Co

8.13 Water OxidationR Bofill, J Garcıa-Anton, L Escriche, and X Sala, Universitat Autonoma de Barcelona, Barcelona, SpainA Llobet, Universitat Autonoma de Barcelona, Barcelona, Spain; Ewha Womans University, Seoul, South Korea; Institute of ChemicalResearch of Catalonia (ICIQ), Tarragona, Spain

ã 2013 Elsevier Ltd. All rights reserved.

8.13.1 Introduction 5058.13.2 Proton-Coupled Electron Transfer and WO Thermodynamics 5068.13.3 O–O Bond Formation Mechanisms 5078.13.4 Molecular WOCs 5088.13.4.1 Ru-Based Polynuclear WOCs 5088.13.4.2 Ru-Based Mononuclear WOCs 5148.13.4.3 Ir-Based WOCs 5168.13.4.4 First-Row Transition Metals as WOCs 5188.13.5 Photoelectrochemically Driven WO Catalysis and First Examples of Water-Splitting Cells 5198.13.6 Conclusion 521Acknowledgments 521References 521

8.13.1 Introduction

The enormous and day-by-day anthropogenic increasing con-

sumption of fossil fuels together with the progressive decrease of

their global reserves1 have put forward the urgent need for a

cheap and sustainable energy source in order to keep the welfare

of our society in the near future. Furthermore, it is essential that

this new source of energy is clean and carbon free in order to

stop global warming, due to the increasing atmospheric CO2

concentration originated in fossil fuel combustion.

During the last 2400–3000 million years of evolution,

Nature has been harvesting sunlight as an energy source through

the photosynthetic processes carried out by green plants, algae,

and cyanobacteria. Throughout the last decade, an enormous

advance in the knowledge of the molecular machinery involved

in photosynthesis has taken place. Two families of electronically

coupled protein complexes, named Photosystem I (PSI) and

Photosystem II (PSII), are involved in photosynthesis. Basically,

during this process four protons and four electrons are removed

in PSII from two water molecules in a thermodynamically unfa-

vorable reaction (E0¼0.94 V vs. SSCE at pH1.0) thanks to the

sunlight energy absorbed by chlorophyll P680 (eqn [1]). This

process generates dioxygen and a gradient of electrons and pro-

tons that ends up with two equivalents of NADPH and three of

adenosine triphosphate (ATP), which constitute the required

reducing equivalents and energy needed for PSI to generate

carbohydrates from CO2 (eqn [2])2:

2H2Oþ 2NADPþ þ 3ADPþ 3Piþ 8hu !O2 þ 2NADPHþ 2Hþ þ 3ATP

[1]

CO2 þ 2NADPHþ 3ATPþ 2Hþ !1=n CH2Oð Þn þ 2NADPþ þ 3ADPþ 3PiþH2O

[2]

One of the most interesting processes occurring during

photosynthesis from a chemical viewpoint occurs at the

Oxygen Evolving Center (OEC) of PSII, where the thermody-

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-09777

namically uphill oxidation of water takes place in the dark in a

Mn4CaO5 cluster. Afterward, the released electrons are trans-

ferred to a TyrZO• radical (Tyr161), which is formed after

oxidative quenching of the excited P680* and in turn is stabi-

lized by the presence of a proximal His residue (His190).3 Later

on, these electrons flow from TyrZ through a channel

of electronic transport that consecutively involves P680, pheo-

phytin (Phe), and quinones A and B (QA, QB), until finally

reaching PSI.

Recently, the structure of the Mn4CaO5 cluster has been

solved at 1.9 A resolution (Figure 1), and it shows the coordi-

nation of five carboxylates from acidic amino acids of the

surroundings binding in a bidentate manner to the Mn and

Ca atoms. Additionally, four water molecules coordinate also

to the cluster, two of which to the Ca ion.4 The presence of

mono- and di-m-oxido ligands completes the first coordination

sphere. The anionic and sigma-donating nature of these

ligands is essential for the accessibility of the higher oxidation

states of the metal centers that will promote the oxygen–

oxygen bond formation, which in turn will finally lead to the

generation of dioxygen. The role of the Ca metal is a subject of

current discussion. It has been suggested that the Ca ion is not

only a structural cofactor, but that it could also play an impor-

tant role as a Lewis acid during the oxidation of water by

affecting the nucleophilic reactivity of its bounded water

molecules.5 In addition, recently, it has also been proposed

to strongly affect the electronic properties of the rest of the Mn

transition metals in the cluster based on related model

complexes.6 The detailed knowledge of how the Mn4CaO5

works at a molecular level is thus of paramount importance

from a biological perspective but is also important for the

design of low-molecular-weight functional analogues.

One of the potential solutions to the current energy crisis

based on fossil fuels is water splitting by sunlight:

2H2O ! O2 þ 2H2 DG� ¼ 113:5kcal mol�1 [3]

4-4.00821-4 505

506 Water Oxidation

From a redox point of view, this reaction can be divided

into water oxidation (WO) and proton reduction as indicated

in the following equations:

2H2O!O2þ4Hþþ4e� E0¼0:94V vs: SSCEð Þ at pH¼1:0 [4]

4Hþþ4e�!2H2 [5]

Incidentally, the WO reaction needed for water splitting is

identical to the one occurring at the OEC-PSII.

8.13.2 Proton-Coupled Electron Transfer andWO Thermodynamics

WO is a challenging task for a catalyst. There are two main

reasons for the difficulty: the first one is the large endothermi-

city of the reaction and the second one is the large molecular

complexity involved from a mechanistic point of view.

Recently, there have been important developments with

respect to the design of new transition metal complexes.

Those molecular complexes are characterized by different sets

of ligand architectures and by their nuclearity. These sets of

D1-D170

CP43-R357

W1O4

CP43-E354

D1-D61D1-E333

D1-H337

D1-H332

D1-D342Mn1

D1-E189

W4W3

W2Ca

D1-A344

O1O5

O2 Mn2Mn4

O3Mn3

Figure 1 x-Ray structure OEC-PSII at 1.8 A resolution. Reprinted withpermission from Macmillan Publishers Ltd: Nature, Umena, Y.;Kawakami, K.; Shen, J. R.; Kamiya, N. Nature 2011, 473, 55–60,copyright (2011).

Table 1 Redox potentials for the oxidation of water at pH 7.0 and 1.0 tog

Redox couple E� (V) (vs. SSCE)

pH¼7.0 pH¼1.0

�OHþ1Hþþ1e�!H2O 2.15 2.50HO–OHþ2Hþþ2e�!2H2O 1.13 1.48HO–O�þ3Hþþ3e�!2H2O 1.02 1.37O¼Oþ4Hþþ4e�!2H2O 0.58 0.94

aBO: O–O bond order.bn is the number of unpaired electrons.cfreq is the range of vibrational frequencies for the O–O bond in cm�1.ddist refers to the range of O–O bond distance in A.

water oxidation catalysts (WOCs) put forward the variety of

strategies that can be used to achieve this goal as well as the

numerous problems associated with this catalytic process.7

A fundamental piece of knowledge that is needed to further

develop this field is the understanding of the reaction mecha-

nism through which this reaction can proceed, since this

knowledge will help to design efficient and rugged catalysts.

Therefore, there is an urgent need to characterize reactive inter-

mediates as well as decomposition pathways to avoid them.

There are several issues that make mechanistic determinations

challenging and that have been holding back their deve-

lopment. One is the intrinsic complexities of the reaction,

where the catalyst is likely to cycle among five different oxi-

dation states, either metal or ligand based or both.8 This

imposes a requirement for transition metal complexes that

need to be sufficiently long lived to be able to perform the

reaction and also to be spectroscopically detectable. Another

fundamental problem is the unavoidable use of water as

solvent, which is a problematic issue due to the limited tem-

perature range at which reactions can be studied and due to its

high absorptivity. One more problem associated with this

reaction is the limited solubility of the catalysts or catalyst

precursors in water. Furthermore, the high thermodynamic

redox potential needed for WO permits the catalyst to oxidize

a broad range of organic and inorganic substrates, and thus

the presence of organic solvents can lead to undesired deacti-

vation pathways.9

The oxidation of water leads to a range of species depending

on the number of electrons removed, whose thermodynamics

are summarized in Table 1, together with interesting features of

the oxidized species. As can be observed in the table, the more

electrons transferred the lower the thermodynamic potential.

