Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.5b01709

A family of tetradentate bis(pyridine-2-sulfonamide) (bpsa) compounds was synthesized as a ligand platform for designing resilient and electronically tunable catalysts capable of performing water oxidation catalysis and other processes in highly oxidizing environments. These wrap-around ligands were coordinated to Ir(III) octahedrally, forming an anionic complex with chloride ions bound to the two remaining coordination sites. NMR spectroscopy documented that the more rigid ligand frameworks-[Ir(bpsa-Cy)Cl₂]^- and [Ir(bpsa-Ph)Cl₂]^--produced C₁-symmetric complexes, while the complex with the more flexible ethylene linker in [Ir(bpsa-en)Cl₂]^- displays C₂ symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the observed electrocatalytic behavior and ruled out the formation of active IrO_x. The electrochemical studies were complemented by work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with thousands of catalyst turnovers was observed with the C₁-symmetric catalysts. Dynamic light scattering experiments ascertained that a molecular species is responsible for the catalytic activity and excluded the formation of IrO_x particles.

Full text of DOI:10.1021/acs.inorgchem.5b01709

Article
pubs.acs.org/IC
Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and
Durable Catalysts for Homogeneous Water Oxidation
†
‡
,§
,†
Mo Li, Kazutake Takada, Jonas I. Goldsmith,* and Stefan Bernhard*
†
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho,
Showa-ku, Nagoya 466-8555, Japan
§
Department of Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, United States
S Supporting Information
*
ABSTRACT: A family of tetradentate bis(pyridine-2-sulfonamide)
bpsa) compounds was synthesized as a ligand platform for designing
(
resilient and electronically tunable catalysts capable of performing
water oxidation catalysis and other processes in highly oxidizing
environments. These wrap-around ligands were coordinated to Ir(III)
octahedrally, forming an anionic complex with chloride ions bound to
the two remaining coordination sites. NMR spectroscopy docu-
−
mented that the more rigid ligand frameworks[Ir(bpsa-Cy)Cl ]
2
−
and [Ir(bpsa-Ph)Cl ] produced C -symmetric complexes, while
2
1
the complex with the more flexible ethylene linker in [Ir(bpsa-
−
en)Cl ] displays C symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in
2
2
nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible
reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation
reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the
observed electrocatalytic behavior and ruled out the formation of active IrO . The electrochemical studies were complemented by
x
work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to
ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics
of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with
thousands of catalyst turnovers was observed with the C -symmetric catalysts. Dynamic light scattering experiments ascertained
1
that a molecular species is responsible for the catalytic activity and excluded the formation of IrO particles.
x
INTRODUCTION
Molecular catalysts with their well-defined and synthetically
tunable structures, readily available characterization methods,
and superior atom efficiency are appealing targets for effecting
■
The use of photons to drive endergonic redox processes, such
as the splitting of water into hydrogen and oxygen, is
considered an attractive approach for storing solar energy as
8
oxygen evolution in homogeneous systems. A number of
1
fuels. While great attention has been given to the fuel-
molecular water oxidation catalysts (WOCs) have been
reported so far, including transition-metal complexes based
2
producing half-reaction, it is arguably the complementary
oxidative half-reaction that is more challenging. Several
9
10
11
12
13
on manganese, ruthenium, iridium, iron, cobalt, and
oxidative processes, such as halogenide to halogen trans-
14
copper. Studies revealed that modifications of the ligand
3
4
formations and the dehydrogenation of organics, are currently
under investigation, but the most desirable product of the
sphere can lead to significant changes in the reactivity and the
longevity of these WOCs. An ideal ligand framework should
bind strongly to the metal center while simultaneously allowing
for its redox properties to be readily tuned. Moreover, a
negatively charged ligand system can enhance the stability of
the high-valent metal oxo species formed during the water
oxidation reaction. Resilience toward demetalation and ligand
oxidation half-reaction is molecular oxygen. Not only is O an
2
environmentally benign product but it also can be used as the
oxidizer in fuel cell or combustion applications. Consequently,
interest in water oxidation is growing, since the resulting
reducing equivalents (4e and 4H per 2H O) can be coupled
not only to hydrogen production but also to other fuel-
producing reactions, including CO reduction and metal
hydroxide to metal transformations. While water oxidation is a
very appealing goal, it is one that is difficult to achieve due to
the high energy barrier for sequentially accomplishing the
transfer of four electrons and four protons as well as the
complexity of the O−O bond formation.
−
+
2
5
6
2
7
Special Issue: Small Molecule Activation: From Biological Principles
to Energy Applications
Received: July 30, 2015
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
degradation processes is also critical, as these complexes are
exposed to highly oxidizing reaction environments.
Scheme 1. Molecular Structures of the Sulfonamide Ligands
H bpsa-Cy, H bpsa-en, and H bpsa-Ph (Top) and the
2
2
2
−
Iridium-based WOCs, due to their robustness as well as
excellent activity, are attractive targets for foundational studies.
Several structural motifs have been explored to date: cyclo-
metalated iridium complexes based on a phenylpyridine
Corresponding Ir(III) Complexes [Ir(bpsa-Cy)Cl ] ,
2
−
−
[Ir(bpsa-en)Cl ] , and [Ir(bpsa-Ph)Cl ] (Bottom)
2
2
1
1a
ligand,
half-sandwich iridium Cp or Cp* (Cp = cyclo-
pentadienyl, Cp* = pentamethylcyclopentadienyl) complex-
1
1b,15
es,
iridium complexes,
found that some of these catalysts degrade to IrO particles,
strongly σ donating N-heterocyclic carbene based
11c,d,f 16
and iridium carboxylates. It was
17
x
but electrochemical quartz crystal nanobalance (EQCN)
1
8
19
experiments and dynamic light scattering (DLS) have
confirmed the molecular nature of catalysts with a robust ligand
environment.
