Meso- and axially-modified IrIIItriarylcorroles with tunable electrocatalytic properties

Article abstract of DOI:10.1016/j.dyepig.2019.108124

The synthesis of three A₂B type Ir(III)triarylcorroles with meso-aryl substituents that provide electron donating (push) and withdrawing (pull) properties and three A₃ type Ir^IIItriphenylcorroles with differing pyridine axial ligands is reported, along with their structural characterization. An analysis of the structure-property relationships in the optical and redox properties has been carried out by comparing their optical spectroscopy and electrochemistry to trends predicted in DFT and TD-DFT calculations. The results demonstrate that A₂B type Ir^IIItriarylcorroles are highly efficient electrocatalyzed oxygen reduction reactions (ORRs) and that their reactivity can be modulated by modulating the electronic structure by changing the nature of the meso-substituent at the B-positions, and even axial pyridine ligands.

Full text of DOI:10.1016/j.dyepig.2019.108124

Dyes and Pigments 175 (2020) 108124
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Meso- and axially-modified IrIIItriarylcorroles with tunable
electrocatalytic properties
,
,
Xifeng Zhanga, Yu Wanga, Weihua Zhua **, John Mackb ***, Rodah C. Soyb, Tebello Nyokongb,
,c,*
Xu Lianga
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China
b Institute for Nanotechnology Innovation, Department of Chemistry, Rhodes University, Makhanda, 6140, South Africa
c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The synthesis of three A2B type Ir(III)triarylcorroles with meso-aryl substituents that provide electron donating
(push) and withdrawing (pull) properties and three A3 type IrIIItriphenylcorroles with differing pyridine axial
ligands is reported, along with their structural characterization. An analysis of the structure-property relation-
ships in the optical and redox properties has been carried out by comparing their optical spectroscopy and
electrochemistry to trends predicted in DFT and TD-DFT calculations. The results demonstrate that A2B type
IrIIItriarylcorroles are highly efficient electrocatalyzed oxygen reduction reactions (ORRs) and that their reac-
tivity can be modulated by modulating the electronic structure by changing the nature of the meso-substituent at
the B-positions, and even axial pyridine ligands.
Ir(III)Corrole
Spectroscopy
Electrochemistry
TD-DFT calculations
Electrocatalysis
1. Introduction
and environmentally friendly properties [16–18]. Recent research has
identified the coordination complexes of transition metal ions as being
Corroles are tetrapyrrolic ligands that provide sterically constrained
environments for coordinating central atoms because in contrast with
porphyrins they have three inner N–H protons, since a trianionic ligand
potentially suitable for electrocatalysis, but their low catalytic effi-
ciencies and insufficient stability in the concentrated acid environments
that are used in this context has limited further development in this field.
For this reason, researchers are currently trying to combine these com-
pounds with inorganic and/or organic nanomaterials to create suitable
catalytic nanocomposites, such as reduced graphene oxides (rGOs),
carbon nanotubes and γ-C3N4, due to their large surface area, high
mechanical strength, structural flexibility and electrical conductivity
[19–21].
is required to form an 18 π-electron system on the inner perimeter when
there are only three meso-carbons [1–4]. Various groups can be intro-
duced at the meso-, β-, and/or axial positions to modify the structures of
the corrole ligands and their metal complexes [5–8]. Interest in corrole
coordination chemistry is not only driven by their unusual structures
and topologies, but also by their interesting properties. The trianionic
corrole ligand stabilizes numerous metals in high oxidation states,
making them suitable for use in bioimaging and biosensors, optical and
magnetic materials, and in catalysis [9–12].
Herein, the synthesis of three A2B type IrIIItriarylcorroles with push-
pull meso-aryl substituents is reported, and characterization of their
properties has been carried out using optical and redox measurements.
