Manganese Compounds as Water Oxidizing Catalysts in Artificial Photosynthesis

Artificial photosynthesis is an umbrella term but it could be introduced as a research field that attempts to mimic the natural process of photosynthesis and uses sunlight to oxidizing and reducing different compounds. In this process, we could assume water as one of the compounds that could be reduced and (or) oxidized to hydrogen and (or) oxygen, respectively. Water splitting is the general term for a chemical reaction in which water is decomposed to oxygen and hydrogen (Pace, 2005). Production of hydrogen fuel from electrolysis of water would become a practical strategy if we could find a ‘’super catalyst’’ for water oxidation reaction (Bockris, 1977). Super catalyst means a stable, low cost, efficient and environmentally friendly catalyst. The water oxidation half reaction in water splitting is overwhelmingly rate limiting and needs high over-voltage (~1V) that results the low conversion efficiencies when working at current densities required, also at this high voltage, other chemicals will be oxidized and this would be environmentally unacceptable for large-scale hydrogen production (Bockris, 1977). Thus, a significant challenge in the sustainable hydrogen economy is to design a water oxidizing catalyst.


Introduction
Artificial photosynthesis is an umbrella term but it could be introduced as a research field that attempts to mimic the natural process of photosynthesis and uses sunlight to oxidizing and reducing different compounds. In this process, we could assume water as one of the compounds that could be reduced and (or) oxidized to hydrogen and (or) oxygen, respectively. Water splitting is the general term for a chemical reaction in which water is decomposed to oxygen and hydrogen (Pace, 2005). Production of hydrogen fuel from electrolysis of water would become a practical strategy if we could find a ''super catalyst'' for water oxidation reaction (Bockris, 1977). Super catalyst means a stable, low cost, efficient and environmentally friendly catalyst. The water oxidation half reaction in water splitting is overwhelmingly rate limiting and needs high over-voltage (~1V) that results the low conversion efficiencies when working at current densities required, also at this high voltage, other chemicals will be oxidized and this would be environmentally unacceptable for large-scale hydrogen production (Bockris, 1977). Thus, a significant challenge in the sustainable hydrogen economy is to design a water oxidizing catalyst.

Water Oxidizing Catalysts in artificial photosynthesis
In past few years, there has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating water oxidizing complex (WOC) of photosystem II (PSII) (Liu et al., 2008;Kanan, M.W. & Nocera, 2008;Cady et al., 2008;Yagi & Kaneko, 2001;Ruttinger & Dismukes, 1997). Of these materials, the Co, Ru and Ir compounds have been shown to be an effective catalyst for water oxidation. However, most of the compounds are expensive and often relate to potentially carcinogenic salts. Particular attention has been given to the manganese compounds aimed at simulating the WOC of PSII (Umena et al., 2011) not only because it has been used by Nature to oxidize water but also because manganese is cheap and environmentally friendly. In this chapter we consider manganese compounds as structural or (and) functional models for the WOC of PSII

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 (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 pK 1 = 6.86 and pK 2 = 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).  (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 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). 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 Mn 2 III,III complex (Lomoth et al., 2006). Similar to PSII, it was shown that upon light absorption, a chain www.intechopen.com 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.

Functional models for biological Water Oxidizing 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 catalytic components for water photolysis and electrolysis.
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-Oradical bridges (Yagi & Narita, 2004). When [(bpy) 2 Mn III (μ-O) 2 Mn IV (bpy) 2 ] 3+ was dissolved in water containing excess (NH 4 ) 2 [(Ce(NO 3 ) 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 (NH 4 ) 2 [(Ce(NO 3 ) 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 [(OH 2 )(terpy)Mn III (μ-O) 2 Mn IV (terpy)(OH 2 )] 3+ has oxygen evolution activity in the presene of KHSO 5 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 [(OH 2 )(terpy)Mn III (μ-O) 2 Mn IV (terpy)(OH 2 )] 3+ onto TiO 2 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 TiO 2 by substituting one of its water ligands by the TiO 2 , as suggested by EPR data (Li et al. 2009).  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) (H 2 O)}][CIO 4 ], 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).

. I t w a s r e p o r t e d t h a t h y d r o g e n p e r o x i d e w a s p r o d u c e d w h e n H C l O 4 was added in stoichiometric amounts to solutions of the Mn(IV) Schiff base dimer [Mn 2 (IV)(BuSalen) 2 (O) 2 ]‚ H 2 O in acetone at 0 °C
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 O 2 per mol of complex) n-C 3 H 7 (0.27)< n-C 8 H 17 (0.2)< n-C 12 H 25 (0.12). These results indicate that the long-chain alkyl groups such as n-C 8 H 17 and n-C 12 H 25 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 www.intechopen.com solution) (Fujiwara et al., 1985). Using 18 O-labeled water showed that indeed water was oxidized in this reaction (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).

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The redox potential of [Mn 4 O 4 (O 2 PR 2 ) 6 ]/[Mn 4 O 4 (O 2 PR 2 ) 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 [Mn 4 O 4 (O 2 PR 2 ) 6 ] cubane complex reacted with the hydrogen-atom donor, phenothiazine in a CH 2 Cl 2 solution, forming [Mn 4 O 4 (O 2 PR 2 ) 6 ] and [Mn 4 O 4 (O 2 PR 2 ) 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 Mn 4 O 4 cubane core was corroborated by the detection of 18 O 2 from [Mn 4 O 4 (O 2 PR 2 ) 6 ] (Maniero et al., 2003).

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 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 manganesecalcium oxides (Zaharieva et al., 2011). One of them results in the formation of Mn 3 Ca 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 www.intechopen.com Fig. 13. Motif of edge-sharing MnO 6 octahedra (di-μ-oxido bridging) in the manganesecalcium oxides (manganese: blue, oxygen: red and calcium: green) (Zaharieva et al., 2011). (Zaharieva et al., 2011). Oxygen evolution formation pathways indicated by 18 O-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 Mn 4 O 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.).

Genetic engineering models
Wydrzynski and co-workers, in a different approach, introduced a reverse engineering approach to build a simple, light-driven photo-catalyst based on the organization and function of the donor side of the PSII reaction centre and a bacterioferretin molecule is being 'coaxed' using genetic engineering to include the chromophores for light absorption and Mn complexes for water oxidation; they could observed oxidation of manganese in this engineered system (Conlan et al., 2007). Recently, Nam and co-workers imtroduced a biologically templated nanostructure for visible light driven water oxidation that uses a genetically engineered virus scaffold to mediate the co-assembly of zinc porphyrins (photosensitizer) and iridium oxide clusters (water oxidizing catalyst) (Nam et al., 2010). Their results suggested that the biotemplated nanoscale assembly of functional components is a promising route to improved photocatalytic water-splitting systems. www.intechopen.com

Acknowledgment
The author is grateful to Institute for Advanced Studies in Basic Sciences for financial support.