**2.1 Structural models for biological Water Oxidizing Complex**

There are many mono, di, tri and tetra nuclear manganese complexes as structural models for the WOC in PSII (Mullins & Pecoraro, 2008).

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 39

**O**

**N N**

**O**

**Mn**

**N**

**O**

**O**

**N**

**O**

(a)

(b) Fig. 2. Schematic structures of a mononuclear Mn(V) (Miller et al., 1998) (a) and a Mn(IV)

Many dinuclear manganese complexes have been studied with different oxo bridge (μ-O) as possible models for the WOC. The relationship between Mn-Mn distance and Mn–O–Mn

In 1988, there was suggestion that WOC had a mononuclear manganes center in close proximity to a trinuclear center and this suggestion was emerged basedon magnetic, spectroscopic, and crystallographic studies in 2000 (Mullins & Pecoraro, 2008). Regarding this issue, study of magnetic, spectroscopic, and crystallographic properties of trinuclear clusters has re-emerged and many trinuclear manganese complexes with linear or bent structure and different oxidation states for manganese ions have been synthesized and

Tetra nuclear manganese complexes are very interesting group of complexes as they could be studied as structural models for the WOC. These complexes show different structures

In 2005, Christou and co-workers have reported the first high oxidation state manganese– calcium cluster. The structure contains [Mn4CaO4] sub-units similar to that found in the WOC in PSII (Fig. 4) (Mishra et al., 2005). Recently, Kotzabasaki et al. (2011) synthesized a heterometallic polymeric complex {[MnIII6Ca2O2(Me-saO)6(prop)6(H2O)2].2MeCN.0.95H2O}n

regarding manganese ions from linear to cubane (Mullins & Pecoraro, 2008) (Fig. 3).

angle has been considered for such complexes (Mullins & Pecoraro, 2008).

complex terminal hydroxo ligands (b) (Yin et al., 2006).

characterized (Mullins & Pecoraro, 2008).

Fig. 1. The WOC and the localization of the substrate water binding sites on the WOC (Umena et al., 2011).

The WOC in PSII is a tetranuclear manganese complex (Fig.1) (Umena et al., 2011). However, mononuclear models are useful and simple complexes for the isolation of highvalent complexes. Regarding these mononuclear manganese complexes, we could obtained many information about spectroscopic properties of Mn(V) compounds as there are no Mn(V) synthetic examples of dinuclear or higher nuclearity structures that have been crystallographically characterized (Fig. 2.) (Mullins & Pecoraro, 2008).

Water and terminal hydroxo ligands in Mn(IV) complexes are very important as there are suggested as one of the substrates for oxygen production in the WOC of PS II. Busch and coworkers reported the first structurally characterized example of a mononuclear Mn (IV) complex with two terminal hydroxo ligands (Fig. 2) (Yin et al., 2006). Using the pH titration of aqueous solutions of the complex, it revealed two acid–base equilibria with pK1 = 6.86 and pK2 = 10, the latter apparently being associated with dimer formation. The complex has shown as a catalyst in olefin epoxidation and hydrogen atom abstraction reactions (Yin et al., 2006).

The Mn(V) = O are very important and it has been proposed as an intermediate in Natural water oxidation. Miller et al have reported the first structurally characterized Mn(V) = O complex (Fig. 2a) (Miller et al., 1998). The complex could not oxidize water but give to us important spectroscopic information of Mn(V) = O group that proposed as an intermediate in biological water oxidation (Miller et al. 1998). However, as considered by Pecoraro, these synthetic Mn(V) = O complexes are stabilized by special ligand(s) and the activity of Mn(V) = O could be completely different from Mn(V) = O of the WOC in PSII (Mullins & Pecoraro, 2008).