Thus a 1e� oxidation process has a prohibitive thermodynamic

barrier of 2.5 V versus SSCE at pH¼1.0, whereas the four-

electron transfer (ET), as it happens at the OEC-PSII,10 has

the lowest one (eqn [4]). This lowering of thermodynamics

contrasts with the increase in molecular complexity. In the 4e�

process, four O–H bonds from two water molecules have to be

broken and an O–O bond has to be formed. Thus, the poten-

tial transition metal complexes that can be considered as can-

didates to carry out this reaction in a catalytic manner are

required to deal with multiple ET processes accompanied also

with proton transfer management.

These requirements are met by Ru–OH2 polypyridyl com-

plexes discovered by Thomas J. Meyer’s group about three

decades ago.11 The capacity of Ru-aqua polypyridyl complexes

ether with remarkable features of the water-oxidized species

H2O oxidized species

BOa nb freqc distd

1 0 740–850 1.491.5 1 1100–1150 1.282 2 1560 1.21

Water Oxidation 507

to lose protons and electrons and easily reach higher oxidation

states is exemplified in the following equations

(L5¼polypyridylic ligand):

L5RuII�OH2 ###/1###

�Hþ�e�

þHþþe�L5Ru

III �OH###/1###�Hþ�e�

þHþþe�L5Ru

IV ¼ O [6]

The higher oxidation states are accessible within a narrow

potential range mainly because of the s-donating character of

the oxide group. From a mechanistic perspective, the simulta-

neous loss of protons and electrons precludes an otherwise

highly destabilized scenario with highly charged species.

A proton-coupled electron transfer (PCET) type of process pro-

vides energetically reasonable reaction pathways that avoid

high-energy intermediates. For instance, for the compropor-

tion reaction of [LRuII–OH2]2þ(L¼(bpy)2(py), bpy is 2,20-

bipyridine and py is pyridine) and [LRuIV¼O]2þ to give two

molecules of [LRuIII–OH]2þ, the energy penalty for a stepwise

process with regard to the PCET is higher than 12.6 kcal mol�1

for the ET-PT process and higher than 13.6 kcal mol�1 for

the PT-ET process whereas the concerted pathway is downhill

by �2.5 kcal mol�1. Furthermore, the energy of activation

for the concerted electron–proton transfer (EPT) process is

10.1 kcal mol�1 and is lower than the thermodynamic value

of any of the stepwise pathways.12

Recently, one more oxidation process has been found in

mononuclear complexes that involves formally the oxidation

of Ru(IV) to Ru(V),13 as indicated in the following equation:

L5RuV ¼ Oþ 1e� ! L5Ru

IV ¼ O [7]

A complex that displays this behavior is [Ru(tpm)(bpy)

(H2O)]2þ, 1, (tpm is the tridentate facial ligand tris-(1-pyrazolyl)

methane),14 whose structure and Pourbaix diagram are shown in

Figure 2. The latter is a graphical representation of the thermo-

dynamic parameters of the species derived from the loss of pro-

tons and electrons from the initial Ru(II)–OH2 complex.

For purposes of simplicity and keeping track with electron

counting, formal metal oxidation states will be used in the

1.6

1.2

0.8

0.4

E�

0

-0.4-2 0

RuIII−OH2

2+

RuN

N

OH2N

N

NN

N

N

H

Figure 2 Left, structure of [Ru(tpm)(bpy)(H2O)]2þ, 1. Right, Pourbaix diag

redox active atoms. However, it has to be borne in mind that

the oxidation can take place both at the metal center and at the

oxygen atom. Their relative electron distribution will depend

on the rest of the auxiliary ligands. As an example, for the Ru–

OH2 complexes, the complete bond description will be repre-

sented as a combination of two extreme resonance forms, such

as [Ru(IV)¼O$Ru(III)–O•].

8.13.3 O–O Bond Formation Mechanisms

The formation of an oxygen–oxygen bond promoted by tran-

sition metal complexes can be classified taking into account

whether an unbound free water molecule participates or not in

the formation of the aforementioned bond. Formally, from

this perspective two possibilities exist: the solvent water nucle-

ophilic attack (WNA) mechanism and the interaction of 2 M-O

entities (I2M), both displayed in Scheme 1.

WNA. As written in Scheme 1, there is a 4-ET demand,

which is quite stringent for a mononuclear complex. A solution

to that is to share the burden with more metal centers provided

there is a bridging ligand (BL) that couples them electronically

and thus allows generating a cooperative effect. Another option

for a metal complex is to cycle up and down in similar oxida-

tion states of different species, as will be shown in the follow-

ing section. It is interesting to point out here that both WNA

and I2M mechanisms have been proposed for the OEC-

PSII.5b,15 The WNA is the inverse reaction that is proposed to

occur in the reduction of dioxygen in the heme-iron protein

Cyt-P450.16 Furthermore, the existence of this mechanism has

been recently shown to occur in a Mn-porphyrin model

complex.17 As will be discussed in the following section, the

nature of the complex can be mononuclear or polynuclear and

it can have one or several Ru–O groups. In the case of the latter,

the job of each Ru–O group will be radically different. While

one of them will be responsible for the O–O bond formation,

the other/s center/s will be responsible for facilitating electron

2

RuII−OH2

RuIII−

RuII−OH

OH

RuIV= 0

RuV= 0

4 6 8 10 12 14pH

ram for 1. E� values are referred to the SSCE electrode.

508 Water Oxidation

trafficking so that the 4e� acceptance process can be shared

among the different metal centers.

I2M. In Scheme 1, this reaction is described as a reductive

elimination but depending on the oxidation states of both the

metal center and the oxygen atoms, the O–O bond forming

step could also be a radical–radical coupling reaction. As in the

previous mechanism, here the nuclearity of the complex can

also be variable.

Additionally, the organic auxiliary ligands can potentially

participate in the electron trafficking process related to the WO

reaction and also in the O–O bond formation step. When this

happens, the ligands are generally termed redox noninnocent

ligands. Examples of these cases will be discussed in the

following section.

8.13.4 Molecular WOCs

This section contains a description of the most significant

structurally characterized WOCs, or their precursors, which

have been described in the literature from the pioneering

work of T. J. Meyer, with the so-called ‘blue dimer’ dinuclear

Ru complex, until the present. Besides their molecular struc-

tures, the most interesting features of these complexes will be

illustrated. The largest number of catalysts studied today are

WNA I2M

O(n + 4)+

M

O

H

H

Mn+ + O–O + 2H+

O M(n + 2)+

O(n + 2)+

M

2Mn+ + O–O

OM

O

H

H O M

OM

Scheme 1 Potential O–O bond formation pathways promoted bytransition metal complexes.

N

ORu

N

N

4+

N

N

Ru

N

N

N OH2

H2O

Figure 3 Drawn structures of dinuclear Ru complexes 2a (left) and 2b (righ

based on Ru complexes and we have classified them depending

on their nuclearity. For complexes that are dinuclear or of

higher nuclearity, the bridging ligand between metal centers

will play a key role, both from the point of view of influencing

the electronic coupling between metal centers and also from

the capacity of the bridging ligand to generate a particular

spatial disposition of the active sites.

8.13.4.1 Ru-Based Polynuclear WOCs

In 1982, Meyer’s group reported the first, molecular, well-

characterized dinuclear Ru complex cis, cis - [(bpy)2(H2O)Ru

(m-O)Ru(H2O)(bpy)2]4þ, 2a (Figure 3(a)), able to oxidize

water to dioxygen, which is also called ‘blue dimer’ because

of its absorption properties at pH 1.0.18 This complex showed a

turnover frequency (TOF) as low as 0.24 min�1 and a turnover

number (TON) of 13.2 when using Ce(IV) as a sacrificial

oxidizing equivalent at pH 1.0.19 The reductive cleavage of

the oxido bridge is one of the deactivation pathways that is

suffered by this catalyst.20

From a mechanistic perspective, this dimer has been studied

experimentally by the groups of Meyer21–23 and Hurst,24 dis-

playing a high degree of complexity. Meyer and coworkers

reported a detailed analysis of the potential mechanism in an

Inorganic Chemistry Forum entitled ‘Making Oxygen,’25 which

has been further extended in other related and more recent

papers.26 As concluding remarks, Meyer proposes that the

catalytically active species [(bpy)2(O)RuVORuV(O)(bpy)2]4þ,

{(O)RuVORuV(O)}4þ, is formed upon PCET oxidation of the

initial {(H2O)RuIIIORuIII(OH2)}4þ compound with Ce(IV).

