The Influences Of Calcium(?) And Chloride On The Oxidative Reactivity Of A Manganese(?) Complex

Available data from different labs have confirmed that both Ca2+ and Cl- are crucial for water oxidation in Photosystem ?. However, their roles are still elusive. Using a manganese(?) complex having a cross-bridged cyclen ligand as a model, the influences of Ca2+ on the oxidative reactivity of the manganese(?) complex and its corresponding manganese(IV) analogue were investigated. It has been found that adding Ca2+ can significantly improve the oxygenation efficiency of the manganese(?) complex in sulfide oxidation. Similar improvements have also been observed for Mg2+, Sr2+ and Ba2+. A new monomeric manganese(IV) complex having two cis hydroxide ligands has also been isolated through oxidation of the corresponding manganese(?) complex with H2O2 in the presence of NH4PF6. This rare species has been well characterized by X-ray crystallography, electrochemistry, EPR, and UV-visible spectroscopy. Notably, using the manganese(IV) complex as a catalyst demonstrates higher activity than the corresponding manganese(?) complex, and adding Ca2+ further improves its catalytic efficiency. However, adding Cl- decreases its catalytic activity. In electrochemical studies of manganese(IV) complexes with no chloride ligand present, adding Ca2+ positively shifted the redox potential of the MnIV/Mn?I couple, but negatively shifted its MnV/MnIV couple. For example, For the MnIV(Me2Bcyclen)(OH)2(PF6)2 complex, the potential of the MnIV/Mn?I couple positively shifts from +0.410 V to +0.455 V(vs SCE), while for the MnV/MnIV couple, it negatively shifts from +0.893 V to +0.855 V(vs SCE). In the manganese(?) complex having chloride ligand, adding Ca2+ shifted both of the MnIV/Mn?I and MnV/MnIV couples in the negative direction. The revealed oxidative reactivity and redox properties of the manganese species affected by Ca2+ and Cl- may provide new clues to understand their roles in the water oxidation process of Photosystem ?.