Photocatalytic water reduction with copper-based photosensitizers: A noble-metal-free system

Article abstract of DOI:10.1002/anie.201205915

Of noble descent: A fully noble-metal-free system for the photocatalytic reduction of water at room temperature has been developed. This system consists of Cu^I complexes as photosensitizers and [Fe₃(CO) ₁₂] as the water-reduction catalyst. The novel Cu-based photosensitizers are relatively inexpensive, readily available from commercial sources, and stable to ambient conditions, thus making them an attractive alternative to the widely used noble-metal based systems.

Full text of DOI:10.1002/anie.201205915

Angewandte
Chemie
DOI: 10.1002/anie.201205915
Hydrogen Generation
Photocatalytic Water Reduction with Copper-Based Photosensitizers:
A Noble-Metal-Free System**
Shu-Ping Luo, Esteban Mejꢀa, Aleksej Friedrich, Alexandra Pazidis, Henrik Junge, Annette-
Enrica Surkus, Ralf Jackstell, Stefania Denurra, Serafino Gladiali, Stefan Lochbrunner, and
Matthias Beller*
The provision of energy has been listed in the “Top Ten of
global concerns” as the single most important issue to be
faced to improve quality of life and ultimately to stop, or at
least to mitigate, the environmental impact of human
activities on the planet.[1] One way in which chemistry can
be used to provide more sustainable and benign energy
technologies is through the development of artificial photo-
synthesis systems, which allow for the conversion of the
energy of sunlight into useful chemical energy by water
splitting.[2] The resulting molecular hydrogen constitutes in
principle a clean, renewable, and carbon-free energy source.[3]
Nowadays, typical photocatalytic systems for the reduc-
tion of protons to hydrogen consist of a suitable photo-
sensitizer (PS), which absorbs the energy of light, a homoge-
neous or heterogeneous catalyst (water reduction catalyst;
WRC), and a sacrificial electron donor (SR). Often the
photosensitizer is the limiting component of the overall
system. Hence, the development of novel photosensitizers
represents a key issue for the advancement in this area. Since
the 1970s, when the use of [Ru(bipy)3]2+ as a photosensitizer
was first reported,[4] other ruthenium complexes,[5] and several
other noble-metal-based systems, including rhenium,[6] plat-
inum,[7] and iridium[8] systems, have been successfully used in
this process. Less expensive noble-metal-free PS are compa-
ratively scarce.[8f,9] The most popular noble-metal-free photo-
sensitizers make use of organic dyes but show low activity and
facile decomposition.[10] In addition, iron hydrogenase
mimics,[11] biomimetic zinc(II) porphyrins,[12] and magnesium
(chlorophyll A)[13] systems have been also investigated.
Based on our interest on the development of a completely
noble-metal-free photocatalytic proton-reduction cata-
lyst,[8b,c,14] we had the idea to use copper-based photosensi-
tizers for this purpose. Notably, copper(I) complexes with
polypyridine ligands show interesting emission properties and
they are considered as active components in organic light-
emitting diodes (OLEDs), light-emitting electrochemical
cells (LECs), luminescence-based sensors, etc.[15]
In this context, we describe herein the synthesis and
characterization of a series of cationic CuI complexes of the
_
_
type [Cu(NN)(PP)]+ and their function as active photosensi-
tizers in the photocatalytic hydrogen generation from water
under basic conditions. To the best of our knowledge, this is
the first efficient noble-metal-free photocatalytic system for
proton reduction using copper-based photosensitizers.
At the start of our investigations we tested around 15
different copper complexes as PSs in the presence of
[Fe3(CO)12] as a WRC and triethylamine as a sacrificial
reductant (Scheme 1). To our delight, the complex
[Cu(bathocuproine)(DPEphos)]PF6 (1; Scheme 2) effectively
promoted proton reduction in the presence of triethylamine
(TEA) and [Fe3(CO)12] with a turnover number (TON) of 477
(equiv of H/equiv of Cu) (Table 1 entry 1). Notably, all three
[*] Dr. S. Luo, Dr. E. Mejꢀa, Dr. H. Junge, Dr. A.-E. Surkus,
Dr. R. Jackstell, S. Denurra, Prof. Dr. S. Gladiali, Prof. Dr. M. Beller
Leibniz-Institut fꢁr Katalyse an der Universitꢂt Rostock e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
Dr. S. Luo
State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology, Zhejiang University of Technology
310014 Hangzhou (China)
S. Denurra, Prof. Dr. S. Gladiali
Universitꢃ di Sassari, Dipartimento di Chimica e Farmacia
Via Vienna 2, 07100 Sassari (Italy)
Scheme 1. Three-component system for the photocatalytic proton
reduction. SR=sacrificial reductant, PS=photosensitizer, WRC=water
reduction catalyst. Pathway A (red): reductive quenching cycle; Path-
way B (blue): oxidative quenching cycle.