Cp* complexes have become the most thoroughly studied
family of iridium-based WOCs recently. These catalysts display
−1
turnover frequencies (TOFs) of up to ∼5 s and turnover
16a
numbers (TONs) in the tens of thousands. Unfortunately,
the Cp* ligand partially degrades under the oxidative stress
caused by the presence of ceric ammonium nitrate (CAN), a
sacrificial oxidant used in O2 evolution studies. This
degradation pathway has recently been documented with a
20
series of in situ NMR experiments by Macchioni et al.
A
double oxidative functionalization of a − C−CH − moiety of
3
the Cp* ligand was observed, which is in some cases followed
by the formation of acetic, formic, and glycolic acids or even the
20b
complete loss of the Cp* ligand.
That the nature of the
catalyst species is changing during the course of the catalytic
process makes it difficult to study the mechanism controlling
the water oxidation reaction.
A new family of tetradentate iridium(III) bis(pyridine-2-
sulfonamide) (bpsa) complexes is presented here to address the
ligand instabilities observed in existing WOCs. The deproto-
nated sulfonamide linkers in combination with the linked
pyridine moieties form a resilient wrap-around ligand sphere.
This robust structure provides a strong electron-donating
environment aimed at stabilizing the highly positively charged
iridium intermediates. Modifications to the linker moiety are
used to fine tune the ligands’ electronic structures. The iridium
complexes within this family have been fully characterized and
demonstrate durable and efficient water oxidation catalysis both
using CAN as a sacrificial oxidant and electrochemically at
neutral pH.
1
Figure 1. H NMR spectra (aromatic region) of H bpsa-Cy, H bpsa-
2
2
en, and H bpsa-Ph (top) and the corresponding Ir(III) complexes
2
−
−
−
[Ir(bpsa-Cy)Cl ] , [Ir(bpsa-en)Cl ] , and [Ir(bpsa-Ph)Cl ] (bot-
tom). The difference in the coordination geometry of the complexes is
evidenced by the doubling of the pyridine peaks observed in the
spectra of the compounds with C geometry.
2
2
2
1
RESULTS AND DISCUSSION
■
arrangement, which is the result of the strong trans effect of
the amido ligands. Furthermore, DFT calculations of the
possible coordination isomers indicate that, for all three
complexes, the two chloride ligands are preferentially in a cis
conformation (Table 1). The three viable coordination
Synthesis. The ligand H bpsa-Cy (Scheme 1) was prepared
2
2
1
by a modified procedure first published by Walsh et al., which
involves the reaction between in situ generated pyridine-2-
sulfonyl chloride and cyclohexanediamine in diethyl ether.
Potassium carbonate solution was replaced with triethylamine,
which was chosen as the base in order to avoid biphasic
reaction conditions. Following the same protocol, H bpsa-en
Scheme 1) was also readily accessible. The limited solubility of
∧
∧
∧
geometries of a tetradentate N N N N ligand with two
ancillary monodentate ligands are depicted in Table 1. NMR
spectroscopy (Figure 1, bottom) established that the more
flexible H bpsa-en coordinated with a C symmetry, while the
2
(
2
2
the monosubstituted 1,2-phenylenediamine product necessi-
tated a switch from diethyl ether to tetrahydrofuran for the
two other, more rigid ligands produced nonsymmetric Ir(III)
1
3
complexes. These findings were corroborated by C NMR
spectroscopy (Figures S7−S12 in the Supporting Information).
The disagreement between the experimentally observed
synthesis of H bpsa-Ph (Scheme 1). All of the ligands exhibit
2
1
C symmetry in their H NMR spectra (Figure 1, top).
2
−
−
The formation of the Ir(III) bpsa complexes (Scheme 1) was
isomers of [Ir(bpsa-Cy)Cl ] and [Ir(bpsa-Ph)Cl ] and the
2
2
carried out by combining equivalent amounts of IrCl ·xH O
results of the DFT modeling can be explained by the fact that
3
2
and the corresponding H bpsa ligands in 2-methoxyethanol and
the energy differences of the C -cis isomer and the C -cis
2
1
2
heating without the addition of any base. The two chloride
ligands for all three complexes are oriented in a cis
isomer are small and that more favorable reaction kinetics could
lead to the formation of the higher energy C -cis isomer.
1
B
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Table 1. Relative and Absolute Heat of Formation Energies
of the Geometry Optimized Ir(III) bpsa Complexes with
Different Ligand Configurations Calculated using DFT with
complexes, which can be assigned to the Ir(III)/Ir(IV) redox
couple. Another irreversible oxidation wave at +1.26 V versus
−
SCE was detected for [Ir(bpsa-en)Cl ] , but only after a
2
b
a B3LYP Functional and a LANL2DZ Basis Set
reductive sweep to potentials past the first observed reduction
(
Figure S15 in the Supporting Information). The appearance of
this new redox feature indicates the instability of the C -cis
2
symmetric complex upon reduction, which likely originates
from the lability of the opposing pyridine moieties due to the
−
trans effect. In comparison to that of [Ir(bpsa-en)Cl ] , the CV
2
of [Ir(bpsa-Ph)Cl2]− shows an additional reversible peak at
+
1.28 V versus SCE during the oxidative sweep (Figure S16 in
the Supporting Information). The rich electrochemistry of the
phenylenediamide derivatives suggests that this additional
22
process is caused by the oxidation of the ligand’s bridging unit
a
The heat of formation calculated for the lowest energy C -cis isomer
2
(
Figure S17 in the Supporting Information). Interestingly, the
b
was used as a reference. See the Experimental Section for details.
involvement of this aromatic sulfonamide group in th−e
electrochemical oxidation process for [Ir(bpsa-Ph)Cl2]
suggests that the linker moiety might not behave innocently
during the catalytic water oxidation reaction and may be
affecting the catalyst’s performance.
