Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals

A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe_3M(μ_4-O)(μ_2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E_(1/2)) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pK_a of the M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity, with a slope of ∼70 mV per pK_a unit. The current study of [Fe_3MO(OH)] and previous ones of [Mn_3MO_n] (n = 2,4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals