Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide complexes as efficient and durable water oxidation catalysts

Article abstract of DOI:10.1016/j.cattod.2016.11.027

A series of phenylene-linked iridium bis-pyridine-2-sulfonamide (bpsa) complexes substituted with electron-donating and electron-withdrawing groups and a new bpsa complex containing naphthalene linkage were synthesized and investigated as efficient and robust water oxidation catalysts. It was found that the oxidation potentials of these substituted complexes are within a broad range, which reflects their effectively tuned electronic structures. Under various water oxidation conditions, diverse kinetics behaviors are observed. The reaction mechanism strongly depends on the structure of the catalyst, and the nature of the oxidant as well as their ratios and concentrations. As evidenced by cyclic voltammetry and DFT calculations, the addition of electron-donating substituents greatly destabilizes the HOMO of the catalyst and thus facilitates the initial oxidation process. On the other hand, the limited driving force provided by the oxidized state of an electron-rich catalyst also impedes efficient water oxidation reactions. The robustness of a water oxidation catalyst is critical, and long-term reactions have exhibited turnover numbers (TONs) of up to 16,200.

Full text of DOI:10.1016/j.cattod.2016.11.027

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Catalytic water splitting
Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide
complexes as efficient and durable water oxidation catalysts
Mo Li, Stefan Bernhard∗
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, 15213, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phenylene-linked iridium bis-pyridine-2-sulfonamide (bpsa) complexes substituted with
electron-donating and electron-withdrawing groups and a new bpsa complex containing naphthalene
linkage were synthesized and investigated as efficient and robust water oxidation catalysts. It was found
that the oxidation potentials of these substituted complexes are within a broad range, which reflects
their effectively tuned electronic structures. Under various water oxidation conditions, diverse kinetics
behaviors are observed. The reaction mechanism strongly depends on the structure of the catalyst, and
the nature of the oxidant as well as their ratios and concentrations. As evidenced by cyclic voltamme-
try and DFT calculations, the addition of electron-donating substituents greatly destabilizes the HOMO
of the catalyst and thus facilitates the initial oxidation process. On the other hand, the limited driving
force provided by the oxidized state of an electron-rich catalyst also impedes efficient water oxidation
reactions. The robustness of a water oxidation catalyst is critical, and long-term reactions have exhibited
turnover numbers (TONs) of up to 16,200.
Received 25 August 2016
Received in revised form 4 November 2016
Accepted 7 November 2016
Available online xxx
Keywords:
Water oxidation
Iridium bpsa complex
Tunability
Electronic structure
HOMO energy
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
ligand structure render molecular water oxidation catalysis an
appealing target [28,29]. Early work on transition metal water oxi-
The constantly growing demand for energy has trigged intensive
and wide-spread research to discover sustainable solar energy con-
version strategies [1–6]. The probably most attractive solution is
the storage of solar energy in chemical bonds to fuels [7–11]. For all
the potential artificial photosynthetic systems such as water split-
ting [12–17], carbon dioxide reduction [18–21] and photobiological
production of fuels [22,23], the complementary oxidative half-
reaction is critical. A desirable oxidative process that can be coupled
to various reductive pathways for fuel production is the produc-
tion of molecular oxygen from water, an abundant and benign
feedstock. Besides, the direct use of atmospheric O2 in fuel cell
or combustion applications makes water oxidation an appealing
goal, rendering it superior to many other currently studied oxida-
tive processes such as halogenide oxidation [24,25] and alcohol
dehydrogenation [26,27]. Significant efforts have been devoted on
developing water oxidation catalysts, great challenges still remain
for overcoming the vast energy barrier and the inherent complexity
of orchestrating the sequential steps of transferring four electrons
and four protons per molecule of formed oxygen.
dation catalysts (WOCs) focused on di- and tetrametallic complexes
[30–36], making use of the metal-metal cooperativity to reduce the
redox demand on each metal center. As more and more single site
catalysts with well-designed ligand framework have been reported
more recently [37–60], it has been well accepted that the presence
of linked metal centers is not required. Among these monometal-
lic complexes, iridium-based WOCs are of great interest because of
their remarkable water oxidation reactivity and robustness under
the harsh oxidizing environments required for driving these reac-
tions.
Up
dine
to
date,
cyclometalated
[42], half-sandwich
iridium
phenylpyri-
iridium Cp*
complexes
(Cp* = pentamethylcyclopentadienyl) complexes [43,44,47,48,61],
iridium carboxylates [62,63] and recently reported wrap-around
iridium bis-pyridine-2-sulfonamide (bpsa) complexes [49] repre-
sent some of the major classes of efficient iridium-based WOCs.
A variety of iridium Cp* complexes with diverse ligand systems
have been shown to catalyze water oxidation with extraordinarily
high activities compared to the iridium phenylpyridine complexes.
However, several degradation pathways of the Cp* ligand leading
to structural modifications or even complete loss of the Cp* ligand
have been documented, which can trigger concerns of molecular
catalyst transforming into active colloids [64–68]. Careful dynamic
light scattering (DLS) studies [48,49,69] have confirmed the molec-
ular nature of some catalysts, but illuminating the mechanism
The advantages of numerous characterization methods avail-
able in homogeneous system and the synthetic tunability of the
∗
Corresponding author.
