Nature Inspired Manganese(III)-Calcium Complexes: Towards Synthetic Models for the WOC of PSII

For some time, the presence of high oxidation state Mn ions and Ca(II) in the active center of Photosystem II has been known. However, coordination complexes that combine both Mn(III) and Ca(II) have been difficult to obtain, with only a handful of examples reported. In this paper we report the synthesis of two new Mn(III)-Ca(II) complexes, 1 [Pr2NH2](3)[Mn6CaO2(OH)(OMe)(3)(SALO)(6) (SALOH)(3)] and 2 [Mn18Ca6O12(OH)(6)(MeO)(12)(PhCOO)(18)(MeOH)(6)]. The complexes have been characterized by single-crystal X-ray diffraction to establish the oxidation state of manganese. The use of salicylato ligands with tert-butyl substituents leads to effective encapsulation of a Ca(II) ion in a cavity that has both hydrophobic and hydrophilic regions, mimicking the enzyme environment.