Field of the invention

The invention is in the field of water splitting, in particular in the field of water oxidation. The invention relates to a process for the oxidation of water under formation of molecular oxygen (0 ₂ ), and to transition metal catalysts for the oxidation of water.

Background of the invention

The conversion of water into molecular oxygen (0 ₂ ) and molecular hydrogen (H ₂ ) is generally referred to as water splitting. Hydrogen is considered to be an ideal fuel. When hydrogen is burned in the presence of oxygen, water is produced as the sole waste product, and therefore hydrogen offers a clean, non-polluting alternative to fossil fuels.

The first step of water splitting involves the oxidation of water under the formation of molecular oxygen and hydrogen ions. In a second step, said hydrogen ions can be reduced to form molecular hydrogen, for example in the presence of a hydrogen evolution catalyst. The half reactions of the water splitting process are depicted below.

2 H ₂ 0 → 0 ₂ (g) + 4 H ⁺ + 4 e ^"

4 H ⁺ + 4 e ^" → 2 H ₂ (g)

An input of energy, e.g. electrical energy, thermal energy or light energy such as for example solar energy is necessary to drive the water splitting process. Water splitting in order to produce hydrogen is of utmost importance to develop new artificial photosynthetic processes, allowing the conversion of solar energy into chemical energy. ¹ Uncatalyzed electrolysis of water requires a large overpotential, which results in a relatively low efficiency. Efficient water oxidation catalysts are required to realize solar to chemical energy conversion at sufficient rates, which would contribute significantly to solve problems around the world' s energy demands. The global awareness of energy related issues has lead to an increased attention to this area of research.

In order to be able to study the oxidation half reaction of the water splitting process, the oxidation half reaction wherein 0 ₂ is formed needs to be separated from the reduction half reaction wherein H ₂ is formed. The formation of H ₂ can be prevented by adding a strong oxidant, such as for example Ce(IV), that accepts the electrons that are liberated in the first half reaction of the water splitting process. A model system for testing a catalyst for water oxidation activity is the chemical oxidation of water in the presence of Ce(IV) as the electron acceptor.

Heterogeneous as well as homogeneous catalysts for the oxidation of water are known in the art. Homogeneous catalysts for the process include mononuclear, dinuclear and polynuclear complexes.

A mononuclear water oxidation catalyst is for example disclosed in D.G.H. Hetterscheid et al., Chem. Commun. 2011, 47, 2712 - 2714, incorporated by reference. The activity of mononuclear iridium complexes 1 and 2 for water oxidation was investigated via chemical oxidation with Ce(IV).

R = C1

R = OH

Complex 2 catalyzes the oxidation of water and displays a turnover frequency (TOF) of 0.38 mol 0 ₂ mol ^"1 s ^"1 and a turnover number (TON) of at least 2000. Catalytic results for 1 were significantly lower than for 2 and required an incubation time, suggesting that dissociation of chloride is required for catalytic reactivity. (Note: according to the authors of Chem. Commun. 2011, 47, 2712 - 2714 the TOF of 1 and 2 in said publication are inadvertently reported in Ce(IV) instead of in 0 ₂ ; a TOF of 1.5 mol mol ^"1 [Ce(IV)] s ^"1 corresponds to a TOF of 0.38 mol mol ^"1 [O ² ] s ^"1 for 2.)

Polynuclear homogeneous polyoxometalate water oxidation catalysts are for example disclosed in WO 2010/107919, incorporated by reference. A polyoxometalate (POM) refers to a polyatomic ion, usually an anion, which includes three or more transition metal oxyanions linked or shared by oxygen atoms. Suitable POM catalysts are catalysts having the formula: [M _p M' _q O _x (OH) _n (H ₂ 0) _m L _r ] ^u [A] _u wherein M is the skeletal POM metal, typically a d center such as tungsten (W ), molybdenum (Mo ^VI ), vanadium (V ^v ), niobium (Nb ^v ), tantalum (Ta ^v ), or combinations thereof; M' is one or more redox active metals selected from f-block elements, d-block elements, or combinations thereof; p is an integer from 0 to 200; q is an integer from 1 to 40; x is an integer from 0 to 450; n is an integer from 0 to 50; m is an integer from 0 to 50; L is an inorganic ligand; r is an integer from 2 to 5; u is a negative integer less than or equal to -1 ; A is a cation or combination of cations such that the sum of the positive charges of the cations balances that negative charge, u, on the polyanion. Examples of a suitable POM include [Co ₄ (H ₂ 0)(PW ₉ 0 ₃₄ ) ₂ ] ^10" , [{Ru ₄ O ₄ (OH) ₂ (H ₂ O) ₄ }(Y-PW ₁₀ O ₃ 6) ₂ ] ^10" and

[{Ru ₄ O ₅ (OH)(H ₂ O) ₄ }(Y-PW ₁₀ O ₃₆ ) ₂ ] ⁹ -.

A dinuclear water oxidation catalyst [(COD)(Cl)Ir(bpi)Ir(COD)](PF ₆ ) [1]PF ₆ , wherein bpi stands for (pyridin-2-ylmethyl)(pyridin-2-ylmethylene)amine), is disclosed by W.I. Dzik et al., Organometallics 2011, 30, 372, incorporated by reference.

Complex [1]PF ₆ displays a TOF of 0.24 mol 0 ₂ mol ^"1 s ^"1 and a TON of at least 1000 in the catalytic oxidation of water. (Note: according to the authors of Organometallics 2011, 30, 372 the TOF of [1]PF ₆ in said publication is inadvertently reported in Ce(IV) instead of in 0 ₂ ; a TOF of 3400 mol mol ^"1 [Ce(IV)] h ^"1 corresponds to a TOF of 0.24 mol mol ^"1 [O ² ] s ^"1 for [1]PF ₆ .)

A mononuclear and a dinuclear ruthenium catalyst for water splitting are disclosed by Bernet et al. , Chem. Commun. 2011, 47, 8058, incorporated by reference.

3a: R = Me

Mononuclear complex 3a, wherein R is Me, catalyses the oxidation of water using Ce(IV) as sacrificial oxidant (1 mM catalyst and 100 mM oxidant in 2.0 ml 0.1M triflic acid solution with a pH of 1.0) with a TOF ₁₀₀ o, i.e. a TOF after 1000 s, of 1080 h ^"1 (i.e. 0.3 s ^"1 ) and a TON of 19. At an 8mM complex concentration (1: 100 catalyst/oxidant) initial turnover numbers as high as 6660 h ^"1 (1.85 s ^"1 ) were observed for 3a. Dinuclear species 5 catalyses the water oxidation less efficiently with a TOFiooo of 7 h ^"1 (0.00194 s ^"1 ) and a TON of 13. Dinuclear complex 5 is kinetically less competent than mononuclear complex 3a.

Mononuclear iridium(III) complex Cp*Ir(H ₂ 0) ₃ S0 ₄ 11 and dinuclear iridium complex [(Cp*Ir) ₂ ( /-OH) ]OH 12 are disclosed in J.D. Blakemore et al. , J. Am. Chem. Soc. 2010, 132, 16017, incorporated by reference.

Cp*Ir(H ₂ 0) ₃ S0 ₄ 11 catalyses the oxidation with a TOF of 5.5 min ^{" 1} (i.e. 0.09 s ^"1 ) and [(Cp*Ir) ₂ ( /-OH) ₃ ]OH 12 with a TOF of 10.4 min ^"1 (i.e. 0.17 s ^"1 ).

In addition, a number of well defined homogeneous cobalt, 2 ruthenium, 3 manganese ⁴ and iron ⁵ based water oxidation catalysts that display reasonable reaction rates are known in the art. However, the observed absolute turnover numbers are relatively low. Robust water oxidation catalysts are known in case of ruthenium 6- ^" 7 and iridium, 8 but in general these robust catalysts have a lower reactivity. In order to successfully demonstrate that hydrogen and oxygen can be produced from water and sunlight in a efficient manner, the reaction rate, i.e. the turnover frequency, as well as the absolute turnover number has to be excellent. ⁹ However, in most cases the turnover number of these systems is quite low.

Therefore it is an object of the invention to provide a hydrolytically and oxidatively stable water oxidation catalyst that is readily synthesized. It is also an object of the invention to provide a water oxidation catalyst having a high turnover frequency. It is a further object of the invention to provide a water oxidation catalyst having a high turnover number. Yet another object of the invention is to provide a dinuclear homogeneous transition metal catalyst that catalyses the oxidation of water with a high turnover frequency and a high turnover number.

Summary of the invention

The invention relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula I is reacted with water, wherein I is defined as:

I wherein:

M 1 and NT 2 are independently selected from the group consisting of Co, Rh, Ir, Fe, Ru and Os;

when M 1 and/or M2 are Co, Rh or Ir, A 1 and/or A2 are independently selected from the group consisting of a cyclopentadienyl ligand [C ₅ (R ) ₅ ] ^~ , a cyclopentadienone ligand

[C ₅ (R 3 ) ₄ (=0)], a cycloheptatrienyl ligand [C ₇ (R 3 ) ₇ ] - ^~ and a 1,4-cyclooctadiene ligand

3 3

[C8(R ) ₁₂ ], wherein R is independently selected from the group consisting of hydrogen, halogen, OH, OR ⁷ , Ci - C ₆ alkyl, C ₆ - C ₁₂ aryl, C ₇ - C ₁₅ alkylaryl, Si(R ⁷ ) ₃ and a linking unit for linkage to a solid material, wherein Ci - C ₆ alkyl, C ₆ - C ₁₂ aryl and C ₇ - C ₁₅ alkylaryl are optionally substituted with OH, C(H)0, COOH, halogen, Si(R ⁷ ) ₃ or a linking unit for linkage to a solid material, and wherein R is independently a Ci - C ₆ alkyl or a C ₆ - C ₁₂ aryl;

1 2 1 2

when M and/or M are Fe, Ru or Os, A and/or A are independently selected from the group consisting of a cyclopentadienyl ligand [C ₅ (R ) ₅ ] ^~ , a cyclopentadienone ligand

3 3 -

[C ₅ (R ) ₄ (=0)], a cycloheptatrienyl ligand [C ₇ (R ) ₇ ] ^~ , a 1,4-cyclooctadiene ligand

3 3 3

[C8(R )i ₂ ] and an arene ligand [C ₆ (R ) ₆ ], wherein R is as defined above;

1 2 B is independently N, O or S, with the proviso that if B is O or S, R and/or R is absent;

X is independently selected from the group consisting of CI, Br, I, O and OH;

D is selected from the group consisting of (i), (ii) and (iii):

wherein n is 1, 2 or 3, t is 0, 1, 2, 3 or 4, u is independently 0, 1, 2 or 3, R ⁴ is independently selected from the group consisting of hydrogen, halogen, a Ci - C ₆ alkyl group, a C - C ₁₂ aryl group, a C ₇ - C ₁₅ alkylaryl group and a linking unit for linkage to a solid material, R ⁵ is independently selected from the group consisting of halogen, Ci - C ₆ alkyl and a linking unit for linkage to a solid material, and (i), (ii) and (iii) optionally comprising one or more heteroatoms Y, wherein Y is selected from the group consisting of N and O for (i) and Y is N for (ii) and (iii);

1 2

R and R" are independently selected from the group consisting of a Ci - C ₆ alkyl group and a linking unit for linkage to a solid material;

R ⁶ is selected from the group consisting of hydrogen, halogen, a Ci - C ₆ alkyl group, a C ₇ - Ci5 alkylaryl group and a linking unit for linkage to a solid material;

E is selected from the group consisting of:

wherein R ⁵ , R ⁶ , t and u are as defined above, p is 1, 2 or 3, and (c), (d) and (e) optionally comprising one or more heteroatoms Y, wherein Y is N or O for (c) and Y is N for (d) and (e);

q is 0, 1, 2, 3, 4, 5 or 6;

Z is selected from the group consisting of anions;

v is 1, 2, or 3; and

w is 0, 1, 2, 3, 4, 5 or 6, with the proviso that (w · v) is equal to q.

The invention also relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula II is reacted with water, wherein II is defined as:

II wherein M ¹ , M ² , A ¹ , A ² , B, D, R ¹ , R ² , R ⁶ and E are as defined above; and X is independently selected from the group consisting of CI, Br, I and OH.

The invention further relates to a compound according to Formula I:

wherein M ¹ , M ² , A ¹ , A ² , B, X, D, R ¹ , R ² , R ⁶ , E, q, Z, v and w are defined as above.

The invention also relates to a compound according to Formula II:

II

1 2 6

wherein M , M , A , A , B, D, R , R , R and E are as defined above and wherein X is independently selected from the group consisting of CI, Br, I and OH.

The invention further relates to a solid material comprising a compound according to Formula I, and to a solid material comprising a compound according to Formula II. In particular, the invention relates to an electrode comprising a compound according to Formula I, and to an electrode comprising a compound according to Formula II.

Detailed description of the invention

Definitions

The verb "to comprise" as is used in this description and in the claims and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded.

In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there is one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

Unsubstituted alkyl groups have the general formula C _n H2 _n+ i and may be linear or branched. Unsubstituted alkyl groups may also contain a cyclic moiety, and thus have the concomitant general formula C _n H _2n _ ₁ . Optionally, the alkyl groups are substituted by one or more substituents further specified in this document, and/or interrupted by heteroatoms selected from the group consisting of oxygen, nitrogen, and sulphur. Examples of alkyl groups include methyl, ethyl, propyl, i-propyl (2-propyl), cyclopentyl, i-butyl, w-hexyl and cyclohexyl.

Unsubstituted alkenyl groups have the general formula C _n H _2n -i, and may be linear or branched. Examples of suitable alkenyl groups include ethenyl, propenyl, i-propenyl, butenyl, pentenyl, decenyl, octadecenyl and eicosenyl and the like. Unsubstituted alkenyl groups may also contain a cyclic moiety, and thus have the concomitant general formula C _n H _2n _3.

Unsubstituted alkenes have the general formula C _n H _2n whereas unsubstituted alkynes have the general formula C _n H _2n -2-

Aryl groups comprise at least six carbon atoms and may include monocyclic, bicyclic and polycyclic structures. Optionally, the aryl groups may be substituted by one or more substituents further specified in this document. Examples of aryl groups include groups such as phenyl, naphthyl and anthracyl.

Arylalkyl groups and alkylaryl groups comprise at least seven carbon atoms and may include monocyclic and bicyclic structures. Optionally, the aryl groups may be substituted by one or more substituents further specified in this document. An arylalkyl group is for example benzyl. An alkylaryl group is for example 4-i-butylphenyl.

Where an aryl group is denoted as a (hetero)aryl group, the notation is meant to include an aryl group and a heteroaryl group. Similarly, an alkyl(hetero)aryl group is meant to include an alkylaryl group and a alkylheteroaryl group, and (hetero)arylalkyl is meant to include an arylalkyl group and a heteroarylalkyl group.

A C ₅ - C ₁₂ heteroaryl group comprises five to twelve carbon atoms wherein one to four carbon atoms are replaced by heteroatoms selected from the group consisting of oxygen, nitrogen, phosphorus and sulphur. A heteroaryl group may have a monocyclic or a bicyclic structure. Optionally, the heteroaryl group may be substituted by one or more substituents further specified in this document. Examples of suitable heteroaryl groups include pyridinyl, quinolinyl, pyrimidinyl, pyrazinyl, pyrazolyl, pyrrolyl, furanyl, benzofuranyl, indolyl, purinyl, benzoxazolyl, thienyl, phospholyl and oxazolyl.

The compounds disclosed in this description and in the claims may comprise one or more asymmetric centres, and different diastereomers and/or enantiomers may exist of the compounds. The description of any compound in this description and in the claims is meant to include all diastereomers, and mixtures thereof, unless stated otherwise. In addition, the description of any compound in this description and in the claims is meant to include both the individual enantiomers, as well as any mixture, racemic or otherwise, of the enantiomers, unless stated otherwise. When the structure of a compound is depicted as a specific enantiomer, it is to be understood that the invention of the present application is not limited to that specific enantiomer.

The compounds may occur in different tautomeric forms. The compounds according to the invention are meant to include all tautomeric forms, unless stated otherwise.

The compounds disclosed in this description and in the claims may further exist as exo and endo regioisomers. Unless stated otherwise, the description of any compound in the description and in the claims is meant to include both the individual exo and the individual endo regioisomer of a compound, as well as mixtures thereof.

Furthermore, the compounds disclosed in this description and in the claims may exist as cis and trans isomers. Unless stated otherwise, the description of any compound in the description and in the claims is meant to include both the individual cis and the individual trans isomer of a compound, as well as mixtures thereof. As an example, when the structure of a compound is depicted as a cis isomer, it is to be understood that the corresponding trans isomer or mixtures of the cis and trans isomer are not excluded from the invention of the present application.

Furthermore, the compounds disclosed in this description and in the claims may exist as facial (fac) and meridional (mer) isomers. Unless stated otherwise, the description of any compound in the description and in the claims is meant to include both the individual fac and the individual mer isomer of a compound, as well as mixtures thereof. As an example, when the structure of a compound is depicted as a fac isomer, it is to be understood that the corresponding mer isomer or mixtures of the fac and mer isomer are not excluded from the invention of the present application.

The term "independently selected" means that a certain substituent on one position in the molecule may differ from the same substituent on a different position in the molecule. As an example, all three R 7 substituents in -Si(R 7 ) ₃ may be different from one another. Similarly, in a compound according to Formula I as defined below, for example all R ⁶ substituents may differ from each other.