E-mail address: bern@cmu.edu (S. Bernhard).
http://dx.doi.org/10.1016/j.cattod.2016.11.027
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Li, S. Bernhard, Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide complexes as efficient
and durable water oxidation catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.027

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involved in water oxidation reaction catalyzed by a variety of
emerging species is difficult and such a transformation greatly
hinders the development of solar fuel systems.
were obtained from commercial sources and used without further
purification. IrCl3·xH2O was purchased from Pressure Chemical
Co. and used as received.
The newly designed iridium bpsa complexes supported by
a resilient wrap-around ligand platform effectively addressed
the ligand instability issues, demonstrating remarkably stable
performance in water oxidation [49]. Electrochemical quartz crys-
tal microbalance (EQCM) and dynamic light scattering (DLS)
experiments conclusively ruled out the formation of active IrOx
nanoparticles [49]. The robust structure of bpsa ligand framework,
capable to stabilize the high valent metal oxo intermediates formed
during water oxidation through extended charge delocalization,
offers a promising direction in future catalysts design.
Based on our previous work on these iridium bpsa complexes
[49], systematic modification of the sulfonamide linker moiety
(Fig. 1) allowed the synthetic tuning of the electronic properties
of these complexes. Phenylene-linked bpsa ligands with electron-
donating and electron-withdrawing groups as well as a new bpsa
ligand H2bpsa-NPTH with naphthalene linkage were synthesized
and used to prepare the corresponding iridium complexes 1-5
(Fig. 1). The catalytic performance of these substituted iridium
bpsa complexes were evaluated under various conditions, high-
lighting the robustness and efficiency of these systems. Through
cyclic voltammetry and DFT calculations, the electronic properties
of these complexes were explored, which implicitly illustrates the
effect of the different substituents on the ability to catalyze water
oxidation.
2.2. Complexes synthesis
2.2.1. Synthesis of pyridine-2-sulfonyl chloride
Pyridine-2-sulfonyl chloride was prepared according to the
previously published procedure [49]. Typically, a solution of 2-
mercaptopyridine (3.24 g, 29.2 mmol) in 20 mL dichloromethane
was combined with concentrated hydrochloric acid (81 mL), and
the mixture was cooled to 0 ◦C in an ice bath. Sodium Hypochlo-
rite (14.5%, 178 mL) was added dropwise and the reaction mixture
which turned yellow gradually was stirred for another 30 min
at 0 ◦C. The resulting solution mixture was extracted twice with
dichloromethane (∼30 mL), and the organic layers were combined
and dried over sodium sulfate for half an hour. The solid was filtered
off, and the solvent was mostly removed in a rotary evaporator,
yielding a colorless liquid (∼3 mL). The resulting pyridine-2-
sulfonyl chloride solution was immediately used for preparing the
following bis(pyridine-2-sulfonamide) ligands with various sub-
stituents due to its moderate stability in the presence of air or water.
Yield: 74%. 1H NMR (500 MHz, CDCl3): ı 8.84 (d, J = 4.5 Hz, 1H), 8.13
(d, J = 7.7 Hz, 1H), 8.07 (td, J = 7.3, 1.5 Hz, 1H), 7.71 (ddd, J = 7.1, 4.8,
1.3 Hz, 1H), 5.30 (s, dichloromethane).
2.2.2. Synthesis of bis(pyridine-2-sulfonamide) (bpsa) ligands
Following a modified procedure based on the method described
in the previous work [49], the appropriate diamine (0.5 equiv) was
added slowly to a solution of the colorless pyridine-2-sulfonyl chlo-
ride resulting from previous steps in 10 mL pyridine, which was
cooled to 0 ◦C in an ice bath. The resulting mixture was stirred
overnight while it gradually warmed up to room temperature. A
large amount of precipitate was formed upon slow addition of puri-
fied water to the reaction mixture. The precipitate was collected via
vacuum filtration and washed with diethyl ether. The crude prod-
uct was dissolved in minimal amount of hot ethanol, and then was
stored in a refrigerator overnight for crystallization.
2. Experimental
2.1. Materials
2-Mercaptopyridine, sodium hypochlorite (14.5%), 1,2-
phenylenediamine,
4,5-difluoro-1,2-phenylenediamine,
4,5-dichloro-1,2-phenylenediamine,
4,5-dimethyl-1,2-
phenylenediamine, 2,3-diaminonaphthalene and all solvents
N,Nꢀ-(1,2-phenylene)bis(pyridine-2-sulfonamide) (H2bpsa-Ph).
Yield: 67.4%, white crystals. 1H NMR (500 MHz, CDCl3): ı 10.37 (s,
2H), 8.77 (d, J = 4.9 Hz, 2H), 8.07 (td, J = 7.9, 1.0 Hz, 2H), 8.00 (dt,
J = 7.9, 1.6 Hz, 2H), 7.59 (ddd, J = 7.6, 4.9, 1.0 Hz, 2H), 7.56 (dd, J = 6.0,
3.6 Hz, 2H), 7.21 (dd, J = 6.1, 3.5 Hz, 2H). 13C NMR (125 MHz, CDCl3):
ı 157.5, 148.4, 139.6, 130.2, 127.8, 127.7, 127.4, 122.7. ESI–MS (m/z,
1% trifluoroacetic acid in MeOH): 413.1 (100%, H2bpsa-Ph-Na+),
391.1 (50%, H2bpsa-Ph).
