Master Thesis

Synthesis of Ruthenium-based Water

Oxidation Catalysts and Mechanistic Study

February 2015

Motoki Yoshida

Division of Organic Chemistry, Department of Chemistry

School of Chemical Science and Engineering

KTH Royal Institute of Technology

Stockholm, Sweden

Abstract

Two series of new mononuclear ruthenium complexes with hydrophobic or hydrophilic

ligands [Ru(bda)L2] and [Ru(pdc)L3] (H2bda = 2,2’-bipyridine-6,6’-dicarboxylic acid;

H2pdc = 2,6-pyridinedicarboxylic acid; L = pyridyl ligands) were synthesized and their

electrochemical properties and catalytic activity toward water oxidation were examined.

It was revealed that the hydrophobic ligands introduced to [Ru(bda)L2] improved the

catalytic performance, almost twofold TON and TOF values were achieved compared to

the [Ru(bda)L2] catalyst with hydrophilic ligands. The cyclic voltammogram of

[Ru(bda)L2] exhibited marginal difference between the catalysts with hydrophobic

ligands and hydrophilic ones, implying that the hydrophobic ligands promoted the

catalytic activity by facilitating formation of a reaction intermediate dimer.

List of Abbreviations

CV cyclic voltammetry

dmso dimethyl sulfoxide

DPV differential pulse voltammetry

GC gas chromatography

H2bda 2,2’-bipyridine-6,6’-dicarboxylic acid

H2pdc 2,6-pyridinedicarboxylic acid

MS mass spectrometry

NHE normal hydrogen electrode

OEC oxygen evolving complex

pic 4-picoline

PCET proton-coupled electron transfer

TFE 2,2,2-trifluoroethanol

TMS tetramethylsilane

TOF turnover frequency

TON turnover number

WOC water oxidation catalyst

Table of Contents

1.1.1.1. Introduction ............................................................................................................. 1

1.1. Background ......................................................................................................... 1

1.2. Water Splitting .................................................................................................... 1

1.3. Water Oxidation Catalysts (WOCs) ................................................................... 3

1.4. Ruthenium-based WOCs .................................................................................... 4

1.5. Aim of Thesis ..................................................................................................... 5

2. Experimental Section .............................................................................................. 6

2.1. Synthesis and characterization............................................................................ 6

2.2. Electrochemistry ............................................................................................... 11

2.3. Water Oxidation ............................................................................................... 11

3. Results and Discussion .......................................................................................... 12

3.1. Cyclic Voltammetry ......................................................................................... 12

3.2. Pourbaix Diagram ............................................................................................. 15

3.3. Water oxidation ................................................................................................ 16

4. Conclusion .............................................................................................................. 17

Acknowledgements ....................................................................................................... 19

Reference ....................................................................................................................... 20

Supporting Information ............................................................................................... 24

1

1.1.1.1. Introduction

1.1. Background

Increasing concern about depletion of fossil fuels, air pollution and global

warming has driven scientists to explore the potential of hydrogen as a fuel of the

future.1 Since the only byproduct during the combustion of molecular hydrogen with

oxygen is water, hydrogen is sometimes considered as an environment-friendly energy

resource. However, hydrogen is a secondary form of energy, and it is promising as an

energy carrier. In other words, since molecular hydrogen scarcely occurs in nature, it is

necessary to produce hydrogen by using energy.2

There are numerous problems to tackle to realize the hydrogen economy, from

distribution and storage of hydrogen to cost and temperature problem of fuel cells.

Particularly, a key problem would be the means to produce hydrogen because most of

the hydrogen for industrial use is currently produced through transformations from

natural gas and oil.3 Although these methods are at present the least expensive processes

to produce hydrogen, adopting hydrogen produced from hydrocarbons as fuel cannot be

a substantial solution to the energy problems. A variety of methods to produce hydrogen

has been investigated, for example, gasification of coal, thermal or biological

conversion of biomass and water splitting using electricity, heat, or light.4 However,

these methods still have problems regarding, in particular, costs for large scale

production and further technical innovation is required.

1.2. Water Splitting

The idea of using water, instead of fossil fuels, as a raw material for hydrogen

generation appears to be the best in terms of environmental burden since the molecules

2

involved in this reaction are very “clean” (Equation 1).

