title: Electrochemical investigation of Mn4O4-cubane water-oxidizing clusters.
abstract: High valence states in manganese clusters are a key feature of the function of one of the most important catalysts found in nature, the water-oxidizing complex of photosystem II. We describe a detailed electrochemical investigation of two bio-inspired manganese-oxo complexes, [Mn(4)O(4)L(6)] (L = diphenylphosphinate (1) and bis(p-methoxyphenyl)phosphinate (2)), in solution, attached to an electrode surface and suspended within a Nafion film. These complexes contain a cubic [Mn(4)O(4)](6+) core stabilized by phosphinate ligands. They have previously been shown to be active and durable photocatalysts for the oxidation of water to dioxygen. A comparison of catalytic photocurrent generated by films deposited by two methods of electrode immobilization reveals that doping of the catalyst in Nafion results in higher photocurrent than was observed for a solid layer of cubane on an electrode surface. In dichloromethane solution, and under conditions of cyclic voltammetry, the one-electron oxidation processes 1/1(+) and 2/2(+) were found to be reversible and quasi-reversible, respectively. Some decomposition of 1(+) and 2(+) was detected on the longer timescale of bulk electrolysis. Both compounds also undergo a two-electron, chemically irreversible reduction in dichloromethane, with a mechanism that is dependent on scan rate and influenced by the presence of a proton donor. When immersed in aqueous electrolyte, the reduction process exhibits a limited level of chemical reversibility. These data provide insights into the catalytic operation of these molecules during photo-assisted electrolysis of water and highlight the importance of the strongly electron-donating ligand environment about the manganese ions in the ability of the cubanes to photocatalyze water oxidation at low overpotentials.
authors.affiliation: School of Chemistry, Monash University, Victoria 3800, Australia.
authors.forename: Robin
authors.initials: R
authors.lastname: Brimblecombe
authors.forename: Alan M
authors.initials: AM
authors.lastname: Bond
authors.forename: G Charles
authors.initials: GC
authors.lastname: Dismukes
authors.forename: Gerhard F
authors.initials: GF
authors.lastname: Swiegers
authors.forename: Leone
authors.initials: L
authors.lastname: Spiccia
mesh_terms: D003460:Crystallization; D004563:Electrochemistry / Q000379:methods*; D005465:Fluorocarbon Polymers / Q000737:chemistry; D008345:Manganese / Q000737:chemistry*; D008422:Materials Testing; D008752:Methylene Chloride / Q000737:chemistry; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry*; D010777:Photochemistry / Q000379:methods; D011522:Protons; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014867:Water / Q000737:chemistry*
chemical_list: D005465:Fluorocarbon Polymers; D011522:Protons; D014867:Water; C040402:perfluorosulfonic acid; D008345:Manganese; D008752:Methylene Chloride; D010100:Oxygen