Towards new supramolecular
biomimetic FeFe-photocatalysts for
hydrogen evolution
H 2O
H 2 + O2
Energy
Vincent Vreeken
22-07-2011
2
Towards new supramolecular
biomimetic FeFe-photocatalysts for
hydrogen evolution
Master thesis by Vincent Vreeken
Daily supervisor:
Sofia Derossi
Supervisor:
Joost N. H. Reek
Second reviewer:
Jarl Ivar van der Vlugt
Master coordinator:
Kees Elsevier
22-07-2011
Renewable energy
FeFe-hydrogenase mimic
Photocatalytic hydrogen evolution
Supramolecular chemistry
Homogeneous catalysis
3
Abstract
The application of FeFe-hydrogenase mimics in photocatalytic systems for hydrogen
evolution is interesting and attractive. Such photocatalytic systems can be divided in three
different classes: covalent dyads, multicomponent systems and supramolecular assemblies.
The supramolecular approach aims to combine advantages of the other classes. Earlier, this
group reported a supramolecular triad capable of photocatalytic proton reduction. In order to
improve the system a new more robust supramolecular linker was recently employed.
For this report, not only the new linker was employed, but also the dithiolate bridge of
the FeFe complex was modified aiming to improve the performance of the system. This
resulted in the design of three new photocatalysts. Two of these catalysts were successfully
synthesized. The employed dithiolate bridges either introduced an amine-group, which could
function as internal base, or an aromatic bridge, which should lead to less negative reduction
potentials.
Characteristics of the obtained photocatalysts were determined by UV/Vis, IR
spectroscopy and cyclic voltammetry. These techniques delivered expected results,
demonstrating the influence of the dithiolate bridge on the properties of the photocatalysts.
Both complexes were active in electrocatalytic reduction of protons. Protonation studies
showed the amine-function indeed could act as an internal base. Unexpectedly, the new
complexes were less active in photocatalytic experiments as compared to the complexes with
unmodified dithiolate bridges. IR spectra followed in time during photocatalysis showed the
FeFe complex was rapidly decomposed. More research on the supramolecular catalysts is
required for better understanding of the systems in order to design improved systems.
4
Samenvatting
Katalytische systemen bestaande uit een model van het FeFe-hydrogenase enzym
en een chromofoor zijn, gebruikmakend van de energie van licht, in staat de reductie van
protonen tot waterstof te katalyseren. Deze fotokatalytische systemen kunnen worden
onderverdeeld in drie soorten: covalente complexen, multi-component systemen en
supramoleculaire structuren. Het toepassen van supramoleculaire strategieën heeft tot doel
de voordelen van de andere systemen te combineren in een systeem. Deze groep heeft
eerder een supramoleculair systeem ontwikkeld waarbij gebruik werd gemaakt van een
supramoleculaire linker om de chromofoor met het FeFe complex te verbinden. Dit systeem
was actief in fotokatalytische proton reductie. Om dit systeem te verbeteren is recentelijk een
nieuwe, robuustere supramoleculaire linker toegepast.
Voor deze thesis is niet alleen deze nieuwe linker toegepast, maar ook is de
dithiolaat brug van het FeFe complex veranderd. Dit heeft geresulteerd in het ontwerp van
drie nieuwe fotokatalysatoren, waarvan er twee succesvol gesynthetiseerd zijn. Van de
verkregen complexen beschikte er een over een amine in de dithiolaat brug, met als doel om
als interne base te fungeren. Het andere complex beschikte over een aromatische brug, om
een positieve verschuiving van de reductie potentiaal van het complex te bewerkstelligen.
De eigenschappen van de gemaakte katalysatoren zijn onderzocht met UV/Vis, IR
spectroscopie en elektrochemie (CV). Deze experimenten gaven resultaten die verwacht
waren: de invloed van de dithiolaat brug is duidelijk zichtbaar in de eigenschappen van de
complexen. Electrokatalyse experimenten toonden aan dat beide complexen de reductie van
protonen katalyseren. Studies waarin de complexen werden geprotoneerd, toonden aan dat
de amine inderdaad kan functioneren als interne base. De nieuwe complexen waren minder
actief in de fotokatalyse experimenten dan de eerdere supramoleculaire systemen. Het
volgen van IR spectra tijdens de fotokatalyse toonde het snelle uiteen vallen van het FeFe
complex. Om in de toekomst verbeterde katalysatoren the ontwerpen, is er meer inzicht nodig
in de processen die plaats vinden tijdens de fotokatalyse.
5
6
Contentlist
Abstract ................................................................................................................... 4
Samenvatting........................................................................................................... 5
1. Introduction ......................................................................................................... 9
1.1 Energy in the future ......................................................................................................... 9
1.2 Production of hydrogen ................................................................................................. 10
1.3 Biomimetic catalysts ...................................................................................................... 11
1.4 Systems for photocatalytic proton reduction ................................................................. 13
1.5 Aim of the thesis ............................................................................................................ 14
2. Synthesis 17
3. Spectroscopic studies ...................................................................................... 23
3.1 UV/Vis studies ............................................................................................................... 23
3.2 Infrared studies.............................................................................................................. 23
3.3 Titration studies of the supramolecular assemblies ...................................................... 27
4. Electrochemistry ............................................................................................... 31
4.1 Cyclic voltammetry ........................................................................................................ 31
4.2 Electrocatalysis ............................................................................................................. 32
5. Photocatalysis ................................................................................................... 38
6. Conclusions....................................................................................................... 43
7. Outlook .............................................................................................................. 45
8. Experimental part .............................................................................................. 46
Abbreviations ........................................................................................................ 50
Acknowledgements............................................................................................... 51
References ............................................................................................................. 52
7
8
1. Introduction
1.1 Energy in the future
Developed and industrialized countries are greatly dependent on energy. It is a first
necessity of life for societies to function. The building of modern countries would not have
been possible without the great amount of cheap fossil fuels that have been mined and drilled
out of Earth’s deeper layers. However, in the past decades it has become more and more
obvious that the use of fossil fuels also has negative effects on our planet. It is now common
knowledge that the use of fossil fuels severely pollutes the atmosphere and that the
greenhouse gasses produced are causing climate change that will directly threaten societies
in certain parts of the world, albeit global warming is a subject of scepticism especially outside
the scientific world. Furthermore, we know that the fossil fuel reserves will not last forever,
while the global energy demand is estimated to double by 2050 and to treble by 2100
compared to the demand in 2001.1 With diminishing reserves and growing demands, it
becomes evident that at some point in the near future the world will face an energy crisis. For
these reasons, pollution, climate change and an imminent energy crisis, it is necessary that
fossil fuels are replaced by energy sources that are sustainable. The transition from a world
dependent on fossil fuels to a world dependent on renewable energies is one of the greatest
and toughest challenges of this century.2
As an alternative for fossil fuels, hydrogen is believed to be an excellent solution. This
idea is attractive, since it is the most abundant element in the universe, not hazardous to the
environment and hydrogen can be used in a combustion engine or in fuel cells only producing
water as a byproduct. However, elemental hydrogen on earth is very rare: hydrogen gas is so
light that it can escape Earth’s gravity and there are therefore no H2 reserves in its crust.
Mining of hydrogen is therefore not an option, which, anyway, would not be sustainable.
Consequently, hydrogen needs to be produced, making it not a primary energy source but an
energy carrier, or vector. From this point of view, the manufactured hydrogen can be ‘black’
and ‘green’, depending on the energy source used in the production. For example, currently,
ca. 90% of the world production of H2 is obtained as byproduct of steam reforming of methane
and therefore is not clean.2 The available alternative energy sources are nuclear fission, wind
energy, hydroelectric power and solar energy.
Nuclear fission is well known, can provide high amounts of energy and does not
produce greenhouse gasses. However, nuclear fission produces radioactive wastes that will
represent a danger for at least ten thousands of years.3 Also, the uranium reserves on Earth
are limited: a sudden worldwide transition to nuclear energy would lead to depletion of all
uranium within a decade and would require the building of an enormous amount of nuclear
reactors thereby magnifying all problems related with the radioactive waste.1 Moreover, the
investments needed for the building of nuclear plants will be at the expenses of investments
in research and development of sustainable energy sources.
9
Wind power has an enormous potential and in some areas of the world it is already
competitive with power generated from fossil fuels. Hydroelectric power is already applied
widely and has a large potential, but because of technical, environmental and legal
restrictions only a small fraction can be exploited.2 Solar energy has the largest potential of all
sustainable energy sources. The sun provides Earth with so much energy that capturing all of
it for one hour would satisfy energetic demands for a whole year. However, many technical
and scientific problems need to be solved before the price of solar energy can compete with
that of fossil fuel energy.1 So far, wind energy and solar energy have the best potential to
replace the current fossil fuel economy for a hydrogen-based one.
1.2 Production of hydrogen
The most ideal source of hydrogen is water, as it is abundant and non-polluting. Also,
using water a hydrogen source allows for a sustainable energy cycle, scheme 1.
Energy
2H2 + O2
2H2O
Energy
Scheme 1: Sustainable water-energy cycle
Several methods are available to split water into oxygen and hydrogen. There is a
photobiological way, in which modified microorganisms produce hydrogen as part of their
photosynthetic process. There is water electrolysis, which is well known and is currently used
to produce hydrogen of high purity. Of course, for a sustainable cycle, clean electricity needs
to be used for this process. Another method is artificial photosynthesis, which takes
inspiration from the natural occurring autotrophic organisms, which “build” energy for
themselves using, indeed, water and sunlight. In this area chemistry plays an important role.
Artificial photosynthesis consists of two steps: a water-oxidation reaction (WOR) and
a proton-reduction reaction (PRR), scheme 2.
2H2O
O2 + 4H+ + 4e-
Water oxidation reaction (WOR)
4H+ + 4e-
2H2
Proton reduction reaction (PRR)
Scheme 2: Steps in hydrogen production by artificial photosynthesis
10
Both the WOR and the PRR require complex catalytic systems that are able to use
energy from light to drive the reaction and that are highly active. The final goal is to develop
devices in which both reactions occur and are combined in such a way that the PRR uses the
protons and electrons produced in the WOR. This report will be mainly concerned with the
proton reduction reaction.