The rapid reaction of the RuVORuV species with water

via WNA creates a peroxidic intermediate, {(HO)

RuIVORuIV(OOH)}4þ, that ends up forming the O–O bond, as

can be seen in Scheme 2. In addition, under excess of Ce(IV) the

peroxidic intermediate can be further oxidized to {(HO)

RuIVORuV(OOH)}5þ, which finally releases dioxygen very

quickly. The performance of the blue dimer 2b has also been

studied electrochemically with In2O3:Sn(ITO)-modified elec-

trodes containing [Ru(4,40 - ((OH)2P(O)CH2)2-bpy)(bpy)2]2þ

attached at the surface. In this case, it has been shown that this

surface binding dramatically increases the rate of surface oxida-

tion of the blue dimer and induces WO catalysis.27

For the blue dimer, Hurst’s group28 proposes different

mechanisms than the ones just described. In particular they

propose that, besides the WNA mechanism described above,

there is a part of the oxidation process that involves the bpy

ligands in the formation of the O–O bond, as is depicted in

Scheme 3. This mechanism is proposed to occur when the

N

NN Ru

N

N

N

3+

NRu N

N

NOH2 H2O

t).

vRu

O IVRu

OHH

N

N

HO

Ru

OH

Ru

OH2

O–OOH2OH2

Ru RuN

NO IIIIII

OH2

RuOIII III

N

N

HHO

O

OIV IVRu

OHHHO

HO H

N

N

H2O

O

OVRu

O O

Ru

N

N

V

H OH2O

v vRu

O O

4H+

4e-

RuO

N

N

H

Scheme 3 Ligand-based mechanism proposed for the dinuclear WOC 2a.

RuIII–O–RuIII

RuIII–O–RuIV

OH2 OH

RuIV–O–RuIV

OH OH

RuV–O–RuIV

O O

RuIV–O–RuIV

OH OOH

RuV–O–RuV

O ORuIV–O–RuV

OH OOH

O−O

2H+, e-

H2O

O–O + H+

H+,e-

H+,e-

e-

e-

H2O

H2O

OH2 OH2

Scheme 2 Metal-based WNA mechanism proposed for the dinuclear WOC 2a.

Water Oxidation 509

dimer reaches its RuV¼O form. At this point it is proposed that

a water solvent molecule adds to one of the pyridyl rings of a

bpy, forming a coordinated bpy radical. This radical

then further adds one more water molecule to form a cis-

dihydroxyl-bpy, which is responsible for the formation of the

O–O bond. Then finally this peroxide intermediate produces

molecular oxygen and the initial Ru complex, closing the

catalytic cycle.

In 2004, more than two decades after the synthesis of 2a, a

new Ru WOC was reported containing the dinucleating

tetradentate bridging ligand Hbpp (3,5-bis-(2-pyridyl)pyra-

zole) and the tridentate meridional 2,20:60,200-terpyridineligand (trpy), in, in-{[RuII(trpy)(H2O)]2(m-bpp)}

3þ (in,in-

Ru–Hbpp) complex 2b (Figure 3).29 The flexible m-oxidobridge of the ‘blue dimer’ was replaced by the anionic and

more rigid bpp� and two bpy were replaced by trpy ligands.

This coordination environment forced a close disposition of

the O atoms of the two aqua groups producing a through-space

supramolecular interaction.30 Addition of excess Ce(IV) to this

complex generates dioxygen efficiently, showing TON and TOF

510 Water Oxidation

values of 512 and 0.78 min�1, respectively, under optimized

conditions.31

Kinetic analysis and 18O labeling studies were then

employed in order to study the WO mechanism taking place

for this complex.31c,32 The {(H2O)RuII–RuII(OH2)}3þ species

is sequentially oxidized by four 1e� processes with Ce(IV)

losing also 4Hþ, up to the IV,IV oxidation state {(O)RuIV–

RuIV(O)}3þ. At this stage, intramolecular O–O bond

formation takes places generating a m-1,2-peroxide species,

{RuIII–(OO)–RuIIIO}3þ, which later on is followed by the

formation of a hydroperoxidic intermediate that finally evolves

oxygen,32 as depicted in Scheme 4. This intramolecular mech-

anistic proposal is further supported by a thorough theoretical

analysis of intermediates and transition states based on density

functional theory (DFT) and CASPT2 calculations.31c More-

over, 18O labeling data together with the first order kinetics

observed for the formation of the intermediate discards

the potential bimolecular nature of the process as well as the

WNA mechanism.

A homolog of catalyst 2b was anchored onto solid sup-

ports in order to get a deeper insight into the potential deac-

tivation pathways that lead to its decomposition and also to

demonstrate the viability of the reaction in the solid state.

From a molecular engineering point of view, this could also

facilitate the introduction of the catalyst into more complex

devices. A pyrrole-substituted derivative of the in,in-Ru–Hbpp

complex [Ru2(m-bpp)(m-OAc)(t-trpy)2]2þ, 2c, (t-trpy¼40-

(p-pyrrolylmethylphenyl)-2,20:60,200-terpyridine) was pre-

pared and anodically electropolymerized. Copolymers were

also generated when monomer 2c was electropolymerized in

the presence of a redox nonactive cobalt carborane complex,

N N NRu

+H2O

O–O

OO

N N NIVRu

O O

N N N NRuRuII II

HH H

HOO

-4 e-

-4 H+

NRuIV

Ru

N

Scheme 4 I2M type of mechanism proposed for the dinuclear complex 2b.

Co-CBN, known to prevent polypyrrole backbone oxidation.33

These new copolymeric materials were generated in the surface

of two different conducting electrodes: vitreous carbon sponge

(VCS) and fluorine-doped tin oxide (FTO) (Figure 4).34 The

new hybrid materials were shown to maintain the intrinsic

electronic properties of their molecular analogue 2b and drasti-

cally improved their catalytic performance by minimizing cata-

lyst–catalyst deactivation pathways. The strategy combining site

isolation and dilution was especially positive: FTO/poly-(2c-Co-

CBN) was capable of oxidizing water to dioxygen with a TON of

250, the highest value reported in the solid state with a chemical

oxidant at that time. A TON value of 120 was obtained after

controlled potential electrolysis at 1.41 V versus the standard

hydrogen electrode (SHE) (TOF of 1.44 min�1).

TiO2 was also used as an oxidatively rugged solid support

to anchor 2b analogues. For this purpose, the carboxylate-

modified 2d derivative in,in-{[RuII(trpy)(H2O)]2(m -bpp-Ra)}3þ

(Hbpp-Ra¼4-((3,5-di(pyridine-2-yl)-1H-pyrazol-4-yl)methyl))

benzoic acid) was synthesized (Figure 5). Compound 2d was

anchored into TiO2-rutile in MeCN. Upon activation of this spe-

cies with Ce(IV) at pH 1.0, coevolution of O2 and CO2 was

observed by on-line mass spectroscopy (MS) together with the

leaching of the catalyst from the solid support.9a,35–37 The oxida-

tive interaction of high-valent Ru¼O species with the benzylic

methylene group of the substituted bpp� ligand has been

suggested as the starting event for CO2 generation. The same

oxidative decomposition process also occurs in related Rumono-

nuclear and tetranuclear complexes.38,39

Finally, two new dinuclear Ru–Hbpp complexes containing

the bis(2-bis(2-pyridyl)ethyl)amine (bpea) ligand have also

been recently synthesized and thoroughly characterized.40

NIIIRu RuIII

+2H2O

H2O

HHO OORu Ru

NN N N

2

Ru Ru

N N N N

HO O

OH

O O

N N N

III III

N RuOH2 H2O

Ru N

N

NNN

N N

3+

N

N

OHO

Figure 5 Structure of the dinuclear Ru complex 2d, containing acarboxylic acid functionalization.