The potential at which the first oxidation process of each
dichloro complex occurs can be correlated with the energy of
each complex’s HOMO. Figure 3 illustrates the DFT-generated
However, considering the error of the heats of formation
obtained by DFT calculations, an accurate prediction of the C1-
cis to C -cis isomer ratio is unfortunately impossible.
Electrochemistry and Electronic Structure. The redox
processes in these catalysts were evaluated via cyclic
voltammetry (CV) in acetonitrile (Table 2 and Figure 2) to
2
a
Table 2. Electrochemical Properties of the Ir(III) bpsa
Complexes
E1/2OX/V (ΔE /mV)
b
complex
EpcRED/V
−1.75
−1.84
−1.70
−
[
[
[
Ir(bpsa-Cy)Cl2]
+0.93 (74)
+0.94 (71)
+0.78 (75)
−
c
c
Ir(bpsa-en)Cl2]
Ir(bpsa-Ph)Cl2]−
+1.26
+1.28 (90)
a
Measured in MeCN containing 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAH) solution as the electrolyte. Scan rate
0.1 V/s. All potentials were referenced to SCE using ferrocene as an
=
+
b
c
internal standard (fc/fc = +0.40 V vs SCE). Peak separation. A
second irreversible oxidation peak, due to the formation of a
degradation product of the original complex, is observed if [Ir(bpsa-
−
en)Cl ] is reoxidized after a reductive sweep to −1.9 V (Figure S15 in
2
the Supporting Information).
−
Figure 3. Frontier orbital diagrams for [Ir(bpsa-Cy)Cl ] , [Ir(bpsa-
2
−
−
en)Cl ] , and [Ir(bpsa-Ph)Cl ] based on DFT calculations (B3LYP/
2
2
LANL2DZ). The calculated trends for the related aquo complexes
Figure S13 in the Supporting Information) are mirroring those
observed for the chloride complexes.
(
−
Figure 2. Cyclic voltammograms of [Ir(bpsa-Cy)Cl ] , [Ir(bpsa-
2
−
−
en)Cl ] , and [Ir(bpsa-Ph)Cl ] in acetonitrile to demonstrate the
2
2
−
−
frontier orbitals for [Ir(bpsa-Cy)Cl ] , [Ir(bpsa-en)Cl ] , and
2
2
first reversible oxidation process of each complex. 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.
−
[
Ir(bpsa-Ph)Cl ] , and their calculated HOMO energies are in
2
resonable agreement with the trends observed for the
electrochemically determined oxidation potential. One can
note that the HOMO of the phenylene-containing complex
−
[
Ir(bpsa-Ph)Cl ] is considerably destabilized in comparison to
2
unravel the effect of the different sulfonamide linkers on the
electronic structures of the different Ir(III) bpsa complexes. A
reversible oxidation wave at potentials ranging from +0.78 V to
the HOMO of the two complexes with aliphatic linkers. The
participation of the phenylene ring in the calculated HOMO is
distinctly visible in Figure 3 and highlights how the
delocalization provided by the aromatic system of the
+
0.94 V versus SCE was observed for each of the three iridium
C
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Inorganic Chemistry
Article
phenylene spacer greatly facilitates the oxidation process. This
further supports the involvement of the ligand in the oxidation
chemistry and likely also in the catalyzed water oxidation
processes. The almost identical oxidation behavior detected via
CV for both of the complexes with aliphatic spacers correlates
well with the very similar electronic structures observed for
these complexes with DFT models. It appears that the different
coordination geometries do not influence the iridium-centered
oxidation processes significantly. On the other hand, a
noteworthy effect of the coordination geometry can be detected
for the electrochemical reduction. The CV shows an irreversible
current response at potentials around −1.8 V vs SCE for all
three Ir(III) bpsa complexes; however, different redox
behaviors upon subsequent oxidative sweeps were detected
second, more significant oxidation wave with an onset potential
at approximately +1.6 V vs SCE was observed. The dramatic
increase in current and the absence of this oxidation wave in
background scans are the signature that electrocatalytic water
oxidation is occurring.
Crabtree and co-workers have reported that the electro-
chemical quartz crystal microbalance (EQCM) can be used to
distinguish heterogeneous water oxidation catalysts from those
18
that are homogeneous. Upon oxidation and activity, the
catalysts that function heterogeneously will deposit upon the
surface of the EQCM and the change in frequency of the
oscillator caused by this loading is easily detected. Initially
EQCM experiments were attempted at gold working electro-
des, as described in the literature. However, at the potential
necessary to observe the oxidation of the iridium complex and
to effect catalytic water oxidation, the redox chemistry (as well
as the dissolution) of the gold surface became quite
(
Figures S14−S16 in the Supporting Information).