Photocatalysed reduction of protons to hydrogen has been investigated for a long
time. Most work has however been focused on heterogeneous catalysts; only more recently
interest in homogeneous photocatalytic systems has grown. These systems use different
metals. The most successful photocatalytic systems reported are based on rhodium, platinum
and palladium complexes.4 Nonetheless, for larger scale applications the use of noble metals
is highly undesirable: noble metals are simply too rare and therefore too expensive for the
production of cheap hydrogen. Therefore catalytic systems with cheap, abundant metals are
currently under heavy investigation. Catalysts containing first row metals are very popular and
in particular cobalt systems have delivered encouraging results.5,6 Recently, also an organic
semiconductor capable of producing H2 was reported, eliminating the need for metals.7
1.3 Biomimetic catalysts
The elucidation of the active site of FeFe and NiFe hydrogenases, found in nature,
has led to another class of catalysts. These enzymes have shown remarkable high activity in
the reversible reduction of protons, reaching turnover frequencies of up to 9000 H2 molecules
per second at neutral pH and at low overpotentials.8 Especially the FeFe-hydrogenases are
highly active and these are the systems that we will be concerned with in this study.
The structure of these enzymes was first reported in 1998.9 The catalytic centre of the
hydrogenase consists of three or more [4Fe-4S]-clusters and a dinuclear FeFe active site, fig.
1. The two Fe centres are bridged by a dithiolate ligand. The bridgehead of this ligand could
not be assigned immediately, with the indecision being between a CH2 group, an oxygen
atom and an NH group, until in 2009,
amine
group.10
14N
HYSCORE experiments gave clear evidence for an
This NH-bridgehead is believed to be crucial for the high turnover frequency
observed in these enzymes, as it can act as an internal base that guides protons close to the
metal centre.11 Other non-proteic ligands around the Fe atoms are CO and CN, which is
noteworthy in biological systems, considering their high toxicity.
11
NH
Cys
[Fe4S4]
S
Fe
OC
NC
S S
Fe
C
O
CN
CO
Fig. 1: Catalytic centre of an FeFe-hydrogenase9
Surprisingly, the active site of these enzymes bears a striking resemblance with
synthetic dinuclear FeIFeI complexes that have been known since the 1920’s,12 fig. 2. This
combined with the high activity of FeFe-hydrogenases in nature, has made these FeFe
complexes extremely attractive in the chemistry of renewable energies, with many groups
spending their efforts in the mimicry of the FeFe-hydrogenase active.
NH
Cys
[Fe4S4]
S
Fe
OC
NC
vs.
S S
Fe
C
O
CN
CO
Fig. 2: Comparison of the active centre of hydrogenase (right) with a synthetic analogue (left)
In order to be good mimics of the natural system, the synthetic complexes require low
overpotentials for proton reduction, high stability during the catalysis and good activity. To
achieve this, most studies involve replacement of the CO groups with other ligands and/or
modification of the dithiolate bridge. This approach has led to numerous mimics of the
enzyme’s active centre. The modification allows, for example, for the introduction of an
internal base, changes in solubility and can influence the electrochemical properties of the
complexes as well as their stability. The reduction potential of the mimics ranges from around
-2.72 V to around -0.59 V vs. Fc+/0.13 Examples of hexacarbonyl FeFe complexes with
different bridges are depicted in fig. 3. The influence of the dithiolate bridge is most clearly
seen in the reduction potential of these complexes. Mimic 1, with a propanedithiolate (pdt)
bridge, is reduced at the most negative potential (values between -1.74 V and -1.59 V vs.
Fc+/0 have been reported).13 The electron withdrawing character of the nitrogen bridgehead in
complex 2 shifts this potential to -1.57 V vs. Fc+/0, while the nitrogen also functions as an
internal base.14 Complex 3 shows the least negative reduction potential, at -1.20 V vs. Fc+/0,
due to the electron-withdrawing effect of the chlorobenzene moiety.15
12
Examples of mimics where one or more carbonyls are replaced are also widely
known. The substitution of CO generally leads to a cathodic shift of the reduction potential,
caused by a higher electron-density at the FeFe site, due to the reduction of π-backdonation
character.
Many mimics are electrocatalytically active in the H+ reduction; an overview of these
systems is given by Felton et al.13 However, until now, all synthetic mimics failed to reproduce
the properties that make the enzymes such good catalysts: high activity and low
overpotentials.
Cl
Cl
N
S S
OC
CO
Fe
Fe
CO
1
CO
Fe
OC
OC
OC
CO
Fe
CO
OC
OC
2
S S
OC
SS
CO
Fe
Fe
CO
OC
CO
CO
OC
3
Fig. 3: Examples of hexacarbonyl FeFe complexes, with different dithiolate bridges
1.4 Systems for photocatalytic proton reduction
The application of FeFe-hydrogenase mimics in photocatalysis requires the addition
of a photosensitizer to the system, functioning to convert the energy of light in a useful
reduction potential. An effective photocatalytic system does not only require an active and
stable FeFe catalyst, but also fast electron transfer from the photosensitizer to the active site.
The photosensitizer therefore needs to generate a reduction potential that is able to reduce
the FeFe site. Reported photocatalytic FeFe systems can be divided into three groups:
covalently linked dyads, multicomponent systems and supramolecular assemblies. 16
The development of covalently linked dyads, in which the photosensitizer is directly
bound to the catalyst, has resulted in photocatalytic systems that either did not work17,18,19 or
showed extremely low turnover numbers (TON< 0,5).20,21 The main advantage of the covalent
approach is the close proximity between photosensitizer and the FeFe site, which should
promote fast electron-transfer. This close contact does, however, also provide a chance for
fast charge-recombination.
Multicomponent systems have delivered the most promising results so far. The
advantages offered by this type of systems are the possibility of screening many
photosensitizer/catalyst combinations without the large synthetic effort required in the
covalent approach and the possibility to easily tune the photosensitizer/catalyst ratio.16
Multicomponent systems have shown good photocatalytic activities in different solvents and
at different values of pH, ranging from acidic to basic.14,22-24 Compared to the covalent dyads,
their performances are by far superior, with TONs up to 466.23
13
Supramolecular assemblies of photosensitizers and FeFe complexes may combine
the advantages of the above-mentioned systems. Also in this case, combinatorial libraries
can be obtained without the need for elaborate covalent syntheses, while, due to the
formation of an assembly, the photosensitizer is kept in close proximity to the FeFe site,
allowing for fast electron-transfer. At the same time, the dynamic exchange of the assembly
might slow down charge recombination once the electron has been transferred to the catalyst.
Sun and co-workers reported an example of such a supramolecular assembly in 2008.25
Although the performance of this photocatalyst was poor it proved the concept of applying
supramolecular strategies to these photocatalytic systems.
1.5 Aim of the thesis
In 2009, our group reported another supramolecular photocatalytic system. It was a
supramolecular triad based on a FeFe complex with a propanedithiolate (pdt) bridge as the
catalyst and Zn-porphyrins as the photosensitizers, scheme 3. The FeFe complex had been
functionalised by replacing one carbonyl group with a pyridyldiphenylphosphine ligand, which
acts as a supramolecular linker between the catalyst and the photosensitizer. Upon
irradiation, the system was capable of producing small amounts of H2. Interestingly, the active
species was found to be a disubstituted complex, formed in situ under irradiation, in which a
second carbonyl group was displaced by a molecule of linker. Another remarkable feature
was that this system only worked when two different Zn-porphyrins were used, suggesting
that the active species requires an asymmetric geometry to function. 26
R
N
R
Zn
N
N
R
N
N
P
SS
R
Fe
OMe
R=
or
OC
OC
CO
Fe
CO
CO
Scheme 3: Supramolecular system for photocatalytic H2 production26
The results obtained with the supramolecular triad encouraged us to develop new
photocatalysts, using the same approach. With this intent, a new photocatalyst was
synthesized, 4, in which a phosphoramidite ligand appended with two pyridyl groups was
selected as the supramolecular linker, fig. 4. This recently developed ligand was chosen for
many reasons: its higher stability, the easier synthesis, the possibility of binding two
chromophores and mainly because phosphoramidites are better π-acceptors than
phosphines, which should decrease the potential at which H 2 evolution occurs. Preliminary
14
results indicate that the new linker improves the performance of the photocatalytic system. 4
appears to be more stable and to produce more H2 gas than the system with the
pyridylphosphine linker. Moreover, 4 does not seem to form disubstituted species during
catalysis and the use of only one Zn-porphyrin is sufficient for activity, all features connected
with its superior robustness.
Zn
N
O
O P
N
SS
CO
Fe
N
Fe
CO
OC
CO
OC
Zn
4
Fig. 4: The recently developed photocatalyst 4
In this report we are aiming at improving 4 by changing the dithiolate bridge. As mentioned
before, the nature of the diothiolate bridge can have a significant influence on the behaviour
of the FeFe catalyst, therefore FeFe complexes 5, 6 and 7 will be studied, which are depicted
in fig. 5 and 6.
N
N
Cl
Cl
N
O
O P
O
O P
N
SS
N
S S
CO
CO
N
Fe
Fe
CO
OC
OC
Fe
N
CO
OC
OC
CO
5
Fe
CO
6
Fig. 5: Complexes 5 and 6 which were synthesized for this thesis
The bridge of 5 is chosen for the benzylamine functionality in the bridgehead, which
may act as internal base. This is in closer resemblance with the natural occurring
hydrogenase. We therefore expect that the introduction of the amine group, bringing protons
in close proximity to the FeFe centre, will increase the turnover frequency of the system. The
bridge had previously been used successfully in other photocatalysts. 4,23 The chlorobenzene
bridge in 6 is chosen for the positive effect on the reduction potential of FeFe complexes it
has earlier displayed.15 Better electron-accepting properties of the FeFe centre should have a
15
positive effect on the electron-transfer and thereby on the outcome of the photocatalysis.
Also, the chlorobenzene bridge reportedly stabilizes the reduced states, possibly making the
photocatalyst more robust in different stages of the catalytic cycle. Photocatalysis with its
hexacarbonyl analogue 3 has already shown promising results.22
For the same reasons, we aim at synthesizing complex 7, depicted in fig. 6. The
naphthalene monoimidedithiolate bridge of this system reportedly stabilizes the complex in
reduced states. An analogue of 7 has been employed successfully in electrochemical studies,
where it was reduced at values that were amongst the most positive reported for FeFe mimics
and photocatalytic evolution of H2 was observed.21
O
O P
N
O
N
N
S
N
O
S
Fe
OC
OC
CO
Fe
CO
CO
7
Fig. 6: Complex 7 as it was designed
In summary, the main goal of this work is to study the effects of different dithiolate
bridges on the properties of FeFe-hydrogenase mimics functionalised with phosphoramidite
ligands: the synthesis of these new complexes will be presented together with their full
spectroscopic, electrochemical and photocatalytic characterization.