N

N RuN N N N

NRu

HH

Electrode

Polypyrrole backbone

RuIIRuII

RuIIIIRu

IIRuIIRu

OO HHHHHH

H H HH

OO

O O

OH2 H2ON N NN

N

3+

Figure 4 Top, structure of the Ru-bis-aqua complex 2c containing pyrrole functionalities. Bottom: schematic drawing of the polymeric materialFTO/poly-2c.

Water Oxidation 511

These complexes possess the general formula trans, fac -{[RunX

(bpea)]2(m -bpp)}mþ (for X¼Cl, n¼ II, m¼1, 2e; for X¼OH,

n¼ III, m¼3, 2f; Figure 6), showing a novel trans-disposition

of the Ru–X groups. The main purpose of this work was to

better understand the effects of the auxiliary ligands on theWO

reactivity as well as to generate new arrangements around the

catalytic Ru centers. This new ligand disposition could impede

the existence of an intramolecular WOmechanism, such as the

one described for 2b,32e and instead could constitute a prom-

ising way of seeking a putative WNA or a bimolecular mecha-

nism. With regard to their x-ray structure, although the flexible

bpea ligand can potentially act either as a meridional or a facial

ligand, in the present work it only acts in a facial way, which

incidentally is the most habitual mode of coordination of bpea

in Ru complexes.40 In addition, although the facial bpea ligand

could potentially engender the cis and trans isomers, only the

trans one is encountered, probably because of the steric repul-

sions between two adjacent bpea ligands that the cis confor-

mation would impose.

Yet, there is still steric repulsion between the bpea ligands in

the trans conformation, and in order to minimize it the bpp�

adopts a nonplanar disposition, which provokes the move-

ment of one Ru atom above and the other below the imaginary

equatorial plane of the complex. Interestingly, for the chloride

complex (2e) p-stacking interactions exist between the pyridyl

rings of each bpea ligand as well as between the pyridyl groups

of neighboring bpea ligands (through space intrasupramole-

cular interactions, Figure 6). When 2e is crystallized with PF6�,

the unit cell contains the two enantiomers of the complex,

which via p-stacking interactions constitute dimers of dimers

(Figure 6). Additionally, the trans nature of this complex in

solution has been confirmed by nuclear magnetic resonance

(NMR), as significant downfield shifts have been observed for

protons that are close to chloride ligands (that would never

occur if a cis disposition existed). Also, NMR has confirmed the

intramolecular p-stacking interactions within the bpea ligands.

Concerning the structure of trans, fac -{[RuIIIOH(bpea)]2(m-bpp)}3þ (2f), it is very similar to the chloride analogue 2e.

However, no through space p-stacking interactions exist in the

solid phase for the hydroxide complex.

The redox properties of 2e and 2f have also been studied and

compared to analogues possessing the trpy ligand instead of

bpea. From these studies, it becomes clear that the higher

s-donation character and lower p-acceptor capacity of the

bpea ligand with respect to trpy provoke a cathodic shift of all

the E1/2 values corresponding to the bpea-containing complexes

and would also be responsible for a higher RuII to RuIII oxida-

tion tendency of these complexes compared to the trpy ones.

This could explain why the bpea-hydroxide complex is obtained

with a 3þ oxidation state. Finally, 2f has been proven to be

catalytically active toward WO at pH 1.0 in the presence of

100 mM Ce(IV), although its efficiency in terms of TON and

rate is lower than its trpy analogue 2b in, in -{[RuII(trpy)

(H2O)]2(m-bpp)}3þ (TON¼11.1 and vi¼8.0 nmol oxygen/s

for 2f vs. TON¼18.0 and vi¼90.0 nmol oxygen/s for 2b at

1 mM catalyst concentration). The differences observed may

be due to different WO mechanisms: while for 2b an

Figure 7 Ortep plot of one of the two enantiomers of trans-{[RuII(H2O)(tpym)]2(m-bpp)}

3þ, 2h. Color code: Ru, violet; N, blue; C, black; O, red.H atoms have been omitted for clarity purposes.

Figure 6 Left, Ortep plot of one of the two enantiomers of trans,fac-{[RuIICl(bpea)]2(m-bpp)}þ, 2e. Color code: Ru, violet; N, blue; C, black; Cl, green.

H atoms have been omitted for clarity purposes. Right, view of the packing structure of the same complex in the presence of PF6� showing the

p-stacking network.

512 Water Oxidation

intramolecular mechanism takes place through the space inter-

action of the two Ru¼O active groups,31c this mechanism is

not possible for the analogue 2f because both putative Ru¼O

groups generated upon achieving higher oxidation states are in

trans disposition.

In order to get further insight into the aspects discussed

above, in a recent work analogues of 2e/f were prepared,

where the bpea ligand has been replaced by the bulkier facial

tpym ligand (tpym¼ tris-(2-pyridyl)-methane), thus gene-

rating the complexes trans-([RuIIX(tpym)]2(m-bpp))mþ (for

X¼Cl, m¼1, 2g; for X¼H2O, m¼3, 2h; Figure 7).41 Analo-

gously as observed for 2e, an inverse relative orientation of the

bpp� pyridyl groups places the Ru atoms above and below the

plane of the pyrazolate moiety of 2h, thus generating two

helical enantiomers in the crystal structure, and p-stackinginteractions exist between the pyridyl groups of neighbor

tpym ligands (Figure 7). Furthermore, because of the higher

sterically demanding character of the tpym ligand with respect

to bpea, 2h shows a RuO–ORu torsion angle of 133.9�, under-lining a more significant distortion of the octahedral symmetry

of both Ru centers than that observed for 2e.

When comparing the redox properties of the tpym-

containing complexes with their respective bpea counterparts

(2g vs. 2e and 2h vs. 2f), the lower s-donating character and

higher p-accepting character of tpym compared to bpea pro-

vokes an anodic shift of all redox potentials.41 Interestingly, for

2h one more wave is observed at 1.52 V, which is attributed

to the IV,IV–IV,III couple and is associated with an electroca-

talytic current linked to the oxidation of water into dioxygen.

Effectively, the addition of 100 mM Ce(IV) at pH 1.0 to a

1.5 mM solution of 2h yields a TON of 9.6 and an initial

TOF of 31�10�3 s�1. Under identical conditions, 2f and 2b

give a TOFi of 3.3�10�3 s�1 and 49�10�3 s�1, respectively,

thus constituting another good example of how the electronic

modification of Ru dinuclear complexes affects the initial oxy-

gen formation rates. Additionally, and most importantly, in

this work we unambiguously demonstrated from 18O- labeling

experiments the mechanism of oxygen formation for 2h, prov-

ing that the O–O bond generation takes place by the interac-

tion of two Ru–O units of two independent catalyst molecules

(bimolecular I2M mechanism).41 Again, this mechanism is in

sharp contrast to the intramolecular I2Mmechanism described

for its trpy analogue 2b.31c

In short, these two last works40,41 show the dramatic influ-

ence of the ligand disposition (either mer or fac) on the elec-

tronic and catalytic properties of dinuclear Ru complexes,

including the molecular mechanism involved in WO.

The strategy of bridging two Ru metal centers by a rigid

bridging ligand was also used by the groups of Thummel and

Sun (Figure 8). A series of symmetrical complexes of the gen-

eral formula trans, trans - [Ru2(m-Cl)(m-binapypyr)(4-R -py)4]

3þ, 3, (binapypyr¼3,6-bis(6-(1,8-naphthyridin-2-yl)

pyridin-2-yl)pyridazine; R¼H, Me, NH2, OCH3, etc.) were

reported using binapypyr as a bridging ligand. The complex

with R¼�OCH3 exhibited the best catalytic performance

with TON values of 689.42 Higher TN values, up to 1690,

were later on obtained by Sun and coworkers using 6,60-(pyridazine-3,6-diyl)dipicolinate (pdd) as a bridging ligand

where the terminal naphthyridyl moieties of binapypyr

are replaced by anionic carboxylates. Surprisingly, complex

[Ru2(m-pdd)(4-Me -py)6]2þ, 4a (Figure 8), is obtained where

one of the carbon atoms of the pyridazine group coordinates

the Ru metal and one of the pyridazinic N remains

uncoordinated.43 Further modification of the system by repla-

cing the central pyridazine by a phthalazine moiety to get

the 6,60-(phthalazine-1,4-diyl)dipicolinate (phdd) ligands,

allows blocking the previous organometallic bond and thus

the expected dinuclear Ru complex, [Ru2(m -phdd)(m -Cl)(4 -Me-py)4]

2þ, 4b (Figure 8), is obtained, giving TON and

TOF values of 10 400 and 72 min�1 respectively.44

R R3+

4+

+

N

N N

N N NRu

O

NN

N

NRu

OO

N

N

ON

NRuNN

ClRu

N

N

N

NN N

R

N

NO

O RuCl

N NRu

Ru

N

N

NN

N

NN

Ru

N

NClCl

N5b

10-

H2O

O

O

O

OHO

Ru RuO

POM

H2O OH2 6b

Ru Ru

OH

O

OO

O

OPOM

OH2

O

O

N

O

N

N

O

N

4b

+

4aR

3

Figure 8 Drawn structures of Ru polynuclear complexes 3–6.