2H2O → 2H2 + O2 (1)

While water the demand of which is expected to grow is also a precious resource, the

amount of water for hydrogen generation to fulfill transportation needs in the U.S would

account for only about 1% of the domestic use of water.5 By applying water as the

source for hydrogen, many countries can reduce their dependence on foreign natural

resources and thus can strengthen their national energy security. Also, generated oxygen

can be utilized for various purposes including manufacture of nitric acid from ammonia,

metal fabrication and healthcare use.1

There are some different forms of energy that can be used for water splitting:

light, heat and electricity. Plants harness sunlight to split water into protons, electrons

and molecular oxygen in the process of photosynthesis. This reaction is catalyzed by the

oxygen evolving complex (OEC) and the core of OEC is proven to be a Mn4CaO5

cluster surrounded by some ligands.6 Considerable effort has been devoted to mimic this

light-driven water splitting system and there has been some progress in fundamental

science, but the conversion efficiency is still too low for practical applications.7 It is

also possible to produce hydrogen by thermal decomposition of water.8 However,

nuclear power plants required to achieve the high temperature for decomposition are

accompanied by dangerous radioactive wastes and a risk of accidents.

Electrolysis of water is potentially the most environment-friendly hydrogen

production method if the electricity is obtained from renewable energy. Nevertheless,

just 4% of hydrogen is industrially produced by electrolysis of water, only when

high-purity hydrogen is required.1 This is because the hydrogen produced by

electrolysis of water is costlier than the one produced from fossil fuels. A catalyst that

3

enables electrolysis of water with a low overpotential is thus highly demanded in order

to embrace the benefit of hydrogen produced by electrolysis.

1.3. Water Oxidation Catalysts (WOCs)

The water splitting process (Equation 1) is composed of two half reactions:

water oxidation (Equation 2) and proton reduction (Equation 3).

2H2O → O2 + 4H+ + 4e- (2)

2H+ + 2e- → H2 (3)

Because both reactions have a large activation barrier, a catalyst that lowers

overpotential and enhances reaction rate is required in order to perform water splitting.

For proton reduction reaction (3), platinum-based complexes have been reported to

show efficient catalytic ability,9 but the scarcity and high price associated with platinum

limit the applications in industrial scale. First-row transition metal catalysts, such as

iron, cobalt and nickel complexes, have also attracted much attention, aiming at

mimicking the functions of so-called hydrogenase enzymes which are responsible for

both oxidation of molecular hydrogen and reduction of protons.10

Meanwhile, the water oxidation reaction (2) is currently recognized as the

bottleneck for the development of the water splitting process.11 The reaction seems to be

simple at an initial glance, but it involves the transfer of four electrons, rearrangement

of multiple bonds and formation of the O-O bond. The search for novel WOCs that

minimize the overpotential, increase the reaction rate and have a high durability has

developed a variety of molecular WOCs consisted of different metal center(s), such as

ruthenium, iridium12 or first row transition metals (e.g. manganese,13 cobalt,14 iron,15 or

copper16).

4

1.4. Ruthenium-based WOCs

Ruthenium-based WOCs are the most extensively studied WOCs since the

discovery of this class as the first homogeneous, non-protein WOCs in the early

1980s.17 While the high price of ruthenium metal is a drawback of ruthenium-based

WOCs, the molecular WOCs that exhibit the highest catalytic efficiency are to date

ruthenium-based. 18,19

[Ru(bda)(pic)2] and [Ru(pdc)(pic)3] (H2bda = 2,2’-bipyridine-6,6’-

dicarboxylic acid; H2pdc = 2,6-pyridinedicarboxylic acid; pic = 4-picoline; Figure 1) are

ruthenium-based WOCs that exhibit relatively high catalytic activity, both reported by

the author’s group.19,20 Both bda2- and pdc2- are strong electron-donating ligands, which

decrease the oxidation potentials of the catalysts and facilitate the electron transfer from

catalysts to an oxidizing equivalent.21 In the case of [Ru(bda)(pic)2], TON of 2000 and

TOF of 2500 min-1 were achieved using CeIV as oxidant.22 A kinetic study of the

catalyst showed that the consumption of CeIV is second order in the concentration of the

catalyst, implying a binuclear catalytic pathway.19 The involvement of a

seven-coordinate ruthenium dimer complex as an intermediate of the water oxidation

reaction was experimentally suggested and the seven-coordinated complex is

characterized by X-ray diffraction.23

Figure 1. Molecular structures of [Ru(bda)(pic)2] and [Ru(pdc)(pic)3]

5

On the other hand, a kinetic study of [Ru(pdc)(pic)3] proved that the catalytic reaction

is first order toward the catalyst, implying mononuclear catalytic pathway is most likely

involved.20 It was shown by NMR spectroscopy and MS analysis that the equatorial pic

is labile under acidic conditions and a solvent molecule alternatively coordinates to the

catalyst at RuIII state. It is proposed that [Ru(pdc)(pic)2(H2O)] is the real WOC. TON

of 550 and TOF of 13.8 min-1 were obtained using CeIV as oxidant.

