Photochemical water oxidation by cyclometalated iridium(iii) complexes: A mechanistic insight

Article abstract of DOI:10.1039/c5cc10520j

The proficiency of the cyclometalated iridium complexes [(η⁵-C₅Me₅)IrCl(L1)] (1), [(η⁵-C₅Me₅)IrCl(L2)] (2) and [(η⁵-C₅Me₅)IrCl(L3)] (3) has been examined towards photochemical water oxidation. The involvement of Ir(iv/v) in the catalytic process has been explored via CV, UV/vis, XPS spectroscopic studies and appreciable TON relative to the theoretical value of 1 from GC.

Full text of DOI:10.1039/c5cc10520j

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Photochemical water oxidation by
cyclometalated iridium(III) complexes: A
mechanistic insight
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Sujay Mukhopadhyay, Roop Shikha Singh, Arnab Biswas and Daya Shankar Pandey*
www.rsc.org/
Proficiency of the cyclometalated iridium complexes [(η5−C5Me5)IrCl(L1)]
(1), [(η5−C5Me5)IrCl(L2)] (2) and [(η5−C5Me5)IrCl(L3)] (3) have been
examined towards photochemical water oxidation. Involvement of Ir(IV/V)
in the catalytic process has been explored by CV, UV/vis, XPS spectro-
scopic studies and appreciable TON relative to theoretical value for 1 from
GC.
electron rich environment, which stabilizes high-valent iridium-oxo
intermediates formed during water oxidation.1b They hypothesized
involvements of a high-valent Ir(IV) and Ir(V) in the cyclic process,
but their existence has not yet been established in WOC. Further,
these complexes are very stable in presence of oxidants due to
strong M−C bond.2c,8 Until now, all the reports related to iridium
complexes focuses on systems like bipyridine (N^N, N^O donor), 2-
phenylpyridine (N^C donor) or its derivatives. However, we were
interested particularly in diversifying the C−H activated IrCp*
complexes involving Schiff bases as these are stable under broad pH
range. These complexes are very rigid to degradation as π−electron
cloud of the negatively charged Cp* moiety stabilizes conjugated
ligands and also high-valent metal complexes via alteration of their
electronic behavior.5
Homogenous water oxidation catalysis (WOC) mimicking natural
photosynthesis is the ultimate quest of research for sustainable
energy resources.1 Over past few years, enormous efforts have
been made by scientific community to overcome challenges in the
development of new alternatives for renewable energy and fuel.1
Hydrogen or oxygen neutral fuels obtained from photocatalytic
splitting of water offers an attractive route for cleaner and green
future.1 But, homo-/heterogeneous splitting of water into H2 and O2
is associated with major challenges towards sustainable energy
production.2 Further, oxidation of water to produce molecular O2
through multi-electron transfer processes is related to high
overpotential.2 To mimic the oxygen evolving complex in
photosystem II, various polyoxometalates, metal oxide nano-
particles, several noble, transition or heavy transition metal
complexes have been used as a homogeneous catalyst with
moderate to high TONs.2d-e Till date, reported processes involved
either electrochemical/ chemical oxidation with sacrificial oxidants
(SO) like Ce(IV), NaIO4, etc. or the most fascinating, photochemical
oxidation in presence of photosensitizers (PS) and SO.2d-3 Major
drawback associated with these systems are the use of SO at very
low or high pH inducing oxidation of water by itself,4 which may
degrade the catalyst and produce unwanted gases like CO2,
formation of nanoparticles and dissociation of ligands to produce
carboxylic acids.1c,2c To outstrip the aforesaid disadvantages,
Crabtree and Bernhard et al., developed some cyclometalated
catalysts based on pentamethylcyclopentadienyl (Cp*) iridium
complexes (IrCp*).1 In these systems the Cp* moiety provides an
Scheme 1. Synthesis of ligands and complexes 1–3.