Complex, uninterpretable, irreversible redox responses
−
following reduction are only observed in [Ir(bpsa-en)Cl ] , a
2
2
3
significant, rendering the interpretation of the EQCM
experiments untenable. As glassy-carbon-coated QCM crystals
are not readily available, subsequent EQCM experiments were
carried out using a quartz crystal with electrodes fabricated
from indium−tin oxide (ITO), which proved to be much more
behavior that can be attributed to significant chemical changes
in the complex upon reduction. The absence of any new,
irreversible redox waves during repeated potential cycling
suggests that the cis arrangement of the pyridine moieties in
−
−
[
Ir(bpsa-Cy)Cl ] and [Ir(bpsa-Ph)Cl ] renders them slightly
2
2
stable under the conditions of the experiment. The results that
more stable, in comparison to the more labile, trans-substituted
−
−
follow clearly demonstrate that the [Ir(bpsa-Cy)Cl ] catalyst
[
Ir(bpsa-en)Cl ] . These differences can be elucidated with the
2
2
is behaving as a molecular, homogeneous species in the process
of water oxidation.
results of the DFT calculations. As shown in Figure 3, [Ir(bpsa-
−
en)Cl2] has the highest LUMO energy among all three
In Figure 5A the voltammetry of [Ir(bpsa-Cy)Cl ]− in
2
complexes, which is in good agreement with the electrochemi-
cally determined reduction potential. Thus, the observed
acetonitrile with 5% water (conditions analogous to those in
Figure 4 where water oxidation catalysis occurs at a glassy-
carbon working electrode) can be seen. It should be noted that
the higher overpotential for water oxidation on ITO, versus
glassy carbon, prevents catalytic water oxidation from taking
place under these conditions. The corresponding QCM trace is
shown below in Figure 5D. The voltammetry shows a well-
defined reversible redox wave corresponding to the Ir(III/IV)
redox process. The rather large peak separation between the
anodic and cathodic waves is likely due to slow electron transfer
−
instability of [Ir(bpsa-en)Cl2] during the electrochemical
reduction process can be correlated with the poor stability of
the LUMO. In comparison, the calculated LUMO energies of
−
[
Ir(bpsa-Cy)Cl2]− and [Ir(bpsa-Ph)Cl2] are significantly
lower, which explains the increased robustness of these
complexes upon reduction.
Water Oxidation Catalysis. The two most robust
−
−
complexes, [Ir(bpsa-Cy)Cl2] and [Ir(bpsa-Ph)Cl ] , were
2
examined for their ability to catalyze water oxidation electro-
chemically. Figure 4 illustrates the CV response with and
24
kinetics attributable to the ITO electrode surface. The QCM
traces show very little difference between the background
−
without H O present for [Ir(bpsa-Cy)Cl ] . The results for
2
2
(
acetonitrile/water) and the solution containing the iridium
−
[
Ir(bpsa-Ph)Cl ] follow the same trends and are reported in
2
complex being oxidized and reduced. In both cases, the change
in the frequency of the QCM is negligible, indicating that there
is no material being deposited (i.e., no precipitation of
heterogeneous catalyst) on the surface of the electrode under
the conditions at which water oxidation electrocatalysis occurs.
As a control, the EQCM experiment was then carried out using
Figure S18 in the Supporting Information. With an addition of
−
5
% water to the acetonitrile solution of [Ir(bpsa-Cy)Cl ] , a
2
2+
the catalyst [Cp*Ir(H O) ] , which was shown, as described
2
3
18
by Crabtree, to behave heterogeneously. As can be seen from
the data in Figure 5B,E, the frequency of the QCM oscillator
decreases dramatically upon oxidation of the Cp*-iridium
complex, replicating the results from the literature and
confirming that the ITO-coated QCM crystal behaves
analogously to one with gold electrodes for the purpose of
discerning whether a WOC is behaving heterogeneously. The
final EQCM experiment, shown in Figure 5C,F, was carried out
−
with the [Ir(bpsa-Cy)Cl ] WOC but in aqueous 0.1 M KNO
2
3
instead of in the nonaqueous acetonitrile solution. While the
voltammetry looks quite similar to that in Figure 5A (a
reversible Ir(III/IV) redox process with a large peak
separation), the QCM behavior is dramatically different from
the results shown in Figure 5D. In aqueous media, upon
oxidation, the iridium complex is deposited on the surface of
the electrode (the large decrease in frequency) and then, as it is
−
Figure 4. Cyclic voltammograms of [Ir(bpsa-Cy)Cl ] in acetonitrile
2
with or without the addition of 5% water (light blue and dark blue,
respectively) and the corresponding baseline scans (gray and black). 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.
D
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Article
−
Figure 5. (A) Cyclic voltammetry of a solution containing 0.9 mM [Ir(bpsa-Cy)Cl ] in 0.1 M TBAH/CH CN with 5% water. (B) Cyclic
2
3
2+
voltammetry of a solution containing 1.2 mM [Cp*Ir(H O) ] in aqueous 0.1 M KNO . (C) Cyclic voltammetry of a solution containing 0.85 mM
Ir(bpsa-Cy)Cl ] in aqueous 0.1 M KNO . (D) QCM response that corresponds to the voltammetry in (A). (E) QCM response that corresponds
2
3
3
−
[
2
3
to the voltammetry in (B). (F) QCM response that corresponds to the voltammetry in (C). In all experiments the potential scan rate is 50 mV/s.
reduced, it desorbs from the electrode, as evidenced by the
frequency increasing back to its original value. This is attributed
to the change in charge state of the complex: the reduced
Ir(III) complex carries a charge of 1− but upon oxidation to
Ir(IV) the complex becomes neutral. As it is anionic, the
reduced catalyst is soluble in aqueous KNO but the oxidized,
3
neutral, complex is not and the latter consequently deposits on
the surface of the electrode until it can be resolublized by
reduction. This behavior is not seen in the EQCM experiments
carried out in acetonitrile because the neutral complex is, not
surprisingly, more soluble in acetonitrile than it is in water.