N,Nꢀ-(4,5-dichloro-1,2-phenylene)bis(pyridine-2-sulfonamide)
(H2bpsa-PhdCl). Yield: 48.2%, white crystals. 1H NMR (500 MHz,
CDCl3): ı 10.41 (s, 2H), 8.75 (d, J = 4.9 Hz, 2H), 8.09 (td, J = 7.9, 0.9 Hz,
2H), 8.05 (dt, J = 7.8, 1.6 Hz, 2H), 7.68 (s, 2H), 7.63 (ddd, J = 7.4,
4.9, 1.3 Hz, 2H). 13C NMR (125 MHz, CDCl3): ı 157.1, 148.7, 139.9,
131.7, 129.7, 129.0, 127.7, 122.8. ESI–MS (m/z, 1% trifluoroacetic
acid in MeOH): 480.9 (100%, H2bpsa-PhdCl-Na+), 458.9 (74%,
H2bpsa-PhdCl).
N,Nꢀ-(4,5-difluoro-1,2-phenylene)bis(pyridine-2-sulfonamide)
(H2bpsa-PhdF). Yield: 26.8%, brown crystals. 1H NMR (500 MHz,
CDCl3): ı 10.41 (s, 2H), 8.75 (d, J = 4.8 Hz, 2H), 8.09 (td, J = 7.8,
0.9 Hz, 2H), 8.04 (dt, J = 7.8, 1.5 Hz, 2H), 7.63 (ddd, J = 7.4, 4.9, 1.3 Hz,
2H), 7.42 (t, J = 9.3 Hz, 2H). 13C NMR (125 MHz, CDCl3): ı 157.1,
148.7, 139.9, 127.7, 126.6, 122.8, 116.6, 116.5. ESI–MS (m/z, 1%
trifluoroacetic acid in MeOH): 449.0 (100%, H2bpsa-PhdF-Na+),
427.0 (58%, H2bpsa-PhdF).
Fig. 1. Schematic structures of bis-pyridine-2-sulfonamide ligands H2bpsa-
Ph, H2bpsa-PhdCl, H2bpsa-PhdF, H2bpsa-PhdMe, H2bpsa-NPTH (top) and the
corresponding Ir(III) complexes [Ir(bpsa-Ph)Cl2]− 1, [Ir(bpsa-PhdCl)Cl2]− 2, [Ir(bpsa-
N,Nꢀ-(4,5-dimethyl-1,2-phenylene)bis(pyridine-2-
PhdF)Cl2]− 3, [Ir(bpsa-PhdMe)Cl2]− 4, [Ir(bpsa-NPTH)Cl2]−
5 (bottom). All the
sulfonamide) (H2bpsa-PhdMe). Yield: 81.8%, off-white crystals.
1H NMR (500 MHz, CDCl3): ı 10.22 (s, 2H), 8.76 (d, J = 4.6 Hz, 2H),
8.06 (d, J = 7.9 Hz, 2H), 7.99 (dt, J = 7.8, 1.5 Hz, 2H), 7.58 (dd, J = 7.6,
complexes exhibit C1 symmetry as characterized by NMR spectroscopy, which is in
agreement with the geometry observed for the previously published parent complex
1 [49].
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3
4.9 Hz, 2H), 7.30 (dd, J = 15.2, 1.3 Hz, 2H), 1.59 (s, 2H). 13C NMR
(125 MHz, CDCl3): ı 157.5, 148.7, 139.5, 136.7, 128.5, 127.5, 127.3,
122.8, 19.4. ESI–MS (m/z, 1% trifluoroacetic acid in MeOH): 441.0
(100%, H2bpsa-PhdMe-Na+), 418.9 (57%, H2bpsa-PhdMe).
N,Nꢀ-(naphthalene-2,3-diyl)bis(pyridine-2-sulfonamide)
(H2bpsa-NPTH). Yield: 36.9%, brown crystals. 1H NMR (500 MHz,
CDCl3): ı 10.46 (s, 2H), 8.79 (d, J = 4.9 Hz, 2H), 8.07 (td, J = 7.9,
0.9 Hz, 2H), 8.04 (s, 2H), 7.98 (dt, J = 7.8, 1.7 Hz, 2H), 7.78 (dd, J = 6.2,
3.2 Hz, 2H), 7.58 (ddd, J = 7.6, 4.9, 1.0 Hz, 2H), 7.44 (dd, J = 6.3,
3.2 Hz, 2H). 13C NMR (125 MHz, CDCl3): ı 157.5, 148.8, 139.5,
132.1, 128.3, 127.7, 127.5, 126.7, 126.4, 122.9. ESI–MS (m/z, 1%
trifluoroacetic acid in MeOH): 463.0 (100%, H2bpsa-NPTH-Na+),
441.0 (60%, H2bpsa-NPTH).
129.4, 129.3, 125.8, 125.2, 118.3, 18.6, 18.2. ESI–MS (m/z, MeOH):
679.0 (100%, [Ir(bpsa-PhdMe)Cl2]−).
Iridium
N,Nꢀ-(naphthalene-2,3-diyl)bis(pyridine-2-
sulfonamide) dichloride sodium salt (Na[Ir(bpsa-NPTH)Cl2])
5. Yield: 32.4%, yellow powder. 1H NMR (500 MHz, CD3CN): ı 9.78
(dd, J = 5.7, 1.5 Hz, 1H), 8.27 (dt, J = 7.8, 1.4 Hz, 1H), 7.93 (m, 3H),
7.86 (ddd, J = 7.7, 5.7, 1.4 Hz, 1H), 7.74 (m, 2H), 7.72 (s, 1H), 7.69
(s, 1H), 7.63 (d, J = 8.2, 1H), 7.33 (m, 2H), 7.25 (ddd, J = 8.1, 6.9,
1.2 Hz, 1H). 13C NMR (125 MHz, D2O): ı 162.1, 158.5, 150.9, 150.7,
145.0, 142.2, 141.4, 139.7, 131.4, 130.6, 129.5, 129.1, 127.2, 126.3,
126.2, 126.1, 126.0, 124.9, 124.7, 113.2. ESI–MS (m/z, MeOH): 701.0
(100%, [Ir(bpsa-NPTH)Cl2]−).
2.3. Methods and techniques
2.2.3. Synthesis of iridium(III) bis(pyridine-2-sulfonamide) (bpsa)
complexes
2.3.1. Physical characterization
1H and 13C NMR spectra were recorded on Bruker Avance
500 MHz spectrometers at room temperature. ESI–MS was per-
formed on Thermo-Fisher LCQ instrument with ∼90 ␮M methanol
solutions. 1% Trifluoroacetic acid was added for mass spectrome-
try of the ligands. Elemental analysis was conducted by Robertson
Microlit Laboratories (Ledgewood, NJ).
Ir(III) bpsa complexes were synthesized by modifying the pre-
viously published procedure [49]. Typically, IrCl3·xH2O (0.10 g,
0.28 mmol), 1 equiv of appropriate H2bpsa ligand (0.28 mmol), and
8/1 (v/v) 2-methoxyethanol/water (∼27 mL) were combined in a
sealed round-bottom flask and degassed with argon for 15 min. The
mixture was heated at 110 ◦C in the dark under an argon atmo-
sphere for 2 h, and the solvent was removed in a rotary evaporator
after the reaction mixture was cooled to room temperature. The
desired Ir(III) bpsa complex was purified by chromatography on
silica gel using 2/1 (v/v) ethyl acetate/acetone eluent, yielding a
yellow powder after drying in the vacuum oven.
2.3.2. Electrochemstry
Cyclic voltammograms were measured on a CH-Instruments
Model 600C Electrochemical Analyzer using
a glassy-carbon
working electrode, a coiled-platinum counter electrode, and a
silver-wire pseudoreference electrode. Solutions were prepared
with 0.1 M tetrabutylammonium hexafluorophosphate (Fluka,
electrochemical grade) as the supporting electrolyte for analysis
in acetonitrile and ∼2 mM corresponding Ir(III) bpsa complex. All
solutions were purged with argon before taking each measurement.
Ferrocene was added as an internal standard, and the oxidation
half-wave potential of ferrocene standard was set to +0.40 V versus
SCE. A scan rate of 0.1 V/s was used, starting from the positive scan
unless further noted.
Iridium N,Nꢀ-(phenylene-1,2-diyl)bis(pyridine-2-sulfonamide)
dichloride sodium salt (Na[Ir(bpsa-Ph)Cl2]) 1. Yield: 35.3%, yellow
powder. 1H NMR (500 MHz, acetone-d6): ı 9.70 (dd, J = 6.3, 1.5 Hz,
1H), 8.40 (dt, J = 7.9, 1.5 Hz, 1H), 8.05 (m, 2H), 7.92 (m, 2H), 7.77
(d, J = 7.9 Hz, 1H), 7.51 (dd, J = 8.0, 1.4 Hz, 1H), 7.45 (ddd, J = 7.6,
6.0, 1.5 Hz, 1H), 7.27 (dd, J = 8.0, 1.3 Hz, 1H), 6.79 (dt, J = 7.8, 1.5 Hz,
1H), 6.67 (dt, J = 7.8, 1.5 Hz, 1H). 13C NMR (125 MHz, D2O): ı 162.3,
161.4, 150.9, 150.8, 142.2, 141.5, 140.4, 130.6, 129.5, 128.9, 125.9,
125.7, 125.2, 122.0, 117.1, 97.3. ESI–MS (m/z, MeOH): 650.1 (100%,
[Ir(bpsa-Ph)Cl2]−).
2.3.3. Dynamic oxygen evolution measurements
Iridium N,Nꢀ-(4,5-dichloro-phenylene-1,2-diyl)bis(pyridine-2-
sulfonamide) dichloride sodium salt (Na[Ir(bpsa-PhdCl)Cl2]) 2.
Yield: 29.7%, dark yellow powder. 1H NMR (500 MHz, CD3CN): ı
9.73 (dd, J = 5.6, 1.2 Hz, 1H), 8.26 (dt, J = 7.9, 1.4 Hz, 1H), 7.97 (dt,
J = 7.7, 1.2 Hz, 1H), 7.90 (m, 2H), 7.85 (ddd, J = 7.8, 5.7, 1.4 Hz, 1H),
7.78 (d, J = 7.8 Hz, 1H), 7.46 (s, 1H), 7.42 (s, 1H), 7.35 (ddd, J = 13.4,
7.7, 1.4 Hz, 1H). 13C NMR (125 MHz, D2O): ı 161.7, 158.1, 151.1,
150.9, 144.6, 142.4, 141.6, 140.6, 130.6, 129.6, 128.9, 127.4, 125.9,
125.2, 123.4, 117.7. ESI–MS (m/z, MeOH): 720.9 (100%, [Ir(bpsa-
PhdCl)Cl2]−).
Typically, a solution of the iridium complex (1 mL) was injected
into a sealed 40 mL EPA vial containing 10 mL of 0.36 M CAN solu-
tion buffered in 1 M HNO3 that was degassed by argon. O2 evolution
was dynamically monitored with pressure transducer equipped on
top of the sealed EPA vials as described in previous literature [42],
and the end point of the reaction was verified by GC analysis.