A complex comprising one transition metal atom is referred to as a mononuclear complex. Similarly, a complex comprising two transition metal atoms is referred to as a dinuclear complex.

As is known to a person skilled in the art, compounds comprising deuterium instead of hydrogen may be more stable, for example towards oxidative C-H activation. Therefore, one or more hydrogen (H) atoms may be replaced by deuterium (D) in the transition metal compounds according to the invention. Unless stated otherwise, the description of any transition metal complex in the description and in the claims is meant to include any derivative of said complex wherein one or more hydrogen atoms are substituted by one or more deuterium atoms.

When the transition metal complexes according to the invention are in solution, said complexes may, depending on the pH of the solution, occur in the form of a protonated or a deprotonated derivative. Unless stated otherwise, the description of any transition metal complex in the description and in the claims is meant to include any protonated or deprotonated form of said complex.

The turnover number (TON) is defined as the number of moles of substrate that a mole of catalyst can convert before being inactivated. The turnover number is calculated by dividing the number of moles of molecular oxygen by the number of moles of catalyst.

The turnover frequency (TOF) refers to the turnover per unit time under turnover conditions. The turnover frequency is herein expressed in mol 0 ₂ per mol catalyst per second (mol mol ^"1 s ^"1 ).

Process for the oxidation of water

The invention relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula I is reacted with water, wherein I is defined as:

The invention also relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula II is reacted with water, wherein II is defined as:

II

1 2

In the compounds according to Formula I or II, M and M are independently

1 2 selected from the group consisting of Co, Rh, Ir, Fe, Ru and Os. Preferably, M and M are independently selected from the group consisting of Fe, Ru, Co, Rh and Ir, more preferably from the group consisting of Ru, Co and Ir, and most preferably from the

1 2

group consisting of Ru and Ir. M and M may be the same or different. In a preferred

1 2

embodiment, M and M are the same.

1 2 1 2

When M and/or M are Co, Rh or Ir, A and/or A are independently selected from the group consisting of a cyclopentadienyl ligand [C ₅ (R ) ₅ ] ^~ , a cyclopentadienone

3 3 - ligand [C ₅ (R ) ₄ (=0)], a cycloheptatrienyl ligand [C ₇ (R ) ₇ ] ^~ and a 1,4-cyclooctadiene

3 3

ligand |¾(R ) ₁₂ ], wherein R is independently selected from the group consisting of hydrogen, halogen, OH, OR ⁷ , Q - C ₆ alkyl, C ₆ - C ₁₂ aryl, C ₇ - C ₁₅ alkylaryl, Si(R ⁷ ) ₃ and a linking unit for linkage to a solid material, wherein C - C ₆ alkyl, C ₆ - C ₁₂ aryl and C ₇ - C ₁₅ alkylaryl are optionally substituted with OH, C(H)0, COOH, halogen,

7 7

Si(R ) ₃ or a linking unit for linkage to a solid material, wherein R is independently a Ci - C ₆ alkyl or a C ₆ - C ₁₂ aryl and wherein said linking unit is as defined below;

1 2 1 2

When M and/or M are Fe, Ru or Os, A and/or A are independently selected from the group consisting of a cyclopentadienyl ligand [C ₅ (R ) ₅ ] ^~ , a cyclopentadienone

3 3 - ligand [C ₅ (R ) ₄ (=0)], a cycloheptatrienyl ligand [C ₇ (R ) ₇ ] ^~ , a 1,4-cyclooctadiene ligand

3 3 3

[C8(R )i ₂ ] and an arene ligand [C ₆ (R ) ₆ ], wherein R is as defined above.

3 3 3

When R is halogen, it is preferred that R is CI or Br. When R is a Ci - C ₆ alkyl group, R is preferably a methyl, ethyl, w-propyl or an i-propyl group, more preferably a methyl group. When R 3 is C ₆ - C ₁₂ aryl, it is preferred that R 3 is a phenyl group.

When R 3 is C ₇ - C ₁₅ alkylaryl, it is preferred that R 3 is a /?-tolyl or a mesityl group. When R is a linking unit for linkage to a solid material, said linking unit is preferably of the formula according to IXa or IXb as defined below. Most preferably, R is hydrogen, a methyl group, an i-propyl group or a phenyl group.

Examples of a [C ₅ (R 3 ) ₅ ] - ^~ ligand include cyclopentadienyl ([C ₅ H ₅ ] ^" or Cp), pentamethylcyclopentadienyl ([CsMes] ^" or Cp*), a hydroxycyclopentadienyl, such as for example [C ₅ (R ³ ) (OH)] ^~ or [C ₅ (R ³ ) (CH ₂ OH)] ^~ , and trimethylsilylcyclopentadienyl [C ₅ H ₄ (SiMe ₃ )] ^~ . Preferably, the cyclopentadienyl ligands are bonded via a rf -hapticity bonding mode. Preferred [C ₅ (R ) ₅ ] ^~ ligands are Cp and Cp*.

Examples of a [C ₅ (R ) ₄ (=0)] ligand include 2,4-cyclopentadien-l-one [C ₅ H ₄ (=0)], tetramethylcyclopentadienone [CsMe ₄ (=0)], 2,5-tetraphenyl-2,4- cyclopentadien-l-one and tetraphenylcyclopentadienone [C ₅ (C ₆ H ₅ ) ₄ (=0)]. Preferably, the cyclopentadienone ligands are bonded via a / -hapticity bonding mode.

Examples of a [C ₆ (R )ό] ligand are benzene (C ₆ H6), hexamethylbenzene (C ₆ Me ₆ ), p-cymene [p-C ₆ H ₄ (Me)(j-C H ₇ )] and mesitylene (C ₆ H ₃ Me ₃ ). Preferably, the [C ₆ (R ³ )6] ligands are bonded via a / -hapticity bonding mode.

Examples of a [C ₇ (R 3 ) ₇ ] - ^~ ligand are cyclohaptatrienyl (C ₇ H ₇ ) ^~ . Preferably the cycloheptatrienyl ligands are bonded via a η -hapticity bonding mode.

Examples of a [Cg(R ) ₁₂ ] ligand are cyclooctadiene (CgH ₁₂ , COD) and cyclooctatetraene (CsHg, COT). The cyclooctadiene ligands may be bonded via a rf - or a / -hapticity bonding mode. Cyclooctatetraene ligands may additionally be bonded via a rf- or a / -hapticity bonding mode. Preferably, the cyclooctadiene ligands are bonded via a //-hapticity bonding mode.

In the compounds according to Formula I or II, B is independently selected from the group consisting of N, O and S, with the proviso that if B is O or S, R 1 and/or R 2 are absent. In a preferred embodiment, B is N.

In the compound according to Formula I, X is independently selected from the group consisting of CI, Br, I, O and OH. In a preferred embodiment, X is CI.

In the compound according to Formula II, X is independently selected from the group consisting of CI, Br, I and OH. In a preferred embodiment, X is CI.

In the compounds according to Formula I or II, D is selected from the group consisting of (i), (ii) and (iii):

wherein n is 1, 2 or 3, t is 0, 1, 2, 3 or 4, u is independently 0, 1, 2 or 3, R ⁴ is independently selected from the group consisting of hydrogen, halogen, a Ci - C ₆ alkyl group, a C ₆ - C ₁₂ aryl group, a C ₇ - C ₁₅ alkylaryl group and a linking unit for linkage to a solid material, R ⁵ is independently selected from the group consisting of halogen, Ci - C alkyl and a linking unit for linkage to a solid material, and (i), (ii) and (iii) optionally comprising one or more heteroatoms Y, wherein Y is selected from the group consisting of N and O for (i) and Y is N for (ii) and (iii).

When R ⁵ is halogen, it is preferred that R ⁵ is CI or Br. When R ⁵ is Ci - C ₆ alkyl, it is preferred that R ⁵ is methyl. Optionally, two R ⁵ substituents together may form a C ₃ - Ci2 cycloalkyl ring or a C ₅ - C ₁₂ (hetero)aryl ring. When R ⁴ and/or R ⁵ is a linking unit for linkage to a solid material, said linking unit is preferably of the formula according to IXa or IXb as defined below.

In a preferred embodiment, D is (i). Preferably, R ⁴ is hydrogen or a methyl group, more preferably hydrogen. In another preferred embodiment, n is 1, i.e. D is -C(R ⁴ ) ₂ -. Most preferably, n is 1 and R ⁴ is hydrogen, i.e. D is -CH ₂ -. When D is selected from (ii) or (iii), t and u are preferably 0.

Optionally, (i), (ii) and (iii) comprise one or more heteroatoms Y. For example,

(i) may comprise one or more N and/or O atoms, i.e. (i) may for example be an ether. Similarly (ii) may for example comprise a pyridine, pyrimidine, pyrazine or triazine moiety, and (iii) may for example comprise a quinoline, isoquinoline, quinazoline or cinnoline moiety.

