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 MnIV/MnIII couple but negatively
shifted its MnV/MnIV couple. In the manganese­(II)
complex having a chloride ligand, adding Ca2+ shifted both
the MnIV/MnIII 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 understanding
their roles in the water oxidation process of Photosystem II.