Considering the usefulness of complexes containing IrCp* moiety,
1-3 involving 4-methyl-ester-(benzylidene-(4-tert-butyl-phenyl)-
amine (L1), 4-cyano-(benzylidene-(4-tert-butyl-phenyl)-amine (L2)
and 4-nitro-(benzylidene-(4-tert-butyl-phenyl)-amine) (L3)5b have
been examined toward photochemical water oxidation. Presence of
both tert-butyl and Cp* groups in these complexes offers a electron
rich milieu at the high-valent metal centre1b and supposed to
stabilize Ir(IV) and Ir(V) species involved in the catalysis. Expectedly,
the occurrence of Ir(V) in photochemical WOC has been
substantiated by CV and UV/vis spectroscopic studies.6 Eudiometric,
manometric, and gas chromatographic studies revealed that the
complex 1 (possessing ester group) serves as superior water
oxidation catalyst relative to the ones having cyano- (2) and nitro-
(3) groups. As electron withdrawing effect of aforesaid groups
follow the order –NO2 > −CN > −CO2Me, catalytic efficiency of
respective complexes may increase with electron density at the
metal centre.1b,7 To best of our knowledge, it is the first report
dealing with cyclometalated IrCp* complexes serving as efficient
WOC photochemically in presence of [Ru(bipy)3]2+ (RuBp)/Na2S2O8
*Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi -
221 005 (U.P.) India
* Corresponding author. Tel.: + 91 542 6702480; fax: + 91 542 2368174.
E-mail: dspbhu@bhu.ac.in
†Electronic Supplementary Information (ESI) available: [CCDC deposition Nos.
1018749, 1018750 (1 and 2) contain the supplementary crystallographic data for
this paper. In addition, Tables and Figures contain crystallographic information,
absorption plot, UV/vis spectra, CV, 1H NMR, and mass isotopic pattern, video of
manometer reading]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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at neutral pH via intermediacy of the high-valent iridium without
degradation of the employed catalysts.
1H, 13C NMR, and mass spectra of ligands and complexes have
been reported elsewhere5b and mass isotropic distribution for 1–3
along with simulated patterns are depicted in Fig. S2, ESI†.
Electronic absorption spectra of 1-3 have been acquired in CH3CN
and CH3CN:H2O, 1:9 (c, 10 µM; v/v; pH ~7.1) (Fig.S6, ESI†). The low
energy bands at ~350−450 nm have been assigned to n−π∗,
whereas high energy bands at ~250−335 nm for mixed LMCT and
π−π∗ transitions. Notably, it displayed decrease in absorbance in
CH3CN:H2O (1:9) as compared to CH3CN. Small changes in UV/vis
spectral features in CH3CN relative to CH3CN:H2O are common and
clearly indicated aquation in presence of water without any change
in chemical identity of 1-3.8 UV/vis photocatalytic irradiation
experiments were performed in a dark chamber in presence of
RuBp (30 μM), Na2S2O8 (300 μM), and catalysts (30 μM) in
CH3CN:H2O (1:9). After addition of RuBp, it showed a strong
absorption at 450 nm characteristic of MLCT for RuBp (Fig. 3, Fig.
S7, ESI†). Mere addition of Na2S2O8 didn’t cause any significant
alteration in the spectral pattern, however after 3 min LED (440 nm)
irradiation and subsequent addition of 1 and 2, absorbance (300 –
600 nm) enhanced with emergence of a new short-lived tiny band
at 760 and 762 nm (ε, 1 × 103 M-1cm-1), respectively. The new bands
at ~760 nm may be ascribed to spin forbidden transition for Ir(V).6
For complex 3 this band was not observed, it simply showed
enhanced absorbance in the range of 300–600 nm. As ester- and
cyano- groups offer weak electron withdrawal relative to the nitro-
group, very short lived Ir(V) may not be stable for 3 and may
substantiate the absence of the minute peak. Further, distinctive
colour change has not been recorded due to presence of highly
absorbing RuBp. UV/vis and CV studies offered excellent support to
propose that complexes do act as water oxidation catalyst and
existence of Ir(IV) and Ir(V) in the catalytic cycle.1b
Fig. 1 ORTEP view of 1 (a) and 2 (b) with 50% ellipsoid probability. H atoms are omitted
for clarity.
