Robust photocatalytic water reduction with cyclometalated Ir(iii) 4-vinyl-2,2′-bipyridine complexes

Article abstract of DOI:10.1039/c0cc01827a

Novel [Ir(CN)₂(NN)]⁺ complexes with NN ligands containing vinyl groups were synthesized resulting in quintupled turn-over numbers for the photocatalytic hydrogen production compared to the analogous non-vinyl compounds.

Full text of DOI:10.1039/c0cc01827a

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Robust photocatalytic water reduction with cyclometalated
0
Ir(III) 4-vinyl-2,2 -bipyridine complexeswz
Stefan Metz and Stefan Bernhard*
Received 10th June 2010, Accepted 25th August 2010
DOI: 10.1039/c0cc01827a
+
2
Novel [Ir(C^N) (N^N)] complexes with N^N ligands containing
vinyl groups were synthesized resulting in quintupled turn-over
numbers for the photocatalytic hydrogen production compared
to the analogous non-vinyl compounds.
In the search for novel renewable energy sources the light-
driven splitting of water in hydrogen and oxygen is a promising
1
approach yielding a potent and storable fuel. Different
strategies have been explored, amongst them are photo-
2
electrochemical systems and molecular photosensitizers, such
3
2 6
Fig. 1 Summary of the synthesized [Ir(C^N) (N^N)][PF ] complexes.
as Ir(III), Pt(II), and Ru(II) complexes. However, there are still
several drawbacks preventing these systems from being suitable
for energy harvesting. For instance, the limited stability of
molecular systems under light-irradiation conditions leads to a
rapid decrease of the turn-over rate, mainly due to the
improved turn-over numbers. Various C^N ligands with electron-
withdrawing or electron-donating substituents were used for these
0
0
experiments. The N^N ligands 4-vinyl-4 -methyl-2,2 -bipyridine
0 0
(
mVbpy) and 4,4 -divinyl-2,2 -bipyridine (dVbpy) incorporated
4
photodegradation of the excited-state photosensitizers. The
one or two polymerizable vinyl moieties (Fig. 1).
deliberate control of the photophysical properties of Ir(III)
compounds by altering the ligand sphere makes them particularly
interesting candidates for optimization experiments. Thorough
studies of our group by parallel screening made a library of
The Ir(III) complexes were prepared according to standard
procedures in 58–89% yield (for experimental details, see ESIz).
+
These new compounds were compared to the [Ir(C^N) (N^N)]
2
0
0
analogs with N^N = 4,4 -dimethyl-2,2 -bipyridine (dMebpy).
As the only difference is the substitution of methyl groups by one
or two vinyl groups, the changes of properties should be a
consequence of the vinyl group’s presence.
+
5
[
Ir(C^N) (N^N)] complexes accessible; the utility of those
2
compounds for the light-driven water reduction and optimization
6
of catalyst and solvent systems resulted in over 2000 turn-overs.
These optimum conditions included the use of a Pt or Pd
precursor that formed a catalytically active metal colloid upon
irradiation.
The photophysical properties of the iridium compounds
described here are summarized in Table 1. All synthesized
Various approaches to modify as well as stabilize photo-
sensitizers with polymers are currently explored. Immobilization
Table 1 Photophysical properties of the Ir(III) complexes. Data
obtained from deaerated 20 mM solutions of the Ir(III) complexes in
THF/water (8/1)
2
+
of [Ru(bpy)
3
]
in a polymer network gave access to a
temperature-controlled photocatalytic hydrogen generating
7
gel system, and the polymerization of a Ru(II) species yielded
a
b
c
C^N
N^N
l
em /nm
t/ms
F
em
a linear copolymer with energy transfer efficiencies reminiscent
8
of those observed in light harvesting dendrimers. As it is
F–mppy
F–mppy
Cl–mppy
Cl–mppy
Cl–mppy
MeO–mppy
MeO–mppy
mppy
mppy
mppy
ppy
ppy
dMebpy
mVbpy
dMebpy
mVbpy
dVbpy
dMebpy
mVbpy
dMebpy
mVbpy
dVbpy
546
556
541
547
571
583
606
581
601
612
583
596
575
599
1.280
1.167
1.465
1.383
0.080
0.383
0.296
0.422
0.332
0.268
0.430
0.349
0.610
0.389
0.299
0.011
0.327
0.014
0.014
0.048
0.025
0.071
0.044
0.028
0.077
0.053
0.085
0.049
common to stabilize metal colloids by the addition of vinyl-
9
derived polymers, Ir(III) complexes containing one or more
vinyl groups at the ligand backbone seemed to be promising
candidates for improving the interaction between photosensitizer
and catalyst. The beneficial effects of this approach include a
stabilization of the Pt colloid as well as a faster electron
transfer from the Ir photosensitizer to the Pt catalyst, which
will result in higher stabilities of the reaction system and
dMebpy
mVbpy
dMebpy
mVbpy
Ph–ppy
Ph–ppy
a
Abbreviation of N^N and deprotonated C^N ligands: 5-methyl-2-(4-
fluorophenyl)pyridine (F–mppy), 5-methyl-2-(4-chlorophenyl)pyridine
Carnegie Mellon University, Department of Chemistry,
4400 5th Ave, Pittsburgh, PA 15213, USA. E-mail: bern@cmu.edu;
Fax: +1 412-268-1061; Tel: +1 412-268-7419
w This article is part of a ChemComm ‘Hydrogen’ web-based themed
issue.
z Electronic supplementary information (ESI) available: Experimental
details for all new compounds, absorption spectra, emission spectra,
and more details about the photocatalytic hydrogen evolution. See
DOI: 10.1039/c0cc01827a
(
5-methyl-2-phenylpyridine (mppy), 2-phenylpyridine (ppy), 2,5-diphenyl-
Cl–mppy), 5-methyl-2-(4-methoxyphenyl)pyridine (MeO–mppy),
0
0
0
pyridine (Ph–ppy), 4,4 -dimethyl-2,2 -bipyridine (dMebpy), 4-vinyl-4 -
0
0
0
methyl-2,2 -bipyridine (mVbpy), 4,4 -divinyl-2,2 -bipyridine (dVbpy).
b
c
Excitation at 400 nm. Reference compound: [Ir(ppy)
2
(bpy)]PF
6
(
F
CH CN = 0.0622).
3
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7551–7553 7551