Thirty years of research on ruthenium-based WOCs have led to significant

improvement in the catalytic efficiency. However, in order to develop WOCs which can

be utilized in industrial scale, further investigation regarding structure-activity

correlation and catalytic mechanism is of great need.24, 25

1.5. Aim of Thesis

In this thesis, four new ruthenium-bda-based or ruthenium-pdc-based WOCs

(A – D; Figure 2), to which hydrophobic or hydrophilic ligands are introduced, are

synthesized and their electrochemical properties and catalytic activity toward water

oxidation is explored.

Figure 2. Molecular structures of A - D

6

The main focus is on how the hydrophobic ligands contribute to enhance catalytic

activity. As previously explained, while ruthenium-bda-based WOCs are said to form a

dimer in the course of water oxidization, ruthenium-pdc-based WOCs are thought to

remain as a monomer during the reaction. Thus, it is anticipated that the introduced

hydrophobic ligands make the bda-based catalyst efficient because the hydrophobic

groups would aggregate in an aqueous solution and assist the formation of dimers. This

research is expected to present a new guideline regarding the design of novel WOCs.

2. Experimental Section

2.1. Synthesis and characterization

The synthesis of the WOCs A - D is shown in Scheme 1.

General: All chemicals and solvents were purchased from Sigma-Aldrich and used

without further purification unless otherwise stated. 1H-NMR spectra were recorded

with 500 MHz of Bruker Avance spectrometer with TMS as internal standard.

Rubda(dmso)2 3, [(η6-cymene)RuCl2]2 4 and phosphonate ligand 6 were prepared

following the literature methods.26–29 Complexations for complexes A - D were

performed on the basis of previous studies. 19,20

7

Scheme 1. The synthetic routes of catalysts A - D

8

Ru(bda)(4-propylpyridine)2 A. Ruthenium (III) chloride trihydrate 1 (6.50 g, 26.5

mmol) was dissolved in dmso (20 mL) with vigorous stirring under nitrogen atmosphere.

After addition of 2-propanol (70 mL), the mixture was heated to 85 oC for 24 h. At

room temperature, the precipitation was filtered off and washed with acetone and

diethyl ether to give 7.89 g of crude Ru-dmso complex 2 as yellowish powder, which

was used without further purification.

The crude product 2 (420 mg), H2bda (200 mg, 819 mmol) and methanol (20 mL) were

combined under nitrogen atmosphere and heated to 70 oC. After all the compounds were

dissolved, triethylamine (3.9 mL, 28 mmol) was added dropwise and the mixture was

heated to reflux for 4 h. After cooling down, the precipitation was filtered off and

washed with diethyl ether to afford 230 mg of Ru-bda complex 3 as brown powder,

which was used without further purification.

The crude product 3 (100 mg), 4-propylpyridine (0.13 mL, 1.00 mmol) and methanol

(20 mL) were combined under nitrogen atmosphere and heated to reflux for 20 h. After

cooling, the solvent was removed under reduced pressure and the crude solid was

purified twice by reprecipitation with methanol/diethyl ether to give brown solid A (88

mg, 0.15 mmol, three steps yield 26%). 1H NMR (500 MHz, Methanol-d4) δ 8.50 (d, J =

8.1 Hz, 2H), 7.94 (d, J = 7.6 Hz, 2H), 7.79 (t, J = 7.9 Hz, 2H), 7.58 (d, J = 5.8 Hz, 4H),

6.96 (d, J = 5.9 Hz, 4H), 2.41 (t, J = 7.7 Hz, 4H), 1.44 (q, J = 7.5 Hz, 4H), 0.76 (t, J =

7.3 Hz, 6H).

Ru(pdc)(4-propylpyridine)3 B. Ruthenium (III) chloride trihydrate 1 (3.70 g, 14.2

mmol), (R)-α-phellandrene (18.5 mL. 115 mmol) and ethanol (150 mL) were combined

under nitrogen atmosphere and heated to reflux for 4 h. After cooling, the solution was

filtered off to afford compound 4. The filtrate was reduced to the half-volume under

9

reduced pressure and the solution was kept in the fridge overnight. The generated

precipitation was filtered off to obtain additional product. In total 3.57 g of Ru-cymene

dimer 4 as dark red micro-crystalline was obtained and used without further

purification.