16
2. Synthesis
This section describes the synthetic steps towards the new FeFe complexes, 5, 6 and
7. First, the hexacarbonyl analogues of these complexes, 2, 3 and 7F. These compounds are
then reacted with the separately synthesized phosphoramide linker, 8, to give the desired
complexes 5, 6 and 7.
a. Hexacarbonyl complex 2
The synthesis of 2 requires two steps, scheme 4. The preparation of 2A posed some
difficulties. Following two different literature procedures resulted in poor yields (~10%) of the
desired product.27,28 A more recent synthesis was finally used,29 involving reaction of Fe(CO)5
with elemental sulphur.
Sulphur flowers
Fe(CO)5
MeOH
S
(OC)3Fe
S
Fe(CO)3
2A
N
1) LiHBEt3, CF3COOH
2) (CH2O)n, PhCH2NH2
S
Fe(CO)3
THF
S
(OC)3Fe
SS
OC
Fe
OC
OC
CO
Fe
CO
CO
2
Scheme 4: Preparation of 230
The resulting crude product was sublimed at room temperature, yielding bright red
crystals of 2A. This process was very slow, yielding only a few hundred milligrams of material
per day. Since the IR spectra of the crude solid and the sublimed crystals are similar,
impurities are assumed to be inorganic and not to contain carbonyl groups. The differences
between the crude solid and the crystals are best seen when dissolved: the solution of the
crude product has a very dark reddish colour, while the sublimed crystals give a bright red
solution.
To attach the benzylamine bridge to complex 2A different methods were tested. An
adapted literature procedure from Rauchfuss gave the best results (71% yield).30 In this
reaction, complex 2A was reduced using LiHBEt3 and CF3COOH. The resulting reduced
species was then added to a mixture of paraformaldehyde and benzylamine in THF. Flash
column chromatography was used for purification.
17
b. Hexacarbonyl complex 3
A literature procedure was used for the preparation of complex 3.15 Commercially
available Fe3(CO)12 and 3,6-dichloro-1,2-benzenedithiolate were refluxed in toluene under
inert atmosphere.
Scheme 5: Preparation of 314
After column chromatography a small impurity was present in the aromatic region of
the
1H-NMR
spectrum, fig. 7. To remove it, the product was recrystallized from a mixture of
toluene and hexanes yielding single crystals. Surprisingly, the NMR spectrum showed the
amount of impurity had increased in these crystals.
Fig. 7: aromatic region of 1H-NMR spectrum of crude 3
On the other hand, IR spectroscopy showed a very clean carbonyl region with peaks
matching the reported values. Therefore, it was decided to use this compound and
functionalise it with the supramolecular linker.
c.
Hexacarbonyl complex 7F
The synthesis of the hexacarbonyl analogue of complex 7 required in six separate
steps, shown in scheme 6, starting from the synthesis of the bridge. For steps 1 to 4,
literature procedures are known, while for steps 5 and 6 literature procedures were adapted.
18
Br
Br
Br
Br
S
NBS
7D
DMF
DMF
7A
Br
S
Na2S2
Br
Br
O
O
S
Br
O
O
O
S
S
CrO3
O
S
Benzylamine
7E
Ac. Anhydride
O
Br
Br
7B
O
Br
O
Br
O
S
N
O
O
O
S
H 2O 2
Fe3(CO)12
Dioxane
O
O
O
O
O
7C
O
N
O
N
O
7F
O
S
(OC)3Fe
S
Fe(CO)3
Scheme 6: Synthetic route towards the hexacarbonyl complex 7F
First, acenaphtene was brominated, using n-bromosuccinimide. This reaction was
rather inefficient, with a yield of 19% (lit. 25%).31 Using the procedure of Vahrenkamp the
brominated product, 7A, was then oxidised using CrO3.32 This reaction needed harsh
conditions and resulted in the light brown solid, 7B. Its 1H-NMR spectrum, fig. 8, revealed a
significant impurity in the aromatic region, which is marked by arrows in the spectrum.
Attempts to recrystallize the product from acetic anhydride and from chlorobenzene were
unsuccessful. Due to the extreme low solubility of 7B column chromatography could not be
performed. Since a large excess of CrO3 was used, the impurity is presumably a result of
over-oxidation. Mass spectrometry indeed showed signals for the desired product and for a
species containing an additional oxygen atom. Since the following step in the synthesis
requires another oxidation, the NMR peaks of the impurity were compared to literature values
of the next product, 7C. These values did not match, leaving the structure of the impurity
unclear.
19
Fig. 8: 1H-NMR spectrum of 7B (left) and 7C (right)
However, since the synthesis of 7C involves another oxidation, which should in
principle not affect the over-oxidised impurity, it was decided to proceed to the next step with
the non purified 7B. For this reaction hydrogen peroxide was used as the oxidizing agent.32
The 1H-NMR spectrum of the product in acetone-d6, fig. 8, results in many peaks and lacks
the doublets at δ = 7.95 and 8.17 ppm, characteristic of the pure anydride. Even changing
solvents, such as dmso-d6, in hopes of helping the solubilisation of the right product, did not
show significant difference. However, mass spectrometry clearly showed signals of the
desired product. Again the product was very poorly soluble, ruling out column
chromatography as a method for purification and all recrystallization attempts from acetic
anhydride failed.
The low solubility of product 7C is probably the cause for the identification problems
in the 1H-NMR spectrum. When NMR samples of 7C were prepared, the solid did not dissolve
completely. This could mean that the signals in the NMR spectrum were just dissolved
impurities, while the desired product was still in the solid state and therefore undetected. For
this reason we decided to proceed with synthesis of 7 and to remove all impurities in a later
stage. The next step in the synthesis, however, required the use of elemental sodium. 32
Safety issues demanded postponing of this reaction. This ultimately led to a halt of the
synthesis of 7.
d. Supramolecular linker
The supramolecular linker, 8, was prepared by a recently reported four-step
synthesis,33 depicted in scheme 7. Starting from S-binol a hydrogenation was performed.
20
Compound 8A was obtained with full conversion. This product was then selectively
brominated with elemental bromine yielding 8B. Subsequently, a Suzuki-coupling was
performed in order to attach two pyridyl-groups. The obtained binol 8C was reacted with
P(NMe2)3 resulting in the phosphoramidite 8. The formation of this product was confirmed by
1H-
and 31P-NMR spectroscopy.
Scheme 7: Synthesis route to the phosphoramidite ligand 832
e. Functionalization of hexacarbonyl complexes with the supramolecular linker
A general procedure was used to functionalise the hexacarbonyl FeFe complexes 2
and 3 with the supramolecular linker 8. The compounds were mixed in presence of Me3NO,
which allows for quick removal of CO ligands. IR spectroscopy was used to follow the
progress of the reaction. Formation of the desired products induced a clear shift of the peaks
in the carbonyl region towards lower wavenumbers. Formation of the products was also
confirmed by NMR experiments.
31P-NMR
spectra show disappearance of the peak for the
free ligand at 140.5 ppm and the formation of a peak around 184-187 ppm confirming that the
ligand is indeed bound via the phosphorus atom. Column chromatography was used to
remove traces of free ligand and other impurities. This resulted in products 5 and 6 in
respectively 47% and 41% yield. The products were also characterised by 1H-NMR and mass
spectrometry.
Previous reports indicate that during functionalization of Fe2(benzenedithiolate)
complexes with PMe3 and CN- formation of mono-Fe(II) complexes also occurs, fig. 9.15
These complexes were not isolated during synthesis of 6. The reason for this is possibly the
use of Me3NO in the synthesis, which was not added in the above mentioned reports, and
21
that might lead to a different reaction pathway, excluding the mono-Fe complex as a byproduct. Another reason might be the use of the phosphoramidite ligand, which stabilizes
higher oxidation states in a lesser extent than phosphines.
Fig. 9: mono-Fe complex found by Ott and co-workers14
In order to gain more insight in the solid structure of 5 and 6, various crystallization
attempts of these products were performed, however in no case X-ray quality single crystals
were obtained. The methods applied in these efforts include layering, slow evaporation,
diffusion and crystallization at low temperature. A reason for these failures might be found in
the hydrogenated backbone of the phosphoramidite ligand: the extended aliphatic backbone
makes the complex extremely soluble, even in hexane, so that crystallization can become a
much harder task.
22
3. Spectroscopic studies
3.1 UV/Vis studies
The FeFe mimics are usually similar in the visible region of UV/Vis spectra: all the
complexes presenting a similar red-brown coloration in a variety of solvents. The absorption
spectra of 5 and 6 in toluene are shown in fig. 10. Both spectra feature an intense UV band at
around 350 nm with a minor absorption in the visible, resulting as a shoulder centred at ~470
nm (ε ~ 1500 M-1 cm-1 for 5 and ~ 1900 M-1 cm-1 for 6). These spectra compare very well with
that of 4 in the same solvent, which shows a visible absorption at ~475 nm (ε ~ 1400 M-1 cm-1)
Fig. 10: Absorption spectra of 5 and 6 in toluene
3.2 Infrared studies
Due to the presence of many carbonyl groups IR spectroscopy provides valuable
information about the electronic characteristics of the FeFe complexes. As pointed out earlier,
substitution of one CO with another ligand induces a pronounced shift of the bands in the
carbonyl region towards lower wavenumbers. This is a result of the reduced π-backbonding
from the FeFe centre to the new ligand as compared to CO (carbonyls are amongst the best
π-acceptors known): the FeFe centre is thus more electron rich, which increases the πbackdonation on the remaining carbonyl groups (and therefore weakening of their stretching
frequencies). An example of this effect is given by the comparison of the IR spectra of 3 and
6, as shown in fig. 11. The electron density around the FeFe centre is intimately related to the
electrochemistry of the complex, therefore, information gained via IR spectroscopy is of
extreme importance to understand the reduction behaviour of the new FeFe mimic along with
its photocatalytic properties.
23
0,4
0,35
0,3
0,25
6
Absorptoin
3
0,2
0,15
0,1
0,05
2100
2080
2060
2040
2020
2000
1980
1960
1940
0
1920
Wavenumber (cm-1 )
Fig. 11: Comparison of IR spectra of 3 and 6
The IR spectra of 4, 5 and 6 in DCM show a standard pattern consisting of an
isolated band followed by three bands, not always well resolved, at lower wavenumbers, fig.
12. Comparison of the spectra displays that the Cl 2bdt (dichlorobenzenedithiolate) bridge of 6
is the most effective at reducing electron density from the FeFe centre, with bands at 2058,
2005, 1987 and 1954 cm -1. This can be attributed to charge delocalisation onto the aromatic
ring combined with the strongly withdrawing Cl substituents on the ring. The amine
functionality in the bridge of 5 only moderately affects the IR spectrum (bands at 2049, 1996
and 1976 cm -1) when compared to the all carbon pdt bridge of 4 (bands at 2047, 1994 1978
and 1962 cm-1). The nitrogen atom is probably positioned too far from the FeFe site to induce
a pronounced peak shift towards higher wavenumbers. Nevertheless the effect of the nitrogen
is visible.
24
5
6
4
2100
2080
2060
2040
2020
2000
Wavenumber (cm-1)
1980
1960
1940
0
1920
Fig. 12: IR spectra of 4, 5 and 6 in DCM
Since photocatalysis involves protonation of the complex, IR spectra of 5 and 6 were
collected in presence of acetic acid and trifluroacetic acid (TFA). Since the IR data mentioned
above indicate that complex 5 has a more electron-rich FeFe centre than 6, and, additionally,
it also contains a nitrogen atom that can act as an internal base, 5 was expected to be more
easily protonated than 6.
Addition of one equivalent of acetic acid induces a little shift of 2 cm-1 towards higher
wavenumbers in the bands in the IR spectra of 5, illustrated in fig. 13. Moreover, the weak
bands at 2073 and 2036 cm -1 suggest that other species are present. Addition of one
equivalent of TFA has a much larger effect, with a drastic change in the IR. Two bands
appear now at higher wavenumbers (2073 and 2016 cm -1, a shift of ca. + 20 cm-1) indicating a
pronounced decrease in the electronic density on the FeFe site. This change is attributed to
protonation of the complex.
Protonation can occur at different positions in the complex: at the FeFe centre, at the
amine function in the dithiolate bridge, at the pyridyl groups and at the thiolate ends. Sun et
al. have previously shown that for an FeFe complex with a N-bridgehead, protonation of a μ-S
can cause a drastic increase of the νCO by about 125 cm -1.34 Similarly Lomoth et al. showed
that formation of a μ-H at the Fe-Fe bond results in an increase of the νCO by about 80 cm -1
while protonation of the N in the bridge caused a shift of about 15 cm -1.35 It is unknown what
change in the IR spectrum, protonation of the pyridyl groups would cause. However, because
of the higher basicity of the tertiary amine in the bridge and the comparable shift found by
Lomoth et al., in the case of 5, protonation at the N-bridgehead seems most likely. A 1H-NMR
study would give more certainty about this. For direct protonation of the FeFe site stronger
acids, like triflic acid are needed.
25
5
5 + 1 eq AcOH
5 + 1 eq TFA
2100
2080
2060
2040
2020
2000
Wavenumber (cm-1 )
1980
1960
1940
0
1920
Fig. 13: IR spectra of 5 with different acids in DCM
The protonation studies with complex 6 gives similar results. Addition of acetic acid
has virtually no effect on the IR spectrum of the complex, fig. 14. TFA on the other hand
clearly affects the spectrum. The shift of the bands in the CO region (ca. +7 cm-1) is however
not as large as for complex 5. The possible protonation sites in complex 6 are the Fe-Fe
bond, the thiolate moieties and the pyridyl groups. Here protonation at the pyridyl groups is
most likely, since Ott et al. have shown that protonation at the μ-S or at the Fe-Fe bond in
benzenedithiolate complexes causes νCO shifts of +74 cm-1 and +63 cm-1 respectively.36
Again, for protonation of the Fe-Fe bond and the μ-S stronger acids are needed.
6
6 + 1eq AcOH
6 + 1 eq TFA
2100
2080
2060
2040
2020
2000
Wavenumber (cm-1 )
1980
1960
1940
0
1920
Fig. 14: IR spectra of 6 with different acids in DCM
26
3.3 Titration studies of the supramolecular assemblies
Formation of supramolecular assemblies is usually assessed via a variety of
techniques. X-ray crystallography can provide information on the structure of the assembly in
the solid state, while in order to understand the dynamic behaviour in solution, NMR or
UV/Vis titrations are typically performed. To study the formation of the assembly between the
photosensitizer, zinc tetraphenylporphyrin (ZnTPP), and the catalysts, complexes 5 and 6,
UV/Vis and luminescence titration experiments were performed. UV/Vis was selected over
NMR spectroscopy, since previous studies confirmed that for similar assemblies with 4, the
Kass was found to be higher than 104 M-1, which requires to perform these titration studies at
the range of UV/Vis concentrations. Luminescence studies, moreover, provide extra
information on the activity of the assembly as a photocatalyst. If electron transfer from the
chromophore to the FeFe centre is observed, which is measure as quenching of the
photosensitizer emission, then the assembly has good chances to function as photocatalytic
dyad.
In these titration experiments, to a stock solution of ZnTPP in DCM, a solution of the
FeFe complexes is added, thereby increasing the concentration of the catalyst stepwise. In
order to keep the concentration of the photosensitizer constant, the solution of the complex is
prepared using the same stock solution of ZnTPP.
Upon addition of 5 to the ZnTPP solution, a clear red-shift in the Q-band of ZnTPP
was observed in the absorption spectrum, fig. 15. This shift is attributed to coordination of one
of the pyridyl groups of the phosphoramidite linker to the zinc. Isosbestic points at 556, 581
and 590 nm are observed, indicating the presence of an equilibrium. Using suitable fitting
software, the association constant, Kass, was determined to be 2.5∙104 M-1. This value is well
comparable to that of 4, for which Kass was calculated to be 6∙104 M-1.
27
0,2
0,18
0,16
0,14
0,12
0,1
0,08
0,06
0,04
0,02
0
500
520
540
560
580
Wavelength (nm)
600
620
640
Fig. 15: UV/Vis titration of 5 in DCM
The steady-state fluorescent measurements were carried out in a similar way. The
solution was excited at the isosbestic point at 555 nm, where the absorption is equal for all
concentrations of added 5, in order to ensure that any change in emission is due exclusively
to quenching of the photosensitizer. These experiments show drastic quenching of the
singlet-excited state of the ZnTPP, fig. 16. The association constant found through these
measurements, Kass= 4.0∙104 M-1, differs slightly from the one found via the UV/Vis titration.
This minor discrepancy is probably caused by some non-systematic error. Repetition of the
titration experiments could provide more accurate values, although the real focus is on the
magnitude of the constant.
28
1,40E+06
1,20E+06
1,00E+06
8,00E+05
6,00E+05
4,00E+05
2,00E+05
0,00E+00
570
590
610
630
650
Wavelength (nm)
670
690
710
Fig. 16: Luminescence tritrations of 5 in DCM
The UV/Vis titrations of 6 provide similar results, fig. 17. A clear red-shift of the Qband of the ZnTPP can be observed, which can be attributed to the supramolecular
coordination of ZnTPP to 6. Isosbestic points can be detected at 527, 554 and 583 nm. Fitting
of the acquired data provides an association constant of 7.1∙104 M-1, therefore higher than the
Kass of 5.
Fig. 17: UV/Vis titrations of 6
The steady-state fluorescent measurements also confirmed the formation of the
supramolecular assembly, fig. 18. Increasing the concentration of 6 while exciting at 555 nm
29
clearly leads to quenching of the singlet-excited state of ZnTPP. Fitting the data provides an
association constant of 6.1∙104 M-1. Again the UV/Vis and luminescence titrations do not give
the same Kass, however both titrations of 6 give association constants that are significantly
higher compared to the values for 5. Possibly, the benzylamine in the dithiolate bridge of 5
causes some steric hindrance, leading to an increased energy-barrier for ZnTPP to
coordinate to the pyridyl groups. Computational modelling might provide more insights on this
matter.
1,13E+06
9,25E+05
7,25E+05
5,25E+05
3,25E+05
1,25E+05
570
590
610
630
-7,50E+04
650
670
690
710
730
750
Wavelength (nm)
Fig. 18: Luminescence titrations of 6 in DCM
The values of association constants found via these studies, are very high, which
points to the formation of robust supramolecular assemblies. However, the most encouraging
piece of information provided by these titrations, is the quenching of the photosensitizer
luminescence upon coordination to the catalyst, which points out to the desired photo-induced
electron transfer, one of the key steps in the photocatalytic production of H 2 by these
systems.
30
4. Electrochemistry
4.1 Cyclic voltammetry
As previously mentioned, the electrochemistry of the FeFe mimics is of essential
importance to understand their ability to undergo electrocatalytic reduction of protons to H 2
and how efficiently these reactions can occur. This information can easily be obtained by
analysing the cathodic portion of their cyclic voltammograms.
Cyclic voltammetry experiments were carried out on the newly synthesized
complexes 5 and 6 to determine their electrochemical properties. In these studies the
complexes were dissolved in acetonitrile solutions and a glassy carbon working electrode was
used. The voltammogram of 5, depicted in fig. 19, shows a typical irreversible reduction peak
at -1.82 V vs. Fc+/0 corresponding to a 1-electron reduction. Compared to 4, which is reduced
at a potential of -1.89 V, the electron-withdrawing N in the dithiolate bridge induces only a
minor anodic shift in the reduction potential. This is in good agreement with the data obtained
from the IR spectroscopy, which showed a small blue-shift of the CO bands due to the effect
of the amine group in the bridge. The irreversibility of the reduction suggests that an unstable
species is formed, which decomposes readily into a product that is electrochemically inactive.
With respect to the hexacarbonyl analogue of 5,37 the phosphoramidite ligand induces a shift
of 250 mV towards a more negative potential attributed to the reduced π-accepting character
of the phosphoramidite ligand compared to the carbonyl.