Catalytic water oxidation studies were performed in 1 M
HNO for all three Ir(III) complexes by driving the reactions
3
chemically with ceric ammonium nitrate (CAN), a one-electron
oxidant incapable of water oxidation. O2 evolution was
monitored using the home-built pressure transducer setup as
described previously. An initial series of experiments was
conducted using a large excess of CAN (0.80 M), while the
concentrations of each complex were kept in the micromolar
range. In Figure 6 the amount of oxygen evolved as a function
of time can be seen, and Table 3 gives the turnover numbers
and frequencies that quantitatively describe the performance of
Figure 6. Plots of O evolution versus time at various concentrations
2
−
−
of catalysts [Ir(bpsa-Cy)Cl ] , [Ir(bpsa-en)Cl ] , and [Ir(bpsa-
2 2
Ph)Cl ] . Reaction conditions: final volume 11 mL, [CAN] = 0.80
−
2
11a
M, 25 °C.
[Ir(bpsa-en)Cl ]− is most likely related to the observed
2
electrochemical instability of this C -symmetric complex. Of
2
−
the two nonsymmetric Ir(III) complexes, [Ir(bpsa-Ph)Cl ]
generated O at a higher rate than [Ir(bpsa-Cy)Cl ] under
2
−
2
2
these WOCs. The amount of O produced in each case was
similar conditions. A maximum TON of 3540 was observed
using an 8.4 μM catalyst concentration during a 10 day
experiment. This value compares well to the TONs measured
for most water oxidation catalysts to date, and the molecular
catalysts in this highly oxidative environment are long-lived.
After completion of the reactions, the gas in the headspace of
2
below the theoretical maximum O yield (2.2 mmol), indicating
2
that the catalysts were deactivated prior to exhausting all of the
−
−
CAN. Both [Ir(bpsa-Cy)Cl ] and [Ir(bpsa-Ph)Cl ] exhibit
2
2
catalytic activity that is significantly higher than that of
−
[
Ir(bpsa-en)Cl ] . The low turnover number (TON) of
2
E
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Inorganic Chemistry
Table 3. Catalytic Performance of [Ir(bpsa-Ph)Cl ] ,
Article
−
in the kinetic plot. The sigmoidal-like O evolution at higher
concentrations is reminiscent of the kinetics observed for the
2
2
−
−
[
Ir(bpsa-Cy)Cl ] , and [Ir(bpsa-en)Cl ] in the Water
2
2
Oxidation Reaction
iridium carbene complexes containing a Cp* unit, suggesting
the possibility of a pathway involving two consecutive first-
complex
concn
regime
CAN/
complex ratio TON
initial TOF
11f
‑
1
order reactions or a redox equilibrium.
Accordingly, O2
(
concn (μM))
(s )
production over time was found to be linear at lower
concentrations, indicating that different concentration regimes
lead to dissimilar catalytic mechanisms. The slower oxygen
evolution during the early periods of these reactions was
initially interpreted as a catalyst degradation phase, which
produced highly active IrOx nanoparticles. However, the
EQCM and DLS results presented here and during earlier
high
[
[
[
Ir(bpsa-Ph)Cl2]−
33.2 μM)
24100
23000
18400
2290
2000
53
7.50 × 10−3
(
−
3.55 × 10−3
Ir(bpsa-Cy)Cl2]
34.8 μM)
(
−
1.94 × 10−4
Ir(bpsa-en)Cl2]
43.5 μM)
(
low
11f
investigations clearly disprove this hypothesis. Slow sub-
[
[
[
Ir(bpsa-Ph)Cl2]−
8.4 μM)
95700
92000
91700
3540
1820
245
3.55 × 10−3
stitution of the chloride ligands with water molecules or a redox
equilibrium seems a more likely explanation for this behavior
(Figure S32 in the Supporting Information). The rate of O2
evolution was observed to be the highest between 30 and 40 h
after the CAN was added, and at high concentrations this rate
(
−
2.80 × 10−3
Ir(bpsa-Cy)Cl2]
8.7 μM)
(
−
9.07 × 10−4
Ir(bpsa-en)Cl2]
8.7 μM)
(
was found to have a linear relationship with the concentration
−
of [Ir(bpsa-Ph)Cl ] , as seen in Figure 7B. The log−log plot of
2
the vials was analyzed by gas chromatography. Carbon dioxide
was not detected in the headspace, confirming the robustness of
the bpsa ligand under these harsh reaction conditions. No
particles were observed by eye during the reaction or at the end
of the process. The homogeneity of the reaction mixture was
further confirmed by dynamic light scattering (DLS) studies,
which eliminated the possibility of iridium oxide nanoparticles
serving as the active catalysts (Figures S19−S29 in the
Supporting Information). It is important to note that the
previously observed formation of gas bubbles was detected by
DLS, but the application of vacuum at the end of the reaction
allowed the evaluation of the solution’s homogeneity without
rate versus concentration (Figure 7C) indicates an apparent
1
.5-order dependence on the catalyst. This fractional reaction
order, which is identical with that observed in earlier studies
with structurally very different Ir-based water oxidation
catalysts, highlights the complexity of such iridium-catalyzed
11f,20b
water oxidation processes.
CONCLUSION
■
A new family of iridium(III) dichloro complexes containing a
tetradentate bis(pyridine-2-sulfonamide) (bpsa) ligand frame-
work was synthesized, and their performance as robust and
competent water oxidation catalysts was documented. Only one
coordination isomer of each of these complexes is isolated in
the synthesis, and the coordination geometries of these
complexes switch from C symmetry to C symmetry as the
1
1f
interference.
The fact that the complex that contains a conjugated
phenylene linker is a more active and robust catalyst is
intriguing. While one might hypothesize that the aromatic
framework may be more susceptible to the oxidative
degradation in comparison to the aliphatic cyclohexane linker,
the ability of the extended delocalized system to facilitate the
stabilization of the highly oxidized iridium center during the
2
1
sulfonamide linkers become more rigid when moving from the
flexible ethylene bridge to the cyclohexane or phenylene ring.