Structures of the complexes 1 and 2 have been authenticated by
X-ray single crystal analyses. Details about data collection, solution
and refinement are summarized in Table S1, and selected
geometrical parameters gathered in Table S2 and S3 (ESI†).
Pertinent views along with partial atom numbering scheme is
shown in Fig. 1. The metal centre in these complexes adopt typical
“piano-stool” geometry and coordination sites about iridium are
occupied by aldimine nitrogen, and carbon from the adjacent
phenyl ring, the chloro group and Cp* ring bonded in η5-manner.
Fig. 2 Cyclic voltammogram of 1 (a) in acetonitrile (c, 1 mM). (b) CV recorded after
addition of 300 µL deaerated water. Inset showing the zoomed reduction wave.
Fig. 3 UV/vis spectra of photocatalytic reaction, inset showing tiny peak at 760
nm.
A typical chemical WO has been realized by mixing a solution of
1−3 (25 µM) to ceric ammonium nitrate (CAN) (4000 μM) at pH 1 in
a sealed vial under stirring at 25 °C. The total reaction volume was
held constant at 2000 μL and kinetics of the WO was studied by
measuring pressure differences, built up by oxygen evolution, with
a manometer (Fig. S19d, ESI†). The manometer consisted of two
sensing ports, one connected to the reaction vial containing both
CAN and catalyst and the other to a reference vial having only CAN.
As the reaction proceeded, pressure difference between reaction
and reference vial enhanced which was recorded by the
manometer with respect to time. After 15 min, pressure difference
attained saturation and hence, oxygen evolution too. The evolved
gas was detected and quantified by GC and to minimize error the
experiments were repeated thrice.
Cyclic voltammograms (CV) of 1-3, acquired in acetonitrile [(n-
Bu)4N]ClO4 (0.1 M) at 50 mV s-1, (Fig. 2a and Fig. S3, ESI†) revealed
one quasi reversible oxidation wave (Epa, Epc; 0.680, 0.615, 1; 0.744,
0.680; 2, 0.753, 0.687 V, 3) assignable to Ir3+/Ir4+ couple and an
irreversible wave at (Epa, 1.065, 1; 1.075, 2; 1.094, 3) due to short
lived Ir(V). Further, these displayed only one irreversible reduction
wave (Epc; -0.279, 1; -0.323, 2; -0.353 V, 3).1a The CV study
indicated that high-valent iridium is generated which might be a key
reactive intermediate for WO.2c After careful addition of 0–300 µL
of deaerated water the reduction wave exhibited negative potential
shift (Epc; -0.659, 1; -0.678, 2; -0.662 V, 3) with emergence of a new
wave at Epc; -1.082, 1; -1.087, 2; -1.091 V, 3, characteristic of
dissolved oxygen (Fig. 2b, Fig. S4-S5, ESI†). Despite solution was
thoroughly purged with N2 and deaerated water added with utmost
care, generated wave clearly indicated that O2 is being produced in
the closed system.
In photochemical catalytic WO reactions, 25 μM 1−3 together
with 500 μM RuBp and 2500 μM Na2S2O8 (SO) at pH 7.2 (phosphate
buffer) were added so that total volume of the reaction mixture and
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head space remains 2 and 3 ml, respectively. The photocatalytic
process was initiated by irradiating reaction mixture with the help
of a 3 W blue LED (λmax, 440 nm) at 25°C (See experimental setup in
Fig. S19, ESI†). Each reaction was run for 15 min and detailed
kinetics, identification and quantification of the gaseous products
were performed by GC following same method as that adopted for
chemical WO with CAN.
µM/min plot gave a straight line and followed first order rate law.
The Kobs = 0.181, 0.169, and 0.153 min-1 for 1, 2, 3 respectively Fig.
S13, ESI†.
Available data indicate that 1-3 act as molecular catalysts and do
not decompose to IrOx nano particles in photocatalytic water
oxidation. TEM analysis revealed lack of nano particles (IrOx) (Fig.