A. Friedrich, Prof. Dr. S. Lochbrunner
Institute of Physics, University of Rostock
Universitꢂtsplatz 3, 18055 Rostock (Germany)
[**] We thank P. Schwarzbach for help in the quantum yield measure-
ments. Financial support by the BMBF through the program
“Innovation und Spitzenforschung in den neuen Lꢂndern” is
gratefully acknowledged.
components, PS, WRC, and SR, are necessary for hydrogen
generation as in the absence of any of them, no activity was
observed. Complex 1 has previously been reported to display
interesting photoluminescence features and was synthesized
according to the described procedure.[15a] Next, several
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205915.
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Scheme 2. Copper(I) complexes containing the ligand DPEphos (bis[2-
_
(diphenylphosphino)phenyl]ether) and different NN ligands.
Table 1: Results for the photocatalytic proton reduction using CuI PS in
the presence of [Fe3(CO)12] as WRC and TEA as SR. Screening of nitrogen
ligands.
Entry[a]
Complex (PS)
t [h]
Vol. H2 [mL]
TON (Cu)[b]
Figure 1. Absorption and luminescence spectra of complex (1) in THF.
1
2
3
4
5
1
2
3
4
5
24
5
21
24
21
20
<1
<1
<1
1.8
477
–
–
–
42
_
With bathocuproine established as a suitable NN ligand
in the copper complex, we next looked for an optimized
_
PP ligand for our system. Hence, several CuI complexes with
[a] Reaction conditions: Cu PS (ca. 3.5 mmol), [Fe3(CO)12] (ca. 5 mmol),
THF/TEA/H2O (4:1:1, 10 mL), pH 11.5, 258C, Xe light irradiation
(output 1.5 W), without light filter, gas evolution measured quantitatively
by automatic gas burettes, gas analysis by GC. All given values are the
averages of at least two experiments. Reaction times were taken when no
further gas evolution was observed. The results differ between 1 to 20%
except for volumes <10 mL (up to 40%); [b] TON(Cu)=n(H)/n(Cu).
different chelating diphosphines were synthesized, character-
ized, and their catalytic activity for hydrogen generation was
assessed with different WRCs.
We screened a range of different diphosphine scaffolds,
such as dppf, binap, naphos, and xantphos (A–D, Scheme 3),
and observed that the replacement of the DPEphos ligand by
the more-rigid xantphos (4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene) confers to the corresponding complex
(9) not only enhanced stability but also improved lumines-
cence performance (for the UV spectrum of 9 see the
Supporting Information, Figure S6).[15d] Based on this obser-
vation, we took xantphos as the lead structure when choosing
other diphosphine ligands to screen (E–K; complexes 10–16;
Scheme 3).
The results obtained for proton reduction with this new
series of complexes are presented in Table 2. Although for the
complexes containing dppf and binap, no and low activity,
respectively, were achieved, with naphos a much higher TON
was observed (Table 2, entries 1–3). Moreover, complex 9,
containing the ligand xantphos, (Table 2, entry 4) displayed
analogues with simple variations on the dinitrogen ligand
scaffold (2–5) were also synthesized and tested. However,
their use led to a significant decrease of activity (see Table 1).
The better performance of the bathocuproine complex (1)
compared with the other bipyridine analogues might be
explained by the improved steric and electronic factors, which
are necessary to get long-lived metal-to-ligand charge-trans-
fer (MLCT) excited states. The methyl groups located at the
2- and 9-positions should not only hamper the expansion of
the coordination sphere of the metal center, thus preventing
exciplex quenching,[16] but also should favor the tetrahedral
geometry as opposed to the preferred square-planar geom-
etry in the formal CuII MLCT excited state, thus slowing
nonradiative decay. This effect, as pointed out by Riesgo
et al.,[17] is due to an increase in the energy gap between the
Cu d orbitals and the ligand p* orbitals in the destabilized
emissive state. Moreover, the presence of electronegative
phenyl substituents and the extended aromaticity of the
phenanthroline backbone provide p* states of decreased
energy and hence should facilitate the MLCT, d10p*0!