The crude product 4 (75.0 mg), H2pdc (41.0 mg, 0.245 mmol) and triethylamine (250

µL, 1.79 mmol) were dissolved in ethanol (8 mL) in a microwave tube under nitrogen

atmosphere. The mixture was heated to 140 oC in the microwave oven for 30 min. After

cooling, 4-propylpyridine (650 µL, 4.99 mmol) was added and the mixture under

nitrogen atmosphere was heated to 140 oC in microwave oven for 30 min. After cooling,

the solvent was removed under reduced pressure and the crude solid was dissolved in

dichloromethane (15 mL) and washed with water (10 mL) for three times. The organic

phase was dried over Na2SO4 and the solvent was removed under reduced pressure. The

crude product was purified by column chromatography on silica gel eluting with

dichloromethane/methanol (10:1, v/v). The obtained solid was further purified by

reprecipitation with dichloromethane/diethyl ether to give black solid B (108 mg, 0.172

mmol, three steps yield 57%). 1H NMR (500 MHz, Chloroform-d) δ 8.77 (d, J = 5.9 Hz,

2H), 8.13 (d, J = 5.8 Hz, 4H), 7.96 (d, J = 7.7 Hz, 2H), 7.55 (t, J = 7.7 Hz, 1H), 7.06 (d,

J = 5.8 Hz, 2H), 6.86 (d, J = 5.9 Hz, 4H), 2.58 (t, J = 7.8 Hz, 2H), 2.45 (t, J = 7.7 Hz,

4H), 1.66 (p, J = 7.5 Hz, 3H), 1.52 (p, J = 7.5 Hz, 7H), 0.93 (t, J = 7.3 Hz, 3H), 0.87 (t,

J = 7.4 Hz, 6H).

Diethyl [(4-pyridinyl)methyl] phosphonate 6. 4-Bromomethylpyridine hydrobromide

5 (1.50 g, 5.93 mmol) and chloroform (50 mL) were combined and treated with an

aqueous solution of K2CO3 (827 mg, 5.98 mmol) to quench the HBr. After phase

separation and drying over Na2SO4, triethyl phosphate (10.0 mL, 58.0 mmol) was added

10

and the solution was purged with nitrogen. The mixture was shielded from light and

heated to reflux for 10 h. After cooling, the solvent was evaporated under reduced

pressure and the resulting mixture was purified by column chromatography on silica gel

eluting with ethyl acetate/methanol (4:1, v/v) to give yellow oil 6 (922 mg, 4.02 mmol,

68%). 1H NMR (500 MHz, Chloroform-d) δ 8.54 (d, J = 5.2 Hz, 2H), 7.25 (dt, J = 4.2,

2.1 Hz, 2H), 4.13 – 3.99 (m, 4H), 3.14 (d, J = 22.3 Hz, 2H), 1.27 (t, J = 7.1 Hz, 6H).

Ru(bda){diethyl [(4-pyridinyl)methyl] phosphonate}2 C. The crude product 3 (40.0

mg), compound 6 (95.5 mg, 0.417 mmol) and methanol (25 mL) were combined under

nitrogen atmosphere and the mixture was heated to reflux for 12 h. After cooling down,

the solvent was removed under reduced pressure and the crude product was purified by

column chromatography on silica gel eluting with dichloromethane/methanol (10:1, v/v)

to afford black solid C (60.0 mg, 0.0748 mmol, yield 93%). 1H NMR (500 MHz,

Methanol-d4) δ 8.62 (d, J = 8.0 Hz, 2H), 8.04 (d, J = 7.7 Hz, 2H), 7.91 (t, J = 7.9 Hz,

2H), 7.77 (d, J = 6.0 Hz, 4H), 7.27 – 7.01 (m, 4H), 4.15 – 3.92 (m, 8H), 3.24 (d, J =

22.8 Hz, 4H), 1.18 (t, J = 7.0 Hz, 12H).

Ru(pdc){diethyl [(4-pyridinyl)methyl] phosphonate}2 D. The crude product 4 (50.0

mg), H2pdc (27.2 mg, 0.163 mmol) and triethylamine (160 µL, 1.15 mmol) were

dissolved in ethanol (8 mL) in a microwave tube under nitrogen atmosphere. The

mixture was heated to 140 oC in the microwave oven for 30 min. After cooling,

compound 6 (560 mg, 2.44 mmol) was added and the mixture under nitrogen

atmosphere was heated to 140 oC in microwave oven for 4.5 h. After cooling, the

solvent was removed under reduced pressure and the crude solid was purified by

column chromatography on silica gel eluting with dichloromethane/methanol (1:1, v/v)

to afford brown crystalline D (145 mg, 0.152 mmol, two steps yield 93%).1H NMR

11

(500 MHz, Chloroform-d) δ 8.82 (d, J = 5.7 Hz, 2H), 8.23 (d, J = 6.1 Hz, 4H), 7.98 (d, J =