Large shifts in the wavelengths of the main spectral bands and in the
redox potentials have been observed for synthetic metallocorroles. In
addition, three A3 type Ir(III)triphenylcorroles with differing p-
substituted pyridine axial ligands have also been prepared. The trends
predicted in the frontier MO energies by theoretical calculations are
compared to those observed experimentally to provide information that
The use of renewable energy sources can be balanced with demand
through the use of fuel cells to decrease the environmental pollution and
greenhouse effect. This has many advantages and is widely considered to
be the energy source that is most likely to replace the use of fossil fuels in
the decades ahead [13–15]. Molecular electrochemical catalysis has
received considerable attention due to its low cost, and its renewable
* Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China. Tel./fax: þ86 511 8879 1928.
** Corresponding author.
*** Corresponding author.
E-mail addresses: sayman@ujs.edu.cn (W. Zhu), j.mack@ru.ac.za (J. Mack), liangxu@ujs.edu.cn (X. Liang).
https://doi.org/10.1016/j.dyepig.2019.108124
Received 22 June 2019; Received in revised form 3 December 2019; Accepted 10 December 2019
Available online 14 December 2019
0143-7208/© 2019 Published by Elsevier Ltd.

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
can guide the design of future group 9 corrole electrocatalysts for use in
ORRs. The electrocatalytic behavior of IrIIItriarylcorrole/rGO-graphene
(IrIIIcorrole/rGO) composites prepared using the three A2B type, and
three A3 type IrIIItriarylcorroles will be described in the context of
electrochemically catalyzed redox reactions.
corroles (4a-c), cyclic voltammetry (CV) measurements were carried out
in low-polar o-dichlorobenzene (o-DCB) containing 0.1 M tetra-n-buty-
lammonium perchlorate ([NBu]ClO4; TBAP) as a supporting electrolytes
(Fig. 5). As shown in Fig. 5 and Table 1, the voltammogram of (py2)
IrIIItriarylcorrole 4a contains a reversible reductive process which can
be
assigned
as
the
ring
reduction
of
[IrIIItriarylcorrole/[IrIIItriarylcorrole]À , while two reversible oxidation
processes are also observed at þ0.52 and þ 1.27 V, respectively. These
two oxidation processes can be assigned to corrole ring oxidation and a
valence change of IrIII/IrIV, respectively [35].
2. Results and discussion
2.1. Structural characterization
Broadly similar redox potential changes are observed for the three
meso-modified IrIIItriarylcorroles 2a-c and the three axially modified
IrIIItriphenylcorroles 4a-c with the exception of the reductive potential
for 4b at À 1.37 and À 1.59 V which can be assigned to the reduction of p-
cyanopyridine. The change of reduction curves of 4b could be assigned
as back-donation from p-cyanopyridine ligand to IrIII center. This was
also spectroscopic observed by expanded soret-band absorption and
usual stabilization of LUMO orbitals from TD-DFT calculations. This is
consistent with the MO energy predictions for the axial ligands of this
complex (Fig. 3). The trends observed in the gaps between the first
reduction and oxidation steps are consistent with the relatively minor
shifts of the main Q and B bands that are observed spectroscopically for
2a-c and 4a-c (Fig. 1) and are predicted in theoretical calculations
(Fig. 4), since the inductive effects of the meso-aryl rings on the energies
of the frontier MOs are similar in each case. Cyclic voltammograms for
2a-c and 4a-c were recorded at scan rates of 50–500 mV/s, and this
demonstrates that the oxidation and reduction processes are diffusion
controlled (Figs. S2–S4, see Supporting Information).
Free base corrole compounds 1 and 3 were synthesized according to
the reported literature procedures [18]. IrIIItriphenylcorroles with
p-functionalized pyridines as axial ligands were prepared through met-
alation and were purified by silica gel column chromatography and
recrystallization. The isolated yield of IrIIItriphenylcorroles are slightly
lower than other first-transition metallocorroles, due to the final prod-
ucts were partially decomposed during purification processes.