d9p*1.[17]
The absorption and fluorescence spectra of 1 are similar to
those of ruthenium[5] and iridium[8g] complexes with pyridine
ligands (see Figure 1). The absorption spectrum exhibits
strong bands in the UV region and a moderate band in the
near UV region at about 400 nm, which results most probably
from a MLCT excited state. The luminescence peaks at
580 nm and the quantum yield in THF is 13%. The lifetime of
the MLCT excited state is 6.9 ms and demonstrates that the
nonradiative decay is indeed slow and does not interfere with
photocatalytic electron-transfer processes.
a higher activity than the DPEphos analogue (1). This result is
_
presumably due to the fact that by making the PP ligand
more rigid (with a larger bite-angle) it is possible to raise the
energy of the CT state, therefore partially inhibiting the
nonradiative quenching, and yielding longer lifetimes.[15b]
Further tuning of the steric and electronic properties of the
PP ligands showed no improvement mainly because of the
_
inherent difficulty of separately assessing both properties
(Table 2, entries 5–9). Nevertheless, when complex 15, con-
taining the ligand DBP-thixantphos (J),[18] which is electron-
enriched compared to xantphos with practically the same
bite-angle, was used, a higher activity was observed (Table 2,
entry 10). Only complex 16 showed better activity (Table 2,
entry 11), with almost the same TON being reached in a much
shorter time. Under our reaction conditions the catalytic
performance of the Cu-based system was better than those of
the well-known photosensitizers [Ru(bipy)3]Cl2 and [Ir(bipy)-
(ppy)2]PF6 (Table 2, entries 12 and 13). The Ru-based system
showed very low efficiency while the Ir-based one gave
relatively good results, as we also previously reported.[14a]
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Table 2: Results for the photocatalytic proton reduction using CuI PS in
the presence of [Fe3(CO)12] as WRC and TEA as SR. Screening of
phosphine ligands.
Entry[a]
Complex (PS)
t [h]
Vol. H2 [mL]
TON (Cu)[b]
1
2
3
4
5
6
7
8
6
7
8
9
10
11
12
13
5
<1
2.2
28
33
<1
17
3.6
<1
6.4
34
34
2.5
25
–
52
20
18
27
5
23
10
8
50
18
6
12
15
646
781
–
397
85
–
9
14
15
16
150
797
804
58
10
11
12
13
[Ru(bipy)3]Cl2
[Ir(bipy)(ppy)2]PF6
576
[a] Reaction conditions: Cu PS (ca. 3.5 mmol), [Fe3(CO)12] (ca. 5 mmol),
THF/TEA/H2O (4:1:1, 10 mL), pH 11.5, 258C, Xe light irradiation
(output 1.5 W), without light filter, gas evolution measured quantitatively
by automatic gas burettes, gas analysis by GC. All given values are the
averages of at least two experiments. Reaction times were taken when no
further gas evolution was observed. The results differ between 1 to 20%
except for volumes <10 mL (up to 40%), [b] TON (Cu)=n(H)/n(Cu).
Scheme 3. Copper(I) complexes containing bathocuproine (2,9-
dimethyl-4,7-diphenyl-1-10-phenantroline) and different PP ligands.
_
Figure 2. Highest productivities (TON) observed for light-driven
proton reduction in the presence of a variety of solvent/SR mixtures
for complexes 9, 15, and, 16 (as PS) after different reaction times
(provided in the Supporting Information, Table 1). Cu PS (3.5 mmol),
[Fe3(CO)12] (5.0 mmol), THF/TEA/H2O (10 mL), 258C, Xe light irradia-
tion (output 1.5 W), without light filter.
To optimize the reaction conditions for this novel system
we used complex 9 as the model PS. First, we investigated the
effect of changing the concentration of PS with a fixed
amount of WRC (see the Supporting Information, Figure S2).
The best compromise between activity and reaction time was
found for a concentration of 2.5 ꢀ 10ꢀ4 m of PS for which an
average TON of 930 (equiv of H/equiv of Cu) was obtained.
Reactions run at lower concentrations require longer reaction
times (i.e. the time required until no further gas evolution is
observed), whereas larger amounts of PS had a detrimental
effect on the activity.