Ir(III) complexes are luminescent in the solid state and in
solution. However, compared to their respective parent
dMebpy species, the emission quantum yields of the mVbpy
and dVbpy complexes are significantly decreased. This
especially holds true for complexes with halide-substituted
C^N ligands (F–mppy and Cl–mppy). In contrast, the life-
times of the mVbpy complexes are only somewhat shorter than
those of the dMebpy parent complexes. The lifetimes of the
dVbpy compounds, on the other hand, are shorter than those
of the dMebpy and mVbpy complexes, with a particular
dramatic decrease for the Cl–mppy series. As expected, the
substitution of one or two methyl groups by vinyl moieties
leads to a small red-shift of the emission maximum.
Table 2 Photocatalytic water reduction experiments. Data obtained
from deaerated 50 mM solutions of the Ir(III) complexes in THF/water/
triethylamine (8/1/1)
a,b
b,c
d
C^N
N^N
TONPt
TONRh
RatioPt/Rh
F–mppy
F–mppy
Cl–mppy
Cl–mppy
Cl–mppy
MeO–mppy
MeO–mppy
mppy
mppy
mppy
ppy
ppy
dMebpy
mVbpy
dMebpy
mVbpy
dVbpy
dMebpy
mVbpy
dMebpy
mVbpy
dVbpy
1300
8500
1625
7950
7225
925
5700
1175
7950
7925
925
1100
1215
1075
1100
1150
1125
1050
1125
1200
1325
1125
1300
1325
1350
1.18
7.00
1.51
7.23
6.28
0.82
5.43
1.04
6.63
5.98
0.82
4.46
1.00
5.20
dMebpy
mVbpy
dMebpy
mVbpy
The hydrogen evolution experiments were performed in
sealed vials with a 16-well LED photoreactor (lem = 460 nm)
on an orbital shaker with a thermostated block (25 1C). Each
sample contained 0.5 mmol of the respective photosensitizer
5800
1325
7025
Ph–ppy
Ph–ppy
a
b
8 h irradiation. TON = 2n(H
c
d
)/n(PS). 6 h irradiation. Ratio
4
2
TONPt/TONRh
.
and 0.375 mmol catalyst (K
(
2
PtCl
8 mL), triethylamine (sacrificial reductant, 1 mL), and H
1 mL). The solutions were degassed and then purged with
4 2 2
or [Rh(bpy) Cl ]Cl) in THF
2
O
(
argon at atmospheric pressure. Pressure transducers were used
to obtain kinetic data, and the head-space was analyzed for H
2
by mass spectrometry (see also ESIz).
All iridium compounds studied functioned as photosensitizer
for the reduction of water (Fig. 2). The turn-over numbers
(
TONs) of the six tested dMebpy derivatives ranged from 925
to 1625, whereas the mVbpy and dVbpy compounds exhibited
dramatically increased stabilities with 5700–8500 turn-overs
(
Table 2). Interestingly, despite the much lower luminescence
quantum yields and shorter lifetimes, all mVbpy complexes
and their dVbpy derivatives as well) produced significantly
+
2
Fig. 3 Photocatalytic hydrogen production of the [Ir(C^N) (N^N)]
(
photosensitizers with in situ generated colloidal Pt during the first
0 min.
more hydrogen than the respective dMebpy derivatives with
TON(mVbpy)/TON(dMebpy) ratios Z 5. The two tested
dVbpy derivatives showed very similar photocatalytic
activities as their mVbpy counterparts. The absorption of
the dMebpy, mVbpy, and dVbpy derivatives at 460 nm is
similar, and can be ruled out as the cause for the increased H2
production.
6
derivatives declined quickly, which occurs most likely via
4
ligand dissociation. This is in strong contrast to the mVbpy
and dVbpy photosensitizers, which, although also showing
lower turn-over rates after 12 h, still exhibit rates around
200 turn-overs per hour for most complexes (see ESIz).
These results suggest that the additional vinyl groups of the
N^N ligand do not significantly change the reaction pathway
or the initial photocatalytic activity, despite the differences in
emission quantum yields and lifetimes of the dMebpy, mVbpy,
and dVbpy derivatives. The stability of the catalytic Ir/Pt
system on the other hand is increased dramatically resulting
in water reduction systems that produce a significant amount
of hydrogen over 16–48 h, contrary to the analogous dMebpy
systems that are mostly deactivated after 4 h. A possible
explanation for this significant difference is the interaction of
the vinyl moieties with the in situ formed colloidal Pt.
The major difference of the vinyl groups for the photo-
catalytic hydrogen generation can be seen when taking a closer
look at the turn-over rates for the first 30 min and after 12 h.
After starting the irradiation of the samples, the traces of the
dMebpy, mVbpy, and dVbpy systems exhibit similar rates
(
Fig. 3). However, the catalytic activity of the dMebpy
Due to the low concentrations involved it is difficult to
determine the interaction of the photosensitizer and the Pt
directly. Therefore, we designed a control experiment using
2 2
molecular [Rh(bpy) Cl ]Cl (Rhcat) as the catalyst instead of
colloidal Pt. Except for the water reduction catalyst, the exact
same conditions were used for the control experiment. If the
high TONs of the mVbpy and dVbpy complexes are due to the
vinyl–Pt interaction, the same experiment with the molecular
Rh catalyst should result in very similar TONs for the
dMebpy, mVbpy, and dVbpy systems. This assumption was
+
Fig. 2 Photocatalytic hydrogen production of the [Ir(C^N)
2
(N^N)]
photosensitizers with in situ generated colloidal Pt.
7
552 Chem. Commun., 2010, 46, 7551–7553
This journal is c The Royal Society of Chemistry 2010