5
2,00E-05
6
4
1,00E-05
E / V vs. Fc+/0
-2,5
-2
-1,5
-1
0,00E+00
-0,5
-1,00E-05
-2,00E-05
-3,00E-05
Fig. 19: Cyclic voltammograms of 4, 5 and 6 in MeCN, reported vs. Fc+/0
31
The dichlorobenzene bridge in complex 6 has a remarkable effect on the
electrochemical behaviour of the complex, as shown in fig. 19. The voltammogram of
complex 6 shows a reduction wave at -1.48 V vs. Fc+/0, much less negative than those of 4
and 5. Moreover, in this case the reduction is quasi-reversible, showing two separate reoxidation events. Electrochemical reversibility of the hexacarbonyl analogue of 6 has earlier
been reported.15 This property has been attributed to interaction between the metal orbitals
with a combination of the filled sulfur p-orbitals and the benzene p-orbitals, allowing
delocalisation of electrons over the benzene ring.38 It is not sure what causes the second reoxidation peak in the voltammogram. A possible explanation could be that the reduced 6∙species is re-oxidized in two steps that occur at different potentials. Capon et al. have
previously shown that the second reduction of a FeFe complex containing the unsubstituted
benzenedithiolate bridge, occurs at a potential less negative than the first reduction. 39 Another
possibility is the formation of an electrochemically active decomposition product from the
reduced 6∙- species.
The comparison of the CV of these different mimics displays the significant influence
that the nature of the dithiolate bridge can have on the electrochemical properties of FeFe
complexes. As previously anticipated these results are in good agreement with the IR data.
4.2 Electrocatalysis
The electrocatalytic activity of complexes 5 and 6 was investigated in presence of
different acids. Titrating increasing concentrations of acetic acid (AcOH) in a solution of 5,
leads to a drastic enhancement in the current intensity (fig. 20), which is typical for H2
evolution. GC (gas chromatography) analysis of the headspace of the electrochemistry cell
after such a reduction cycle confirmed formation of H2 gas.
32
E / V vs. Fc+/0
-2,5
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
0 mM
17,5 mM
35 mM
70 mM
140 mM
210 mM
Fig. 20: Electrocatalytic reduction of AcOH by 5 in MeCN, reported vs. Fc+/0
The addition of the stronger trifluoroacetic acid (TFA) also induces increased current
intensity, as displayed in fig. 21.
E / V vs. Fc+/0
-2,5
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
0
18 mM
37 mM
73 mM
Fig. 21: Electrocatalytic reduction of TFA by 5 in MeCN, reported vs. Fc+/0
The effect of the lower pKa of TFA becomes very clear when the voltammograms are
compared, fig. 22. Addition of AcOH causes catalytic reduction of protons, with a wave at
around -2.1 V vs. Fc+/0. A small shoulder can be observed at around -1.8 V vs. Fc+/0, which is
the potential at which 5 is reduced in absence of acid. Reduction of 5 is therefore a likely first
step in the catalytic mechanism, then followed by protonation. When TFA is used, a large
anodic shift of approximately 400 mV is observed. Literature reports indicate that such a shift
33
can implicate protonation of the amine function in the bridge. 11 Given this and the IR data (fig.
13 and the related discussion) it is very likely that TFA changes the mechanism of the H 2
production in the case of 5. Instead of an initial reduction, 5 is now protonated first and then
reduced, a mechanism earlier observed by Capon et al. for a complex with a NHCH2CH2OMe
bridgehead.40
E / V vs. Fc+/0
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
5
5 + 35 mM AcOH
5 + 37 mM TFA
-0,9
Current
-2,5
Fig. 22: Comparison of different acids reduced by 5 in MeCN, reported vs. Fc+/0
Complex 6 is also capable of catalysing the reduction of protons, fig. 23. Increasing
the AcOH concentration leads to enhancement of the current intensity. The two-electron
reduction process around -1.4 V vs. Fc+/0 is still observed in MeCN. Two other reduction
events are then observed around -1.95 V vs. Fc+/0 and -2.25 V vs. Fc+/0, respectively. At high
acid concentrations, these reduction waves merge.
34
E / V vs. Fc+/0
-2,5
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
Current
0
17,5 mM
35 mM
70 mM
140 mM
210 mM
Fig. 23: Electrocatalyis reduction of AcOH by 6 in MeCN, reported vs. Fc+/0
Similar considerations are observed in presence of TFA, fig. 24.
E / V vs. Fc+/0
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
0 mM
18 mM
-0,9
Current
-2,5
37 mM
73 mM
146 mM
220 mM
Fig. 24: Electrocatalytic reduction of TFA by 6 in MeCN, reported vs. Fc+/0
A comparison of the electrocatalytic reduction of AcOH and TFA by 6, clearly
demonstrates once more the effect of the strength of the acid on the catalytic potential, as
depicted in fig. 25. TFA is reduced at a potential much more positive than AcOH (1.7 V vs. 2.0
V, respectively).
35
E / V vs. Fc+/0
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
Current
-2,5
6
6 + 35 mM AcOH
6 + 37 mM TFA
-7,00E-04
Fig. 25: Comparison of different acids reduced by 6 in MeCN, reported vs. Fc+/0
Despite the marked difference in the reduction potential of the complexes 5 and 6,
the potential at which they both catalyse reduction of AcOH is much more similar (2.09 V vs.
1.99 V, respectively), depicted in fig. 26. The positive effect of the chlorobenzene bridge on
the reduction of 6, appears not to be entirely reflected in the electrocatalytic reduction of
AcOH, suggesting different mechanisms in the catalysis.
E / V vs. Fc+/0
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
Current
-2,5
5 + AcOH
6 + AcOH
-3,60E-04
Fig. 26: Comparison of reduction of AcOH by 5 and 6 in MeCN, reported vs. Fc+/0
Similar observations can be made in the comparison of the reduction of TFA by 5 and
6, fig. 27, even though in this case an interesting remark can be made: H2 evolutions occurs
at more positive potentials for 5 than for 6 (1.68 V vs. 1.71 V, respectively). Thus, the
36
protonation at the amine-bridgehead of 5, shown in the IR data (fig. 13), clearly has a positive
influence also on its the electrocatalytic properties.
E / V vs. Fc+/0
-2,3
-2,1
-1,9
-1,7
-1,5
-1,3
-1,1
-0,9
Current
-2,5
5 + TFA
6 + TFA
-7,00E-04
Fig. 27: Comparison of reduction of TFA by 5 and 6 in MeCN, reported vs. Fc+/0
37
5. Photocatalysis
The newly prepared FeFe complexes 5 and 6 and their supramolecular assemblies
were employed in photocatalytic experiments to investigate their catalytic activity, table 1. In
these experiments the photocatalyst is combined with a photosensitizer and a sacrificial
electron donor in acidic conditions. The solution was irradiated continuously and wavelengths
shorter than 530 nm were filtered off, because the FeFe complexes absorb these, as shown
by the UV/Vis experiments, which might lead to their photodecomposition. Also, a water-flow
cuvette was used to filter off the heat of the lamp. After 3 hours a sample of the headspace
was taken and measured by gas chromatography. Different electron donors, proton sources
and solvents were used to investigate the impact of these conditions on the performance of
the catalysts.
Table 1: Results of photocatalytic experiments with 5 and 6
Entry
FeFe complex
Substratea (mM)
Amount of H2 (μL)
Ratio N2/O2
Area of H2b
1
5
A (4)
3.1
3.5
572
2
6
A (4)
7.5
4.6
1382
3
5
A (104)
1.7
4.4
306
4
5
B (4)
1.3
3.5
241
5c
5
B (4)
1.8
3.4
336
6d
5
C (100)
1.4
7.8
254
7d
6
C (100)
0
3.7
0
8e
6
A (4)
2.6
3.3
475
9
4
A (4)
40
The photocatalyses were performed in 50 mL Schlenks with degassed toluene solutions (5 mL) 1 mM in FeFe
complex, 2 mM in ZnTPP and 2 mM in ZnTPP(p-OMe) and kept under continuous irradiation by a 160-W Xe highpressure lamp. The irradiation time was 3 hours. A cut-off filter was used to exclude wavelengths <530 nm and
infrared radiation of the lamp was removed by absorption in a water flow cell. The reaction temperature was 293 K.
a: different substrates were used, A= [NiPr2EtH][OAc], B= [NiPr2EtH][CF3COO] and C= NEt3
b: calibration of GC: area= 184.22 V[H2] (μL)
c: additionally 5 μmol (1 eq) of degassed CF3COOH was added
d: instead of toluene, degassed acetone (4.5 mL) and degassed water (0.5 mL) were used as solvent
e: 3.5 mM [Ru(bpy)3][BF4]2 was used as photosensitizer instead of ZnTPP and ZnTPP(p-OMe)
Initially, the ionic liquid [NiPr2EtH][OAc] (DIPEAc) was chosen as the proton and
electron source, as for the reported experiments with 4 (entry 9). Under these conditions 6,
proves to be much more active than 5, producing more than twice the amount of H2.
38
However, the amount of H2 generated is very small compared to other photocatalytic
systems: e.g. the photocatalyst 4 produces 40 μL of H2 under the same circumstances.
In entry 1 and 2 the substrate : catalyst ratio is 4, so not very high. It was then
decided to change this ratio to 100 (see entry 3), reasoning that the increased availability of
protons and electrons might reduce the time between steps in the catalytic cycle thereby
reducing the possibility for decomposition due to instability in reduced and protonated states.
This approach proved to be unsuccessful, since the amount of H2 produced decreased in this
experiment.
Since the results with CF3COOH in the CV studies of 5 were promising, with
significantly lowered potentials at which electrocatalysis started compared to the acetic acid,
the electron and proton donor [NiPr2EtH][OOCCF3] was prepared. The use of this sacrificial
donor did however not improve the performance of the photocatalyst, as depicted in entry 4.
Since a first explanation for this result was that this new proton and electron source is not
enough acidic, it was then thought that addition of an equivalent of free CF 3COOH could
improve the performance of the system. Indeed, the amount of produced hydrogen increased
(entry 5), remaining however much less than for entry 1. Possibly, the reduced performance
of photocatalyst 5 in entry 4 and 5 is caused by demetallation of the ZnTPP and ZnTPP(pOMe) photosensitizers, induced by the stronger acidic conditions. This might also explain the
formation of a dark solid on the walls of the Schlenk during the catalysis. This solid was not
found in any other entry.
