Chemistry of a series of heterobimetallic complexes MnIII2(Ca/Sr)X (X = Cl, Br).

This manuscript describes the syntheses, structures and magnetism of Mn-Ca/Sr complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions Ca or Sr were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the Ca/Sr ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes.