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 cis-hydroxide ligands has also been isolated through oxidation of the corresponding manganese(II) complex with H2O2 in the presence of NH4PF6. This rare cis-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.