7.7 Hz, 2H), 7.61 (t, J = 7.7 Hz, 1H), 7.23 – 7.13 (m, 2H), 7.06 – 6.89 (m, 4H), 4.01 (ddq,

J = 19.7, 8.4, 7.1 Hz, 12H), 3.11 (d, J = 22.6 Hz, 2H), 2.99 (d, J = 22.5 Hz, 4H), 1.19 (dt,

J = 14.0, 7.0 Hz, 18H).

2.2.Electrochemistry

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)

measurements were performed with an Autolab potentiostat with a GPES

electrochemical interface (Eco Chemie), using a pyrolytic graphite electrode (basal

plane, diameter 3 mm) (PGBE) as working electrode and a platinum column as counter

electrode and measured versus Ag/ AgCl reference electrode (3 M KCl aqueous

solution). All potentials measured herein were reported against the Ag/AgCl reference

electrode. The cyclic voltammograms were obtained in a phosphate buffer solution (0.1

M) or HNO3 aqueous solution (pH 1; trace metal grade, 70%, purified by redistillation,

99.999%; Fisher Scientific) containing a catalyst (1 mM) and TFE

(2,2,2-trifluoroethanol, >99%; 20 or 25 volume %). TFE was added to improve the

solubility of the catalysts to the solution.

2.3.Water Oxidation

The oxygen evolution was recorded with a pressure transducer (Omega

PX138-030A5 V) driven versus time by a power supply (TTi-PL601) at 8.00 V and

calibrated by gas chromatography (GC) (GC-2014 Shimadzu). First, (NH4)2[Ce(NO3)6]

(99.5%) was dissolved in HNO3 aqueous solution (initial pH 1.0) and the solution was

stirred in a 25 mL flask. Next, catalyst solution (HNO3 aqueous solution/TFE= 1/1, v/v)

12

was rapidly injected to the flask and started the measurement. Finally, the amount of

generated oxygen was calibrated by GC.

(NH4)2[Ce(NO3)6] (CeIV) is a strong one-electron transfer oxidant (E ≈ 1.72 V

vs NHE) which is soluble in water, and used as a sacrificial electron acceptor (Equation

4).

WOC

4CeIV + 2H2O O2 + 4H+ + 4CeIII (4)

The conditions used in the measurements were as follows:

A and C for TOF: HNO3 pH 1 aqueous solution (3.4 mL) containing CeIV (0.346 M)

and catalyst (1.14 × 10-4 M) and TFE (0.1 mL).

A and C for TON: HNO3 pH 1 aqueous solution (3.475 mL) containing CeIV (0.346 M)

and catalyst (2.86 × 10-5 M) and TFE (25 µL).

B and D: HNO3 pH 1 aqueous solution (2.85 mL) containing CeIV (8.33 × 10-2 M),

catalyst (6.67 × 10-5 M) and TFE (0.15 mL).

3. Results and Discussion

3.1. Cyclic Voltammetry

Figure 3 displays the cyclic voltammograms (CVs) of Ru-bda based complexes

A and C under pH 1 nitric acid condition (upper) or pH 7 phosphate buffer condition

(lower), and Figure 4 displays the CVs of Ru-pdc based complexes B and D under the

same conditions. In Table 1 the oxidation potentials, onset potential for catalytic current

acquired from DPV spectrum and catalytic current at 1.5 V acquired from CV spectrum

are summarized.

13

Figure 3. CV data of complex A at pH 1 (upper left), C at pH 1 (upper right), A at pH 7

(lower left) and C at pH 7 (lower right). [(NH4)2[Ce(NO3)6]]= 0.1 mM.

Figure 4. CV data of complex B at pH 1 (upper left), D at pH 1 (upper right), B at pH 7

(lower left) and D at pH 7 (lower right). [(NH4)2[Ce(NO3)6]]= 0.1 mM.