**O**

**Mn**

**O**

**O**

**Mn**

**O**

**N N**

**O**

**O**

**O**

**O**

**Mn**

**O**

Fig. 1. The WOC and the localization of the substrate water binding sites on the WOC

The WOC in PSII is a tetranuclear manganese complex (Fig.1) (Umena et al., 2011). However, mononuclear models are useful and simple complexes for the isolation of highvalent complexes. Regarding these mononuclear manganese complexes, we could obtained many information about spectroscopic properties of Mn(V) compounds as there are no Mn(V) synthetic examples of dinuclear or higher nuclearity structures that have been

Water and terminal hydroxo ligands in Mn(IV) complexes are very important as there are suggested as one of the substrates for oxygen production in the WOC of PS II. Busch and coworkers reported the first structurally characterized example of a mononuclear Mn (IV) complex with two terminal hydroxo ligands (Fig. 2) (Yin et al., 2006). Using the pH titration of aqueous solutions of the complex, it revealed two acid–base equilibria with pK1 = 6.86 and pK2 = 10, the latter apparently being associated with dimer formation. The complex has shown as a catalyst in olefin epoxidation and hydrogen atom abstraction reactions (Yin et

The Mn(V) = O are very important and it has been proposed as an intermediate in Natural water oxidation. Miller et al have reported the first structurally characterized Mn(V) = O complex (Fig. 2a) (Miller et al., 1998). The complex could not oxidize water but give to us important spectroscopic information of Mn(V) = O group that proposed as an intermediate in biological water oxidation (Miller et al. 1998). However, as considered by Pecoraro, these synthetic Mn(V) = O complexes are stabilized by special ligand(s) and the activity of Mn(V) = O could be completely different from Mn(V) = O of the WOC in PSII (Mullins & Pecoraro,

**Ca**

**OH2**

**H2O**

**Mn**

**OH2**

**O**

**H2O**

H O H

(Umena et al., 2011).

al., 2006).

2008).

**O**

**O**

**O O**

**O**

O

O

crystallographically characterized (Fig. 2.) (Mullins & Pecoraro, 2008).

Fig. 2. Schematic structures of a mononuclear Mn(V) (Miller et al., 1998) (a) and a Mn(IV) complex terminal hydroxo ligands (b) (Yin et al., 2006).

Many dinuclear manganese complexes have been studied with different oxo bridge (μ-O) as possible models for the WOC. The relationship between Mn-Mn distance and Mn–O–Mn angle has been considered for such complexes (Mullins & Pecoraro, 2008).

In 1988, there was suggestion that WOC had a mononuclear manganes center in close proximity to a trinuclear center and this suggestion was emerged basedon magnetic, spectroscopic, and crystallographic studies in 2000 (Mullins & Pecoraro, 2008). Regarding this issue, study of magnetic, spectroscopic, and crystallographic properties of trinuclear clusters has re-emerged and many trinuclear manganese complexes with linear or bent structure and different oxidation states for manganese ions have been synthesized and characterized (Mullins & Pecoraro, 2008).

Tetra nuclear manganese complexes are very interesting group of complexes as they could be studied as structural models for the WOC. These complexes show different structures regarding manganese ions from linear to cubane (Mullins & Pecoraro, 2008) (Fig. 3).

In 2005, Christou and co-workers have reported the first high oxidation state manganese– calcium cluster. The structure contains [Mn4CaO4] sub-units similar to that found in the WOC in PSII (Fig. 4) (Mishra et al., 2005). Recently, Kotzabasaki et al. (2011) synthesized a heterometallic polymeric complex {[MnIII6Ca2O2(Me-saO)6(prop)6(H2O)2].2MeCN.0.95H2O}n

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 41

reaction of first electron transfer could be achieved whereby the Ru(III) species, obtained in presence of an electron acceptor, was quenched through an intermolecular electron transfer

Fig. 5. Molecule containing a sensitizer covalently linked to a manganese complex.

There are a few manganese complexes that produce oxygen in the presence of different oxidants (Cady et al., 2008; Yagi & Kaneko, 2001; Ruttinger & Dismukes 1997). It is important to know that even if the oxidants may do donate O-atoms that end up in the product oxygen, can nevertheless play an important role in identifying potentially useful

