Synthetic strategies towards a Mn_4CaO_n model complex of the oxygen-evolving complex of photosystem II

The oxygen-evolving complex (OEC) of photosystem II is a Mn_4CaO_5 heterometallic cluster. In the past, we synthesized a structural model (1) of the Mn_3CaO_4 subsite of the OEC using a hexapyridyl, trialkoxide ligand. Addn. of the fourth, dangling manganese has been targeted to more fully mimic the structure of the OEC. The original complex 1, stabilized across three faces of the cubane by carboxylates, has been desymmetrized using a chelating bisoximate ligand by replacing two of the κ2-carboxylates. The properties and reactivity of the desymmetrized Mn_3CaO_4 complex has been compared to the original, pseudo-C3 sym. cluster, and its redox and acid/base chem. has been studied, as both are pertinent to the OEC mechanism. Strategies for synthesizing a Mn_4CaO-_n(n=4, 5) OEC model from the new, desymmetrized complex will be discussed.