For water oxidation experiments using sodium periodate
(NaIO4) as a sacrificial oxidant, 30 mL 19 mM unbuffered solution
of the oxidant was placed in a sealed 40 mL EPA vial. A solution
of the iridium complex (1 mL) was then injected into the EPA vial,
and the reaction was monitored immediately after the addition of
the catalyst. Similarly, the components of the headspace in each
reaction vial were checked with GC at the end of the experiments.
Iridium N,Nꢀ-(4,5-difluoro-phenylene-1,2-diyl)bis(pyridine-2-
sulfonamide) dichloride sodium salt (Na[Ir(bpsa-PhdF)Cl2]) 3.
Yield: 27.7%, dark yellow powder. 1H NMR (500 MHz, CD3CN):
ı 9.73 (dd, J = 5.7, 1.0 Hz, 1H), 8.26 (dt, J = 7.8, 1.4 Hz, 1H), 7.96
(dt, J = 7.8, 1.2 Hz, 1H), 7.90 (m, 2H), 7.84 (ddd, J = 7.7, 5.7, 1.4 Hz,
1H), 7.76 (d, J = 7.9 Hz, 1H), 7.34 (ddd, J = 7.6, 5.8, 1.4 Hz, 1H), 7.27
(dd, J = 12.2, 8.0 Hz, 1H), 7.23 (dd, J = 12.3, 8.4 Hz, 1H). 13C NMR
(125 MHz, D2O): ı 161.7, 158.1, 151.2, 151.0, 142.3, 141.6, 130.6,
129.6, 125.8, 125.2, 116.4, 116.2, 105.1, 104.9. ESI–MS (m/z, MeOH):
687.0 (100%, [Ir(bpsa-PhdF)Cl2]−).
2.3.4. DFT calculations
All DFT calculations were performed with Gaussian 09 using the
B3LYP functional and LANL2DZ as the basis set. Default parameters
and thresholds were used for gradient convergence. Geometry opti-
mizations for all five Ir(III) bpsa complexes were carried out under
no constraints in vacuum condition, yielding reasonable optimal
coordinates and orbital energies.
Iridium N,Nꢀ-(4,5-dimethyl-phenylene-1,2-diyl)bis(pyridine-2-
sulfonamide) dichloride sodium salt (Na[Ir(bpsa-PhdMe)Cl2]) 4.
Yield: 34.3%, yellow powder. 1H NMR (500 MHz, CD3CN): ı 9.72
(dd, J = 5.7, 1.5 Hz, 1H), 8.24 (dt, J = 7.8, 1.5 Hz, 1H), 7.92 (dt, J = 7.7,
1.4 Hz, 1H), 7.88 (ddd, J = 7.9, 1.4, 0.6 Hz, 1H), 7.82 (m, 2H), 7.72 (d,
J = 7.9 Hz, 1H), 7.31 (ddd, J = 7.6, 5.8, 1.5 Hz, 1H), 7.20 (s, 1H), 7.08
(s, 1H), 2.21 (s, 3H), 2.14 (s, 1H). 13C NMR (125 MHz, D2O): ı 162.3,
158.6, 150.8, 150.7, 142.1, 142.0, 141.3, 137.8, 134.4, 130.6, 130.4,
3. Results and discussion
3.1. Catalytic activities
The structurally altered derivatives 2–5 as well as the parent
complex 1 were explored side by side for catalytic water oxi-
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3.2. Kinetics and mechanistic insights
The ratio of the Ce(IV) oxidant to the catalyst appear to have a
strong effect on the catalytic mechanism [48,70,73]. Consequently,
complexes 1–5 were further investigated as water oxidation cata-
lysts with a large excess of CAN by varying the concentration of the
catalyst over a wide range from 5 to 50 ␮M (Table S1, Fig. S1–S3).
In contrast to the aforementioned results with a high catalyst load
(50 ␮M), the amount of O2 produced with all these complexes
at relatively low catalyst concentrations was below the stoichio-
metric maximum yield, indicating a deactivation of the catalyst
prior to the consumption of all CAN. Overall, the parent complex 1
exhibits superior performance compared to the structurally mod-
ified derivatives 2–5 as water oxidation catalysts. This is clearly
documented by the fact that the TOFmax of 1 was up to one order of
magnitude higher than those of 2–5. Close inspection of the behav-
iors of these catalysts revealed that at lower concentrations the O2
evolution lost the sigmoidal-like feature and appeared close to be
linear over time, likely indicating a change in mechanisms when
different CAN/catalyst ratios are involved.
Fig. 2. O2 evolution traces of complexes 1–5 at 50 ␮M catalyst and 0.36 M CAN (final
volume = 11 mL). The total amount of O2 produced with all catalysts is consistent
with the stoichiometric limit of the added CAN.
For all complexes, the maximum O2 evolution rate was found
to be linearly correlated with catalyst concentration (Fig. 4A). A
conventional log–log plot of maximum rate against the catalyst
concentration exhibits an apparent order of one in catalyst for
all these complexes (Fig. 4B), unlike the fractional reaction order
observed in previous work with 0.80 M CAN [49]. Furthermore, the
rate of oxygen evolution exhibited a notable increase (almost 10-
fold) as compared to the results obtained for the parent complex
1 previously at high CAN concentration (Table S1) [49]. The con-
centration dependence on CAN as well as the variation in apparent
reaction order in catalyst is consistent with mechanisms involv-
ing the formation of an intermediate containing Ce(IV) fragment
as described in other iridium, ruthenium and iron water oxidation
systems, [38,70,74–76] which further proves the non-innocence of
CAN as a sacrificial oxidant under conditions involving high oxidant
concentrations.