4a, Fig. S14−S15, ESI†) in the solution containing 1−3 (25 μM), RuBp
(500 µM) and SO (2500 μM) after 1 h irradiation. The pH of solution
decreased from 7.2−6.5 after irradiation and at this pH if any IrOx
nanoparticles formed would remain insoluble and detectable by
TEM. The detected solid in the TEM grid is possibly a consequence
of dried inorganic materials. To compare the mechanism of action
of the catalysts and hydrated iridium chloride, we used IrCl3.xH2O as
catalyst under analogous conditions which resulted in nanoparticles
of average size 5–10 nm (Fig. 4b). It strongly suggested that 1−3 act
as molecular WOC.
After 15 min of catalytic photochemical WO, excess pressure
created in the reaction vial was released by a eudiometer2d setup
that displaced approximately 250 μL of water by catalyst 1. Pressure
difference between the reference and reaction vial was also
recorded by a manometer and video of the manometer reading is
given in ESI†. Similar experiments have been performed for CAN
mediated WO and nearly 350 μL water displacement was recorded
at pH 1.0. Headspace specimen gas (100 μL) was taken by a gas
tight syringe from the reaction vial and injected into the GC. To
calculate total oxygen evolved in the reaction, headspace volume
was calculated after 1 h assuming completion of the reaction.2d The
GC was calibrated using standard oxygen (98%) in a Tedlar bag from
Sigma-Aldrich, and calibration curve used to calculate the quantity
of oxygen evolved from the reaction mixtures.
All the three complexes have been found to be efficient WOC to
generate O2 (Fig. S8−S11, ESI†). However, efficiency of the catalyst
to regenerate after completion of a cycle is also very important. In
this context, TON and TOF of the catalyst have been calculated for
both chemical and photochemical reactions. For chemical process
160 equiv. of CAN was used with respect to catalyst. Since Ce4+
serves as a 1e– oxidant, maximum TON possible was 40.3a From the
GC data net O2 release from the system for each catalyst has been
calculated.2d Catalyst 1 showed highest efficiency (TON, 26, 17, 12;
TOF, 1.73, 1.13, 0.8/min; 1, 2, 3 respectively). But, TON has been
found to be much lower than expected; it may be due to
degradation of the catalyst at very low pH by highly oxidising CAN.
At the same time, unwanted nitric/nitrous oxide gases evolved from
the system may also give an erroneous result. Considering this
photochemical water oxidation pathway has been attempted which
gave superior results in comparison to chemical process. For
photochemical process 20 equiv. of PS and 100 equiv. SO were used
with respect to the catalyst. Since, SO presents a 2e- oxidant
maximum theoretical TON should be 50. In this case too, 1 showed
highest TON (TON, 47, 42, 40; TOF, 3.13, 2.8, 2.67/min; 1, 2, 3
respectively). The photochemical water oxidation is much more
reliable (purity of evolved gas as O2) than CAN mediated process.
Again, when we used 100 equiv. of PS, and 500 equiv. of SO with
respect to the catalysts for 1 h of LED irradiation, TON increases,
signifying homogeneity of the catalyst (TON = ~220, 190, 180 for 1,
2, 3 respectively). Use of higher equivalents of PS and SO lowered
the pH of medium below 5, which reduces their efficiency and
oxygen evolution stopped, suggesting the condition to be optimum.
Overall O2 yield after careful analysis of the evolved gases by GC did
not show evolution of CO2 and indicated robust nature of the
catalysts. In neutral solution (pH 7.2) reported TON is very much
encouraging as during reaction pH of the solution becomes lower
and efficiency of SO goes down with lower pH.9 Kinetic
measurements have also been made with the help of manometer
reading vs time. It showed 2 min time lag, sharp increase in the
manometer reading (O2 evolution) and a plateau at the end of the
reaction indicating saturation. The time vs TON plot is shown in Fig.
S12, ESI†. Controlled experiments showed that all the three
components, i.e. the PS, SO, and catalysts are must for water
oxidation. The concentration of the catalysts vs. oxygen evolution in
Fig. 4 (a) TEM images of 1 after 1 h photocatalytic reaction (b) TEM image showing IrOx
nanoparticles after 1 h photocatalytic reaction with hydrated iridium chloride
Dynamic light scattering (DLS) has become a very valuable
procedure for determining the particle size. To affirm that IrOx type
nanoparticles are not formed during the course of reaction, DLS
experiment has been performed with the catalyst in presence of PS
(500 µM) and SO (2500 µM), and LED irradiation of 15 min. Particles
of nano dimension were not detected for 1, 2, and 3 (average
particle sizes ~1.6 µm, 1.4 µm, and 1.9 µm respectively).
