**2.2.3 Porphyrin complexes**

44 Artificial Photosynthesis

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)

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

(Fujiwara et al., 1985).

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

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

Fig. 10. Dimeric tetraarylporphyrins linked by 1,2-phenylene bridge as a model for the WOC in PSII (Shimazaki et al., 2004).

#### **2.2.4 Cubane like model**

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) (Maniero et al., 2003).

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., 2003).

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 47

is concurrent with an oxygen evolution with turnovers of up to 104 mol of oxygen per mol of [Mn] and calculated rate constants from two series of experiments of 0.039 and 0.026 mol [O2] s-1 M-2. A 1:1 reaction of tert-Butyl hydroperoxide with [Mn] is rate determining and the resultant species is proposed to be the mononuclear, catalytically competent, Mn(IV)=O

Oxides and Hydroxides of transition metals cations like Fe(III), Co(III), Mn(III), Ru(IV), and Ir(IV) appear to be efficient catalysts for water oxidation in the presence of Ce(IV), S2O8-2

Shilov and Shafirovich in 1965 have shown that colloidal MnO2 catalyzes the oxidation of

**Mn <sup>O</sup>**

Scheme 1. Suggested mechanism for water oxidation by Oxides and Hydroxides of

Recently, we introduced amorphous calcium - manganese oxide as efficient and biomimetic catalysts for water oxidation (Najafpour et al., 2010). These oxides are very closely related to the WOC in PSII not only because of similarity in the elemental composition and oxidation number of manganese ions but also because of similarity of structure and function

The structure of these amorphous powders have been evaluated, using extended-range Xray absorption spectroscopy (XAS), X-ray absorption near-edge structure (XANES) and Extended X-Ray Absorption Fine Structure (EXAFS) (Zaharieva et al., 2011). These results reveal similarities between the amorphous powders and the water oxidizing complex of PSII. Two different Ca-containing motifs were identified in these amorphous manganese – calcium oxides (Zaharieva et al., 2011). One of them results in the formation of Mn3Ca cubes, as also proposed for the WOC of PSII. Other calcium ions likely interconnect oxide-layer fragments. It was concluded that these readily synthesized manganese-calcium oxides are the closest structural and functional analogs to the native the WOC of PSII found so far

**<sup>O</sup> Mn**

**O**

**H2O**

**n**

3+) (Shilov &

water to dioxygen in the presence of strong oxidants like Ce(IV) and Ru(bpy)3

Shafirovich, 1979 (translation)). Suggested mechanism is shown in Scheme 1.

(Seidler-Egdal et. al., 2011).

and Fe(bpy)33+ as oxidants.

transition metals.

**2.2.6 Manganese Oxides and Hydroxides** 

**Mn**

(Najafpour 2011a,b; Zaharieva et al., 2011) (Fig. 13).

**O**

**O**

The redox potential of [Mn4O4(O2PR2)6]/[Mn4O4(O2PR2)6]+ is 1.38 V vs NHE, which is considerably greater than those found for the dimanganese(III,IV)/(III,III) couple and the majority of known (IV,IV)/(III,IV) couples. The [Mn4O4(O2PR2)6] cubane complex reacted with the hydrogen-atom donor, phenothiazine in a CH2Cl2 solution, forming [Mn4O4 (O2PR2)6] and [Mn4O4(O2PR2)6]+ as well as releasing two water molecules from the core. This result shows that two of the corner oxos of the cubane can be converted into two labile water molecules. The evolution of oxygen molecule from Mn4O4 cubane core was corroborated by the detection of 18O2 from [Mn4O4(O2PR2)6] (Maniero et al., 2003).

Fig. 11. Synthetic complexes Mn4O4 (O2PR2)6, R = Ph and 4–MePh (only one O2PR2 is shown).

### **2.2.5 Mn(II) complexes of monoanionic pentadentate ligands**

McKenzie's group (Seidler-Egdal et. al., 2011) reported that tert-Butyl hydroperoxide oxidation of Mn(II) complexes of **1** (Fig. 12), in large excesses of the tert-Butyl hydroperoxide,

Fig. 12. The Mn(II) complex (**1**) of monoanionic pentadentate ligands reported by McKenzie's group.

is concurrent with an oxygen evolution with turnovers of up to 104 mol of oxygen per mol of [Mn] and calculated rate constants from two series of experiments of 0.039 and 0.026 mol [O2] s-1 M-2. A 1:1 reaction of tert-Butyl hydroperoxide with [Mn] is rate determining and the resultant species is proposed to be the mononuclear, catalytically competent, Mn(IV)=O (Seidler-Egdal et. al., 2011).