In the compounds according to Formula I or II, R 1 and R 2 are independently selected from the group consisting of a Ci - C ₆ alkyl group and a linking unit for linkage to a solid material, for example a solid support or the surface material of an electrode. In one embodiment, R 1 and R 2 are independently a Ci - C ₆ alkyl group, for example a methyl, ethyl, w-propyl, i-propyl, w-butyl, i-butyl, i-butyl, w-pentyl, i-pentyl, w-hexyl or cyclohexyl group, preferably a methyl, ethyl, propyl or i-propyl group. In 1 2

another embodiment R and/or R is a linking unit for linkage to a solid material, for example a solid support or the surface material of an electrode. Said linking units are preferably of the formula according to IXa or IXb as defined below.

In the compounds according to Formula I or II, R ⁶ is selected from the group consisting of hydrogen, halogen, a Ci - C ₆ alkyl group, a C ₇ - C ₁₅ alkylaryl group and a linking unit for linkage to a solid material. Said linking unit is described in more detail below. In a preferred embodiment, R ⁶ is selected from the group consisting of hydrogen, methyl, ethyl and w-propyl. More preferably, R ⁶ is hydrogen.

In the compounds according to Formula I or II, E is selected from the group consisting of:

wherein R ⁵ , R ⁶ , t and u are as defined above, p is 1, 2 or 3, and (c), (d) and (e) optionally comprising one or more heteroatoms Y, wherein Y is N or O for (c) and Y is N for (d) and (e). In a preferred embodiment of the process according to the invention, E is (b), wherein R ⁶ is as defined above. In a further preferred embodiment E is (b) and R ⁶ is hydrogen. Most preferably, E is (b), R ⁶ is hydrogen and B is N.

Examples of the carbene moieties of compounds I and II wherein various possibilities for (a) - (e) and B are incorporated are depicted below.

For a compound according to Formula I, q is 0, 1, 2, 3, 4, 5 or 6, preferably 0, 1, 2, 3 or 4. As will be clear to the person skilled in the art, the value of q in I depends on the oxidation state of the metal centers M 1 and M2 , on the nature of the ligands A 1 and

A 2 and on the nature of the bridging ligands X. As an example, when M 1 and M2 are both Ir(III), A ¹ and A ² are both an rf -cyclopentadienyl ligand [Cs(R ³ )5] ^~ and X is CI, then q is 2. Also when M ¹ and M ² are both Ru(II), A ¹ and A ² are both an jf -arene ligand [C ₆ (R ³ )6] and X is CI, q is 2. When M ¹ and M ² are both Ru(II) and A ¹ and A ² are both an /^-cyclopentadienyl ligand [C ₅ (R ³ )5] ^~ and X is CI, then q is 0. When M ¹ and M ² are both Ir(III), A ¹ and A ² are both an / -cyclooctadiene ligand [C ₈ (R ³ ) ₁₂ ] and X is CI, then q is 4. When M 1 and M2 are both Ru(II), A 1 and A2 are both p-cymene, the first X is O and the second X is OH, then q is 1. When M ¹ and M ² are both Ir(III), A ¹ and A ² are an / -cyclooctadiene ligand [C ₈ (R ³ )i ₂ ], the first X is O and the second X is OH, then q is 3.

Z in compound I is selected from the group consisting of anions, and the charge v of the anion may be 1, 2 or 3. Suitable types of anions are known to the person skilled in the art, and include for example chloride (CI ), iodide (Γ), bromide (Br ), hydroxide (OH ^" ), phosphate (P0 ₄ ^3~ ), sulphate (S0 ₄ ^2~ ), nitrate (N0 ₃ ^~ ), periodate (I0 ₄ ^~ ) and perchlorate (C10 ₄ ^~ ). Another type of anions are the so called weakly coordinating or non-coordinating anions, such as for example disclosed in WO 2008/095933 and WO 2006/052427, both incorporated by reference. Examples of weakly or non-coordinating anions are tetrafluoroborate (BF ₄ ^" ), hexafluorophosphate (PF ₆ ^~ ), hexafluoroantimonate (SbF ₆ ^" ), triflate (OTf) and tetrakis(pentafluorophenyl)borate [B(C ₆ F5) ₄ ] . In a perferred

3- embodiment, the anion is selected from the group consisting of CI ^" , Γ, Br ^" , OH ^" , P0 ₄ ^" , S0 ₄ ^2" , N0 ₃ ^" , I0 ₄ ^" , C10 ₄ ^" , PF ₆ ^" , OTf and SbF ₆ ^" .

As will be clear to a person skilled in the art the overall charge in compound I equals 0. The positive charge q of the cation needs to be compensated by the negative charge of the one or more anions Z, hence w is 0, 1, 2, 3, 4, 5 or 6, with the proviso that (w · v) is equal to q. As an example, when q is 2 and Z is PF ₆ ^" , i.e. v is 1, then w is 2, and when q is 0 then w is 0.

As was mentioned above, in a preferred embodiment B is N. When B is N, R ¹ and R ² may together form a -C(R ⁶ )2-D-C(R ⁶ ) ₂ - group, wherein D and R ⁶ are as defined above, as is depicted in compounds IX and X below. R ⁴ , R ⁵ , R ⁶ , n and t are as defined above. MAX ₂ may for example be Cp*Ir(Cl) ₂ .

In one embodiment, M 1 is different from M2. In a preferred embodiment, M 1 and/or M is iridium or ruthenium, more preferably Ir(III), Ru(II) or Ru(III). Even more preferably M 1 and/or M 2 is iridium, and most preferably Ir(III). In another embodiment

M 1 and 2 are the same, preferably M 1 and M2 are both iridium or ruthenium, and more preferably M 1 and M 2 are both Ir(III), Ru(II) or Ru(III). Therefore, in a preferred embodiment of the process according to the invention, M 1 and/or M 2 is iridium or ruthenium, preferably Ir(III), Ru(II) or Ru(III), more preferably iridium, most preferably Ir(III). In a preferred embodiment of the process according to the invention the compound according to Formula I is an iridium compound according to Formula III as depicted below, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

In another preferred embodiment of the process according to the invention the compound according to Formula II is an iridium compound according to Formula IV, wherein R ¹ , R ² , R ⁴ , R ⁶ and X are as defined above for a compound according to Formula II.

In yet another preferred embodiment of the process according to the invention the compound according to Formula I is a ruthenium compound according to Formula V, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

In yet another preferred embodiment of the process according to the invention the compound according to Formula I is a ruthenium compound according to Formula VI, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

VI

In a particularly preferred embodiment of the process according to the invention the compound according to Formula I is an iridium compound according to Formula VII, as shown below. In VII, (Z ^v" ) _w is for example (PF6 ^~ ) ₂ -

In another particularly preferred embodiment of the process according to the invention the compound according to Formula II is an iridium compound according to Formula VIII, as shown below.

VII VIII

Compounds according to Formulas I and II are closely interrelated. The open structure II may be converted into the bridged structure I by reaction with for example a silver salt such as for example AgPF ₆ . In addition, when dissolved I and II are in equilibrium, as is shown below. When II is for example dissolved in water, bridged structure I is formed.

II I

Whether I or II prevails in solution is depending on the solvent, on the nature of M ¹ , M ² and X and on the concentration of I and/or II and free X ^v~ . As a result of this equilibrium, compounds with an open structure II as well as compounds with a bridged structure I can be applied as a catalyst in the process according to the invention.

As an example, the equilibrium between iridium compounds VII and VIII is depicted below. When VIII is dissolved in water, the open compound VIII is in equilibrium with the bridged compound VII, whereby the large majority (about 95% or more) is in the bridged form VII.

VTII VTT

The invention relates to a process for the oxidation of water, in other words, to a process for the production of molecular oxygen (0 ₂ ) from water. The process is catalysed by a compound of the structure I or II. As was described above, the oxidation of water under the formation of 0 ₂ can be considered as the first half reaction of the water splitting process, wherein water is converted into 0 ₂ and H ₂ .

The second half reaction of the water splitting process is the reduction of hydrogen ions into molecular hydrogen, H ₂ . This step takes place in the presence of a hydrogen evolution catalyst. Many catalysts for this reaction are known in the art, including homogeneous and heterogeneous catalysts as well as enzymes. Examples of hydrogen evolution catalysts include catalysts comprising metals from the platinum group (i.e. a platinum electrode), hydrogenase catalysts such as for example di-iron carbonyl as disclosed in A.M. Kluwer et al., PNAS 2009, 106, 10460 (incorporated by reference), and iron-nickel complexes such as for example disclosed in B. E. Barton et al., J. Am. Chem. Soc. 2010, 132, 14877 (incorporated by reference), Dubois nickel catalyst such as for example disclosed in M.L. Helm et al., Science 2011, 333, 863 (incorporated by reference) or cobalt oxime hydrogenase catalysts. ¹⁰

In a preferred embodiment, the process according to the invention further comprises the step of reduction of hydrogen ions under formation of molecular hydrogen (H ₂ ). This step is preferably executed in the presence of a catalyst, for example a hydrogen evolution catalyst or a hydrogenase catalyst.