Water Oxidation 513

The direct deposit of a dinuclear WOC onto a solid surface

(adsorbed mainly through van der Waals interactions) was

reported by Tanaka and coworkers more than 10 years ago by

immobilizing their dinuclear bis-hydroxide complex [Ru2(OH)2(3,6- tBu2sq)2(m-btpyan)]

2þ, 5a (3,6- tBu2sq¼3,6-di- tert -

butyl -1,2 -semiquinone; btpyan¼1,8-bis(2,20 :60200) - terpyri-dylanthracene) into an indium tin oxide (ITO) electrode.45 This

hybrid material was able to generate molecular oxygen with an

outstanding TON of 33 500 (phosphate buffer, pH 4) at an

applied potential of 1.7 V versus Ag/AgCl for 40 h. Very recently,

the same group has reported a related Ru–Cl complex replacing

the cathechol ligands by bpy, {[Ru(Cl)(bpy)]2(m-btpyan)}2þ,

5b (Figure 8), andmanaged to get a crystal structure.46 Complex

5b upon treatment with Ce(IV) in acidic solution generates

molecular oxygen up to 400 TON. It is proposed that the O–O

bond formation takes places in an intramolecular fashion.

The first POM complex capable of oxidizing water to dioxy-

gen electrochemically was reported by Shannon and coworkers

with a molecular formula of [Zn2Ru2(OH2)2(ZnW9O34)2]14�,

6a.47 They showed that, for this particular complex, the replace-

ment of the Ru metals by Zn totally inhibited the WO capacity

of the molecule. Later on and simultaneously, the Bonchio

and Hill groups independently reported in 2008 the synthesis

of a tetranuclear Ru complex containing a dinucleating

tetradentate polyoxometalate ligand, [Ru4O4(OH)2(H2O)4(g - SiW10O36)2]

10�, 6b (POM¼g-SiW10O36, Figure 8).48 The

geometry of 6b consists of a central adamantane {Ru4O6} unit

where the four Ru atoms are situated in a tetrahedral disposi-

tion alternated with O atoms, which in turn are situated in the

vertex of a hypothetical octahedron. Then, the POMunits act as

tetradentate bridging ligands between two Ru centers and

finally each Ru atom completes its octahedral coordination

514 Water Oxidation

with a terminal aqua ligand. This catalyst is very stable against

degradation and it is reported that its TON is limited by the

availability of Ce(IV). Its TOF value is 7.5 min�1. The high

stability of this complex is attributed to the absence of organic

ligands that avoids the putative existence of bimolecular deac-

tivation pathways. The mechanistic pathway of 6b was

reported in 2009 including a DFT study. In this work, it is

proposed that the Ru4 core is sequentially oxidized by 4e�

and 4Hþ as indicated in the following equation:

RuIV4 OH2ð Þ4

� �10� � 4Hþ � 4e� ! RuV4 OHð Þ4

� �10�[8]

Once the four Ru atoms reach formally oxidation state V, one

of the RuV–OH groups suffers a WNA to generate a RuIII–OOH

species that finally evolves dioxygen.49 Recently, ab initio DFT

calculations by Piccinin and Fabris50 have shown that the free

energy difference between the initial and final oxidation states

are significantly lower than the thermodynamic limit for WO.

This might suggest that either the DFT model does not ade-

quately describe the electronic properties of this complex or

that oxidation states higher than RuV or oxidized intermediates

such as RuIV–OO are involved. Very recently, Bonchio and

coworkers have reported the electrostatic immobilization of

the all-inorganic and robust Ru-POM WOC 6b into modified

ITO electrodes.51–53 Functionalized multiple- and single-walled

carbon nanotubes (MWCNTs and SWCNTs) bearing polycatio-

nic dendrimeric substituents were used in order to improve the

electrical contact between complex 6b and the electrode. The

resulting ITO nano-hybrid material is able to electrocatalytically

oxidize water at pH 7 with a maximum TOF of 5.1 min�1 and at

relatively low overpotentials (0.35–0.60 V vs. SHE).

8.13.4.2 Ru-Based Mononuclear WOCs

In 2005, Thummel’s group reported for the first time the

capacity of mononuclear Ru complexes to act as WOCs.42a

They prepared a family of Ru–OH2 complexes based on the

tridentate N3 meridional ligand 2,20-(4-(tert-butyl)pyridine-2,6-diyl)bis(1,8-naphthyridine) (tpn) of the general formula

trans - [Ru(tpn)(4-R-py)2(H2O)]2þ, (R¼Me, 10) with various

substituted pyridines. In these complexes, the aqua ligand was

situated in the same plane as tpn, as can be seen in Figure 9,

and with the axial coordination positions occupied by the

pyridines. Later on, in 2008, Thummel et al. further expanded

the number of active Ru-mononuclear WOCs with a N6 type of

coordination, as is the case of complex [Ru(trpy)(4-Me-

py)3]2þ, 11a,54 with TONs ranging from 20 to 1170. In paral-

lel, Meyer’s group offered a mechanistic explanation for the

behavior of mononuclear N5Ru–OH2 complexes as WOCs

based on complexes such as [Ru(trpy)(bpm)(OH2)]2þ, 7,

(bpm is 2,20-bipyrimidine) and [Ru(trpy)(bpz)(OH2)]2þ, 8,

(bpz is 2,20-bipyrazine) (Figure 9).13,55 It is proposed that

these complexes can lose two protons and two electrons to

reach oxidation state IV, and that they can further lose one

more electron to reach the highly reactive oxidation state V.

Then, Ru(V) suffers a nucleophilic attack of a solvent water

molecule, forming a {RuIIIN5(OOH)} intermediate that then

undergoes a rapid one electron oxidation accompanied by a

proton loss to form a {RuIVN5(OO)} derivative, as depicted in

Scheme 5. Finally, this Ru(IV) peroxide complex evolves O2

to regenerate the initial Ru(II) complex. Alternatively,

the {RuIVN5(OO)} intermediate can be further oxidized to

{RuVN5(OO)}, which then releases O2 and generates a

(RuIII–OH) species. A thorough mechanistic description of the

WO pathways in similar lines but more complete has been

recently reported for complex 10 by Polyanski and Fujita.56

Additionally, Sakai and Berlinguette and coworkers57 have also

lately reported the WO mechanism of related mononuclear [Ru

(trpy)(bpy)(OH2)]2þ derivatives, 9, highlighting its dependence

on the electronic characteristics of the bpy substituents.58 Llobet

and coworkers have also reported newmononuclear isomeric Ru

complexes in- and out - [Ru(trpy)(bpp)(OH2)]2þ (in-14 and out-

15), where the bpp– ligand acts in a chelate manner and have put

forward the different reactivity of these isomeric species asWOCs.