14

Table 1. DPV and CV data of complexes A - D

complex Eox III/ II (V) Eox IV/ III (V) Eox V/IV (V) Eonset (V) I (µA)a

A (pH=1) 0.43 0.92 -- 1.10 10.0

B (pH=1) 0.29 1.04 1.20 1.23 9.1

C (pH=1) 0.46 0.98 -- 1.18 6.8

D (pH=1) 0.31 0.70 -- 1.23 7.9

A (pH=7) 0.40 0.78 -- 1.14 61.9

B (pH=7) 0.28 0.78 1.28 1.39 42.7

C (pH=7) 0.39 0.82 -- 0.95 38.2

D (pH=7) -- 0.62 -- 1.27 17.4

a catalytic current at 1.5 V

Catalytic current was observed for all the complexes A – D at both pH 1 and pH 7. The

difference of corresponding oxidation potential between A and C were small (the

differences were smaller than 0.1 V), implying that the hydrophobic/hydrophilic part of

the ligands have little effects on electrochemical properties of the complexes.

The observed catalytic current was larger at high pH than low pH. This is

explained from Nernst equation (Equation 5):

� = ��+

��

��

��

� (5)

(where � is a cell potential, �� is a standard cell potential, �is the gas constant, �

is absolute temperature, is number of moles of electrons transferred in the cell

reaction, � is the Faraday constant, �� is chemical activity for oxidants, �� is

chemical activity for reductants.)

For the water oxidation reaction (2), E0 = 1.02 V vs Ag/ AgCl (3M KCl), so the

15

equation (5) is expressed as below:

E = (1.02－0.059 pH) V vs Ag/ AgCl (3M KCl) (6)

It is clear from the equation (6) that voltage required for water oxidation is lowered

under high pH condition and this is why the catalytic current is larger at high pH.

3.2. Pourbaix Diagram

The DPVs of complex A and C at various pH values were recorded and the

Pourbaix diagram was drown (Figure 5).

Figure 5. Plots of voltage versus pH (Pourbaix diagram) for complex A (left) and

complex C (right)

The diagram shows the thermodynamically most stable species at certain pH and

voltage. Slopes in the diagram (-0.059, which can be calculated in the same way as

equation 6 from Nernst equation) indicate that the equilibrium potential between two

species is dependent on pH value of the solution i.e. proton coupled electron transfer

(PCET) is involved. On the other hand, horizontal lines indicate that the equilibrium

potential is independent of pH value i.e. neither protons nor hydroxides are involved in

A C

16

the reaction. From this diagram, it is suggested that Ru(V)=O is involved in water

oxidation process. The voltage required for the reaction is depicted in the equation (6).

3.3.Water oxidation

Chemical water oxidation was demonstrated by using CeIV as a sacrificial

oxidant in an aqueous solution at pH 1. A typical oxygen evolving trace by using A - D

as a catalyst was depicted in Figure 6. TON and TOF were calculated as an average

value of three measurements.

Figure 6. Oxygen evolving trace by catalysts A and C (left) and B and D (right)

Table 2. Catalytic data of complexes A - D

complex TON TOFinitial (s-1)

A 1929 32

B 219a 3 × 10-2

C 1044 18

D 85a 9 × 10-3

a after 8 hours

17

As expected, bda-based complex with hydrophobic ligands A showed superior

catalytic activity compared to bda-based complex with hydrophilic ligands C. The

lifetime of these two complexes approximately appeared to be the same, but the TON

and TOF of A increased almost twofold compared to C. This is possibly because the

formation of intermediate dimers is promoted by the hydrophobic ligands in aqueous

solution, considering that the CV measurement revealed the ligands have little influence

on the electrochemical properties of the catalysts.

Meanwhile, the pdc-based complex with hydrophobic ligands B also exhibited

better catalytic performance compared to the other pdc-based complex with hydrophilic

ligands D. This result was not anticipated since the hydrophobic ligands were not

supposed to enhance the catalytic activity of the pdc-based complex that works as a

monomer catalyst. Further investigation is required to clarify the influence of

hydrophobic ligands on catalytic performance of pdc-based catalysts.

4. Conclusion

Catalysts A – D were successfully synthesized and their electrochemical properties and

catalytic activities were revealed. All the catalysts exhibited catalytic activities toward

the water oxidation reaction. It was shown that the bda-catalyst with hydrophobic

ligands A exhibited better catalytic performance compared to the one with hydrophilic

ligands C. There was marginal difference in the CV diagrams of A and C, suggesting

that the hydrophobic ligands help form an intermediate dimer and this could be why the

catalytic activity improved. However, pdc-catalyst with hydrophobic ligands B also

unexpectedly showed better catalytic performance compared to the one with

18

hydrophobic ligands D. Further research on the effect of hydrophobicity of WOCs is

underway.