As was explained above, in order to be able to study the oxidation half reaction of the water splitting process, the oxidation half reaction wherein 0 ₂ is formed needs to be separated from the reduction half reaction wherein H ₂ is formed. The formation of H ₂ is prevented by adding a strong oxidant, such as for example Ce(IV), that accepts the electrons that are liberated in the first half reaction of the water splitting process. A model system for testing a catalyst for water oxidation activity is the chemical oxidation of water in the presence of Ce(IV) as the electron acceptor. The invention therefore also relates to a process for the chemical oxidation of water in the presence of an electron acceptor, wherein the process for the water oxidation as defined above is performed in the presence of an oxidant. An example of such an oxidant is Ce(IV), such as for example cerium ammonium nitrate (NH ₄ ) ₂ Ce(N03)6.

An input of energy is needed to drive the water oxidation process, and ultimately the water splitting process. In the process according to the invention, energy is therefore provided to the reaction mixture. In a preferred embodiment, the reaction mixture is exposed to electrical energy. In a more preferred embodiment, the reaction mixture is exposed to solar energy. As is known to the person skilled in the art, solar energy comprises solar light (solar radiation, i.e. electromagnetic radiation) and heat from the sun. The spectrum of solar radiation striking the earth's atmosphere spans a range of 100 nm to about 1 mm and can be divided into the following ranges, in increasing order of wavelengths:

- Ultraviolet C or (UVC) range: about 100 to about 280 nm. However, due too absorption by the atmosphere very little reaches the earth's surface.

- Ultraviolet B or (UVB) range: about 280 to about 315 nm. Also UVB radiation is mostly absorbed by the atmosphere before reaching the earth's surface.

- Ultraviolet A or (UVA) range: about 315 to about 400 nm.

- Visible range or visible light: about 380 to about 780 nm.

- Infrared range: about 700 nm to about 10 ⁶ nm (1 mm). It is responsible for an important part of the electromagnetic radiation that reaches the earth. It is also divided into three types on the basis of wavelength:

o Infrared-A: 700 nm to 1400 nm

o Infrared-B: 1400 nm to 3000 nm

o Infrared-C: 3000 nm to 1 mm.

In a preferred embodiment, the reaction mixture is exposed to electromagnetic radiation, preferably to electromagentic radiation having a wavelength of about 100 nm to about 10 ⁶ nm (i.e. about 1 mm). More preferably, the reaction mixture is exposed to electromagnetic radiation having a wavelength of about 280 nm to about 1 mm. The reaction mixture may for example be exposed to electromagnetic radiation in the ultraviolet range, in the visible range, in the infrared range, or combinations thereof, wherein said ranges are as defined above. In a further preferred embodiment, the reaction mixture is exposed to electromagnetic radiation in the range of about 315 nm to about 1 mm. In a most preferable embodiment, the reaction mixture is exposed to solar radiation.

The process for the oxidation of water according to the invention comprises a step wherein a compound according to Formula I or II is reacted with water, wherein I and II are described in more detail above. The concentration of the compound according to Formula I, or of the compound according to Formula II, may for example be in the range of about 10 ^"10 M to about 10 ^"1 M, preferably in the range of about 10 ^"9 M to about 10 - ^" 2 M, more preferably in the range of about 10 - ^" 8 M to about 10 - ^" 3 M and most preferably in the range of about 10 ^"7 M to about 10 ^"4 M.

In a preferred embodiment, water is used as the solvent in the process according to the invention. Optionally, one or more additional solvents may be present, such as for example acetonitrile (MeCN) or nitromethane (MeN0 ₂ ).

Optionally, the process according to the invention may be executed in the presence of one or more additives that enhance the nucleophilicity of water. Examples of such additives include Ag ⁺ , Ca ²⁺ , Zn ²⁺ , etc. The presence of Ag ⁺ is particularly preferred. In a preferred embodiment, the process according to the invention is therefore executed in the presence of Ag ⁺ . Ag ⁺ may be added to the reaction mixture in the form of a silver salt, preferably Ag ₂ 0.

The process according to the invention may for example be performed at a temperature in the range of about 0°C to about 100°C, preferably about 5°C to about 90°C, more preferably about 10°C to about 70°C, even more preferably of about 15°C to about 50°C. Most preferably, the process according to the invention is executed at ambient temperature, i.e. without additional heating or cooling. Ambient temperature may range from about 15°C to about 35°C.

The pH at which the process according to the invention is executed strongly depends on the nature of the compound according to Formula I or according to Formula II, particularly on the choice of metal and the choice of ligands. For example when the metal is ruthenium, a pH of 8 or more is preferred, whereas when the metal is iridium, a pH of 2 or lower is preferred.

The process according to the invention is catalysed by compounds according to Formula I or II. For example, VIII catalyses the oxidation of water in the presence of Ce(IV) as an electron acceptor with a turnover frequency TOF of 0.58 mol mol ^"1 0 ₂ s ^"1 and with a turnover number TON of 20000.

As described above, the process according to the invention may be executed in solution, wherein compund I or II is dissolved in water and, optionally, one or more additional solvents such as for example acetonitrile (MeCN) or nitromethane (MeN0 ₂ ). As is known to a person skilled in the art, an electrochemical cell is a device capable of either deriving (electrical) energy from chemical reactions, or facilitating chemical reactions through the introduction of (electrical) energy.

The process according to the invention may be executed in an electrochemical cell comprising said solution of I or II. Alternatively, the process according to the invention may be executed in an electrochemical cell whereby the electrochemical cell comprises an electrode comprising a compound according to Formula I or II (see below for a more detailed description of the electrode comprising a compound according to Formula I or II).

An example of a chemical cell is an electrolysing device. An electrolysing device comprises a positive and a negative electrode. In an electrolysing device electrical energy, preferably derived from renewable sources, is used to produce hydrogen, in other words, electricity is converted into hydrogen. On the positively charged electrode water is oxidized to molecular oxygen (0 ₂ ) and protons (H ⁺ ). The electrons that are formed are transported through the electrolysing device from the positive to the negative electrode. On the negative electrode protons are reduced by a hydrogen evolution catalyst or a hydrogenase catalyst to form molecular hydrogen (H ₂ ). Said protons are transferred from one side of the electrolysing device to the other side, where they are reduced. The negative electrode comprises said hydrogen evolution catalyst or hydrogenase catalyst. As was described above in more detail, many of such catalysts are known in the art and commercially available. In a preferred embodiment the process according to the invention is executed in an electrolysing device. In a further preferred embodiment the process according to the invention is executed in an electrolysing device, wherein I or II is comprised by or bonded to the positive electrode of the electrolysing device.

Another example of an electrochemical cell is a solar fuel cell. In a solar fuel cell light is captured by a chromophore, a sensitizer or a light harvesting antenna resulting in a charge separation. Said chromophores, sensitizers and light harvesting antennae are well known in the art, and many types are commercially available. The charge separation is used to oxidize water at one side of the device, and to reduce protons on the other side of the device, similar to an electrolysing device. Electrons are transported through the charge separation compartment in order to allow the reaction to continue. In a preferred embodiment the process according to the invention is executed in a solar fuel cell, under the influence of electromagnetic radiation.

The invention therefore also relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula I is reacted with water, wherein I is defined as above, and wherein the process is executed in an electrochemical cell. In a preferred embodiment, the electrochemical cell is an electrolysing device or a fuel cell, in particular a solar fuel cell.

The invention further relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula II is reacted with water, wherein II is defined as above, and wherein the process is executed in an electochemical cell. In a preferred embodiment, the electrochemical cell is an electrolysing device or a fuel cell, in particular a solar fuel cell.

In addition, the invention relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula I is reacted with water, wherein I is defined as above, and wherein the process is executed in a device wherein water is converted into H ₂ and 0 ₂ . Preferably, said process is executed under irradiation with electromagnetic radiation, more preferably with solar radiation.

The invention also relates to a process for the oxidation of water under formation of molecular oxygen, comprising a step wherein a compound according to Formula II is reacted with water, wherein II is defined as above, and wherein the process is executed in a device wherein water is converted into H ₂ and 0 ₂ . Preferably, said process is executed under irradiation with electromagnetic radiation, more preferably with solar radiation.