The C -symmetric structures, which can effectively avoid the
1
destabilizing trans effect of the two opposing pyridine moieties
catalytic process may be of greater significance. To investigate
found in the C -symmetric structure, have much greater
stability, as evidenced by electrochemical studies and DFT
2
−
the behavior of the [Ir(bpsa-Ph)Cl ] WOC in greater depth,
2
CAN oxygen evolution experiments were carried out over a
wider range of catalyst concentrations, as can be seen in Figure
calculations. The two electrochemically more stable C -
1
−
−
symmetric complexes [Ir(bpsa-Cy)Cl ] and [Ir(bpsa-Ph)Cl ]
2
2
7
A.
also demonstrate performance much higher than that of
[Ir(bpsa-en)Cl2]− in the Ce(IV) driven water oxidation,
reaching TONs in the thousands during a 10 day experiment.
EQCM and dynamic light scattering studies excluded the
−
At higher concentrations of [Ir(bpsa-Ph)Cl ] , O evolution
2
2
was observed to proceed at a faster rate, but the catalyst also
stopped producing oxygen earlier, as evidenced by the plateau
−
Figure 7. (A) Plots of O evolution versus time at various concentrations of [Ir(bpsa-Ph)Cl ] . The reactions were performed in 11 mL of CAN at
2
2
−
0
.80 M. (B) Plot of maximum rates of O evolution observed between 30 and 40 h versus concentrations of [Ir(bpsa-Ph)Cl ] . (C) log (rate)−log
2
2
−
(
concentration) plot of [Ir(bpsa-Ph)Cl ] and its linear fit.
2
F
DOI: 10.1021/acs.inorgchem.5b01709
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
presence of IrO nanoparticles during the oxidation of water,
strongly supporting the homogeneity of these long-lived
Synthesis of N,N′-(Ethane-1,2-diyl)bis(pyridine-2-sulfona-
x
mide) (H bpsa-en). The synthetic method was the same as that
2
described above except using ethylenediamine (0.21 g, 3.50 mmol)
instead of (1R,2R)-cyclohexane-1,2-diamine (0.40 g, 3.50 mmol). An
ivory-colored solid (0.90 g, 2.63 mmol) was collected after the solvent
iridium water oxidation catalysts under highly oxidative
−
conditions. Additionally, [Ir(bpsa-Cy)Cl ] and [Ir(bpsa-Ph)-
2
−
Cl2] were also found to be competent electrocatalysts for
was removed in a rotary evaporator, and the resulting solid was dried
water oxidation. DFT calculations indicate that the conjugated
1
in a vacuum oven. Yield: 79%. H NMR (300 MHz, CDCl ): δ 8.72
3
−
phenylene ring of [Ir(bpsa-Ph)Cl ] participates in its HOMO,
2
(d, J = 5.0 Hz, 2H), 8.01 (dt, J = 7.9, 1.0 Hz, 2H), 7.95 (td, J = 7.7, 1.7
suggesting that the phenylene linker is not innocent in the
Hz, 2H), 7.54 (ddd, J = 7.5, 4.8, 1.0 Hz, 2H), 6.46 (s, 2H), 3.40 (s,
4H). 13C NMR (125 MHz, CDCl ): δ 157.9, 149.7, 138.5, 126.9,
oxidation process and can enhance the water oxidation activity
3
−
of [Ir(bpsa-Ph)Cl ] by stabilizing the high-valent iridium oxo
122.2, 43.4. MS-ESI (m/z, 1% trifluoroacetic acid in MeOH): 343.1
2
(
100%, H bpsa-en).
intermediate through extended charge delocalization. Future
work will involve the further exploration of this unique Ir(III)
bpsa motif, both through ligand modifications and through
changes in the metal center, for the design of even more
efficient and robust water oxidation catalysts.
2
Synthesis of N,N′-(1,2-Phenylene)bis(pyridine-2-sulfona-
mide) (H bpsa-Ph). In a dried and sealed round-bottom flask, 1,2-
2
phenylenediamine (0.39 g, 3.60 mmol) was dissolved in dried and
degassed tetrahydrofuran (∼100 mL). A 2 equiv amount of N-
ethyldiisopropylamine (0.93 g, 7.2 mmol) was also added to the
reaction mixture. The solution was sonicated for 30 min at room
temperature and then cooled to 0 °C. The colorless pyridine-2-
sulfonyl chloride solution (∼3 mL) resulting from previous steps was
injected slowly into the above solution. The reaction mixture was
stirred at 0 °C for several hours and then left overnight at room
temperature. The undissolved solid was discarded. The solvent was
removed in a rotary evaporator. Methanol was added to the resulting
tarlike oil to initiate precipitation. The off-white precipitate was
collected through filtration and was dissolved in a minimal amount of a
hot methanol and dichloromethane mixture. The solution was settled
in the refrigerator to allow recrystallization. The resulting solids were
collected and recrystallized the second time from a hot methanol and
dichloromethane mixture. White crystals (0.42 g, 1.09 mmol) were
EXPERIMENTAL SECTION
■
General Considerations. 2-Mercaptopyridine, sodium hypochlor-
ite (14.5%), (1R,2R)-cyclohexane-1,2-diamine, ethylenediamine, 1,2-
phenylenediamine, triethylamine, N-ethyldiisopropylamine, and all
solvents were used without further purification from commercial
sources. [Cp*Ir(H O) ]SO was synthesized as described in the
2
3
4
2
5
literature. IrCl ·xH O was purchased from Pressure Chemical Co.
