title: Redox-inactive metals modulate the reduction potential in heterometallic manganese-oxido clusters.
abstract: Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidation-state tetranuclear-dioxido clusters comprising three manganese centres and a redox-inactive metal (M). Crystallographic studies show an unprecedented Mn3M(µ4-O)(µ2-O) core that remains intact on changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds demonstrate a linear dependence between reduction potential and acidity with a slope of ∼100 mV per pKa unit. The Sr(2+) and Ca(2+) compounds show similar potentials, an observation that correlates with the behaviour of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard MC 127-72, Pasadena, California 91125, USA.
authors.forename: Emily Y
authors.initials: EY
authors.lastname: Tsui
authors.forename: Rosalie
authors.initials: R
authors.lastname: Tran
authors.forename: Junko
authors.initials: J
authors.lastname: Yano
authors.forename: Theodor
authors.initials: T
authors.lastname: Agapie
mesh_terms: D002118:Calcium / Q000737:chemistry; D002384:Catalysis; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D017895:Manganese Compounds / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D013324:Strontium / Q000737:chemistry; D014867:Water / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry
chemical_list: D017895:Manganese Compounds; D009942:Organometallic Compounds; D014867:Water; D015032:Zinc; D010100:Oxygen; D002118:Calcium; D013324:Strontium