MALDI-TOF MS data for 2a contains an intense parent peak at m/z ¼
1010.44 (Calcd. [M]þ ¼ 1010.40) providing direct evidence that the
(py)2Ir(III)-5,15-(p-trifluoromethylphenyl)-10-phenylcorrole was suc-
cessfully synthesized. Similar MALDI-TOF MS peaks were observed for
IrIIItriarylcorroles 2b-c and 4a-c. The proton signals for the meso-sub-
stituents and pyrrole rings of 4a mainly lie beyond 7.40 ppm in the
aromatic region.
2.2. Electrochemical and electrocatalytic characterizations
A series of IrIIIcorrole/rGO composites were prepared and examined
as electrocatalysts for ORRs (Fig. 6). Firstly, in the case of electro-
catalyzed ORRs, it is noteworthy that the voltammograms of the stan-
dard 4a/rGO composites contain significant catalytic peaks beyond the
IrIIIcorrole/[IrIIIcorrole]À step with an overpotential of À 393 mV. Upon
introducing electron-withdrawing trifluoromethylphenyl groups at
5,15-positions, there is a clear positive shift of the catalytic peak for the
IrIIIcorrole/rGO for 2a with an overpotential of À 285 mV. More positive
The redox properties of a large number of group 9 metallocorrole
derivatives have been reported previously, including those with Co(III),
Rh(III) and Ir(III) central ions [29–31]. The investigation of the struc-
tural chemistry of group 9 corroles has usually provided results that are
consistent with a classic low-spin d6 configuration description with
low-energy open-shell states also readily accessible for many of these
complexes, and the four-coordinate group 9 corroles are only observed
as intermediates in solution [32–34]. In order to gain further insight into
the electronic structures of the meso- (2a-c) and axially-modified Ir(III)
Fig. 1. Magnetic circular dichroism (top) and UV–vis absorption spectra (bottom) of 2a-c and 4a-c in CH2Cl2.
2

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
methoxypyridine causes a significant negative shift of the overpotential
to À 446 mV due to the destabilization of the π-MOs.
It is clear from the investigation of the electrocatalytic properties of
the nanocomposites that the introduction of electron donating and/or
withdrawing substituents at the meso- and axial positions of group 9
corroles has a much larger effect on the electrocatalytic properties than
it does on their optical and redox properties. In the context of nano-
composites, such as porphyrinoid-functionalized rGOs, electron transfer
Fig. 2. Angular nodal patterns and MO energies of the frontier-MOs of model
complex 5 at the CAM-B3LYP/SDD level of theory.
Fig. 4. TD-DFT calculated spectral of model complex 5, 2a-c and 4a-c at the
CAM-B3LYP/SDD level of theory. Red and black diamonds are used to highlight
the Q and B bands and MLCT bands, respectively. The green and blue diamonds
denote transitions into MOs localized on the axial ligands and meso-aryl rings,
respectively. The Chemcraft program was used to generate the simulated
spectra by using a fixed bandwidth of 2000 cmÀ 1 [25–28]. (For interpretation
of the references to colour in this figure legend, the reader is referred to the
Web version of this article.)
overpotentials are also observed for the IrIIIcorrole/rGO composites of
2b and 2c at À 243 and À 178 mV, respectively. When the electronic
structure was modulated by introducing p-functionalized pyridines at
the axial positions, modified electrocatalyzed ORR behavior was also
observed. In the presence of electron-withdrawing p-cyanopyridine was
introduced, the significant stabilization of the
π-MOs causes an over-
potential at À 387 mV, while introducing electron donating p-
Fig. 3. MO energies of the frontier-MOs of model
complex 5, 2a-c and 4a-c at the CAM-B3LYP/SDD
level of theory. The a, s, -a and -s MOs are high-
lighted with thicker black lines, while blue lines are
used for the 3dxz and 3dyz MOs. The other unoccu-
pied MOs shown are localized on the axial or phenyl
rings. Small black diamonds denote occupied MOs,
while black circles are used to highlight the s and -s
MOs. The average HOMOÀ LUMO gap values that
take into account all four frontier π-MOs are high-
lighted with red diamonds and are plotted against a
secondary axis. (For interpretation of the references
to colour in this figure legend, the reader is referred
to the Web version of this article.)