ꢀ1
Fig. 5 Cyclic voltammogram of multiple scans (100 mV s ) of
[Ir(Cl–mppy) (mVbpy)]PF in acetonitrile containing 0.1
Fig. 4 Photocatalytic hydrogen production of the photosensitizers
+
M
2
6
[
Ir(mppy)
2
(N^N)] (N^N = dMebpy (green), mVbpy (red), or dVbpy
(nBu) NPF at a Pt electrode (red lines: first scans; green lines: last
4
6
(
blue)), using either in situ generated colloidal Pt (solid lines) or
scans).
molecular [Rh(bpy)
2
Cl
2
]Cl (broken lines) as catalyst.
These studies should prove very useful in optimizing existing
and developing novel visible-light driven water-reduction
systems of high stability.
+
2
confirmed and all [Ir(C^N) (N^N)] /Rhcat catalyst systems
turned out to produce a similar amount of hydrogen with
TONs of 1050 to 1350 (Table 2). The amount of hydrogen
This work was supported through an NSF CAREER award
produced is also in the same range as observed for the
+
(
CHE-0949238). S.M. gratefully acknowledges a postdoctoral
[
Ir(C^N) (dMebpy)] /Pt photoreactions (925–1625 TONs) and
+
2
fellowship from the DFG (German Research Association).
2
significantly lower than the TONs of the [Ir(C^N) (mVbpy)] /Pt
systems (5700–8500; Fig. 4). To exclude that the amount of Rh
catalyst is the limiting factor of these reactions, concentration
Notes and references
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+
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2
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2
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+
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moieties with the Pt catalyst during the photocatalytic water
reduction experiments.
5
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iridium complexes with vinyl moieties. Although the emission
quantum yields were lower than those of their non-vinyl
derivatives, these compounds showed superior activities for
the photocatalytic water reduction. The combination of vinyl
moieties at the iridium photosensitizer and a colloidal Pt
catalyst dramatically enhanced the longevity of the photo-
catalytic system: consequently a fivefold increase of hydrogen
evolution compared to the methyl parent compounds resulted,
and the most robust vinyl system exceeded 8500 turn-overs.
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This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7551–7553 7553