Further, they have also reported the performance of their ana-

logues, replacing the Hbpp by the 2-(5-phenyl-1H-pyrazol-3-yl)

pyridine (H3p) ligand.59

Due to the straightforward and versatile synthesis of mono-

nuclear species, which contrasts with the much more demand-

ing preparation of their dinuclear counterparts, dozens of these

complexes have been recently tested as WOCs. Especially inter-

esting are the complexes [Ru(bdc)(4-Me-py)2], 12a, (bdc is

(2,20-bipyridine)-6,60-dicarboxylate) and [Ru(pdc)(4-Me-

py)3], 13, (pdc is pyridine-2,6-dicarboxylate) reported by Sun

and coworkers, which contain anionic carboxylate ligands

(Figure 9).60 These {RuIIN4O2} WOCs achieve spectacular

TON and TOF values of 2000 and 2500 min�1 for 13 and

550 and 13.8 min�1 for 12a. This impressive performance is

ascribed to the strong s-donating effect of the anionic carbox-

ylate ligands that allow reaching higher oxidation states

easily.61 Mechanistically, it is proposed that complex 12a can

easily reach oxidation state V, that undergoes a bimolecular

I2M mechanism upon dimerization of the {RuV¼O} species

to form a peroxo species {RuIV–OO–RuIV}, which under stoi-

chiometric conditions evolves oxygen. However, under cata-

lytic conditions it can be further oxidized to {RuIV–OO–RuV}

or to its superoxide analogue, which finally can also release

dioxygen62,63 (Scheme 6). The replacement of 4-Me-py by

isoquinoline in 12a, to obtain [Ru(bdc)(isoquinoline)2],

12b, generates the best WOCs ever reported approaching the

performance of the OEC-PSII with TOF higher than 300 s�1,64

thanks to a p–p interaction of the isoquinoline ligands in the

formation of the peroxide dimer intermediate.

In sharp contrast, it has been observed that in complex [Ru

(phdc)(4-Me-py)2], 12b, (phdc is 1,10-phenanthroline-2,9-

dicarboxylate), the replacement of the bpy moiety of 12a by a

phenanthroline moiety shifts the mechanism from I2M to

WNA.65 This result thus puts forward how subtle changes in

ligand design can dramatically influence the reactivity of a

WOC. In this line we have recently reported a series of mono-

nuclear Ru complexes with triazolylidene carbenes (tac), [Ru

(tac)(MeCN)4]2þ, 11b, where the carbene is coordinated in an

abnormal manner, which carry out the oxidation of water to

oxygen in an extremely fast manner, at a TOF close to

120 min�1.66 Interestingly, related Ru complexes with carbenes

coordinating in a normal fashion also act as WOCs, but their

performance is very poor. Another example of the subtle influ-

ence of electronic and steric effects over the WO capacity of Ru

catalysts has been recently reported by Yagi and coworkers.67

N

N

2+

N

N

N

N7

N

Ru

OH2

N

NN

N

N

NN

N

Me

N

NMe

N

N N

11b11aMe

Me

NN

N

2+ 2+2+

N

NN

N N

NO

O OO

O

O

O

O

N

N

N

N N

N

N

N

N

N

NN N

NN

H2O

H2ON

Ru

N

16

N

N

2+

HX

N

OH2

RuNN

N

N NNHX

14 15

N

2+OH2

11c 12a13

2+ 2+

H

HO

10

NNRu

Ru

RuH2O

Ru Ru

Ru Ru

N

2+

N

N

N

N8

N

Ru

OH2 N

2+

N

N

N

R2

R2

R1

OH2

9

NRu

Figure 9 Drawn structures of Ru mononuclear complexes 7–16.

Water Oxidation 515

They have prepared and isolated the two geometrical isomers

of the complex [Ru(trpy)(pnaph)(OH2)]2þ, 11c (pnaph is

2-(pyridin-2-yl)-1,8-naphthyridine), Figure 9. Whereas the iso-

mer that has the naphthyridine moiety trans to the Ru–OH2

group is highly active (TON higher than 50), the other isomer

with the py moiety trans to Ru–OH2 is practically inactive.

A mononuclear Ru–OH2 complex that contains a pentaden-

tate ligand based on a heteroundecatungstate containing addi-

tional Si and germanium, [RuIII(H2O)SiW11O39]5� and

[RuIII(H2O)GeW11O39]5�, has also been reported to act as

WOCs. Mechanistic investigations suggest that the O–O bond

formation takes place through a WNA pathway.68 Finally, the

O

RuVNN

N

-H+

H2O

O H

NN

N

-2e-

-2H+

-H+

-H+

-e-

-e-

-e--e-

O

HONNRuIIIN

NN

H2OO

O

RuV N

NN

N

O–O + H+

N

NNRuIV

NN

N

O

O

H2O

O–O

OH2NNRuII

NN

N

NNRuIV

NN

N

RuIII

O

NN

NN

Scheme 5 General WNA mechanism proposed for mononuclear Ru complexes.

2Ru(IV)

2Ru(V) 2Ru(II) 2Ru(III)

OH2 O2

O2

+2H2O

(IV)RuRu(IV)

O xs Ce(IV) (IV)RuO

O Ru(V)-1e-O

O OH2

OH-2H+, -2e--2H+, -2e-

+2H2O, -H+

-2e-

Scheme 6 Bimolecular I2M mechanism proposed for complex 12a,under stoichiometric and catalytic regimes.

516 Water Oxidation

WO activity and mechanistic pathway of the cis - [Ru

(bpy)2(OH2)2]2þ complex 16, Figure 9,69 has been studied.

18O-labeling experiments combined with DFT analysis (MO6-L

DFT and CASSCF/CASPT2 calculations) have provided evidence

for the existence of a WNAmechanism operating in this case.70

A number of mononuclear Ru WOCs have been anchored in

conductive solid surfaces and their activity has been tested elec-

trochemically. For this purpose, the original ligands have been

functionalized so that the RuWOC can be attached at the surface

of an electrode without modifying the intrinsic coordination

properties of its parent compound. One example that illustrates

this strategy is the preparation of the complex [Ru(Mebimpy)

(4,40 -(CH2PO3)2bpy)(OH2)]2þ, A1 (Mebimpy is 2,6-bis

(1-methylbenzimidazol-2-yl)pyridine; 4,40 -(CH2PO3)2 bpy is

([2,20-bipyridine]-4,40-diylbis(methylene))diphosphonate). The

phosphonate-bpy can be attached at the surface of the electrode

and thus acts as a bridge between the electrode and the Ru metal

center,71 as depicted in Figure 10, to generate the new hybrid

material FTO-A1. With an applied potential of 1.85 V versus SHE

at pH 5, the FTO-A1 derivative is reported to sustain electrocata-

lytic oxidation of water for at least 8 h, reaching TON values

around 11000 with TOF of 21.6 min�1. Additionally, Meyer’s

group reported a mononuclear Ru-aqua complex containing

a Ru-based redox mediator linked by a bridging bpm ligand

of the formula, [(4,40 -(CH2P(O)(OH)2)2bpy)2Ru(m-bpm)Ru

(Mebimpy)(OH2)]4þ, A2 (bpm is 2,20-bipyrimidine). They have

attached it to the surface or to conductive solid supports such as

ITO, FTO or FTO–TiO2 (Figure 10). The ITO-A2-modified elec-

trode showed constant catalytic currents giving a TON of at least

28 000, with a TOF of 36 min�1 and an efficiency around 98%

after 8 h of continuous O2 evolution at an applied potential of

1.8 V versus SHE.

8.13.4.3 Ir-Based WOCs

In 2008, Bernhard and coworkers described a family of cyclo-

metalated Ir(III) complexes of the general formula [Ir(5-R1,40 -

R2,2-phenylpyridine)2(OH2)2]þ 17a–e (Figure 11), able to

catalytically oxidize water to O2 in the presence of Ce(IV) at pH

0.7.72 Experiments performed with 30 000 equivalents of Ce(IV)

yielded a maximum TON of 2760 (approximately one-third of

total conversion) after 1 week of reaction, thus indicating an

P

O

OH2

Ru

N N N

2+

N

N

N

N

P

OO

O

OO

OP

NRu

N

N

NN

NN

N

N

N

N

N

N

PO3H2

PO3H2

OH2

Ru

4+

O

O

O

OP

Electrod

e

Electrod

e

O

FTO-A1 FTO-A2

Figure 10 Left, mononuclear Ru complex heterogenized at the surface of an electrode via phosphonate functionalization. Right, as in the left case butwith the addition of a redox mediator.

R1

H2ON

+

R2

17a R1 = R2 = H

17c R1 = CH3, R2 = Ph17b R1 = CH3, R2 = H

17d R1 = CH3, R2 = F17e R1 = CH3, R2 = ClR2

H2O

N

Ir

R1

Figure 11 Drawn structure of Ir mononuclear di-aqua complexes 17a–e.

