**2.2.1 Terpy and bpy manganese complexes**

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

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 43

active (Ashmawy et al., 1985). All the active complexes exhibit a band at 590 nm in the electronic spectrum, which is absent for the inactive complexes (Ashmawy et al., 1985). The rate of dioxygen evolution is dependent on the manganese (III) complex (first order) and quinone concentrations (order, 0.5) and the pH of the reaction medium, but is independent of solvent. The activation energy for dioxygen evolution in [{Mn(salpd) (H2O)}][CIO4], is 80 kJ mol-1, and the evidence points to homolytic, rather than heterolytic, fission of water (Ashmawy et al., 1985). The wavelength dependence of the reaction rate shows a maximum for photolysis in the 450-600 nm region where the quinone does not absorbed (proposal mechanism is shown

Fig. 7. Proposal mechanism for water oxidation by reduce p-benzoquinone to

dioxygen would evolve from solution (Pecoraro et al, 1994).

Oxygen evolution was also observed upon mixing solid manganese(III) bidentate Schiff base complexes with aqueous solutions of (NH4)2[(Ce(NO3)6] (Najafpour & Boghaei, 2009). However, oxygen evolution was not observed upon mixing solutions of the complexes (in acetonitrile) with Ce(IV). Electron-withdrawing substituents on the Schiff base ligands (NO2, Br) enhanced the reactivity of the manganese complexes toward oxygen evolution. Oxygen evolution was also affected by R groups on the ligands, in the order Me>Et > Bz (Najafpour

It was reported that hydrogen peroxide was produced when HClO4 was added in stoichiometric amounts to solutions of the Mn(IV) Schiff base dimer [Mn2(IV)(BuSalen)2(O)2]' H2O in acetone at 0 °C (Fig. 8) (Boucher & Coe 1975). The complex was converted in 62% yield to the Mn(III) monomer from which the dimer was initially synthesized by air oxidation in chloroform solution. However, Pecoraro suggested that as [Mn2IV(BuSalen)2(O)2]' H2O has catalase activity, if hydrogen peroxide was produced,

in Fig. 7) (Ashmawy et al., 1985).

Hydroquinone (Ashmawy et al., 1985).

& Boghaei, 2009).

The unimolecular oxygen evolution might be explained by either O-O coupling of di-*μ*-O bridges or attack of outer-sphere water onto a *μ*-O bridge in high oxidation species, probably including *μ*-O radical bridges (Yagi & Narita, 2004). When [(bpy)2MnIII(*μ*-O)2MnIV(bpy)2]3+ was dissolved in water containing excess (NH4)2[(Ce(NO3)6], no evolution of gas was observed, and analysis of the gas phase confirmed that no oxygen was formed; however oxygen evolution was observed when these complex as a solid was added to a (NH4)2[(Ce(NO3)6] solution (Yagi & Narita, 2004). Isotopic labeling of the solvent water showed that indeed water was oxidized (Yagi & Narita, 2004). It was also reported that [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ has oxygen evolution activity in the presene of KHSO5 or NaOCl, as primary oxidants. A proposal mechanism was reported by Brudvig group for the reaction (Fig. 6) (Cady et al., 2008). Recently, Brudvig's group have reported the attachment of [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ onto TiO2 nanoparticles via direct adsorption. The resulting surface complexes were characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. Their results showed that the complex attaches to near-amorphous TiO2 by substituting one of its water ligands by the TiO2, as suggested by EPR data (Li et al. 2009).

Fig. 6. Schematic diagram of the proposed mechanism of water oxidation by [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ (figure was reproduced from Cady et al. (2008)).

