Tetranuclear Mn_4O_4 complexes as models of the oxygen evolving complex of photosystem II

Abstract

To better understand the biol. water-oxidn. catalyst, the pentanuclear oxygen-evolving complex (OEC) of Photosystem II (PSII), synthetic model compds. have been utilized to benchmark spectroscopic signatures and proposed mechanisms of the enzyme. Due to the complexity and low symmetry of the OEC, accurate models remain rare. Although systematic changes to the coordination environment of metal centers in clusters can be informative regarding effects on spectroscopy and reactivity, synthetic methods to a series of structurally related clusters are limited. Starting from Mn_4O_4 cubanes supported by a multinucleating triarylbenzene ligand framework and acetates, a family of related complexes was prepd. through ligand-exchange with phosphinate and tethered diamidate ligands. These clusters, in the MnIIIMnIV_3 redox state, show EPR features reminiscent of the S_2 state of the OEC. SQUID magnetometry, EPR spectroscopy, and reactivity studies of these cubane clusters will be presented in the context of modeling the properties of the oxygen-evolving complex in Photosystem II.