Heterobimetallic complexes with MIII-(μ-OH)-MII cores (MIII = Fe, Mn, Ga; MII = Ca, Sr, and Ba): structural, kinetic, and redox properties

Abstract

The effects of redox-inactive metal ions on dioxygen activation were explored using a new Fe^II complex containing a tripodal ligand with 3 sulfonamido groups. This iron complex exhibited a faster initial rate for the reduction of O₂ than its Mn^II analog. Increases in initial rates were also observed in the presence of group 2 metal ions for both the Fe^II and Mn^II complexes, which followed the trend NMe₄⁺ < Ba^II < Ca^II = Sr^II. These studies led to the isolation of heterobimetallic complexes containing Fe^III-(μ-OH)-M^II cores (M^II = Ca, Sr, and Ba) and one with a [Sr^II(OH)Mn^III]⁺ motif. The analogous [Ca^II(OH)Ga^III]⁺ complex was also prepared and its solid state molecular structure is nearly identical to that of the [Ca^II(OH)Fe^III]⁺ system. Nuclear magnetic resonance studies indicated that the diamagnetic [Ca^II(OH)Ga^III]⁺ complex retained its structure in solution. Electrochemical measurements on the heterobimetallic systems revealed similar one-electron reduction potentials for the [Ca^II(OH)Fe^III]⁺ and [Sr^II(OH)Fe^III]⁺ complexes, which were more positive than the potential observed for [Ba^II(OH)Fe^III]⁺. Similar results were obtained for the heterobimetallic Mn^II 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.