In 2010 Sun and co-workers published a multicomponent photocatalytic system
reaching turnover numbers up to 466 in basic conditions using a mixture of water and
acetone as solvent.23 In this system triethylamine functions as the electron donor, while water
provides the protons. These interesting findings prompted us to test photocatalysts 5 and 6
under the same conditions. However, the outcome of these experiments was poor (see entry
6 and 7): 5 did produce some H2, while 6 did not evolve a detectable amount. At the end of
the photocatalysis a purple precipitate was found in the Schlenk.
The possibility of using complex 6 in a multicomponent system instead of a
supramolecular assembly was also investigated. An experiment where [Ru(bpy)3]2+ was used
as the photosensitizer showed that the complex still catalyses reduction of protons, although
the amount of hydrogen evolved is lower than for the supramolecular assembly (entry 8 vs.
2). However, considering that the absorption maximum of [Ru(bpy) 3]2+ is 452 nm41 and the
cut-off filter employed only allows wavelengths longer than 530 nm, the photosensitizer was
probably not excited optimally in our setup. Removal of the cut-off filter may therefore improve
the performance of the system, although it could, at the same time, lead to faster
decomposition of the FeFe complex.
Neither photocatalyst 5 nor 6 perform well in any of the conditions tested. This is
surprising because their synthesis was motivated by the expectation that the modified bridges
should have positive effects on the photocatalysis: in the case of 5 by offering an extra
protonation site close to the reactive centre and in the case of 6 by reducing the potential at
39
which H2 evolution occurs. Instead, both 5 and 6 give lower turnover numbers than 4. Maybe
fast decomposition of the FeFe complexes is the reason for this. This decomposition is
evidenced in the GC measurements, which revealed a considerable amount of free CO for
both 5 and 6. IR spectroscopy of the irradiated samples confirmed this: the CO bands that are
typical for the FeFe complexes were not detected in the spectra after photocatalysis. Rather
unexpectedly, for all entries, the ratio of N2/O2 peak areas had also changed during the
photocatalysis. N2 and O2 are always present in the chromatograms for the set-up used in this
GC. During calibrations this ratio had been consistently found to be 3. All entries display an
increase in the ratio after irradiation. An increase of the N2/O2 ratio was also found upon
injection of samples containing pure CO2. This suggests that the retention time of CO2 is the
same as for N2. These findings implicate that apart from CO, also CO2 might be formed
during the experiments. The question as to how CO2 would be formed remains unsolved.
Since decomposition appears to play an important role during the photocatalysis, the
IR spectrum of the reaction mixture of a photocatalysis experiment with 4 was followed over
time. This should reveal any structural changes of the complex.
These experiments were carried out placing the photocatalytic mixture in an IR cell.
This was then irradiated in the usual way. Every 10 minutes an IR spectrum was recorded.
We are aware that the results might not be very representative, as the IR cell present some
limitations to the usual Schlenk set-up (no stirring is involved and the cell is not fully airtight),
but some of the results might give interesting insights.
0,5
0,4
0
10
20
30
40
80
105
150
0,3
0,2
0,1
0
2160
2140
2120
2100
2080
2060
2040
2020
2000
1980
1960
wavenumber (cm-1)
Fig. 28: Time profile (in minutes) of IR spectra of 4 during photocatalysis,in toluene
40
The IR spectra of 4 over time (see experimental section for actual conditions) show
rapid decomposition of the complex in the first 10 minutes (fig. 28). After this period the
decomposition appears to slow down, but not to stop entirely. Furthermore, there is clear
growth of bands at 2033 and 2073 cm -1, indicating formation of the hexacarbonyl analogue 1
and of a broad band centred at around 2135 cm -1 belonging to an unknown species, although
it could possibly be assigned to free CO in toluene. The formation of 1 is noteworthy, as it can
mean that the bond between 8 and the Fe may not be as strong under the photocatalytic
conditions as it was expected. On the other hand, the decomposition of 4 leads to release of
a vast amount of CO, as evidenced in the GC analyses; so an alternative explanation for the
formation of 1 could be that the rapid increase in the concentration of CO in solution leads to
substitution of 8 by a CO.
With these results in mind, we thought it would be interesting to study the effect of
addition of an extra equivalent of the free phosphoramidite ligand, 8, to the mixture during
photocatalysis, hoping it would prevent recombination of CO to the complex. The results of
this experiment are shown in fig. 29.
0,5
0,4
0,3
0
10
20
30
40
50
60
80
100
0,2
0,1
2160
2110
2060
wavenumber
2010
0
1960
(cm-1)
Fig. 29: IR spectra followed in time (minutes) of 4 and excess ligand, 8, during photocatalysis, in toluene
The spectra once more show fast decomposition of 4. However, in this case, there is not as
much of hexacarbonyl analogue formed. The band at ~2035 cm-1, on the other hand, is much
more pronounced compared to the previous experiment. If this band can indeed be assigned
to gaseous CO, the reduced formation of the hexacarbonyl would perfectly explain it, since it
would mean a higher concentration of CO in the solution, whose binding to 4 is impaired by
the presence of extra ligand. Remarkably, the spectra also reveal the formation of a band at
2014 cm-1, which can be attributed to a double substituted species, fig. 30. The formation of a
double substituted species has earlier been witnessed in the case of the pyridylphosphine
41
ligands, where it was found to be the real photocatalytic active species.26 With this in mind,
the appearance of the absorption band could be very interesting, although the intensity of the
band is very weak. Despite the addition of extra free ligand the vast majority of 4 is still
decomposed.
N
N
O
O P
N
N
N
SS
Fe
OC
OC
O
PO
Fe
CO
CO
N
Fig. 30: Double substituted analogue of 4
42
6. Conclusions
In order to improve photocatalyst 4, this contribution has aimed at modifying this
complex at the dithiolate bridge. This has led to the design of three new supramolecular FeFe
photocatalysts, 5, 6 and 7. Complexes 5 and 6 were prepared successfully while the
synthesis of 7 failed due to synthetic problems.
UV/Vis
absorption
spectra,
IR
spectra
and
cyclic
voltammograms
show
unambiguously that the modifications on the dithiolate bridge deeply affect the spectroscopic
and electronic properties of these catalysts. Titration studies show that both 5 and 6 form
supramolecular assemblies with ZnTPP. Furthermore, as expected, a blue-shift is observed in
the νCO of the IR spectra of 5 and 6 compared to the one of 4. For 5, however, this effect is
not as large as was expected. The N-bridgehead might be positioned too far from the FeFe
centre to have a significant electronic effect. Comparable observations have been made in
cyclic voltammograms. Complex 6 is reduced at a potential much more positive than 4, while
the difference in reduction potential of 5 and 4 is not significant.
Both 5 and 6 exhibit electrocatalytic activity in the reduction of acetic acid to
hydrogen. For both catalysts this event occurs at potentials more negative than the reduction
of the complex itself. This negative shift is largest for 6.
IR studies on 5 and 6 show that acetic acid is too weak to protonate the complexes.
Trifluoroacetic acid, on the other hand, is sufficiently acidic to protonate 5 at the aminebridgehead, causing significant changes in its electrochemical properties. In case 6, instead,
the complex is merely protonated at the pyridyl groups of the phosphoramidite ligand, which
only slightly affects its electrochemical properties. The observations of the protonation studies
are also reflected in electrocatalytic reduction of trifluoroacetic acid by 5 and 6. Evolution of
H2 occurs at more positive potentials for 5 than for 6 demonstrating the positive effect of
protonation of the amine-bridgehead.
The performed photocatalytic studies give poor results. Under a variety of conditions,
i.e. different proton and electron donors as well as different solvents, 5 and 6 produce very
little volumes of H2. Under the same conditions 6 is more active than 5., however, their
performances are much lower, when compared to 4.
Following the IR spectra of 4 in time during photocatalysis has provided interesting
insights. In the first 10 minutes of irradiation of the photocatalytic mixture, 4 is decomposed
very rapidly and its hexacarbonyl analogue, 1, is formed. Addition of extra free
phosphoramidite ligand, partly prevents recombination of CO with 4 and also leads to
formation of the double substituted analogue of 4, which could be of possible interest.
In conclusion, the studies have shown that the dithiolate bridge significantly affects
the properties of the supramolecular photocatalysts. The trend of these effects can be
explained for the IR and CV studies. However, the expected positive effects of the employed
dithiolate bridges on the photocatalytic performances of the supramolecular systems have not
43
been observed; instead the effects have been negative. The question as to why 5 and 6
perform badly in photocatalysis cannot be answered with the current data in hands and
requires further studies.
44
7. Outlook
The new photocatalysts presented in this report did not result in the expected improvement of
the supramolecular system, but they offer interesting starting points for future studies.
First of all, it would be very interesting to study the FeFe complexes during photocatalysis by
in-situ IR spectroscopy (e.g. directly in the Schlenk). Compared to the IR study presented in
this report, such studies would provide more accurate knowledge on the decomposition of the
complexes. This knowledge then could be used for the design of future FeFe complexes. For
the same reason spectro-electrochemistry on these complexes would be very interesting.
Furthermore, obtaining crystal structures of the complexes and the assemblies would provide
valuable structural information.
The electrochemistry presented in this report has provided useful information on the
new catalysts. However, the conditions in these experiments do not optimally reflect the
conditions
during
photocatalysis,
because
they
were
performed
in
absence
of
photosensitizers. So the experiments give information about the complexes and not about the
supramolecular assemblies. It would therefore be interesting to study what effect the addition
of a photosensitizer would have on the cyclic voltammogram and on the electrocatalytic
properties of the catalysts.
The photocatalytic conditions could be optimized by additional screening of, for
example, different proton and electron donors, other solvents, other photosensitizers with
different absorption spectra and different ranges of the wavelength. It would, additionally, be
very interesting to test the photocatalytic activity of 5 and 6 when ascorbic acid is used as the
sacrificial electron and proton donor. Ascorbic acid is widely used in literature reports, which
enables better comparison with other reported photocatalysts. Furthermore, since
multicomponent systems have delivered results that are still the best reported, it can be
interesting to explore the possibility of employing 5 and 6 in a multicomponent system instead
of a supramolecular one.