H and C NMR spectra were recorded on Bruker Avance 300 and
3
2
1
13
500 MHz spectrometers at room temperature. ESI-MS was performed
on a Thermo-Fisher LCQ instrument with ∼90 μM methanol
solutions. A 1% trifluoroacetic acid solution was added for mass
spectrometry of the ligands.
collected after washing with diethyl ether several times and dried in a
Synthesis of Pyridine-2-sulfonyl Chloride. Pyridine-2-sulfonyl
1
vacuum oven. Yield: 33%. H NMR (300 MHz, CDCl ): δ 10.36 (s,
chloride was prepared by a modified method published by Walsh et
3
21
2
8
H), 8.76 (ddd, J = 4.9, 1.6, 0.9 Hz, 2H), 8.07 (td, J = 7.9, 1.0 Hz, 2H),
.00 (dt, J = 7.9, 1.6 Hz, 2H), 7.58 (m, 4H), 7.20 (dd, J = 6.1, 3.5 Hz,
al. Typically, 2-mercaptopyridine (1.00 g, 9.00 mmol) was combined
with 20 mL of dichloromethane in a 250 mL round-bottom flask.
Concentrated hydrochloric acid (25 mL) was added, and the mixture
was cooled to 0 °C. The dropwise addition of sodium hypochlorite
1
3
2H). C NMR (125 MHz, CDCl
127.8, 127.7, 127.4, 122.7. MS-ESI (m/z, 1% trifluoroacetic acid in
): δ 157.5, 148.8, 139.6, 130.2,
3
+
(
14.5%, 55 mL) was exothermic and caused the solution to turn yellow
MeOH): 413.1 (100%, H bpsa-Ph-Na ), 391.1 (46%, H bpsa-Ph).
2
2
gradually. The reaction mixture was stirred for another 30 min at 0 °C,
and the resulting solution was extracted twice with dichloromethane
Synthesis of Iridium N,N′-((1R,2R)-Cyclohexane-1,2-diyl)bis-
(
pyridine-2-sulfonamide) Dichloride Sodium Salt (Na[Ir(bpsa-
Cy)Cl ]·0.5H O). IrCl ·xH O (0.10 g, 0.28 mmol), 1 equiv of H bpsa-
(
∼20 mL). The organic layers were combined and dried over sodium
2
2
3
2
2
Cy (0.11 g, 0.28 mmol), and 2-methoxyethanol (∼20 mL) were
sulfate for 1/2 h. The solid was filtered off, and the solvent was mostly
removed in a rotary evaporator, producing a colorless liquid (∼3 mL).
The resulting pyridine-2-sulfonyl chloride solution was used without
combined in a sealed round-bottom flask and degassed with argon for
1
5 min. The mixture was heated at 110 °C in the dark under an argon
atmosphere for 2 h. After the mixture was cooled to room
temperature, the solvent was removed under reduced pressure. The
desired iridium compound was separated by column chromatography
on silica gel using 14/1 (v/v) dichloromethane/2-methoxyethanol as
the eluent. A yellow powder (0.11 g, 0.17 mmol) was collected after
further purification, since it is only moderately stable in the presence of
1
air or water. Yield: 74%. H NMR (300 MHz, CDCl ): δ 8.84 (d, J =
3
4
7
.5 Hz, 1H), 8.13 (d, J = 7.7 Hz, 1H), 8.07 (td, J = 7.3, 1.5 Hz, 1H),
.71 (ddd, J = 7.1, 4.8, 1.3 Hz, 1H), 5.30 (s, dichloromethane).
Synthesis of N,N′-((1R,2R)-Cyclohexane-1,2-diyl)bis-
(pyridine-2-sulfonamide) (H bpsa-Cy). The pyridine-2-sulfonyl
the solvents were removed in a rotary evaporator and vacuum oven.
2
1
chloride solution obtained by the aforementioned procedure was
Yield: 59%. H NMR (300 MHz, acetone-d
6
): δ 9.68 (dd, J = 6.4, 1.3
diluted with diethyl ether (∼20 mL). Separately, (1R,2R)-cyclohexane-
Hz, 1H), 8.15 (td, J = 8.0, 1.5 Hz, 1H), 7.94 (td, J = 7.7, 1.2 Hz, 1H),
7.79 (d, J = 7.8 Hz, 1H), 7.72 (m, 2H), 7.53 (d, J = 5.7 Hz, 1H), 7.31
(ddd, J = 7.5, 5.8, 1.5 Hz, 1H), 3.62 (dt, J = 10.2, 3.3 Hz, 1H), 2.73 (dt,
J = 10.4, 3.5 Hz, 1H), 2.48 (m, 1H), 2.27 (m, 1H), 1.58 (m, 2H), 1.28
1
,2-diamine (0.40 g, 3.50 mmol) was dissolved in diethyl ether (∼20
mL), to which triethylamine (2 equiv) was also added. The diethyl
ether solution of (1R,2R)-cyclohexane-1,2-diamine and triethylamine
was then slowly injected into the pyridine-2-sulfonyl chloride solution,
and a white precipitate was formed almost immediately. The reaction
mixture was stirred at 0 °C for several hours, and after that the crude
product was separated by vacuum filtration and washed with diethyl
ether. The resulting powder was extracted with ethyl acetate. The
product was taken up in ethyl acetate, with the impurities left as an
insoluble residue. The mixture was filtered, and a light yellow solid
(m, 2H). 13C NMR (125 MHz, CD CN): δ 167.8, 162.8, 150.1, 149.1,
3
139.8, 138.8, 128.2, 127.7, 125.1, 124.8, 75.1, 64.9, 36.8, 33.5, 25.4,
−
24.5. MS-ESI (m/z, MeOH): 657.1 (100%, [Ir(bpsa-Cy)Cl ] ). Anal.
2
Calcd for Na[Ir(bpsa-Cy)Cl ]·0.5H O: C, 27.87; H, 2.78; N, 8.12.