It is shown that [(bpy)2MnIII(*μ*-O)2MnIV(bpy)2]3+ and [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ (bpy: 2,2'-Bipyridine; terpy: 2,2';6',2"-terpyridine) (Tagore et al., 2008; Yagi & Narita, 2004) have water oxidation activity in the presence of (NH4)2[(Ce(NO3)6], NaClO and KHSO5. Yagi and Narita (2004) observed when comparable amount of [(bpy)2MnIII(*μ*-O)2MnIV(bpy)2]3+ or [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ were adsorbed onto Kaolin clay, the addition of a large excess of (NH4)2[(Ce(NO3)6] to its aqueous suspension produced a significant amount of oxygen. The rate of oxygen evolution increased linearly with the amount of [(bpy)2MnIII(*μ*-O)2MnIV(bpy)2]3+ indicating unimolecular oxygen evolution in contrast with bimolecular catalysis of

**2.2 Functional models for biological Water Oxidizing Complex** 

catalytic components for water photolysis and electrolysis.

**2.2.1 Terpy and bpy manganese complexes** 

[(OH2)(terpy)MnIII(*μ*-O)2 MnIV(terpy)(OH2)]3+.

leading to the formation of the tyrosyl radical.

(prop = propionate; Me-saOH2 = 2-hydroxyphenylethanone oxime) (Fig. 4). Nayak et al. (2011) have also synthesized two new polynuclear heterometallic cluster complexes with [MnIII3MIINa] (M= Mn, Ca) core were synthesized using two in situ formed Schiff bases. This compound appeared to catalyse water oxidation in the presence of NaOCl which was followed by using Clark electrode and online mass spectrometry (Nayak et al., 2011).

Fig. 3. Different core types observed in Mn-oxo tetramers.

Fig. 4. The first high oxidation state manganese–calcium cluster reported by Christou and co-workers. The structure contains [Mn4CaO4] sub-units similar to that found in the WOC in PSII (a). The heterometallic polymeric complex

{[MnIII6Ca2O2(MesaO)6(prop)6(H2O)2]. 2MeCN. 0.95H2O}n reported by Kotzabasaki et al. (2011**)** (b) (figure was reproduced from Kotzabasaki et al. (2011)**).**

In 1997, Styring's group reported a molecule containing a sensitizer covalently linked to a manganese complex (Fig. 5). In this compound, the ruthenium chromophore could donate an electron to an external acceptor and consequently oxidize a coordinated manganese ion with rate constants ∼ 50 ns to 10 μs. Then this group showed that how a Ru–Tyr molecular dyad could be used to power the light driven oxidation of a dinuclear Mn2 III,III complex (Lomoth et al., 2006). Similar to PSII, it was shown that upon light absorption, a chain

(prop = propionate; Me-saOH2 = 2-hydroxyphenylethanone oxime) (Fig. 4). Nayak et al. (2011) have also synthesized two new polynuclear heterometallic cluster complexes with [MnIII3MIINa] (M= Mn, Ca) core were synthesized using two in situ formed Schiff bases. This compound appeared to catalyse water oxidation in the presence of NaOCl which was

followed by using Clark electrode and online mass spectrometry (Nayak et al., 2011).

Fig. 3. Different core types observed in Mn-oxo tetramers.

(a) (b)

(b) (figure was reproduced from Kotzabasaki et al. (2011)**).**

PSII (a). The heterometallic polymeric complex

{[MnIII6Ca2O2(MesaO)6(prop)6(H2O)2].

Fig. 4. The first high oxidation state manganese–calcium cluster reported by Christou and co-workers. The structure contains [Mn4CaO4] sub-units similar to that found in the WOC in

In 1997, Styring's group reported a molecule containing a sensitizer covalently linked to a manganese complex (Fig. 5). In this compound, the ruthenium chromophore could donate an electron to an external acceptor and consequently oxidize a coordinated manganese ion with rate constants ∼ 50 ns to 10 μs. Then this group showed that how a Ru–Tyr molecular

(Lomoth et al., 2006). Similar to PSII, it was shown that upon light absorption, a chain

0.95H2O}n reported by Kotzabasaki et al. (2011**)**

III,III complex

2MeCN.

dyad could be used to power the light driven oxidation of a dinuclear Mn2

reaction of first electron transfer could be achieved whereby the Ru(III) species, obtained in presence of an electron acceptor, was quenched through an intermolecular electron transfer leading to the formation of the tyrosyl radical.

Fig. 5. Molecule containing a sensitizer covalently linked to a manganese complex.
