title: Water oxidation by manganese oxides formed from tetranuclear precursor complexes: the influence of phosphate on structure and activity.
abstract: 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 O2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-μ-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 S2O8(2-) at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-μ-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-μ-oxo bridges.
authors.affiliation: Analytical Chemistry, Department of Chemistry - Biomedical Center, Uppsala University, P.O. Box 599, S-75124 Uppsala, Sweden.
authors.forename: Denys
authors.initials: D
authors.lastname: Shevchenko
authors.forename: Magnus F
authors.initials: MF
authors.lastname: Anderlund
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.forename: Ivelina
authors.initials: I
authors.lastname: Zaharieva
authors.forename: Anders
authors.initials: A
authors.lastname: Thapper
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D017895:Manganese Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D017895:Manganese Compounds; D010087:Oxides; D010710:Phosphates; D014867:Water; C027424:manganese oxide