In a preferred embodiment, the process according to the invention is executed under exposure to electromagnetic radiation, particularly solar radiation, preferably to electromagentic radiation having a wavelength of about 100 nm to about 10 ⁶ nm (i.e. about 1 mm). More preferably, the reaction mixture is exposed to electromagnetic radiation having a wavelength of about 280 nm to about 1 mm. The reaction mixture may for example be exposed to electromagnetic radiation in the ultraviolet range, in the visible range, in the infrared range, or combinations thereof, wherein said ranges are as defined above. In a further preferred embodiment, the reaction mixture is exposed to electromagnetic radiation in the range of about 315 nm to about 1 mm.

Compounds of the structure I and II

The invention also relates to a compound according to Formula I:

I wherein M ¹ , M ² , A ¹ , A ² , B, X, D, R ¹ , R ² , R ⁶ , E, q, Z, v and w are defined as above for a compound according to Formula I. Preferred embodiments for the compound according to Formula I are also described in more detail above.

In a specific embodiment, the invention relates to a compound according to Formula I as defined above, with the proviso that I is not:

- [(Cp*RhCl) ₂ [l, -(l,8-dimethylnaphthalene)-3,3'-(dimethyldiimadazol-2- ylidene)](PF ₆ ) ₂ (3a-Rh);

- [(Cp*IrCl) ₂ [l, -(l,8-dimethylnaphthalene)-3,3'-(dimethyldiimadazol-2- ylidene)](PF ₆ ) ₂ (3a-Ir); or

- [(Cp*IrCl) ₂ [l, -(l,8-dimethylnaphthalene)-3,3'-(diisopropyldiimadazol-2- ylidene)](PF ₆ ) ₂ (3b-Ir).

The invention further relates to a compound according to Formula II: wherein M ¹ , M ² , A ¹ , A ² , B, X, D, R ¹ , R ² , R ⁶ and E are as defined as above for a compound according to Formula II. Preferred embodiments of the compound according to Formula II are also described in more detail above.

In a specific embodiment, the invention relates to a compound according to Formula II as defined above, with the proviso that II is not the compound according to Formula XXII:

The compound according to Formula XXII, as depicted above, was disclosed in B. Cetinkaya et al., J of Mol. Catalysis: Chemical 118, 1997, L1-L4, incorporated by reference. This compound was applied in the synthesis of 2,3-dimethylfuran. No potential use this compound for the oxidation of water is mentioned in Cetinkaya et al..

In a specific embodiment, the invention relates to a compound according to Formula II as defined above, with the proviso that II is not:

- [(Cp*RhCl ₂ ) ₂ [l, -(l,8-dimethylnaphthalene)-3,3'-(dimethyldiimadazol-2- ylidene)] (2a-Rh);

- [(Cp*IrCl ₂ ) ₂ [l,l'-(l,8-dimethylnaphthalene)-3,3'-(dimethyldiimadaz ol-2- ylidene)] (2a-Ir); or

- [(Cp*IrCl ₂ ) ₂ [l,l'-(l,8-dimethylnaphthalene)-3,3'-(diisopropyldiima dazol- 2-ylidene)] (2b-Ir).

Compounds 2a-Rh, 2a-Ir, 2b-Ir, 3a-Rh, 3a-Ir and 3b-Ir, as depicted below, were disclosed in K. Ogata et al., J. Organometal. Chem. 2010, 695, 1675, incorporated by reference.

2a-Ir (R = Me, M = Ir) 3a-Ir (R = Me, M = Ir)

2b-Ir (R = 'Pr, M = Ir) 3b-Ir (R = 'Pr, M = Ir)

2a-Rh (R = Me, M = Rh) 3a-Rh (R = Me, M = Rh)

These compounds were applied in the intramolecular hydroamination reaction of various 2-ethynylanilines to give the corresponding indole compounds. No other potential use of these compounds is mentioned in Ogata et al.. A preferred compound according to Formula I is an iridium compound according to Formula III as depicted below, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

A preferred compound according to Formula II is an iridium compound according to Formula IV, wherein R ¹ , R ² , R ⁴ , R ⁶ and X are as defined above for a compound according to Formula II.

Preferably, R 1 and/or R2 are methyl and more preferably R 1 and R2 are both methyl. Preferably, R ⁴ and/or R ⁶ is hydrogen and more preferably R ⁴ and R ⁶ are both hydrogen. More preferably, R ¹ and R ² are methyl and R ⁴ or R ⁶ are hydrogen, and most preferably R ¹ and R ² are both methyl and R ⁴ and R ⁶ are both hydrogen. Therefore iridium compounds according to Formula VII and VIII as shown below are particularly preferred. In VII, (Z ^v" ) _w is for example (PF6 ^~ ) ₂ -

VII V III

Another preferred compound according to Formula I is a ruthenium compound according to Formula V, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

Yet another preferred compound according to Formula I is a ruthenium compound according to Formula VI, wherein R ¹ , R ² , R ⁴ , R ⁶ , X, Z, v and w are as defined above for a compound according to Formula I.

Preferably, R 1 and/or R2 are methyl and more preferably R 1 and R2 are both methyl. Preferably, R ⁴ and/or R ⁶ is hydrogen and more preferably R ⁴ and R ⁶ are both hydrogen. More preferably, R ¹ and R ² are methyl and R ⁴ or R ⁶ are hydrogen, and most preferably R ¹ and R ² are both methyl and R ⁴ and R ⁶ are both hydrogen.

As was described above, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and/or R V may also be a linking unit for linkage to a solid material. A wide variety of linking units for attaching a molecule to a solid material, for example a solid support or the surface material of an electrode, are known in the art. A general structure for a linking unit is IXa:

-S-L,

IXa wherein:

L is a functional group selected from the group consisting of hydrogen, halogen, R , -CH=C(R ⁸ ) ₂ , -C≡CR ⁸ , -[C(R ⁸ ) ₂ C(R ⁸ ) ₂ 0] _q -R ⁸ , wherein q is in the range of 1 to 200, -CN, -N ₃ , -NCG, -GCN, -GR ⁸ , -N(R ⁸ ) ₂ , - ⁺ N(R ⁸ ) _3, -C(G)N(R ⁸ ) ₂ , -C(R ⁸ ) ₂ GR ⁸ , -C(G)R ⁸ , -C(G)GR ⁶ , -S(0)R ⁸ , -S(0) ₂ R ⁸ , -S(0)OR ⁸ , -S(0) ₂ OR ⁸ , -S(0)N(R ⁸ ) ₂ , -S(0) ₂ N(R ⁸ ) _2, -OS(0)R ⁸ , -OS(0) ₂ R ⁸ , -OS(0)OR ⁸ , -OS(0) ₂ OR ⁸ , -P(0)(R ⁸ )(OR ⁸ ), -P(0)(OR ⁸ ) ₂ , -OP(0)(OR ⁸ ) ₂ , -Si(R ⁸ ) ₃ , -GC(G)R ⁸ , -GC(G)GR ⁸ , -GC(G)N(R ⁸ ) ₂ , -N(R ⁸ )C(G)R ⁸ , -N(R ⁸ )C(G)GR ⁸ and -N(R ⁸ )C(G)N(R ⁸ ) ₂ , wherein G is oxygen or sulphur and wherein R is independently selected from the group consisting of hydrogen, halogen, d - C ₂₄ alkyl groups, C ₆ - C ₂₄ (hetero)aryl groups, C ₇ - C ₂₄ alkyl(hetero)aryl groups and C ₇ - C ₂₄ (hetero)arylalkyl groups;

S is selected from the group consisting of linear or branched CrC ₂ oo alkylene groups, C ₂ -C ₂ oo alkenylene groups, C ₂ -C ₂ oo alkynylene groups, C ₃ -C ₂ oo cycloalkylene groups, C ₅ -C ₂ oo cycloalkenylene groups, Cg-C ₂ oo cycloalkynylene groups, C ₇ -C ₂ oo alkylarylene groups, C ₇ -C ₂₀ o arylalkylene groups, C ₈ -C ₂₀ o arylalkenylene groups, C ₉ -C ₂₀ o arylalkynylene groups; and

optionally the alkylene groups, alkenylene groups, alkynylene groups, cycloalkylene groups, cycloalkenylene groups, cycloalkynylene groups, alkylarylene groups, arylalkylene groups, arylalkenylene groups and arylalkynylene groups may be substituted, and optionally said groups may be interrupted by one or more heteroatoms, preferably 1 to 100 heteroatoms, said heteroatoms preferably being selected from the

8 8

group consisting of O, S and NR , wherein R is defined as above. Most preferably, the heteroatom is O.

Preferably, G is oxygen. More preferably, L is selected from the group consisting of -S(0)R ⁸ , -S(0) ₂ R ⁸ , -S(0)OR ⁸ , -S(0) ₂ OR ⁸ , -S(0)N(R ⁸ ) ₂ , -S(0) ₂ N(R ⁸ ) _2, -OS(0)R ⁸ , -OS(0) ₂ R ⁸ , -OS(0)OR ⁸ , -OS(0) ₂ OR ⁸ , -P(0)(R ⁸ )(OR ⁸ ), -P(0)(OR ⁸ ) ₂ , -OP(0)(OR ⁸ ) ₂

8 8

and -Si(R ) ₃ , wherein R is as defined above. Furthermore, the functional group L may optionally be masked or protected. The R groups may be selected independently from each other, which means that the three R 8 groups present in, for example, a -Si(R 8 ) ₃ substituent may be different from each other.