#### **2.2.2 Schiff base complex**

A number of manganese (III) complexes of the type [{MnL(H2O)}]2+ (L = dianion of O,N,tetradentate Schiff base), in aqueous solution, have been shown to liberate dioxygen and reduce p-benzoquinone to hydroquinone when irradiated with visible light (Ashmawy et al., 1985). The photoactivity is critically dependent on the structure of the ligand, the complex [{Mn(salpd) (H2O)}][CIO4], (salpd = propane-l,3-diylbis(salicylideneiminate)) being the most

The unimolecular oxygen evolution might be explained by either O-O coupling of di-*μ*-O bridges or attack of outer-sphere water onto a *μ*-O bridge in high oxidation species, probably

was dissolved in water containing excess (NH4)2[(Ce(NO3)6], no evolution of gas was observed, and analysis of the gas phase confirmed that no oxygen was formed; however oxygen evolution was observed when these complex as a solid was added to a (NH4)2[(Ce(NO3)6] solution (Yagi & Narita, 2004). Isotopic labeling of the solvent water showed that indeed water was oxidized (Yagi & Narita, 2004). It was also reported that [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ has oxygen evolution activity in the presene of KHSO5 or NaOCl, as primary oxidants. A proposal mechanism was reported by Brudvig group for the reaction (Fig. 6) (Cady et al., 2008). Recently, Brudvig's group have reported the attachment of [(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ onto TiO2 nanoparticles via direct adsorption. The resulting surface complexes were characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. Their results showed that the complex attaches to near-amorphous TiO2 by substituting one of its water

ligands by the TiO2, as suggested by EPR data (Li et al. 2009).

Fig. 6. Schematic diagram of the proposed mechanism of water oxidation by

[(OH2)(terpy)MnIII(*μ*-O)2MnIV(terpy)(OH2)]3+ (figure was reproduced from Cady et al.

A number of manganese (III) complexes of the type [{MnL(H2O)}]2+ (L = dianion of O,N,tetradentate Schiff base), in aqueous solution, have been shown to liberate dioxygen and reduce p-benzoquinone to hydroquinone when irradiated with visible light (Ashmawy et al., 1985). The photoactivity is critically dependent on the structure of the ligand, the complex [{Mn(salpd) (H2O)}][CIO4], (salpd = propane-l,3-diylbis(salicylideneiminate)) being the most

radical bridges (Yagi & Narita, 2004). When [(bpy)2MnIII(*μ*-O)2MnIV(bpy)2]3+

including *μ*-O-

(2008)).

**2.2.2 Schiff base complex** 

active (Ashmawy et al., 1985). All the active complexes exhibit a band at 590 nm in the electronic spectrum, which is absent for the inactive complexes (Ashmawy et al., 1985). The rate of dioxygen evolution is dependent on the manganese (III) complex (first order) and quinone concentrations (order, 0.5) and the pH of the reaction medium, but is independent of solvent. The activation energy for dioxygen evolution in [{Mn(salpd) (H2O)}][CIO4], is 80 kJ mol-1, and the evidence points to homolytic, rather than heterolytic, fission of water (Ashmawy et al., 1985). The wavelength dependence of the reaction rate shows a maximum for photolysis in the 450-600 nm region where the quinone does not absorbed (proposal mechanism is shown in Fig. 7) (Ashmawy et al., 1985).

Fig. 7. Proposal mechanism for water oxidation by reduce p-benzoquinone to Hydroquinone (Ashmawy et al., 1985).

Oxygen evolution was also observed upon mixing solid manganese(III) bidentate Schiff base complexes with aqueous solutions of (NH4)2[(Ce(NO3)6] (Najafpour & Boghaei, 2009). However, oxygen evolution was not observed upon mixing solutions of the complexes (in acetonitrile) with Ce(IV). Electron-withdrawing substituents on the Schiff base ligands (NO2, Br) enhanced the reactivity of the manganese complexes toward oxygen evolution. Oxygen evolution was also affected by R groups on the ligands, in the order Me>Et > Bz (Najafpour & Boghaei, 2009).

It was reported that hydrogen peroxide was produced when HClO4 was added in stoichiometric amounts to solutions of the Mn(IV) Schiff base dimer [Mn2(IV)(BuSalen)2(O)2]' H2O in acetone at 0 °C (Fig. 8) (Boucher & Coe 1975). The complex was converted in 62% yield to the Mn(III) monomer from which the dimer was initially synthesized by air oxidation in chloroform solution. However, Pecoraro suggested that as [Mn2IV(BuSalen)2(O)2]' H2O has catalase activity, if hydrogen peroxide was produced, dioxygen would evolve from solution (Pecoraro et al, 1994).

