**2.2.6 Manganese Oxides and Hydroxides**

46 Artificial Photosynthesis

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).

P

Ph Ph

O

Mn

O

O

Mn <sup>O</sup>

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

O

O

Mn

Mn

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

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,

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

shown).

McKenzie's group.

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 and Fe(bpy)3 3+ as oxidants.

Shilov and Shafirovich in 1965 have shown that colloidal MnO2 catalyzes the oxidation of water to dioxygen in the presence of strong oxidants like Ce(IV) and Ru(bpy)3 3+) (Shilov & Shafirovich, 1979 (translation)). Suggested mechanism is shown in Scheme 1.

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

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

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

Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis 49

Fig. 14. Scanning electron microscope (SEM) (a) and transmission electron microscopy (TEM) (b,c) images of amorphous calcium - manganese oxides. These readily synthesized manganese-calcium oxides are the closest structural and functional analogs to the native the

WOC of PSII found so far.

Fig. 13. Motif of edge-sharing MnO6 octahedra (di-μ-oxido bridging) in the manganese – calcium oxides (manganese: blue, oxygen: red and calcium: green) (Zaharieva et al., 2011).

(Zaharieva et al., 2011). Oxygen evolution formation pathways indicated by 18O-labelling studies showed that water oxidation by these layer manganese oxides in presence of cerium (IV) ammonium or [Ru(bpy)3]+3 is a ''real'' water oxidation reaction and both oxygen atoms of formed dioxygen molecules originated from water (Shevela et al., 2011). In continuation of our efforts to synthesize an efficient and biomimetic catalyst for water oxidation, we synthesized nano - size amorphous calcium - manganese oxides that are very closely related to the WOC in PSII not only because of similarity in the elemental composition, oxidation number of manganese ions and similarity of structure and function to Mn4O*5*Ca cluster in PSII but also because of nearer on the of catalyst particle size as compared to previously reported micro - size amorphous calcium manganese oxides (Fig. 14.).