Examples of suitable linking units include, but are not limited to, (poly)ethylene glycol diamines (such as for example l,8-diamino-3,6-dioxaoctane or equivalents comprising longer ethylene glycol chains), polyethylene glycol or polyethylene oxide chains, polypropylene glycol or polypropylene oxide chains, l,x-diaminoalkanes wherein x is the number of carbon atoms in the alkane, and the like.

In one embodiment of the invention, the linker is according to Formula Xlb:

IXb wherein:

R ₆ is as defined above;

K is selected from the group consisting of C(R ₆ ) ₂ , O, S and N(R ⁶ ) wherein R ⁶ is as defined above;

L is selected from the group consising of -Si(R ⁷ ) ₃ , -S(0) ₂ (OR ⁸ ) and -P(0)(OR ⁸ ) ₂ ;

R is as defined above;

m is 0 - 200; and

optionally, the [C(R ₆ ) ₂ ] _m moiety is interrupted by one of more heteroatoms selected from the group consisting of N, O and S.

Preferably, K is C(R ₆ ) ₂ or O. R may for example be methyl, ethyl, propyl, i- propyl, w-butyl, s-butyl, i-butyl, i-butyl, w-pentyl, i-pentyl, cyclopentyl, w-hexyl, s- hexyl or cyclohexyl. Preferably, R is hydrogen.

As an example, a compound according to Formula I and a compound according to Formula II wherein R is a linker -[C(R6) ₂ ] _m -K-L IXb as defined above, are shown below.

Several examples of a linker according to Formula IX attached to the carbene part of a compound according to Formula I or II are depicted below.

Particularly preferred are compounds according to Formula III and IV as described above, wherein R 1 and/or R 2 are a linker according to Formula IXa, more preferably IXb, as defined above.

The invention thus also relates to a compound of Formula I wherein R 1 , R2 , R 3 , R ⁴ , R ⁵ , R ⁶ and/or R ⁷ is a linking unit for linkage to a solid material, wherein the linking unit is according to Formula IXa or IXb as defined above, and to a compound of Formula II wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and/or R V is a linking unit for linkage to a solid material, wherein the linking unit is according to Formula IXa or IXb as defined above.

In a preferred embodiment, the solid material is a solid support or the surface material of an electrode. Surface materials of an electrode are known in the art and include for example carbon, silicon, indium tin oxide (ΓΓΟ), iron oxide (Fe _x O _y ) and titanium oxide (Ti0 ₂ ). A large variety of solid supports are known in the art. These include for example silica such as Si0 ₂ , alumina such as A1 ₂ 0 ₃ and zeolites.

The invention further relates to a solid material, such as for example a solid support or the surface material of an electrode, comprising a compound according to Formula I, and to a solid material comprising a compound according to Formula II.

The compound according to Formula I or II may for example be attached to the solid material via a linking unit, for example a linking unit as defined above. The invention therefore further relates to a solid material, such as for example a solid support or the surface material of an electrode, wherein the solid material is attached to a compound according to Formula I via a linking unit. The invention also relates to a solid material, such as for example a solid support or the surface material of an electrode, wherein the solid material is attached to a compound according to Formula II via a linking unit. Preferably, the linking unit is according to Formula IXa or IXb as defined above. When a compound according to Formula I or II is attached to a solid material via a linking unit according to Formula IXa or IXb, the attachment to the solid material occurs via the L-group of the linking unit.

As is known to a person skilled in the art, an electrochemical cell is a device capable of deriving (electrical) energy from chemical reactions or facilitating chemical reactions through the introduction of (electrical) energy. An electrochemical cell comprises two half-cells and each half-cell comprises an electrode and an electrolyte. Electrodes are well known in the art materials and include for example a carbon electrode, a silicon electrode, an indium tin oxide (ΓΓΟ) electrode, an iron oxide (Fe _x O _y ) electrode and a titanium oxide (Ti0 ₂ ) electrode. In particular, the invention relates to an electrode, in particular an electrode for an electrochemical cell, wherein the electrode comprises a compound according to Formula I, and to an electrode, in particular an electrode for an electrochemical cell, wherein the electrode comprises a compound according to Formula II. The invention further relates to an electrochemical cell comprising an electrode, wherein the electrode comprises a compound according to Formula I, and to an electrochemical cell comprising an electrode, wherein the electrode comprises a compound according to Formula II. In a preferred embodiment, the electrochemical cell is an electrolysing device or a fuel cell, in particular a solar fuell cell. An electrochemical cell, an electrolysing device and a fuel cell were described in more detail above.

Compounds according to Formula II wherein M 1 and M2 are the same and A 1 and A are the same can be prepared by reacting carbene salt XII with a transmetallation reagent, preferably Ag ₂ 0, to form the corresponding carbenoid complex, or with a base to form a carbene compound, followed by complexation of said carbenoid complex or carbene compound with [A ¹ M ¹ Q ₂ ] ₂ , wherein Q is halogen, preferably Br or CI, as is shown in Scheme 1.

Xll 11

Scheme 1

The invention therefore also relates to a process for the preparation of a compound according to Formula II, preferably a process wherein M 1 = M2 and A 1 = A , the process comprising the steps of:

(a) reacting a carbene salt according to Formula XII:

wherein R ¹ , R ² , R ⁶ , B, D, E, Z, v and w are as defined above, with the proviso that (w · v) is 2;

with a transmetallation agent to form the corresponding carbenoid complex or with a base to form the corresponding carbene compound, followed by

(b)reacting said carbenoid complex or said carbene compound with [A ¹ M ¹ Q ₂ ] ₂ ,

wherein A ¹ and M ¹ are as defined above and wherein Q is selected from the group consisting of F, CI, Br and I, preferably Br or CI.

In a preferred embodiment, the carbene salt according to Formula XII is a carbene salt according to Formula XIII wherein R ¹ , R ² , R ⁶ , B, D, Z, v and w are as defined above. More preferably the carbene salt is according to Formula XIV, wherein Z ^v" is preferably Br ^" or CI ^" .

XIII XIV

Compounds according to Formula II wherein M 1 and M 2 and A 1 and A2 differ from each other can be prepared by reacting a carbene salt according to Formula XV with a transmetallation reagent, preferably Ag ₂ 0, to form the corresponding carbenoid complex, or with a base to form the corresponding carbene compound, followed by complexation of said carbenoid complex or said carbene compound with [Α ^Χ Μ^ ₂ ] ₂ , wherein Q is halogen, preferably Br, to form mononuclear M ¹ metal complex XVI. Reaction of XVI with a compound XVII gives mononuclear M ¹ metal complex carbene salt XVIII, which is transformed into the dinuclear M 1 -M 2 compound according to Formula II via reaction with a base or a transmetallation reagent, preferably Ag ₂ 0, followed by complexation with [A 2 M 2 Q ₂ ] ₂ to form II. This sequence is shown in Scheme 2.

II XVIII

Scheme 2

The invention therefore relates to a process for the preparation of a compound according to Formula II, the process comprising the steps of:

(a) reacting a carbene salt according to Formula XV: wherein R ¹ , R ⁶ , E, D and B are as defined above and Q is selected from the group consisting of F, CI, Br and I, preferably, Br,

with a transmetallation reagent to form the corresponding carbenoid complex or with a base to form the corresponding carbene compound;

(b) complexation of said carbenoid complex or carbene compound with

[A ¹ M ¹ Q ₂ ] ₂ , wherein A ¹ , M ¹ and Q are as defined above, to form XVI;

(c) reaction of XVI with a compound according to Formula XVII:

XVII wherein R 2 , B and E are as defined above, to form mononuclear M 1 metal complex carbene salt XVIII;

(d) reaction of XVIII with a transmetallation reagent to form the corresponding carbenoid complex or with a base to form the corresponding carbene compound;

(e) complexation of said carbenoid complex or carbene compound with

[A 2"M 2"Q ₂ ] ₂ , wherein A 2", 2 and Q are as defined above, to form II.

As an example the preparation of a compound according to Formula II wherein M ¹ is iridium and M is ruthenium is shown below in Scheme 3.

1. Ag ₂ 0

2. [Ru(p-cymene)CI ₂ ] ₂

Scheme 3

As was described in more detail above, a compound according to Formula II can be converted into a compound according to Formula I by dissolving the compound according to Formula II in water, by reacting the compound according to Formula II with a silver salt such as for example AgPF ₆ , or a thallium salt, or by salt metathesis.