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 45

solution) (Fujiwara et al., 1985). Using 18O-labeled water showed that indeed water was

Shimazaki et al. (2004) have reported dimanganese complexes of dimeric tetraarylporphyrins linked by 1,2-phenylene bridge (Fig. 10). The catalyst can oxidize olefins such as cyclooctene to form epoxide with stiochiometric amount of m-chloroperbenzoic acid. It is proposed that the oxidation of a dimanganese (III) tetraarylporphyrin dimer could give the corresponding high valent Mn(V)=O complex, which is the active species in these oxidation. They reported on the oxidation of the dimanganese porphyrin dimer by employing meta-Chloroperoxybenzoic acid as an oxidant, and the characterization of the

Furthermore, oxygen evolution was observed from the Mn(V)=O species when a small excess of trifluoromethanesulfonic acid was added (Shimazaki et al., 2004). Mn(V)=O was

oxidized in this reaction (Fujiwara et al., 1985).

resulting Mn(V)=O species by spectroscopic methods.

detected by EPR, UV/VIS, and Raman spectrum (Shimazaki et al., 2004).

Mn

<sup>N</sup> <sup>N</sup>

N

Fig. 10. Dimeric tetraarylporphyrins linked by 1,2-phenylene bridge as a model for the WOC

Several types of experimental evidence have demonstrated that the synthetic complexes Mn4O4(O2PR2)6, R = Ph and 4–MePh, containing the [Mn4O4]6+ core surrounded by six facially bridging bidentate phosphinate anions, produce dioxygen following removal of one phosphinate ligand to form the reactive butterfly complex [Mn4O4(O2PR2)5] (Fig. 11)

Dissociation of a phosphinate ligand is achieved using light absorbed by a charge transfer O-Mn transition, producing dioxygen in high quantum high yield (46–100%) (Maniero et al.,

O

O

N

N N

in PSII (Shimazaki et al., 2004).

**2.2.4 Cubane like model** 

(Maniero et al., 2003).

2003).

**OH2**

Mn <sup>N</sup> N

**2.2.3 Porphyrin complexes** 

Fig. 8. Proposal mechanism for hydrogen peroxide producing when HClO4 was added in stoichiometric amounts to solutions of the Mn(IV) Schiff base dimmer (Boucher & Coe 1975).

Fujiwara et al. have reported the preparation and characterization of a series of dichloromanganese (IV) Schiff base complexes (Fujiwara et al., 1985). They have shown that the manganese(IV) complex dichlorobis(N-R-3-nitrosalicylideneaminato) manganese(IV) reacts with water to liberate molecular oxygen (Fig. 9) (Fujiwara et al., 1985).

Fig. 9. Structure of the complex *trans*-Mn(IV)L2Cl2 (L = *N*-alkyl-3-nitrosalicylimide) (Fujiwara et al., 1985).

Absorbed spectrometry using an alkaline pyrogallol solution and measurement of dissolved oxygen by an oxygen electrode were employed to detect and determine dioxygen liberated during the reaction of manganese(IV) complexes with water (Fujiwara et al., 1985). It could be seen that the reactivity is affected by the alkyl groups of the complexes: the reaction with water is retarded in order of (mol of O2 per mol of complex) n-C3H7 (0.27)< n-C8H17 (0.2)< n-C12H25 (0.12). These results indicate that the long-chain alkyl groups such as n-C8H17 and n-C12H25 can protect the central manganese(IV) ion from attack by water molecules (Fujiwara et al., 1985). This may arise from hydrophobicity of these groups. In other words, the reactivity of the manganese(IV) complexes with water can be controlled by the choice of alkyl groups. Also they have found that the pH values of reaction decrease in the course of the reaction of the manganese(IV) complexes in the presence of water (without any buffer solution) (Fujiwara et al., 1985). Using 18O-labeled water showed that indeed water was oxidized in this reaction (Fujiwara et al., 1985).
