Redox-Inactive Metals Modulate the Reduction Potential in Heterometallic Manganese-Oxido Clusters

Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but their roles in catalysis are currently not well understood. A series of high oxidation state tetranuclear-dioxido clusters comprised of three manganese centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Mn(3)M(μ(4)-O)(μ(2)-O) core that remains intact upon changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV, dependent upon the Lewis acidity of the second metal. With the pK(a) of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between reduction potential and acidity with a slope of ca. 100 mV per pK(a) unit. The Sr(2+) and Ca(2+) compounds show similar potentials, an observation that correlates with the behavior of the OEC, which is active only in the presence of one of these two metals.