19

Acknowledgements

First of all, I would like to express my sincere gratitude to Professor Licheng

Sun for giving me the opportunity to perform my degree project as a member of his

research group at KTH. It was truly great experience for me to conduct research at his

renowned laboratory in the field of artificial photosynthesis. He always provided me

with incisive comments that led me to the right direction.

I would also like to express my deepest appreciation to my supervisor, Quentin

Daniel, who had supported me throughout my work by giving me detailed instructions

and proper advice. This project would not have completed without his assistance and

encouragement. One could not wish for a better supervisor than him.

I am grateful to all the other members in the laboratory and in Department of

Organic Chemistry as well, particularly Doctor Lele Duan, Lei Wang and Fusheng Li,

for giving me useful advice and kindly explaining me instructions for laboratory

equipment. Whole my stay in the lab became fruitful thanks to these people.

Last but not least, I thank the love and patience of my family. I would like to

dedicate this thesis to my grandmother, Haruko, who always cheered me up during my

stay in Sweden by writing letters to me, but passed away three days after I returned to

Japan.

20

Reference

1. Armaroli, N. & Balzani, V. The hydrogen issue. ChemSusChem. 4, 21–36 (2011).

2. Rausch, B., Symes, M. D., Chisholm, G. & Cronin, L. Decoupled catalytic

hydrogen evolution from a molecular metal oxide redox mediator in water

splitting. Science. 345, 1326–1330 (2014).

3. Häussinger, P., Lohmüller, R. & Watson, A. M. Ullmann’s Encycl. Ind. Chem.

249–307 (2012).

4. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting.

Chem. Soc. Rev. 38, 253–278 (2009).

5. Turner, J. A., Williams, M. C. & Rajeshwar, K. Hydrogen Economy based on

Renewable Energy Sources. Electrochem. Soc. Interface 13, 24–30 (2004).

6. Umena, Y., Kawakami, K., Shen, J.-R. & Kamiya, N. Crystal structure of

oxygen-evolving photosystem II at a resolution of 1.9 Å. Nature. 473, 55–60

(2011).

7. Geletii, Y. V., Huang, Z., Hou, Y., Musaev, D. G., Lian, T., & Hill, C. L.

Homogeneous light-driven water oxidation catalyzed by a tetraruthenium

complex with all inorganic ligands. J. Am. Chem. Soc. 131, 7522–7523 (2009).

8. Kodama, T. & Gokon, N. Thermochemical Cycles for High-Temperature Solar

Hydrogen Production. Chem. Rev. 107, 4048–4077 (2007).

9. Sakai, K. & Ozawa, H. Homogeneous catalysis of platinum(II) complexes in

photochemical hydrogen production from water. Coord. Chem. Rev. 251, 2753–

2766 (2007).

10. Le Goff, A., Artero, V., Jousselme, B., Tran, P. D., Guillet, N., Métayé, R., Fihri,

A., Palacin, S. & Fontecave, M. From Hydrogenases to Noble Metal-Free

Catalytic Nanomaterials for H2 Production and Uptake. Science. 326, 1384–1387

(2009).

21

11. Kärkäs, M. D., Verho, O., Johnston, E. V & Åkermark, B. Artificial

Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev.

114, 11863–12001 (2013).

12. McDaniel, N. D., Coughlin, F. J., Tinker, L. L. & Bernhard, S. Cyclometalated

Iridium ( III ) Aquo Complexes : Efficient and Tunable Catalysts for the

Homogeneous Oxidation of Water. J. Am. Chem. Soc. 130, 210–217 (2008).

13. Mukhopadhyay, S., Mandal, S. K., Bhaduri, S. & Armstrong, W. H. Manganese

clusters with relevance to photosystem II. Chem. Rev. 104, 3981–4026 (2004).

14. Zhou, X., Li, F., Li, H., Zhang, B., Yu, F., & Sun, L. Photocatalytic Water

Oxidation by Molecular Assemblies Based on Cobalt Catalysts. ChemSusChem.

7, 2453–2456 (2014).

15. Panda, C., Debgupta, J., Díaz Díaz, D., Singh, K. K., Sen Gupta, S., & Dhar, B.

B. Homogeneous Photochemical Water Oxidation by Biuret-Modified

Fe-TAML: Evidence of FeV(O) Intermediate. J. Am. Chem. Soc. 136, 12273–

12282 (2014).