Fig. 3. O2 evolution rates of complexes 1–5 over time at 50 ␮M catalyst and 0.36 M
CAN (final volume = 11 mL).
To evaluate the robustness of these complexes in water
oxidation catalysis, long-term experiments were conducted for
complexes 1–5 with a large excess of CAN at low catalyst con-
centrations (Fig. 5A, Fig. S3). Although these complexes generated
oxygen below the stoichiometric limit, they remained constantly
active for a week or more. 1 achieved the highest turnovers of up
to 16,200 during this time, which was obtained in the presence
of 71,210 equivalents of CAN (Table S1). 2 proved to be almost
as durable and efficient as 1, while 3, which is also substituted
with electron-withdrawing substituents, accomplished only about
80% of the turnovers of 2 (Fig. 5A). Complex 4 with the methyl
substituents and 5 with the naphthalene linkage demonstrated
robustness comparable to 3 (Fig. S3), with a slightly higher reac-
tivity of observed for 4 under these conditions. Apparently, the
kinetics profiles discussed above are not sufficient to understand
the different TONs and evaluate the reactivity observed with differ-
ent water oxidation catalysts. Nevertheless, interesting trends were
found for complexes 1–5 when inspecting the relationship between
the TOFmax and the molar ratio of CAN and catalyst (Fig. 5B, Fig. S4)
[72]. Similar to the parent complex 1, the TOFmax of 2 also peaked
when the molar ratio between CAN and catalyst is around 18,000
with a 20 ␮M catalyst concentration, and it decreased slightly as
the catalyst load was further reduced. The slightly lower turnovers
obtained by 2 is likely due to the small reactivity difference between
1 and 2 (Fig. 5B). The TOFmax of 3 also reached a maximum at the
same CAN/catalyst ratio as 1 and 2, but a significant drop in reactiv-
ity was observed when the CAN/catalyst ratio was large (Fig. 5B).
This difference in kinetics behaviors highlights the significant effect
of the fluorine substituents on the reactivity of the catalyst. This
decrease in reactivity is contrasted by the more robust nature of
dation that was chemically driven by cerium ammonium nitrate
(CAN) buffered in 1 M HNO3. O2 evolution was monitored with the
homebuilt pressure transducer setup as described previously [42],
and the GC-verified results of these reactions are plotted in Fig. 2.
Essentially all these reactions proceeded to completion, producing
stoichiometric oxygen within 4% of the theoretical maximum yield.
This finding is contrasted by recent work on iridium Cp* water oxi-
dation catalysts that exhibit non-stoichiometric oxygen evolution
at high catalyst loading [70–72]. No carbon dioxide was detected
at the endpoint of these reactions. A short induction period when
O2 is initially evolved at a relatively slower rate was observed for
all five complexes, corresponding to the exchange of the ancil-
lary chloride ligands with labile water molecules from the solvent
which was discussed in previously published work [49]. Monitor-
ing the rate of O2 evolution revealed that regardless of the added
tuning groups, all substituted complexes 2–4 and complex 5 with
the naphthalene linkage are similarly or slightly less active as water
oxidation catalyst compared to the parent complex 1 (Fig. 3, Table
S1). Complexes 2 and 3 with electro-withdrawing substituents (Cl
and F respectively) catalyze water oxidation faster than 4 with
the electron-donating substituents (Me); all the phenylene-linked
complexes outperform 5 with the naphthalene linkage containing
the extended conjugation system. Moreover, the maximum rate for
5 was achieved much later than for 1–4, at a stage of the reaction
when almost 40% CAN was already consumed. The distinct behav-
iors of these complexes observed in O2 evolution trigger further
in-depth studies to gain a better understanding of the effects of
these structurally diverse ligands and, as a consequence, the mech-
anism of catalyzed water oxidation reactions.
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Fig. 4. A) Plots of maximum rates of O2 evolution versus concentrations of complexes 1–5 at 0.36 M CAN (final volume = 11 mL). B) Log-log plot of complexes 1–5 and their
linear fits. The slopes indicate the apparent orders of O2 evolution on each catalyst.
Fig. 5. A) Long-term O2 evolution from 11 mL aqueous solutions of 5 ␮M complexes 1–3 and 0.36 M CAN. B) Trends of maximum TOF versus the molar ratio between CAN
and catalyst for complexes 1–3. The results for complex 4 and complex 5 are included in the Supporting information.
Fig. 6. A) O2 evolution traces of complexes 1–5 at 5 ␮M catalyst and 19 mM NaIO4 (final volume = 31 mL). Stoichiometric amount of oxygen was produced for all these
complexes after an incubation time of various lengths. B) O2 evolution rates of complexes 1–5 over time at 5 ␮M catalyst and 19 mM NaIO4 (final volume = 31 mL).
the fluorinated version of the original [Ir(ppy)2(H2O)2]+ water oxi-
dation catalyst [42], indicating a difference in the formation of
the active catalytic species. Examining the dependence of TOFmax
on CAN/catalyst ratio for complex 4 and 5 also revealed a similar
prominent decrease in O2 evolution rate as observed for 3 when
CAN was present in great excess (Fig. S4). The decreased stability
can easily be attributed to the addition of the methyl substituents
or the extension of conjugation system in the linkage moiety that
decreases its resilience to oxidative degradation. Moreover, 4 and 5
reached their maximum TOFmax at larger CAN/catalyst ratios than
1, 2 and 3, suggesting that two different catalyst degradation path-
ways may be involved for these five bpsa complexes.