3

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
Fig. 5. Cyclic voltammograms for IrIIItriarylcorroles 2a-2c and IrIIItriphenylcorroles 4a-c in o-dichlorobenzene (o-DCB) containing 0.1 M TBAP as a supporting
electrolyte.
channels may be the most important factor [36,37].
3. Conclusions
redox properties of three A2B type IrIIItriarylcorroles with push-pull
meso-aryl substituents and three A3 type IrIIItriphenylcorroles with
differing pyridine axial ligands have been described. An analysis of the
electronic structure and structure-property relationships has been car-
ried out by comparing their optical spectroscopy and electrochemistry to
In summary, the synthesis and characterization of the optical and
4

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
recommended by Piepho and Schatz are used to describe the sign of the
Faraday terms, so the sign of the B0 terms matches that of the MCD
signal [22].
Table 1
Redox potentials observed for IrIIItriarylcorroles 2a-c and 4a-c in volts.
E1/2 OxII
E1/2 OxI
E1/2 RedI
E1/2 RedII
E1/2 OxI À RedI
2a
2b
2c
4a
4b
4c
þ1.26
þ1.29
þ1.22
þ1.27
þ1.21
þ1.25
þ0.58
þ0.59
þ0.57
þ0.52
þ0.59
þ0.51
À 1.71
À 1.68
À 1.71
À 1.95
À 1.37
À 1.99
–
2.29
2.27
2.28
2.47
–
4.2. Preparation of modified electrodes
–
–
–
1.0 mg of rGO was mixed with 1 mL of isopropyl alcohol containing
0.2% Nafion and the mixture sonicated in an ultrasonic bath for 30 min
to produce a homogeneous mixture of concentration 1 mg/mL. The
À 1.59
–
2.50
surface of the glassy carbon electrode (GCE) was polished with 0.05 μm
alumina and rinsed with doubly distilled water in the ultrasonic bath to
remove any adhered Al2O3 particles. The electrodes were rinsed with
ethanol and dried under room temperature for ca. 5 min. Three 3 μL of
the rGO/isopropyl alcohol/Nafion suspensions was drop cast on the
surface of the GC electrode and allowed to dry at room temperature. 10
μ
L aliquots of 0.2 mM dichloromethane solutions of 2a-c and 4a-c were
added dropwise to the rGO/Nafion-coated electrodes and dried at room
temperature for 1 h. The electrodes were stored in MilliQ water in the
dark.
4.3. Computational methods
The Gaussian 09 software package [23] was used to carry out DFT
geometry optimizations for 2a-c and 4a-c and a model complex 5
(Scheme S1, see ESI) with meso-trifluoromethylphenyl rings at both the
B- and A2-positions by using the B3LYP functional with SDD basis sets.
TD-DFT calculations were carried out by using CAM-B3LYP functional,
which includes a long-range correction of the exchange potential, since
this provides more accurate results for complexes that have excited
states with significant intramolecular charge transfer character. The
Chemcraft program [24] was used to generate simulated spectra.
