Strategies for synthesizing Mn_4CaOn models relevant to biological water oxidation

The biol. site of water oxidn.-the oxygen-evolving complex (OEC) of photosystem II-is a Mn_4CaO_5 heterometallic cluster, proposed to contain a Mn-_3CaO_4 cubane / open cubane unit with a fourth, dangling Mn bridged to the cubane through two oxido ligands. In past work we synthesized a Mn_3CaO_4 cubane supported by three acetates and a 1, 3,5-triarylbenzene framework contg. six pyridines and three alkoxides. More recently, incorporation of a fifth metal center has been targeted to complete the Mn_4Ca stoichiometry. A desymmetrized Mn_3CaO_4 complex has been made with reactivity complementary to the parent, sym. complex; synthetic strategies from this desymmetrized complex toward more accurate Mn4Ca models of the OEC will be discussed. Redox and acid/base reactivity will also be discussed, as both are relevant to the mechanism of water oxidn