Of course, ultimately it would be very interesting to employ a H 2O-splitting
photocatalyst together with a H+-reduction photocatalyst in a device. Successful development
of an active device could provide a significant contribution to the solution for future energy
problems.
45
8. Experimental part
All solvents employed were dried, either distilled or from the SPS (Solvent Purification
System) and they were always degassed prior use. All experiments were carried out under an
inert-gas atmosphere using standard Schlenk techniques. Unless described otherwise the
chemicals used were commercially available and used without further purification. The 1H and
31P
NMR spectra were recorded at 400 and 162 MHz, respectively, on a Bruker av400 and
calibrated to the residual proton signals of the solvent. High resolution mass spectra were
recorded on a JEOL JMS SX/SX102A four sector mass spectrometer; for FAB-MS 3nitrobenzyl alcohol was used as a matrix. IR spectra (FT-IR) were recorded with a Bruker
Alpha-p FT-IR spectrometer. The UV/Vis spectra were obtained by scanning between 180900 nm on a Hewlett-Packard 8453 UV-Visible Spectrophotometer.
The phosphoramidite linker (8),33 Fe2(µ-S)2(CO)6 (2A),29 and Fe2(μ-3,6-dichloro-1,2benzenedithiolate)(CO)6 (3)15 were prepared according to reported methods.
Synthesis of Fe2[(SCH2)2NCH2Ph](CO)6 (2)
Under an argon atmosphere, Fe2(µ-S)2(CO)6 (1.72 mg, 5 mmol) was dissolved in 80 mL of
THF. The clear dark red solution was then cooled down to -78° C. To this solution 10.1 mL of
1M LiHBEt3 in THF (10.1 mmol) was added, causing a change of colour to dark green. Then
argon purged CF3COOH (1.04 mL, 13.6 mmol) was added, resulting in a reddish solution. In
a separate flask, a mixture of paraformaldehyde (6.01 g, 200 mmol) and benzylamine (10.9
mL, 100 mmol) in 200 mL THF, had been previously stirring for 4 hours. The Fe containing
solution was transferred to this mixture via cannula and simultaneously filtered, to remove the
lithium salts formed in the first step. The resulting mixture was stirred overnight. It was then
filtered under inert atmosphere and the solvent was removed in vacuo. The resulting product
was purified on a silica gel column, under N2, using a mixture of hexanes and EtOAc as
eluents (9:1), yielding a red solid. Yield: 1,7 g (71%). IR (MeCN): νCO= 2074 (m), 2037 (s),
1997 (s). 1H NMR (CD3CN): δ 7.33-7.19 (m, 5H), 3,76 (s, 2H), 3.42 (s, 4H) ppm.
Synthesis of 5
Under a nitrogen atmosphere 2 (267 mg, 0.56 mmol), the phosphoramidite 8 (292 mg, 0.56
mmol) and Me3NO (47 mg, 0.624 mmol) were dissolved in 15 mL MeCN. After 1h30, the
reaction mixture was sampled via IR, showing that the hexacarbonyl precursor, 2, was still
present. The mixture was thus treated with some excess Me3NO until IR showed that all the
starting material, 2, had been consumed. The solvent was removed in vacuo and a silica gel
column under N2 atmosphere was used for purification. Elution with DCM and THF (5:3) gave
the product as the first red band. The product was dried with Et2O 3 times. IR (DCM): νCO=
46
2049, 1996 and 1976 cm -1.1H-NMR (CD2Cl2): δ 8.88 (d, J = 14.9 Hz, 2H), 8.55 (s, 1H), 8.29
(d, J = 3.9 Hz, 1H), 8.13 (d, J = 7.6 Hz, 1H), 7.92 (d, J = 7.9 Hz, 1H), 7.36-7.21 (m, 7H), 7.097.07 (m, 2H), 3.01-2.86 (m, 5H), 2.72-2.61 (m, 3H), 2.47-2.40 (m, 2H), 2.34-2.27 (m, 2H), 2.2
(d, J = 10.4 Hz, 6H) 1.91-1.78 (m, 7H), 1.70-1.63 (m, 3H) ppm.
ppm. FAB-MS: 971.1 (m/z) (calcd
MH+:
31P-NMR
(CD2Cl2): δ 187.1
971.1)
Synthesis of 6
Fe2(μ-3,6-dichloro-1,2-benzenedithiolate)(CO)6 (244 mg, 0.5 mmol) was placed in a Schlenk
under nitrogen atmosphere with phosphoramidite, 8, (261 mg, 0.50 mmol) and Me3NO (45
mg, 0.60 mmol) and 15 mL of MeCN was added. The mixture was stirred and after 10 min the
solution changed colour from clear dark colour to a turbid brown mixture. After 35 min, IR
spectrum showed hardly any conversion, so extra Me 3NO was added until full conversion had
been reached. The solvent was removed in vacuo. A silica gel column under inert conditions
was used for purification of the product. Elution with DCM and THF (5:1) yielded 206 mg of
the product, a red solid (41%). IR (DCM): νCO= 2058, 2005, 1987 and 1954 cm -1.1H NMR
(CD2Cl2): δ 8.88 (d, J = 19.4 Hz, 2H), 8.56 (s, 1H), 8.44 (d, J = 3.6 Hz, 1H), 8.12 (d, J = 5.2
Hz, 1H), 7.95 (d, J = 7.7 Hz, 1H), 7.40-7.26 (m, 3H), 7.13 (s, 1H), 6.43 (dd, J = 5.4, 6.0 Hz,
2H), 3.01-2.86 (dd, J = 7.2, 7.4 Hz, 4H), 2.65-2.56 (m, 2H), 2.33-2.2 (m, 2H), 2.11 (d, J = 10.2
Hz, 6H), 1.88-1.79 (m, 5H), 1.71-1.59 (m, 3H) ppm.
31P-NMR
(CD2Cl2): δ 184.3 ppm. FAB-
MS: 982.0 (m/z) (calcd MH+: 982.0)
Electrochemistry
The cyclic voltammetry experiments were carried with an Autolabpgstad 10. A glassy carbon
work electrode, a platinum counter electrode and a silver reference electrode were used. The
experiments were performed under nitrogen atmosphere. Distilled, degassed acetonitrile (5
mL) was used as solvent,
nBu NPF
4
6
as electrolyte (0.1 M) and ferrocene as internal
reference. The concentration of the FeFe complexes was 1 mM. Acetic acid and
trifluoroacetic acid were degassed by bubbling nitrogen through before addition to the
solution.
Photocatalysis
A 180 W Xe-lamp was used for irradiation in the photocatalytic experiments. A water-flow
filter was used to filter IR radiation of the lamp and a cut-off filter filtered off wavelengths
shorter than 530 nm. The composition of the headspace was analyzed by taking a 1 mL
sample with an argon-flushed airtight syringe, which was injected directly in a Global Analyser
Solutions CompactGC equipped with a TCD detector. The GC has been calibrated by
injection of different volumes of H2. The ionic liquid [NiPr2EtH][OAc] was prepared by addition
of ethyldiisopropylamine to acetic acid. The liquid was then dried overnight over vacuum. The
density of the liquid was reported in literature (value maybe). 42 The salt [NiPr2EtH][OOCCF3]
47
was similarly prepared by addition of ethyldiisopropylamine to trifluoroacetic acid. The solid
was then dried overnight over vacuum.
The photocatalysis experiments were carried out in the following way:
Entry 1,2: In a 50 mL Schlenk under argon atmosphere the FeFe complex,5 or 6, (5 μmol),
ZnTPP (10 μmol) and ZnTPP(p-OMe) (10 μmol) were dissolved in 5 mL toluene. The ionic
liquid [NiPr2EtH][OAc](4μL, 20 μmol) was added and the solution was degassed by bubbling
argon through it for 5 minutes, keeping the Schlenk was covered from light The solution was
irradiated continuously for 3 hours, and then the headspace in the Schlenk was analyzed by
GC.
Entry 3: In a 50 mL Schlenk under argon atmosphere the FeFe complex, 5 or 6, (5 μmol),
ZnTPP (10 μmol) and ZnTPP(p-OMe) (10 μmol) were dissolved in 5 mL toluene. The ionic
liquid [NiPr2EtH][OAc] (100μL, 500 μmol) was added and while the Schlenk was covered from
light, argon was bubbled through the solution for 5 minutes. The solution was irradiated
continuously for 3 hours, after this time the atmosphere in the Schlenk was analyzed by GC.
Entry 4: In a 50 mL Schlenk under argon atmosphere the FeFe complex 5 (5 μmol), ZnTPP
(10 μmol), ZnTPP(p-OMe) (10 μmol) and [NiPr2EtH][OOCCF3] (20 μmol) were dissolved in 5
mL toluene. While the Schlenk was covered from light, argon was bubbled through the
solution for 5 minutes. The solution was irradiated continuously for 3 hours, and then the
headspace in the Schlenk was analyzed by GC.
Entry 5: In a 50 mL Schlenk under argon atmosphere the FeFe complex 5 (5 μmol), ZnTPP
(10 μmol), ZnTPP(p-OMe) (10 μmol) and [NiPr2EtH][OOCCF3] (20 μmol) were dissolved in 5
mL toluene. CF3COOH (5 μmol) was added and while the Schlenk was covered from light,
argon was bubbled through the solution for 5 minutes. The solution was irradiated
continuously for 3 hours, and then the headspace in the Schlenk was analyzed by GC.
Entry 6, 7:In a 50 mL Schlenk under argon atmosphere the FeFe complex, 5 or 6, (5 μmol),
ZnTPP (10 μmol) and ZnTPP(p-OMe) (10 μmol) were dissolved in 4.5 mL acetone and 0.5
mL water. Triethylamine (70μL, 100 μmol) was added and while the Schlenk was covered
from light, argon was bubbled through the solution for 5 minutes. The solution was irradiated
continuously for 3 hours, and then the headspace in the Schlenk was analyzed by GC.
Entry 8: In a 50 mL Schlenk under argon atmosphere the FeFe complex 6 (5 μmol) and
[(bpy)3Ru](BF4)2 (19μmol) were dissolved in 5 mL toluene. The ionic liquid [NiPr2EtH][OAc]
(4μL, 20 μmol) was added and while the Schlenk was covered from light, argon was bubbled
through the solution for 5 minutes. The solution was irradiated continuously for 3 hours, and
then the headspace in the Schlenk was analyzed by GC.