Established as efficient and durable water oxidation catalysts
operating under highly oxidative and acidic conditions, complexes
1–5 were also investigated for water oxidation catalysis with NaIO4
acting as a two-electron oxidant. While it typically requires isotope
study to determine the source of oxygen during water oxidation
processes, periodate is stable over a wide pH range (approximately
from 2 to 7.5), which allows us to explore the behaviors of these
water oxidation catalysts in a near-neutral reaction medium [77].
Under conditions of low catalyst concentration and small excess of
oxidant, the maximum TOFs achieved for complexes 1–5 are gen-
erally up to three times as fast as those observed under similar
conditions with CAN as the primary oxidant (Fig. 6, Table S2). It
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Fig. 7. A) Cyclic voltammograms of complexes 1, 3 and 4 in acetonitrile to demonstrate the reversible oxidation process associated with Ir(IV)/Ir(III) redox couple of each
complex in the absence of water. The results for complex 2 and complex 5 are included in the Supporting information. B) Cyclic voltammograms of complex 1 in acetonitrile
with a more positive limit. For all CVs, a glassy carbon working electrode was used with a 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAH) supporting electrolyte
solution. Scan rate = 0.1 V/s.
Fig. 8. Cyclic voltammograms of complex 2 (A) and complex 3 (B) in acetonitrile. A glassy carbon working electrode was used with a 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAH) supporting electrolyte solution. Scan rate = 0.1 V/s.
dation catalysts, which is roughly one order of magnitude less than
the typical heterogeneous systems, [78,79] also points toward the
molecular nature of these water oxidation catalysts. An alternative
explanation for the delayed onset of these reactions could be the
differences in Cl−/H2O/IO4− substitution initiated by periodate oxi-
dation. The sigmoid shape of the O2 evolution traces observed using
NaIO4 as the oxidant is typical for autocatalytic reactions, [80–82]
which evokes complimentary insights into water oxidation mech-
anisms and further highlights the distinct roles of the sacrificial
oxidants in the catalytic cycle.
3.3. Electrochemistry and DFT modeling
Cyclic voltammetry (CV) in acetonitrile was used to evaluate
Fig. 9. Frontier orbitals for complexes 1–5 based on DFT calculations
(B3LYP/LANL2DZ). The HOMO orbital energies of these complexes can be
tuned by addition of electron-donating or electron-withdrawing substituents due
to the mixed metal/ligand character of these orbitals. The enlarged ␲-system of
5 changes the electronic structure of this complex dramatically, and thus a direct
comparison with the phenylene-bridged system is impossible.
the redox properties of complexes 1-5, providing valuable insights
into the effect of various substituents on the electronic structure
of Ir(III) bpsa complexes. The reversible oxidation wave, which can
be assigned to the Ir(IV)/Ir(III) redox couple, was observed for all
five complexes at potentials ranging from +0.64 V to +0.92 V versus
SCE (Fig. 7A, Fig. S5, Table 1). Analogous to the parent complex 1
(Fig. 7B), the CVs of 2, 3 and 4 show a second reversible oxidation
process at more positive potentials that was assigned to the oxida-
tion of the phenylene linkage as discussed in the previous work [49].
Moreover, 3 demonstrates an additional reversible oxidation wave
at a potential lower than the Ir(IV)/Ir(III) redox couple after a com-
plete oxidative sweep to potential past the second oxidation. This
is likely due to the formation of a degradation product caused by
the introduction of more labile fluorine substituents to the bridging
unit (Fig. 8B). For complex 4, the methyl substituents are believed to
undergo some irreversible transformation at high oxidation poten-
tial as evidenced by the irreversible oxidation peak at +1.63 V versus
is notable that the time of incubation was markedly longer com-
pared to CAN-driven water oxidation, especially in the case of 3 and
5, as evidenced by the bifurcation time tB extracted from the O2
evolution traces. The extremely long induction period followed by
rapid O2 generation could be an indication of the formation of irid-
ium oxide nanoparticles. However, clear differences in bifurcation
time as well as O2 evolution rate among these bpsa complexes with
diverse substituents were observed, suggesting a ligand effect on
an active molecular species. Besides, the fact that the reaction rates
are of the same order as the analogous molecular iridium water oxi-
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Fig. 10. A) Relationship between the oxidation potentials of complexes 1–5 in acetonitrile measured by cyclic voltammetry and calculated HOMO energies from DFT calculation
(B3LYP/LANL2DZ). B) Relationship between the oxidation potentials of complexes 1–5 in acetonitrile measured by cyclic voltammetry and maximum TOFs determined at
50 ␮M catalyst and 0.36 M CAN.
Table 1
allows the fine-tuning of the energy of HOMO, which is strongly
involved in water oxidation catalysis. The addition of electron-
Electrochemical propertiesa of complexes 1–5.
Complex
E1/2OX/V (ꢀEb/mV)
EpcRED/V
withdrawing substituents was found to stabilize the HOMO, which
greatly impedes the oxidation process as evidenced by the excep-
tionally high redox potential of the Ir(IV)/Ir(III) couple. On the other
hand, the addition of electron-donating substituents causes the
opposite effect (Fig. 10A).