The optimal ratio for the solvent/SR/water mixture was
also investigated (Figure 2). Best results were obtained when
a mixture of THF/TEA/H2O with a volumetric composition
of 4:3:1 was used. With the optimized conditions a turnover
number of 804 (equiv of H/equiv of Cu) in 6 h was obtained
when 16 was used as the PS. Alternative SRs to TEA (i.e.
ascorbic acid and triethanolamine) were also tried and
K2PtCl6 was tried as the as the WRC. Notably, also under
these conditions hydrogen generation is observed, thus
demonstrating the general applicability of the Cu PS. How-
ever, the use of either ascorbic acid or triethanolamine led to
a decrease in the activity compared to the optimized system
described above. By using [HNEt3][HFe3(CO)11] as the WRC
we observed the same behavior as with [Fe3(CO)12]. The
addition of the electron-deficient tris(3,5-bis(trifluorome-
thyl)phenyl)phosphine, which had a positive influence in the
Angew. Chem. Int. Ed. 2013, 52, 419 –423
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421

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Communications
Ir PS/Fe WRC system,[8b] led only to a slightly decreased
TON.
suppliers and used as received. Copper(I) complexes (1–16) were
synthesized according to a literature procedure (see the Supporting
information).[15a] The diphosphine ligands were either purchased or
synthesized following the reported procedures.[18,19] The amount of
gas liberated was measured by an automatic gas burette. Details on
the equipment and the experimental set-up have been published
elsewhere.[8b] The relative composition of the evolved gas was
determined by GC (gas chromatograph HP 6890N, carboxen 1000,
TCD, external calibration). The light source was a 300W Xe lamp.
Typical procedure for light-driven water reduction: A double-
walled thermostatically controlled reaction vessel was evacuated and
purged with argon. The copper photosensitizer and [Fe3(CO)12] were
added as solids. The corresponding solvent mixture (THF/TEA/H2O)
was added and the temperature of the system was fixed at 258C
before switching on the light source. The reaction mixture is then
stirred at 258C until no further gas evolution is observed. All given
values are the averages of at least two experiments. The results differ
between 1 and 20% except for volumes of less than 10 mL (up to
40%).
It is worth mentioning, that the proton reduction also
takes place, although at a lower rate, that is 688 TON after
65 h for complex 9, in the absence of UV light by application
of a 395 nm cut-off filter (see the Supporting Information,
Table S1, entry 7). On the other hand, under these conditions
the system shows an enhanced stability of up to three days.
To obtain mechanistic insights into the action of the Cu-
based PS we measured the lifetime of 1 in a 4:1:1 mixture of
THF/TEA/H2O in the presence and absence of 0.5 mm of
[Fe3(CO)12]. With TEA in the absence of WRC the decay
time of the luminescence is 200 ns, thus reflecting the
quenching rate by TEA and thereby most probably the
electron-transfer rate. If the WRC is also added, the decay
time of the luminescence reduces further to 100 ns. This
finding shows that the presence of the WRC provides
a second quenching pathway that is equally fast as the
reductive electron transfer and the total quenching rate is
twice as high. We assign this second pathway to the oxidative
electron transfer from the sensitizer to the WRC. In both
cases the strongly accelerated luminescence decay compared
to that in pure THF (6.9 ms, see above) indicates that after
optical excitation electron transfer occurs with a high effi-
ciency. In addition, CV measurements of the reduction
potential of the Cu PS 9 (ꢀ1.54 V vs. Ag/AgCl/LiClsat) show
the general feasibility to reduce the WRC (see the Supporting
Information, Figure S7).
The deactivation of the catalytic system is expected to be
due either to the decomposition of the photosensitizer,
decomposition of [Fe3(CO)12],[14b] or by reaction with the
decomposition products of TEA.[4b] Deactivation resulting
from changes in the pH of the reaction mixture can be ruled
out since the pH remains almost constant (from initial 11.5 to
ca. 11.4 after reaction for example, with 16; Table 2, entry 11).
In summary, we have developed a completely noble-
metal-free molecular-defined system for the photocatalytic
reduction of protons from water at room temperature. Our
novel system makes use of CuI complexes as photosensitizers
and [Fe3(CO)12] as a water reduction catalyst in the presence
of TEA as a sacrificial reductant. It should be noted that the
new copper-based photosensitizers are relatively inexpensive,
readily available from commercial sources, and stable to
ambient conditions making them an attractive alternative to
the widely used iridium (and other noble-metal-based) PS
systems. Owing to the obvious possibilities of tuning the
properties of the copper center with related ligands (carbenes,
other P- and N-ligands) as well as potential heterogenization
of the systems further improvements can be anticipated.
Finally, it should be noted that these systems are also of
interest for other applications (OLEDs, LEC, etc.) because of
their photochemical behaviour.
Received: July 24, 2012
Published online: October 9, 2012
Keywords: copper · hydrogen · photochemistry ·
.
sustainable chemistry · water splitting
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