Cl

+

+

lr

lrN

N

HO

Olr lr

lr

20

2+

NCCH3

21

N lrO

O

CF3

2+

NCCH3

N

NN

N

2322

lr

OH

H

N

N N

N

O

18 19

H2O

OH2

OH2

2+

Figure 12 Drawn structures of Ir mononuclear complexes 18–23.

Water Oxidation 517

excellent robustness. However, traces of CO2 were also detected,

suggesting the partial oxidation of the organic ligands.

After Bernhard’s initial proposal, Crabtree and coworkers

reported a wide range of active Ir(III) pentamethylcyclopenta-

dienyl (Cp*) WOCs containing diverse cyclometalated or N,N-

bidentate ligands, including [IrCl(Cp*)(bpy)]þ, 18, [Ir(Cp*)(H2O)3]

2þ, 19, [(Ir(Cp*))2(m-OH)3]þ , 20, and [Ir(Cp*)(pp)

(CF3COO)], 21 (pp is 2-(pyridin-2-yl)propan-2-olate),

Figure 12.73,74 After 8 h, a TON of 320 and TOF of 0.7 min�1

were obtained for complex 18 using Ce(IV) as oxidant at pH

0.9.73b Under optimized conditions, a TOF of 14.4 min�1

could be obtained for this complex. In the same work, com-

plexes [Ir(Cp*)(H2O)3]SO4, 19, and [(Ir(Cp*))2(m -OH)3]OH,

20, were also tested, showing the highest TOF (20 min�1for 19

and 25 min�1for 20) among this series of Ir catalysts tested.

From a mechanistic perspective, both experimental and DFT

calculations point toward a WNA type of mechanism where

the highly active {Ir(V)¼O} species reacts with a solvent water

molecule to generate the corresponding {Ir(III)–OOH} species,

which further evolves to finally generate dioxygen.

In 2010, Albrecht and coworkers reported Ir complexes

containing abnormally bound N-heterocyclic carbenes,75

which are expected to stabilize higher oxidation states given

their zwitterionic resonance forms. In this context, complexes

[Ir(Cp*)(AC)(MeCN)]2þ, 22 and 23, where AC is the abnormal

carbene ligand bonded in two different manners (Figure 12),

were synthesized, proving to be the most active and stable

chemically triggered Ir complexes for WO with a TOF of

1.4 min�1 and a TON around 10000 after 5 days of continu-

ous O2 evolution.76

However, despite the interesting performance of the above-

described catalysts, a recent publication by Grotjahn and

coworkers introduces some controversy about their molecular

nature. These authors demonstrate that, after Ce(IV) addition

to several of the Ir WOCs previously reported, iridium oxide

(IrOx) nanoparticles end up being responsible for oxygen

518 Water Oxidation

evolution.77 The capacity of IrO2 to act as a heteorogeneous

WOC under photochemical conditions has been known for a

long time.78 Further, Macchioni et al.79 have studied the

decomposition pathways that the Ir-Cp* type of complexes

can undergo, which involve C–H activation of one of the Me

groups of the Cp* ligand.

In addition, the molecular nature of the Ir catalysts 19 and

21 was also studied based on cyclic voltammetry (CV) and

piezoelectric gravimetry (electrochemical quartz crystal nano-

balance, EQCN) experiments.80 These studies indicate that

upon electrochemical activation of 19, anodic deposition and

amorphous iridium oxide formation take place. This new

material is remarkably active for WO at low overpotentials

(�200 mVat0.5 mAcm�2) and shows continuous operation

for periods of days without loss of activity. In sharp contrast,

no sign of deposition is found for 21. Nevertheless, the data

provided do not exclude the potential formation of soluble or

suspended products such as iridium oxide nanoparticles,

although all indications seem to point out the homogeneous

nature of the catalytic process.

8.13.4.4 First-Row Transition Metals as WOCs

Manganese, the transition metal selected by Nature in the OEC

at PSII, has attracted much attention. However, active and

robust WOCs based on this transition metal are still elusive.

Several oxide-bridged Mn complexes, such as {[(trpy)(H2O)

Mn]2(m -O)2}3þ, 24, have been synthesized and tested as

WOCs using oxygen-transfer sacrificial oxidants such as

N

3+

N

N

24

OMn

H2O

N

N

N

OMn

OH2

Figure 13 Drawn structure of dinuclear Mn complexes 24 and 25.

CoCoCoCo

O O O O O O

POM¢ 10-

26POM¢

O

OH2OOOOOOO

H2O

Figure 14 Drawn structure of the Co complexes 26, containing a polyoxom

oxone or sodium hypochlorite. The activity of these complexes

is not free from controversy given the difficulty in differentiat-

ing from catalase-only activity.81–85 A dinuclear Mn complex

with a dinucleating N2O3 trianionic ligand bridging both

metal centers has recently been reported by Akermark

with the formula [Mn2(mop)(m -OMe)(m -OAc)}3þ, 25

(mop¼2,20-(5-methyl-2-oxido-1,3-phenylene)bis(1H-benzo

[d]imidazole-4-carboxylate) (Figure 13). This complex acts as

a WOC at pH 7.2 using an outer sphere electron transfer agent

like [Ru(bpy)3]3þ with TONs higher than 20.

Following the successful strategy previously employed for Ru

with the anionic polyoxometallate ligand, a new Co complex

bearing this type of ligands was also reported:

[Co4(H2O)2(PW9O34)2]10�, 26 (Figure 14). This complex con-

tains an a -PW9O34 POM skeleton and two {Co–OH2} active

sites.When treatedwith [Ru(bpy)3]3þ as sacrificial oxidant, a TN

of 1000 and a TOF of 354 min�1 can be achieved.86 However, in

a recent work under electrocatalytic conditions, Finke proposed

that the real active species is actually CoOx from de-

coordination of the POM complex.87 Co salts have been

known for a long time to act as WOCs,88,89 and the develop-

ment of CoOx has been extensively carried out lately by Nocera’s

group.90 Very recently, the group of Berlinguette prepared a new

Co complex91 [CoII(py5)(OH2)](ClO4)2, 27, containing a pen-

tacoordinated py5 ligand (2,6-bis(methoxydi(pyridin-2-yl)

methyl)pyridine), which has also been shown to be an active

WOC based mainly on electrochemical experiments.

The use of iron as a WOC was first reported in 2010 by

Bernhard and coworkers, showing a set of Fe(III) complexes

Mn MnO

OO O O O

O

ON N

NH

25

NH

27

MeO OMe

NCo

N

N

NN

OH2 2+

etallate ligand, and 27 containing a pentadentate py5 type of ligand.

Water Oxidation 519

containing highly oxidatively rugged tetraamido macrocyclic

ligands that upon reacting with Ce(IV) in water at pH 0.7

catalyze WO at various rates. The best results were obtained

with complex [Fe(ddtt)(H2O)]�, 28 (ddtt is the tetraanionic

ligand 13,14-dichloro-6,6-difluoro-3,3,9,9-tetramethyl-2,5,7,10-

tetraoxo-3,5,6,7,9,10-hexahydro-2H-benzo[e][1,4,7,10]tetraaza-

cyclotridecine-1,4,8,11-tetraide) (Figure 15), which produces

fast release of O2 for 20 s (TOF above 78 min�1) and then

continues evolving at a much slower rate, with overall TON

values above 16.92 A DFT study has been carried out to charac-

terize the catalytic cycle undergone by this complex. It is pro-

posed that a WNA mechanism is operating upon reaching a {Fe

(V)¼O} species. However, the energy barriers obtained for the

proposed catalytic cycle could not account for the fast oxygen

evolution found experimentally.93 Recently, it has also been

claimed that several Fe(II) complexes containing easily oxidiz-

able tetradentate ligands can also oxidize water to dioxygen.94

8.13.5 Photoelectrochemically Driven WO Catalysisand First Examples of Water-Splitting Cells

Three main components are needed for carrying out light-

driven WO in the homogeneous phase: a photosensitizer able

to harvest sunlight energy, a WOC, and a sacrificial electron

acceptor. [Ru(bpy)3]2þ and derivatives are the most commonly

used photosensitizers due to their strong absorbance in the

visible spectrum, microsecond excited-state lifetimes at room

temperature, and high redox potentials. When light with ade-

quate wavelength is shone over [Ru(bpy)3]2þ, an excited state

is generated, [Ru(bpy)3]2þ*, which is capable of transferring an

electron to a sacrificial electron acceptor such as

P

hv

P* EA

EA-

x4

P+

2H2O WOCA

WOCO–O

x4

e- flow

Scheme 7 Combination of reactions involved in light-induced WO.WOC, catalyst in a non-active oxidation state. WOCA, catalyst in its activeoxidation state. P, photosensitizer. P*, photosensitizer in its excited state.EA, sacrificial electron acceptor.

