3, 7.8 Hz, 1F), -143.90 (dd,
J = 22.4, 7.9 Hz,1F), -162.52 (dt, J = 22.3, 7.2 Hz, 2F), -162.65 (m, 4F).
HRMS-ESI ([M+H]+): 1238.2214; calcd for C61H42CoF12N7O3P: 1238.2222.
UV-Vis (CH2Cl2) λmax/nm (log ε): 407 (4.74), 556 (3.93), 583 (3.85).
Synthesis of 5,10,15-tris(4-(2-hydroxyethyl)-amino-2,3,5,6-tetrafluoro-
phenyl)corrole (F12C). The corrole ligand was obtained according to
reported procedure of our group.[32] In a 10 mL round bottom flask, the
mixture of 20 mg (0.025 mmol) F15C, 0.2 mL ethanolamine, 2 mL DMSO
was stirred at 150 C for 2 h. Until UV or TLC analysis shows all F15C had
been consumed. Then the reaction mixture was cooling to room
temperature. The mixture was extracted with CH2Cl2/water (1: 1, v: v) and
wash three times with water. Organic layer was collected and the solvent
was removed by rotatable evaporation under vacuum. The crude product
was purified via column chromatography with silica gel (300-400 mesh)
using dichloromethane/ethanol (93: 7, v: v) as eluent to afford a purple
solid in 82% yield (18.9 mg). 1H NMR (400 MHz, CD3OD, δ, ppm): 9.08 (s,
2H), 8.85 (s, 2H), 8.63 (s, 2H), 8.54 (s, 2H), 6.27 (s, 2H), 6.15 (s, 1H), 4.99
(s, 3H), 3.75 (s, 6H), 3.63 (s, 6H). HRMS-ESI ([M+H]+): 920.2230; calcd
for C43H30F12N7O3: 920.2213. UV-Vis (CH2Cl2) λmax/nm (log ε): 416 (4.74),
567 (3.97), 608 (3.72).
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-phenyl)corrole Co(III)-
PPh3 (F10CCo). The desired cobalt(III) complex was prepared by using the
same method to F15CCo. Crude product was purified by column
chromatography on silica gel (300-400 mesh) using dichloromethane/ n-
hexane (1: 8, v: v) as eluent. Black product was obtained with a yield of
92% (24 mg). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.71 (d, J = 4.3 Hz, 2H),
8.29 (d, J = 4.6 Hz, 2H), 8.23 (d, J = 4.4 Hz, 2H), 8.03 (d, J = 4.3 Hz, 2H),
7.68 (m, 2H), 7.52 (m, 2H), 7.04 (t, J = 7.3 Hz, 3H), 6.69 (t, J = 7.3 Hz, 6H),
4.65 (m, 6H), 4.13 (q, J = 7.1 Hz, 1H). 19F NMR (400 MHz, CDCl3, δ, ppm):
-136.88 (dd, J = 23.8, 6.1 Hz, 2F), -137.87 (dd, J = 24.4, 6.2 Hz, 2F), -
154.21 (t, J = 20.9 Hz, 2F), -162.21 (m, 2F), -162.63 (m, 2F). HRMS-ESI
([M+H]+): 1025.1284; calcd for C55H29CoF10N4P: 1025.1297. UV-Vis
(CH2Cl2) λmax/nm (log ε): 381 (4.52), 557 (3.82), 588 (3.70).
Synthesis of meso-5,15-bis(pentafluorophenyl)-10-phenyl)corrole (F10C).
The compound was prepared according to the reported procedure.[33] 1H
NMR (400 MHz, CDCl3, δ, ppm): 9.11 (t, J = 4.2Hz, 2H), 8.71 (s, J = 3.8Hz,
4H), 8.57 (d, J=3.8Hz, 2H), 8.23-8.14(m, 2H), 7.80-7.73(m, 3H). HRMS-
Synthesis of 5,15-bis(4-(2-hydroxyethyl)-amino-2,3,5,6-tetrafluorophenyl)-
10-phenyl)corrole Co(III)-PPh3 (F8CCo). F8CCo was prepared by using the
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10.1002/ejic.201900996
European Journal of Inorganic Chemistry
FULL PAPER
same method to F15CCo. Purple product were obtained with a yield of 89%
(25 mg). 1H NMR (400 MHz, CD3OD, δ, ppm): 9.12 (s, 2H), 8.89 (s, 2H),
8.75 (s, 2H), 8.62 (s, 2H), 8.19 (s, 2H), 8.05 (d, J = 26.5 Hz, 2H), 8.00 (m,
4H), 7.49 (s, 3H), 7.17 (s, 6H), 5.23 (s, 6H), 4.48 (s, 4H), 4.29 (s, 4H), 3.96
(s, 1H). 19F NMR (400 MHz, CD3OD, δ, ppm): -142.34 (s, 4F), -160.30 (q,
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=
11.2 Hz, 4F). HRMS-ESI ([M+H]+): 1107.2213; calcd for
C59H41CoF8N6O2P: 1107.2227. UV-Vis (CH2Cl2) λmax/nm (log ε): 404 (4.69),
560 (3.99), 580 (3.86).
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This research was funded by the National Natural Science
Foundation of China (No. 21671068).
Keywords: corrole • cobalt • electrochemistry • hydrogen
evolution • molecular catalyst
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FULL PAPER
Electrocatalytic hydrogen evolution
Four free-base corroles bearing
hydroxyethyl amino groups and their
cobalt(III) complexes were served as
Ying Chen, Qi-Hang Fan, Md Sahadat
Hossain, Shu-Zhong Zhan and Hai-Yang
Liu, * Li-Ping Si.*
promising
electrocatalysts
for
hydrogen evolution in homogeneous
system using acetic acid, trifluoroacetic
acid and water as proton sources.
Free-base corrole was firstly reported
as electrocatalyst for H2 evolution.
Page No.1– Page No.8
Electrocatalytic Hydrogen Evolution
of Cobalt and Free-base Triaryl
Corrole Bearing Hydroxyethyl Amino
Groups
*corrole, hydrogen evolution
9
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