2
2
Found: C, 28.07; H, 2.95; N, 7.95.
Synthesis of Iridium N,N′-(Ethane-1,2-diyl)bis(pyridine-2-
sulfonamide) Dichloride Sodium Salt (Na[Ir(bpsa-en)Cl ]·
2
(
1.14 g, 2.86 mmol) was collected after the ethyl acetate was removed
H O). IrCl ·xH O (0.10 g, 0.28 mmol), 1 equiv of H bpsa-en (0.097
2
3
2
2
1
in a rotary evaporator and vacuum oven. Yield: 86%. H NMR (300
g, 0.28 mmol), and 2-methoxyethanol (∼20 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 atmosphere
for 2 h. After the mixture was cooled to room temperature, the solvent
was removed under reduced pressure. The desired iridium compound
was separated by column chromatography on silica gel using 6/1 (v/v)
dichloromethane/methanol as eluent. A yellow powder (0.073 g, 0.12
mmol) was collected after the solvents were removed in a rotary
MHz, CDCl ): δ 8.89 (s, 2H), 8.66 (dt, J = 4.9, 1.4 Hz, 2H), 8.01 (td, J
3
=
7.8, 1.3 Hz, 2H), 7.98 (dt, J = 7.2, 1.3 Hz, 2H), 7.54 (ddd, J = 7.2,
4.9, 1.5 Hz, 2H), 3.34 (dd, J = 8.9, 3.3 Hz, 2H), 2.45 (dt, J = 13.2, 2.5
13
Hz, 2H), 1.71 (m, 2H), 1.45 (m, 2H), 1.27 (m, 2H). C NMR (125
MHz, CD CN): δ 158.1, 149.3, 139.3, 127.2, 122.1, 57.1, 34.5, 24.0.
3
MS-ESI (m/z, 1% trifluoroacetic acid in MeOH): 397.1 (100%,
H bpsa-Cy).
2
G
DOI: 10.1021/acs.inorgchem.5b01709
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
evaporator and vacuum oven. Yield: 43%. 1H NMR (300 MHz,
convergence. Geometry optimizations for all three possible isomers
of each iridium complex were carried out under no constraint, yielding
reasonable optimal coordinates and orbital energies.
acetone-d ): δ 9.82 (dd, J = 5.8, 0.6 Hz, 2H), 8.10 (dt, J = 7.8, 1.5 Hz,
6
2
2
1
H), 7.75 (dd, J = 8.0, 0.6 Hz, 2H), 7.62 (ddd, J = 7.4, 5.8, 1.5 Hz,
H), 3.33 (m, 4H). 13C NMR (125 MHz, D O): δ 161.1, 152.0, 141.1,
2
28.8, 123.6, 54.9. MS-ESI (m/z, MeOH): 603.0 (100%, [Ir(bpsa-
ASSOCIATED CONTENT
Supporting Information
−
■
en)Cl ] ). Anal. Calcd for Na[Ir(bpsa-en)Cl ]·H O: C, 22.36; H,
2
2
2
2
*
S
.19; N, 8.69. Found: C, 22.78; H, 2.17; N, 8.22.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01709.
Synthesis of Iridium N,N′-(Phenylene-1,2-diyl)bis(pyridine-
2
-sulfonamide) Dichloride Sodium Salt (Na[Ir(bpsa-Ph)Cl ]·
2
2
.5H O). IrCl ·xH O (0.05 g, 0.14 mmol), 1 equiv of H bpsa-Ph
2
3
2
2
(
0.055 g, 0.14 mmol), and 2-methoxyethanol (∼20 mL) were
All NMR spectra, the electrochemical analysis of the
catalysts, and in operando DLS studies for all catalysts
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 atmosphere for 2 h. After the reaction mixture was cooled to
room temperature, the solvent was removed under reduced pressure.
The desired iridium compound was separated by chromatography on
silica gel using 2/1 (v/v) ethyl acetate/acetone as the eluent. A yellow
powder (0.033 g, 0.05 mmol) was collected after the solvents were
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for J.I.G.: jigoldsmit@brynmawr.edu.
E-mail for S.B.: bern@cmu.edu.
■
1
removed in a rotary evaporator and vacuum oven. Yield: 36%. H
*
NMR (300 MHz, acetone-d ): δ 9.70 (dd, J = 6.3, 1.5 Hz, 1H), 8.40
6
(
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, 5.9, 1.5 Hz,
H), 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, D O): δ 162.3, 161.4,
Funding
U.S. National Science Foundation CHE-1362629.
Notes
The authors declare no competing financial interest.
1
(
1
1
2
50.9, 150.8, 142.2, 141.5, 140.4, 130.6, 129.5, 128.9, 125.9, 125.7,
25.2, 122.0, 117.1, 97.3. MS-ESI (m/z, MeOH): 651.0 (100%,
−
ACKNOWLEDGMENTS
[
2
Ir(bpsa-Ph)Cl ] ). Anal. Calcd for Na[Ir(bpsa-Ph)Cl ]·2.5H O: C,
■
2
2
2
6.71; H, 2.38; N, 7.79. Found: C, 26.75; H, 2.31; N, 7.49.
Electrochemistry. Cyclic voltammograms were measured on a
The authors from CMU acknowledge support from the
National Science Foundation through CHE-1362629. The
authors acknowledge MicroVacuum for generously providing
samples of the ITO-coated QCM crystals.
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, electro-
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V/s was employed, starting from the positive scan unless further
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2
26
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H
DOI: 10.1021/acs.inorgchem.5b01709
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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