1
2
3
4
5
+0.78 (75)
+0.92 (69)
+0.64 (73)c
+0.67 (71)
+0.69 (49)e
+1.28 (90)
+1.41 (67)
+0.86 (68)
+1.15 (92)
+1.31e
–
–
−1.70
−1.64
−1.66
−1.71
−1.86
+1.32 (95)
+1.63d
+1.50e
It is expected that electron-donating substituents greatly facil-
itate the oxidation process through the electronic destabilization
of the HOMO of the bpsa complexes. Accordingly, complex 4 with
the higher HOMO energy is expected to be more easily oxidized
and thus catalyze water oxidation faster than the parent complex
1 due to the easier accessibility of the highly oxidized species that
are required for the abstraction of electrons from water. In reality,
poor activity was observed for 4 (Fig. 10B), which can be attributed
to the limited driving force provided by the oxidized species as
well as the electronic instability caused by the possible destructive
transformation of the methyl groups under the highly oxidative
conditions (Fig. S6). For complexes 2 and 3, which are decorated
with electron-withdrawing substituents, the stabilization of the
HOMO of the complex effectively enlarges the energy difference
between the ground state and the first oxidized state, rendering it
a stronger oxidant and thus potentially a more efficient water oxi-
dation catalyst compared to the parent complex 1. However, the
high energy barrier needed for reaching the required highly oxi-
dized state of the catalyst significantly impedes the water oxidation
process, and as a result, both 2 and 3 perform worse than 1 (Fig. 10).
It is worth mentioning that the reactivity of complex 5 appears to
be the worst among all these complexes, likely due to the fact that
the bridging unit of 5 is most susceptible to oxidation as illustrated
by the irreversibility of the oxidative processes evidenced by the
measured CV (Fig. S7).
a
Measured in MeCN containing 0.1 M tetra-n-butylammonium hexafluorophos-
phate (TBAH) solution as the electrolyte. Scan rate = 0.1 V/s. All potentials were
referenced to SCE using ferrocene as an internal standard (fc/fc+ = +0.40 V vs SCE).
b
Peak separation.
c
A reversible oxidation peak at +0.64 V, due to the formation of a degradation
product of the original complex, is observed if 3 is oxidized after an oxidative sweep
to +1.9 V (Fig. 8B).
d
The irreversible oxidation peak corresponding to the oxidation of the methyl
substituents appears if 4 is oxidized to +1.9 V (Fig. S6).
e
The redox behaviors of 5 with naphthalene linkage differ from 1 containing
phenylene linkage: two adjacent, irreversible oxidation peaks due to the oxidation
of the ligand’s bridging unit are observed (Fig. S7). Also, the relatively small peak
separation of the oxidative process of Ir(IV)/Ir(III) redox couple indicates a difference
in oxidation mechanism.
SCE (Fig. S6). The susceptibility of the methyl groups to degrada-
tion has also been observed in other molecular water oxidation
catalysts with various ligand scaffolds. It is noteworthy that 2 did
not undergo irreversible transformations upon the oxidative sweep
as evidenced by the two fully reversible oxidation peaks in CV
(Fig. 8A). The observed electronic stability caused by the addition of
the chlorine substituents to the phenylene bridging unit is consis-
tent with the previously observed great robustness of 2 as a water
oxidation catalyst, which explains the high TONs in the longevity
experiments under highly acidic and oxidative conditions. Not sur-
prisingly, complex 5, containing the naphthalene linkage, exhibits
electrochemical properties distinctly different than its cousin com-
plexes 1–4. The oxidation wave corresponding to Ir(IV)/Ir(III) redox
couple at +0.69 V versus SCE turned out to be quasi-reversible with
much smaller peak separation which was followed by two adjoin-
ing irreversible oxidation peaks caused by the oxidation of the
ligand’s bridging unit (Fig. S5, Fig. S7) [83].
The observed effect of the redox behavior of these bpsa
complexes caused by adding electron-donating or electron-
withdrawing substituents to the linkage moiety is reasonably
predictable by correlation with the DFT calculated HOMO energy
levels of these complexes. As illustrated in Fig. 9, the calculated
HOMOs for complexes 1-5 all exhibit a significant distribution of
electron density over both the metal center and the bridging unit,
which is in good agreement with the presence of an oxidation
process beyond the Ir(IV)/Ir(III) redox couple observed electro-
chemically. The involvement of the ligand in the oxidation process
4. Conclusions
A variety of iridium bis-pyridine-2-sulfonamide (bpsa) com-
plexes with diverse substituents on the phenylene bridging unit
as well as an iridium complex with a new bpsa ligand con-
taining the naphthalene linkage were synthesized and studied
in the homogeneous, chemically driven oxidation of water. The
electronic structure of these iridium water oxidation catalysts
is highly tunable through substitution with electron-donating or
electron-withdrawing substituents, which is documented by cyclic
voltammetry and DFT calculations. Such differences in the elec-
tronic structures of these complexes lead to significantly different
activity in water oxidation catalysis. Very different kinetic behav-
iors are observed for these complexes, suggesting that various
Please cite this article in press as: M. Li, S. Bernhard, Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide complexes as efficient
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mechanistic pathways are involved. Moreover, it has been found
that the reactivity of these water oxidation catalysts is affected
by many other factors including the driving force provided by the
oxidized catalyst, the accessibility of highly oxidized species, and
the electronic stability of the involved catalyst. These factors need
be balanced when designing a feasible water oxidation catalyst in
order to achieve optimal performance. Future work will involve fur-
ther exploration of these water oxidation catalysts under various
conditions and precise detection of the key intermediates to gain
an in-depth understanding of the catalytic mechanisms.
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