The invention thus also relates to a process for the preparation of a compound according to Formula I, wherein a compound according to Formula II is:

(a) dissolved in water, or

(b) reacted with a silver salt or a thallium salt, or

(c) subjected to a salt metathesis process

to form a compound according to Formula I. As was described in more detail above, a compound according to Formula I or II may be used as a catalyst in a process for the oxidation of water under formation of molecular oxygen. Therefore the invention also relates to the use of a compound according to Formula I in an oxidation of water under formation of molecular oxygen (0 ₂ ), and to the use of a compound according to Formula II in an oxidation of water under formation of molecular oxygen.

As was described above, the oxidation of water under formation of 0 ₂ may be regarded as the first half reaction of a water splitting process. The invention thus also relates to the use of a compound according to Formula I in a water splitting process, and to the use of a compound according to Formula II in a water splitting process.

Examples

General procedures

All syntheses were carried out under an argon or a dinitrogen atmosphere. Every solution addition or transfer was performed via syringe. Nuclear Magnetic Resonance (NMR) experiments were performed on a Varian Inova spectrometer ( ¹ H: 500 MHz), a Varian Mercury (1H: 300 MHz) or a Bruker ARX-400 (1H: 400 MHz). Chemical shifts are referenced to the solvent signal. [IrCp*Cl ₂ ] ₂ and 3,3'-(propane-l,3-diyl)bis(l- methyl-imidazolium)bromide were prepared via literature procedures. 11 ' 12

Electrochemistry was recorded on an autolab PGSTAT10; oxygen levels were measured by an interscience compact GC with TCD detector and a Hansatech Oxygraph Clark electrode. Elemental analysis were sent to Mikroanalytisches Labor Kolbe.

Example 1: Synthesis of a VIII:

A 2.8 equivalent excess of 3,3'-(propane-l,3-diyl)bis(l-methyl- imidazolium)bromide and 100 mg Ag ₂ 0 were dissolved in MeCN and stirred overnight in the dark. The white grayish solid was filtered and washed with MeCN. 58 mg of this solid and 200 mg [IrCp*Cl ₂ ] ₂ were dissolved in dichloromethane and stirred for two hours. The solution was filtered over Celite, concentrated and precipitated with diethylether. Filtration of the yellow solid gave 182 mg of VIII in 60 % yield. Crystals suitable for X-ray diffraction were grown from slow evaporation of a solution of VIII in chloroform.

1H NMR (CDCI ₃ ): 7.05 (s, 2H, NHC), 6.88 (s, 2H, NHC), 4.70 (m, 2H, CH ₂ ), 3.92 (s, 6Η, Me), 3.90 (d, 2Η, CH ₂ ), 2.5 (m, 2Η, CH ₂ ), 1.61 (s, 30Η, Cp*). 1H NMR (D ₂ 0): 7.50 (s, 2H, NHC), 7.25 (s, 2H, NHC), 4.84 (t, 2H, CH ₂ ), 4.41 (s, 6Η, Me), 3.80 (d, 2Η, CH ₂ ), 2.8 (m, 1Η, CH ₂ ), 2.2 (m, 1Η, CH ₂ ), 1.63 (s, 30Η, Cp*).

Elemental analysis of VIII found (calculated) C: 37.18 (37.20), Η: 4.37 (4.63), N: 5.35 (5.60).

Example 2: Synthesis of VII [(Z ^v~ ) _w is (OTf) ₂ ]

100 mg of VIII and 50 mg AgOTf were dissolved in acetone. The mixture was stirred for 2 hours and filtered over Celite. The solution was evaporated yielding VII(OTf) ₂ as a orange solid. Crystals suitable for X-ray diffraction were obtained by slow evaporation of an acetone solution of VII (OTf) ₂ (30 % isolated yield).

1H NMR (acetone-d ₆ ): 7.50 (s, 2Η, NHC), 7.26 (s, 2H, NHC), 4.85 (t, 2H, CH ₂ ), 4.39 (d, 2Η, CH ₂ ), 3.79 (s, 6Η, Me), 2.9 (m, 1Η, CH ₂ ), 2.2 (m, 1Η, CH ₂ ), 1.61 (s, 30Η, Cp*).

¹⁹ F NMR (acetone-d ₆ ): -78.81 (OTf).

Example 3: Synthesis of VII(OAc)4

50 mg of VIII was treated with 36 mg of AgOAc in chloroform. The solution was filtered over Celite and evaporated yielding VII(OAc) ₄ as a yellow solid.

1H NMR (CD ₂ C1 ₂ ): 7.31 (s, 2Η, NHC), 6.89 (s, 2H, NHC), 4.70 (m, 2H, CH ₂ ), 3.92 (m, 2Η, CH ₂ ), 3.82 (s, 6Η, Me), 2.30 (m, 2Η, CH ₂ ), 1.98 (s, 12Η, OAc), 1.62 (s, 30Η, Cp*).

Example 4: Synthesis of XX

XX A 2.8 equivalent excess of 3,3'-(propane-l,3-diyl)bis(l-methyl-imidazolium) bromide and 500 mg Ag ₂ 0 were dissolved in MeCN and stirred overnight in the dark. The white grayish solid was filtered and washed with MeCN. 150 mg of this solid and 160 mg [Ru(p-cymene)*Cl ₂ ] ₂ were dissolved in dichloromethane and stirred for two hours. The solution was filtered over Celite, concentrated and precipitated with diethylether.

1H NMR (CDC1 ₃ ): 7.14 (s, 2H, NHC), 6.94 (s, 2H, NHC), 5.39 (m, 6H, cymene and CH ₂ ), 5.09 (d, 4H, cymene), 4.75 (bs, 2H, CH ₂ ), 3.94 (s, 6H, Me), 2.92 (m, 2H, iPr), 2.34 (m, 2H, CH ₂ ), 1.99 (s, 6H, Me), 1.24 (d, 12H, iPr).

Example 5: Synthesis of X

2 CI ^"

XXI

Compound XX was dissolved in water resulting in spontaneous formation of the bridged structure XXI.

1H NMR (CDCI ₃ ): 7.27 (s, 2H, NHC), 7.05 (s, 2H, NHC), 5.86 (m, 2H, cymene), 5.57 (d, 2H, cymene), 5.51 (d, 2H, cymene), 5.24 (d, 2H, cymene), 4.51 (bs, 2H, CH ₂ ), 4.11 (bs, 2H, CH ₂ ) 3.50 (s, 6H, Me), 2.82 (m, 2H, iPr), 2.63 (m, 1H, CH ₂ ), 2.02 (m, 1H, CH ₂ ) 1.86 (s, 6H, Me), 1.26 (d, 12H, iPr).

Example 6: Catalytic experiments

In a typical catalytic run, 548 mg of cerium ammonium nitrate (1 mmol) was dissolved in 2 mL of demineralized water and placed with a stirring bar in a roundbottom Schlenk flask equipped with a septum. The Schlenk flask was connected to needle through which overpressure can escape into a burette, from where the amount of gas production can be determined very precisely, as is described in more detail in D.G.H. Hetterscheid and J.N.H. Reek, Chem. Commun. 2011, 47, 2712, incorporated by reference. The appropriate amount of catalyst (7 x 10 ^"5 mmole - 7 x 10 ^"4 mmole) was dissolved in 1 mL demineralized water and carefully added through the septum into the reaction flask via a syringe. The mixture was vigorously stirred throughout the entire reaction time and the volume of produced oxygen was monitored and recorded in time. The headspace of the setup was led over a Clark electrode or injected into a GC with TCD detector, showing that the headspace was significantly enriched with dioxygen. Control experiments that were carried out under standard conditions without catalyst did not produce more than 0.1 mL of gas, even after hours. For example VIII and VII(OAc) ₄ (which is defined as VII wherein all four X ligands are acetate (OAc) instead of CI) were evaluated for water oxidation activity upon chemical oxidation with 0.33 M Ce ^IV (pH = 0.4). The kinetic profile of compound VIII shows a first order dependence in catalyst and a second order dependence in [Ce ^IV ] (or pseudo-zeroth order at very low catalyst concentrations). The kinetic profile of VIII is the same when Z ^v" is CI ^" and OTf . In addition, the kinetic profile of VII is the same as that of VIII. When all four chlorides of VII are displaced by for example acetate to form VII(OAc) ₄ , the reaction rate drops and the rate order changes to a first order dependency in [Ce(IV)] .

Due to a seemingly second order rate law in [Ce(IV)], the turnover frequency of VIII is strongly dependent on the Ce(IV) concentration. For example the turnover frequency TOF of 0.58 mol mol ^"1 0 ₂ s ^"1 at 0.33 M solutions of Ce(IV) is unprecedentedly high for iridium water oxidation catalysts with a turnover number TON of 20000. Compounds XX and XXI catalysed the oxidation of water with a TOF of 1.8 mol mol ^"1 0 ₂ s ^"1 .

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