Interestingly, the particle size was found to be pressure dependent.
They contract to 180-190 nm upon applying vacuum for 15 min and
enlarge again under normal pressure. Further, we performed DLS
for all the three photochemical reactions after 24 h the irradiation.
At this stage the solution looks clear and homogeneous under
naked eye, and the result shows absence of any micron sized
particles. The DLS peaks below
5 nm clearly indicate that
photochemical reactions are homogeneous. So from the DLS results
we conclude that in reality the observed micron sized particles are
air bubbles as suggested by Bernhard and Albrecht.10 After 24 h the,
air bubbles completely comes out from the reaction mixture or
remained homogeneously solvated. These findings confirmed the
molecularity of catalyst and resulting spectra is depicted in Fig. S16,
ESI†. The nature of iridium species present in solution after
photocatalytic water oxidation has been followed by 1H NMR
studies. After 1 h irradiation solution containing PS, SO and
catalysts were dried and washed thoroughly thrice with water,
dried in a desiccator and subjected to 1H NMR analysis. The
resonances due to various protons for 1-3 did not show any shift
except emergence of one detectable peak for water which may be
attributed to replacement of the -Cl by H2O (Fig. S1, ESI†). It
strongly supported that complexes under study (1−3) are stable
WOC. Above results are consistent with 1 being an efficient and
stable molecular WOC exhibiting maximum TON of 220 in a
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,
photochemical water oxidation. Further, the TON reported through
photochemical WO is superior to many other molecular iridium
catalysts at neutral pH.1c-d,11
4
5
6
Further to identify and confirm the proposed high-valent
Ir(IV)/Ir(V) species, XPS analyses (X-ray photoelectron spectroscopy)
has been performed (Fig. S17, ESI†). After 15 min of LED irradiation
the solution containing catalysts, PS and SO, were immediately
freeze dried under liquid nitrogen and utilized for further study. XPS
spectra of Ir-4f lines (doublet) were utilized to investigate the
formation of high-valent oxo- and peroxo- species involved in the
photochemical process proposed in the mechanism (Fig. S18, ESI†).
The doublets corresponds to Ir-4f7/2 and Ir-4f5/2 lines at 61.45,
and 64.40 eV for 1 and 3 and 61.65, and 64.55 eV for 2,
respectively. The Ir-4f7/2 binding energy is obviously higher than
that for metallic Ir (60.9 eV), indicating a higher oxidation level of
the iridium species. From the literature values for Ir-4f7/2 and Ir-
4f5/2 lines it is clear that high-valent Ir(IV) oxidation state exists in
the concerned samples.12
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Conclusions
Conclusively, three proficient cyclometalated Ir based catalysts
for homogeneous WO have been developed for the pursuit of
renewable energy fuels and efficacy of 1−3 has been examined
toward both chemical and photochemical WO. These exhibited
good stability at extremely low pH and to LED light (440 nm)
irradiation. CV, UV/vis and XPS spectral studies indicated existence
of Ir(IV) and Ir(V) species in catalytic cycle. Further, these catalysts
exhibited impressive TON for photochemical water oxidation at
neutral pH. The molecular nature of the catalysts remained intact
−
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1
despite photochemical irradiation which has been supported by H
NMR, TEM and DLS analysis. Owing to molecular nature of the
catalysts these can be further improved by variation of ligands to
have efficient catalytic systems both in terms of TON and TOF.
Notes and references
‡ S.M. thanks the University Grants Commission, New Delhi,
India for the award of a Senior Research Fellowship [19-
6/2011(i) EU-IV]. We are also grateful to Dr. Sayam Sen Gupta
and Mr. Kundan K. Singh, National Chemical Laboratory, Pune,
India, for extending some facilities, and helpful discussions.
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