48
Conditions for IR measurements on 4 during photocatalysis:
In a 50 mL Schlenk under argon atmosphere the FeFe complex 4, (5 μmol), ZnTPP (10 μmol)
and ZnTPP(p-OMe) (10 μmol) were dissolved in 5 mL toluene. The ionic liquid
[NiPr2EtH][OAc] (4μL, 20 μmol) was added and while the Schlenk was covered from light,
argon was bubbled through the solution for 5 minutes. A sample of the solution was
transferred to a one mm IR cell, which was then irradiated. Spectra were taken at indicated
times.
Conditions for time-resolved IR measurements on 4 with additional phosphoramidite 8:
In a 50 mL Schlenk under argon atmosphere the FeFe complex 4, (5 μmol), phosphoramidite,
8, (5 μmol), ZnTPP (10 μmol) and ZnTPP(p-OMe) (10 μmol) were dissolved in 5 mL toluene.
The ionic liquid [NiPr2EtH][OAc] (4μL, 20 μmol) was added and while the Schlenk was
covered from light, argon was bubbled through the solution for 5 minutes. A sample of the
solution was transferred to a one mm IR cell, which was then irradiated. Spectra were taken
at indicated times.
UV/Vis titrations
ZnTPP (8∙10-7 mol) was dissolved in 10 mL DCM. Catalyst 5 or 6 (8∙10-6 mol) was dissolved in
5 mL of the fresh prepared ZnTPP solution.
Starting with the ZnTPP solution, the solution containing the catalyst was added gradually.
Titrations were carried out in a 1 mm cuvet.
The binding constants were determined with fitting software developed by C. A. Hunter et al.
Steady-state fluorescence titrations
ZnTPP (8∙10-7 mol) was dissolved in 10 mL DCM. Catalyst 5 or 6 (8∙10-6 mol) was dissolved in
5 mL of the fresh prepared ZnTPP solution.
Starting with the ZnTPP solution, the solution containing the catalyst was added gradually.
Titrations were carried out in a 10 mm cuvet. The solutions were excited at 555 nm.
The binding constants were determined with fitting software developed by C. A. Hunter et al.
49
Abbreviations
UV/Vis
Ultraviolet/visible
IR
Infrared
CV
Cyclic voltammetry
WOR
Water oxidation reaction
PRR
Proton reduction reaction
Fc
Ferrocene
Pdt
Propanedithiolate
TON
Turnover number
THF
Tetrahydrofuran
NMR
Nuclear Magnetic Resonance
ε
Extinction coefficient
DCM
Dichloromethane
Cl2bdt
Dichlorobenzenedithiolate
AcOH
Acetic acid
TFA
Trifluoroacetic acid
ZnTPP
Zinc tetraphenylporphyrin
Kass
Association constant
GC
Gas chromatography
ZnTPP(p-OMe)
Zinc tetra(p-methoxyphenyl)porphyrin
SPS
Solvent Purification System
FAB-MS
Fast atom bombardment mass spectroscopy
50
Acknowledgements
Sofia Derossi, for being my daily supervisor, for teaching me a lot, all the help provided during
the writing of this thesis and for the great time I had doing my research.
Joost Reek, for being my supervisor and allowing me to do this research.
Jarl Ivar van der Vlugt, for being my second reviewer, for all input in the research and for the
steady stream of new articles on hydrogenases.
Bart van den Bosch, for all help in the lab and for his great taste of music.
Carien van Aubel, for providing me with the [Ru(bpy)3][(BF4)2].
The members in the water-oxidation/proton-reduction minimeeting group, for their advices
and input in my research.
The homkat group, for not only being great a group during work hours, but also beyond these
hours.
All other people who contributed, in whatever way, to my research and this thesis.
51
References
(1)
Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729-15735.
(2)
Armaroli, N.; Balzani, V. ChemSusChem 2011, 4, 21-36.
(3)
Ewing, R. C. Weber, W. J.; Lian, J. J. Appl. Phys. 2004, 95, 5949.
(4)
Wang, M. Na, Y. Gorlov, M.; Sun, L. Dalton Trans. 2009, 6458.
(5)
Losse, S. Vos, J. G.; Rau, S. Coor. Chem. Rev. 2010, 254, 2492-2504.
(6)
McLaughlin, M. P. McCormick, T. M. Eisenberg, R.; Holland, P. L. Chem. Commun. 2011.
(7)
Zhang, Z. Long, J. Yang, L. Chen, W. Dai, W. Fu, X.; Wang, X. Chem. Sci. 2011.
(8)
Frey, M. ChemBioChem 2002, 3, 153-160.
(9)
Peters, J. W. Science 1998, 282, 1853-1858.
(10) Silakov, A. Wenk, B. Reijerse, E.; Lubitz, W. Phys. Chem. Chem. Phys. 2009, 11, 6592.
(11) Capon, J.-F. Gloaguen, F. Pétillon, F. Y. Schollhammer, P.; Talarmin, J. Coor. Chem. Rev.
2009, 253, 1476-1494.
(12) Reihlen, H. Friedolsheim, A. V.; Oswald, W. Liebigs Ann. Chem. 1928, 465, 72.
(13) Felton, G. A. N. Mebi, C. A. Petro, B. J. Vannucci, A. K. Evans, D. H. Glass, R. S.;
Lichtenberger, D. L. J. Organomet. Chem. 2009, 694, 2681-2699.
(14) Na, Y. Wang, M. Pan, J. Zhang, P. Åkermark, B.; Sun, L. Inorg. Chem. 2008, 47, 2805-2810.
(15) Schwartz, L. Singh, P. S. Eriksson, L. Lomoth, R.; Ott, S. CR. Chim. 2008, 11, 875-889.
(16) Lomoth, R.; Ott, S. Dalton Trans. 2009, 9952.
(17) Ott, S. Borgström, M. Kritikos, M. Lomoth, R. Bergquist, J. Åkermark, B. Hammarström, L.;
Sun, L. Inorg. Chem. 2004, 43, 4683-4692.
(18) Ekstrom, J. Abrahamsson, M. Olson, C. Bergquist, J. Kaynak, F. B. Eriksson, L. Sun, L.
Becker, H.-C. Akermark, B. Hammarström, L.; Ott, S. Dalton Trans. 2006, 4599.
(19) Wang, H.-Y. Si, G. Cao, W.-N. Wang, W.-G. Li, Z.-J. Wang, F. Tung, C.-H.; Wu, L.-Z. Chem.
Commun. 2011.
(20) Song, L.-C. Wang, L.-X. Tang, M.-Y. Li, C.-G. Song, H.-B.; Hu, Q.-M. Organometallics 2009,
28, 3834-3841.
(21) Samuel, A. P. S. Co, D. T. Stern, C. L.; Wasielewski, M. R. J. Am. Chem. Soc. 2010, 132,
8813-8815.
(22) Streich, D. Astuti, Y. Orlandi, M. Schwartz, L. Lomoth, R. Hammarström, L.; Ott, S. Chem.
Eur. J. 2010, 16, 60-63.
(23) Zhang, P. Wang, M. Na, Y. Li, X. Jiang, Y.; Sun, L. Dalton Trans. 2010, 39, 1204.
(24) Wang, F. Wang, W.-G. Wang, X.-J. Wang, H.-Y. Tung, C.-H.; Wu, L.-Z. Angew. Chem. Int.
Ed. 2011, 50, 3193-3197.
(25) Li, X. Wang, M. Zhang, S. Pan, J. Na, Y. Liu, J. Åkermark, B.; Sun, L. J. Phys. Chem. B 2008,
112, 8198-8202.
52
(26) Kluwer, A. M. Kapre, R. Hartl, F. Lutz, M. Spek, A. L. Brouwer, A. M. van Leeuwen, P. W. N.
M.; Reek, J. N. H. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 10460-10465.
(27) Seyferth, D. Henderson, R. S.; Song, L. C. Organometallics 1982, 1, 125-133.
(28) Cowley, A. Inorganic syntheses. Wiley: New York ;;Chichester, 1997; Vol. 31.
(29) Stanley, J. L. Rauchfuss, T. B.; Wilson, S. R. Organometallics 2007, 26, 1907-1911.
(30) Li, H.; Rauchfuss, T. B. J. Am. Chem. Soc. 2002, 124, 726-727.
(31) Neudorff, W. D. Lentz, D. Anibarro, M.; Schlüter, A. D. Chem. Eur. J. 2003, 9, 2745-2757.
(32) Tesmer, M.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2001, 2001, 1183-1188.
(33) Bellini, R. Chikkali, S. H. Berthon-Gelloz, G.; Reek, J. N. H. Angew. Chem. Int. Ed. 2011, n/an/a.
(34) Dong, W. Wang, M. Liu, X. Jin, K. Li, G. Wang, F.; Sun, L. Chem. Commun. 2006, 305.
(35) Eilers, G. Schwartz, L. Stein, M. Zampella, G. de Gioia, L. Ott, S.; Lomoth, R. Chem. Eur. J.
2007, 13, 7075-7084.
(36) Ezzaher, S. Gogoll, A. Bruhn, C.; Ott, S. Chem. Commun. 2010, 46, 5775.
(37) Na, Y. Pan, J. Wang, M.; Sun, L. Inorg. Chem. 2007, 46, 3813-3815.
(38) Felton, G. A. N. Vannucci, A. K. Chen, J. Lockett, L. T. Okumura, N. Petro, B. J. Zakai, U. I.
Evans, D. H. Glass, R. S.; Lichtenberger, D. L. J. Am. Chem. Soc. 2007, 129, 12521-12530.
(39) Capon, J.-F. Gloaguen, F. Schollhammer, P.; Talarmin, J. J. Electroanal. Chem 2004, 566,
241-247.
(40) Capon, J.-F. Ezzaher, S. Gloaguen, F. Pétillon, F. Y. Schollhammer, P.; Talarmin, J. Chem.
Eur. J. 2008, 14, 1954-1964.
(41) Kalyanasundaram, K. Photochemistry of polypyridine and porphyrin complexes; Academic
Press: London ;;San Diego, 1992.
(42) Anouti, M. Caillon-Caravanier, M. Le Floch, C.; Lemordant, D. J. Phys. Chem. B 2008, 112,
9406-9411.
53