Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a Manganese(II) Complex of a Cross-Bridged Cyclen Ligand

Available data from different laboratories have confirmed that both Ca²⁺ and Cl^– are crucial for water oxidation in Photosystem II. However, their roles are still elusive. Using a manganese­(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca²⁺ on the oxidative reactivity of the manganese­(II) 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(II) complex in sulfide oxidation and further accelerate the oxidation of sulfoxide to sulfone. Similar improvements have also been observed for Mg²⁺, Sr²⁺, and Ba²⁺. A new monomeric manganese­(IV) complex having two <i>cis</i>-hydroxide ligands has also been isolated through oxidation of the corresponding manganese­(II) complex with H₂O₂ in the presence of NH₄PF₆. This rare <i>cis</i>-dihydroxomanganese­(IV) species has been well characterized by X-ray crystallography, electrochemistry, electron paramagnetic resonance, and UV–vis spectroscopy. Notably, using the manganese­(IV) complex as a catalyst demonstrates higher activity than the corresponding manganese­(II) 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 Mn^IV/Mn^III couple but negatively shifted its Mn^V/Mn^IV couple. In the manganese­(II) complex having a chloride ligand, adding Ca²⁺ shifted both the Mn^IV/Mn^III and Mn^V/Mn^IV 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 understanding their roles in the water oxidation process of Photosystem II