The effects of redox-inactive metal ions on dioxygen activation were explored using a new Fe complex containing a tripodal ligand with 3 sulfonamido groups. This iron complex exhibited a faster initial rate for the reduction of O than its Mn analog. Increases in initial rates were also observed in the presence of group 2 metal ions for both the Fe and Mn complexes, which followed the trend NMe < Ba < Ca = Sr. These studies led to the isolation of heterobimetallic complexes containing Fe-(-OH)-M cores (M = Ca, Sr, and Ba) and one with a [Sr(OH)Mn] motif. The analogous [Ca(OH)Ga] complex was also prepared and its solid state molecular structure is nearly identical to that of the [Ca(OH)Fe] system. Nuclear magnetic resonance studies indicated that the diamagnetic [Ca(OH)Ga] complex retained its structure in solution. Electrochemical measurements on the heterobimetallic systems revealed similar one-electron reduction potentials for the [Ca(OH)Fe] and [Sr(OH)Fe] complexes, which were more positive than the potential observed for [Ba(OH)Fe]. Similar results were obtained for the heterobimetallic Mn complexes. These findings suggest that Lewis acidity is not the only factor to consider when evaluating the effects of group 2 ions on redox processes, including those within the oxygen-evolving complex of Photosystem II.
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