Water oxidation by manganese oxides formed from tetranuclear precursor complexes : the influence of phosphate on structure and activity

• Two types of manganese oxides have been prepared by hydrolysis of tetranuclear Mn(III) complexes in the presence or absence of phosphate ions. The oxides have been characterized structurally using X-ray absorption spectroscopy and functionally by O-2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-mu-oxo bridged manganese ions that form layers with limited long-range order, consisting of edge-sharing MnO6 octahedra. The average manganese oxidation state is +3.5. The structure of these oxides is closely related to other manganese oxides reported as water oxidation catalysts. They show high oxygen evolution activity in a light-driven system containing [Ru(bpy)(3)](2+) and S2O82- at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-mu-oxo bridged manganese ions. Instead the phosphate groups are acting as bridges between the manganese ions. The average oxidation state of manganese ions is +3. This type of oxide has much lower water oxidation activity in the light-driven system. Correlations between different structural motifs and the function as a water oxidation catalyst are discussed and the lower activity in the phosphate containing oxide is linked to the absence of protonable di-mu-oxo bridges.

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NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

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