16. Zhang, T., Wang, C., Liu, S., Wang, J. L. & Lin, W. A Biomimetic Copper Water

Oxidation Catalyst with Low Overpotential. J. Am. Chem. Soc. 136, 273–281

(2014).

17. Duan, L., Tong, L., Xu, Y. & Sun, L. Visible light-driven water oxidation—from

molecular catalysts to photoelectrochemical cells. Energy Environ. Sci. 4, 3296–

3313 (2011).

18. Xu, Y., Fischer, A., Duan, L., Tong, L., Gabrielsson, E., Åkermark, B., & Sun, L.

Chemical and light-driven oxidation of water catalyzed by an efficient dinuclear

ruthenium complex. Angew. Chem. Int. Ed. 49, 8934–8937 (2010).

19. Duan, L., Bozoglian, F., Mandal, S., Stewart, B., Privalov, T., Llobet, A., & Sun,

L. A molecular ruthenium catalyst with water-oxidation activity comparable to

that of photosystem II. Nat. Chem. 4, 418–423 (2012).

22

20. Duan, L., Xu, Y., Gorlov, M., Tong, L., Andersson, S., & Sun, L. Chemical and

photochemical water oxidation catalyzed by mononuclear ruthenium complexes

with a negatively charged tridentate ligand. Chem. Eur. J. 16, 4659–4668 (2010).

21. An, J., Duan, L. & Sun, L. Ru complexes containing pyridine dicarboxylate

ligands: electronic effects on their catalytic activity toward water oxidation.

Faraday Discuss. 155, 267–275 (2012).

22. Duan, L., Araujo, C. M., Ahlquist, M. S. G. & Sun, L. Highly efficient and robust

molecular ruthenium catalysts for water oxidation. Proc. Natl. Acad. Sci. 109,

15584–15588 (2012).

23. Duan, L., Fischer, A., Xu, Y. & Sun, L. Isolated seven-coordinate Ru (IV) dimer

complex with [HOHOH]- bridging ligand as an intermediate for catalytic water

oxidation. J. Am. Chem. Soc. 131, 10397–10399 (2009).

24. Wang, L., Duan, L., Stewart, B., Pu, M., Liu, J., Privalov, T., & Sun, L. Towards

Controlling Water Oxidation Catalysis : Tunable Activity of Ruthenium

Complexes with Axial Imidazole / DMSO Ligands. J. Am. Chem. Soc. 134,

18868–18880 (2012).

25. Duan, L., Wang, L., Inge, A. & Fischer, A. Insights into Ru-Based Molecular

Water Oxidation Catalysts: Electronic and Noncovalent-Interaction Effects on

Their Catalytic Activities. Inorg. Chem. 52, 7844–7852 (2013).

26. Dulie, E., Devillers, M. & Marchand-brynaert, J. Novel Phosphinite-Ruthenium

( II ) Complexes Covalently Bound on Silica: Synthesis, Characterization, and

Catalytic Behavior versus Oxidation Reactions of Alcohols into Aldehydes.

Organometallics. 22, 804–811 (2003).

27. Casini, A., Gabbiani, C., Sorrentino, F., Rigobello, M. P., Bindoli, A., Geldbach,

T. J., Marrone, A., Re, N., Hartinger, C. G., Dyson, P. J. & Messori, L. Emerging

protein targets for anticancer metallodrugs: Inhibition of thioredoxin reductase

and cathepsin B by antitumor ruthenium(II)-arene compounds. J. Med. Chem. 51,

6773–6781 (2008).

23

28. Fischer, T. & Riedl, R. Strategic targeting of multiple water-mediated

interactions: A concise and rational structure-based design approach to potent and

selective MMP-13 inhibitors. ChemMedChem. 8, 1457–1461 (2013).

29. Kodama, K., Kobayashi, A. & Hirose, T. Synthesis and spectral properties of

ruthenium(II) complexes based on 2,2’-bipyridines modified by a perylene

chromophore. Tetrahedron Lett. 54, 5514–5517 (2013).

24

Supporting Information

Figure S1. 1H NMR spectrum of A

Figure S2. 1H NMR spectrum of B

25

Figure S3. 1H NMR spectrum of C

Figure S4. 1H NMR spectrum of D

NRu

N

N

N

OO

OO

P

P

O

O

O

OO

O

26

Figure S5. DPV data of complex A and C

Figure S6. DPV data of complex B and D

27

Figure S7. Oxygen evolving trace by catalysts A and C for TOF