Fig. 6. CV measurements for the 2a-c/rGO and 4a-c/rGO nanocomposites in
0.1 M NaOH aqueous solution.
4.4. Synthesis and characterizations
trends predicted in DFT and TD-DFT calculations. Further investigations
with IrIIIcorrole/rGO composites demonstrated that while push-pull A2B
-
type IrIIItriarylcorroles and axially-modified A3 type IrIII
4.4.1. Synthesis of bis(pyridine)Ir(III)-5,15-(p-trifluoromethylphenyl)-10-
phenylcorrole, 2a
triphenylcorroles have broadly similar redox and optical properties,
large differences are observed in their electrocatalytic properties of the
rGO nanocomposites. Their reactivity in this regard can be modulated by
changing the nature of the meso-substituent at the B-positions or by
Synthesis of meso-p-trifluoromethylphenyl-dipyrromethane followed
the literature reported procedure. meso-p-trifluoromethylphenyl-dipyr-
romethane (2 mmol, 0.6106 g) and benzaldehyde acid (1 mmol, 0.2 g)
were stirred in a mixed methanol/5% HCl acid (v:v ¼ 1:1) solution for 1
h. The organic components were extracted with CH2Cl2, and the solvent
was fully evaporated. The residue was dissolved in 10 mL CH2Cl2 and p-
chloranil was added and the solution was heated to reflux at 55 �C for 5 h
to give the 5,15-(p-trifluoromethylphenyl)-10-phenylcorrole 1a in
67.2% yield (0.1546 g). 1a (0.0331 g, 0.05 mmol), [Ir(cod)Cl]2 (0.1679
g, 0.25 mmol) and anhydrous K2CO3 (0.0691 g, 0.5 mmol) were dis-
solved in a 40 mL anhydrous THF, and refluxed at 85 �C under N2 for 90
min. Once fluorescence was no longer observed, pyridine (0.4746 g, 0.6
mmol) was added. After removal of the organic solvent, purification by
silica gel column chromatography gave the deep purple 2a target
compound in 6.9% yield (0.0519 g). MALDI-TOF-mass: m/z ¼ 1010.44
(Calcd. [M]þ ¼ 1010.40). 1H NMR (400 MHz, CDCl3, 298K): δ , 8.89 (s,
2H), 8.65 (d, J ¼ 4.9 Hz, 2H), 8.43 (s, 2H), 8.36 (d, J ¼ 7.2 Hz,H4H), 8.20
(s, 5H), 8.01 (d, J ¼ 7.9 Hz, 4H), 7.95 (d, J ¼ 8.0 Hz, 2H), 6.13 (d, J ¼
7.5 Hz, 2H), 5.19 (d, J ¼ 7.4 Hz, 4H), 1.72 (d, J ¼ 5.8 Hz, 4H).
-
changing the axial pyridine ligands in the context of A3 type IrIII
triphenylcorroles. These properties suggest that the electrocatalytic
properties of group 9 corrole nanocomposites merit further in-depth
study since it is apparent that a fine-tuning of the electrocatalytic
properties of ORRs can be achieved in a rational manner on this basis
once the key structure-property relationships have been identified.
4. Experimental section
4.1. General considerations
1H NMR spectra were recorded on a Bruker AVANCE 400 spec-
trometer (400.03 MHz). Residual solvent peaks were used to provide
internal references (δ ¼ 7.26 ppm for CDCl3). All reagents and solvents
used were of reagent grade and were used as received unless noted
otherwise. Cyclic voltammetry was carried out on a Chi-730D electro-
chemistry station with a three-electrode cell. A glassy carbon disk, a
platinum wire and an Ag/AgCl electrode were used as the working,
counter and reference electrodes, respectively. An inert nitrogen atmo-
sphere was introduced during all of the electrochemical measurements,
which were carried out at room temperature. The UV and visible regions
of the electronic absorption spectra were recorded with an HP 8453A
diode array spectrophotometer. A JASCO J-815 spectrodichrometer
equipped with a JASCO permanent magnet (1.6 T) was used to measure
magnetic circular dichroism (MCD) spectra. Spectra were recorded
using both parallel and antiparallel fields. The conventions
4.4.2. Synthesis of bis(pyridine)Ir(III)-5,15-(p-trifluoromethylphenyl)-10-
pentafluorophenyl-corrole, 2b
The synthetic procedure is the same as for 2a, with penta-
fluorobenzaldehyde used instead of benzaldehyde, and the target com-
pound was obtained in a 30.3% yield (0.053 g). MALDI-TOF-MS: m/z ¼
1099.97 (Calcd. [M]þ ¼ 1099.99). 1H NMR (400 MHz, CDCl3, 298K): δ ,
H
8.93 (dd, J ¼ 14.4, 4.2 Hz, 2H), 8.70–8.58 (m, 2H), 8.47 (s, 2H), 8.39 (d,
J ¼ 7.6 Hz, 2H), 8.34–8.23 (m, 4H), 8.03 (d, J ¼ 8.1 Hz, 2H), 7.97 (d, J ¼
6.5 Hz, 2H), 6.17 (dd, J ¼ 14.6, 7.3 Hz, 2H), 5.23 (dt, J ¼ 10.5, 7.2 Hz,
4H), 1.78 (dd, J ¼ 39.1, 5.5 Hz, 4H).