O

O

F

-

F

O

O

NN

Cl

Cl

N N

Fe

OH2

Figure 15 Drawn structure of the mononuclear Fe complex 28.

[CoIII(NH3)5Cl]2þ. This generates a Co(II) complex that

decomposes and [Ru(bpy)3]3þ. The latter then should be capa-

ble of oxidizing a WOC from its low oxidation state to a higher

oxidation state, WOCA, which in turn oxidizes water to dioxy-

gen, as depicted in Scheme 7.

Several mono- and dinuclear Ru WOCs have been tested

following this light-driven strategy. Catalysts 4a,954b,44 11 -

OH2,61 12,96 and 1360 have been tested by Sun’s group obtain-

ing TON values of 1270, 60, 84, 100, and 62, respectively. The

in-14 and out-15 mononuclear Ru–Hbpp complexes reported

by Llobet’s group (Figure 9) have also been tested via photo-

chemical triggering.59 Despite the viability of the concept, the

TON numbers achieved are always about a third with regard

to the same system but using a chemical oxidant, which is

indicative of the increased complexity of the photoelectro-

chemical system.

The tetranuclear Ru-POM catalyst 6b has also been proven

efficient in light-driven WO catalysis. A TON of 350 and a

quantum yield (F) of 0.09 under optimized conditions using

[Ru(bpy)3]2þ as photosensitizer and persulfate as electron

acceptor have been obtained by Hill and coworkers.97 On the

other hand, Bonchio, Campagna, and Puntoriero reported a

TON of 80 and an outstanding F of 0.30 when using a tetra-

nuclear Ru dendrimer {[Ru(bpy)2]3m3- [(Ru(dpypy)3]}8þ, 29

(dpypy is 2,3-di(pyridin-2-yl)pyrazine), as photosensitizer

(Figure 16) and persulfate as electron acceptor for the same 6b

complex.98 Moreover, these authors have also shown the high

photocatalytic activity of 6b adsorbed onto a sensitized nano-

crystalline TiO2 surface.99 Furthermore, Hill and coworkers have

reported that the Co4-POM catalyst 26 (Figure 14) is also very

active for light-driven WO, with TON values above 220 and a Fof 0.15 when using persulfate as electron acceptor and [Ru

(bpy)3]2þ as photosensitizer at pH 8.0.100 Very recently, Sakai

and coworkers have presented visible light-induced WO cata-

lyzed by molybdenum-based POMs with mono- and dicobalt

(III) cores.101 Finally, Sun and coworkers have reported a new

trinuclear Ru complex, 30 (Figure 17), which contains both

catalyst and photosensitizer within the same molecule, and

NN

8+

N

NN

N

NN

RuN

N

NRu

NN

N

N

N

N

N

NRu

NN

N

N

NRu

Figure 16 Drawn structure for the light absorber tetranuclear Rucomplex 29 with a dendritic configuration.

520 Water Oxidation

that upon shining light is capable of oxidizing water to dioxy-

gen, giving at least a TON of 40.102

One of the first photo-electrochemical cells (PECs) for

water splitting was built by Moore and collaborators, which

established the proof of principle for this new technology.

They used a porphyrin-sensitized nanoparticulated TiO2

photoanode and a hydrogenase-modified carbon felt

cathode.103 Later on, Mallouck and coworkers described

another PEC104 using phosphonate and carboxylate [Ru

(bpy)3]2þ derivatives anchored onto TiO2 as photosensitizers

and IrO2 nanoparticles as WOCs in the anode. The cell used a

Pt wire as a cathode and needed a small external potential

input to generate H2 in the cathode. In this manner, TONs of

16 and quantum yields of 0.09 were obtained. More recently,

O

N

H N N

Ru

N

O

O

N

N

N

N

N

N

Ru

Figure 17 Drawn structure of the trinuclear Ru complex 30, which contains

Naf

ion

e-

RuP

sunlight

FTO

TiO

2

O

O

O

O

N

N

P

O

P

O

Ru

N

N

N

N

Figure 18 Photo-electrochemical cell (PEC) designed for water splitting wit

Crabtree and Brudvig have developed a PEC containing a high-

potential porphyrin sensitizer and a Cp*-Ir catalyst analogous

to 18 (Figure 12), both containing a carboxylate group for

attachment to TiO2 nanoparticles in the photoanode and a

standard Pt cathode.105 This cell generates a photocurrent

after illumination when working under a small external bias,

although no H2 concentrations have been measured.

The groups of Dismukes and Spiccia obtained interesting

results by employing a tetranuclear Mn-oxide cluster

[Mn4O4L6]þ (L is diarylphosphinate) embedded in a Nafion

membrane as WOC combined with [Ru(bpy)2(4,40 - (COO)2-

bpy)]2þ onto FTO–TiO2 as photosensitizer.106 This photoa-

node was connected to a Pt cathode via an aqueous electrolyte

solution and an external circuit, and a TON of 13 and a TOF of

O

O

N H

N

N

N

N

N

N

N

Ru

4+

O

both a WOC and a light harvesting unit in a single molecule.

H2O

O2

2H+

H2

12a

V

Pt

h sunlight, using 12a as a WOC. See text for further details.

1980 1990

2a2b

11b12a

6b

11c

12b

OEC

2000

Year

3

1

-1

-3

Log

TOF

(s-1

)

2010 2020

Figure 19 Plot of log of initial TOF for the catalytic water oxidationreaction versus time (the year reported), for a series of representativeWOCs.

Water Oxidation 521

0.78 min�1 were obtained in this manner. However, Spiccia

and coworkers have recently reported that the Mn4 cluster

decomposes upon light irradiation, generating MnOx in

mixed oxidation states III and IV.107

Finally, Sun and coworkers have recently reported a PEC

employing a molecular Ru WOC.108 In this work, the catalyst

[Ru(bdc)(4 -Me-py)2], 12a, was immobilized into a Nafion

membrane and the [Ru(bpy)2(4,40 - (PO3H2)2 -bpy)]

2þ photo-

sensitizer was anchored onto a FTO–TiO2 film. Further, a Pt

foil was used as cathode (Figure 18). Again, an external elec-

trical bias was needed and a TON of 16 and a TOF 0.45 min�1

were achieved.

8.13.6 Conclusion

The urgent need for a clean and renewable energy source to

replace the exhausting and contaminating fossil fuels has pro-

moted intense research in light-driven water splitting. Within

this context, the WO process has been traditionally recognized

as one of the bottleneck processes that hamper the development

of a technology based on water splitting by sunshine.

However, during the last 7 years tremendous advancements

have taken place in this field. From the ‘blue dimer’ as the only

catalyst back in 1982, literally dozens of active WOCs have

been recently described with increasing TONs and TOFs. An

interesting graph that shows this degree of activity is presented

in Figure 19, where the log values of TOF of a selected number

of molecular WOCs, including the OEC-PSII, are plotted

against the year they have been reported. It is impressive to

see that in the last 4 years the reported TOF has increased by

more than 4 orders of magnitude and that we are quickly

approaching the values of Nature. Impressive advancements

have also been taking place at a mechanistic level and a few

O–O bond formation pathways have been nicely elucidated.

As a consequence of all these efforts, the construction of robust

PECs seems now feasible and a few examples have been already

described. The main challenge that we still have ahead con-

cerns the creation of a PEC for water splitting using only

sunlight, where all the reactions occur in a harmonious

manner to generate hydrogen and oxygen. For a related chapter

in this Comprehensive, we refer to Chapter 3.15.

Acknowledgments

Support from Generalitat de Catalunya (2009 SGR-69),

SOLAR-H2 (EU 212508), ACS (PRF 46819-AC3), and MICINN

(Consolider Ingenio 2006–0003, CTQ2011-26440, CTQ2011-

60476, CTQ2010-21532-C02-02, and CTQ-2010-21497) are

gratefully acknowledged.

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