5

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
Scheme 1. Synthesis of Ir(III)corroles 2a-c and 4a-c.
4.4.3. Synthesis of bis(pyridine)Ir(III)-5,15-(p-trifluoromethylphenyl)-10-
methoxyphenyl-corrole, 2c
4.4.4. Synthesis of bis(pyridine)iridium(III)5,10,15-triphenylcorrole, 4a
Synthesis of 5,10,15-triphenylcorrole 3 followed the literature re-
ported procedure. Free base corroles 3 (0.0263 g, 0.05 mmol), [Ir(cod)
Cl]2 (0.1679 g, 0.25 mmol) and anhydrous K2CO3 (0.0691 g, 0.5 mmol)
were dissolved in 40 mL of anhydrous THF and refluxed at 85 �C under
N2 for 90 min. After the disappearance of fluorescence, pyridine (0.4746
g, 0.6 mmol) was added. After removal of the organic solvent, purifi-
cation by silica gel column chromatography gave the pure deep solid-
state compound 4a in 25% yield (0.0519 g). MALDI-TOF-MS: m/z ¼
874.72 (Calcd. [M]þ ¼ 874.04). 1H NMR (400 MHz, CDCl3, 298K): δ ,
8.83 (d, J ¼ 4.1 Hz, 2H), 8.66 (d, J ¼ 4.8 Hz, 2H), 8.42 (d, J ¼ 4.7 HHz,
The synthetic procedure is the same as 2a, with 4-methoxybenzalde-
hyde used instead of benzaldehyde. The target compound was obtained
in a 32.5% yield (0.0930 g). MALDI-TOF-MS: m/z ¼ 1040.15 (Calcd.
[M]þ ¼ 1040.06). 1H NMR (400 MHz, CDCl3, 298K): δ , 8.88 (d, J ¼ 4.2
Hz, 2dcH), 8.64 (d, J ¼ 4.8 Hz, 2H), 8.45 (d, J ¼ 4.8 HzH, 2H), 8.38 (d, J ¼
7.9 Hz, 4H), 8.28 (d, J ¼ 4.2 Hz, 2H), 8.23 (d, J ¼ 7.9 Hz, 2H), 8.00 (d, J
¼ 8.0 Hz, 4H), 7.94 (d, J ¼ 8.0 Hz, 2H), 6.13 (t, J ¼ 7.6 Hz, 2H), 5.19 (t,
J ¼ 7.2 Hz, 4H), 3.75 (q, J ¼ 7.2 Hz, 6H), 1.70 (d, J ¼ 5.5 Hz, 4H).
6

X. Zhang et al.
Dyes and Pigments 175 (2020) 108124
2H), 8.25 (d, J ¼ 7.1 Hz, 5H), 8.07 (d, J ¼ 7.0 Hz, 2H), 7.74 (t, J ¼ 7.5
Hz, 4H), 7.69–7.50 (m, 6H), 6.08 (ddd, J ¼ 7.6, 4.6, 1.5 Hz, 2H),
5.22–5.12 (m, 4H), 1.81 (dd, J ¼ 6.7, 1.3 Hz, 4H).
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Declaration of competing interest
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This work was financially supported by the National Natural Science
Foundation of China (21701058), the Natural Science Foundation of
Jiangsu Province (BK20160499), the State Key Laboratory of Coordi-
nation Chemistry (SKLCC1817), the Key Laboratory of Functional
Inorganic Material Chemistry (Heilongjiang University) of Ministry of
Education, the China post-doc foundation (2018M642183), the Lanzhou
High Talent Innovation and Entrepreneurship Project (2018-RC-105)
and the Jiangsu University (17JDG035). Theoretical calculations were
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Appendix A. Supplementary data
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