Synthesis, Electronic Structure, and Spectroscopy of Multinuclear Mn Complexes Relevant to the Oxygen Evolving Complex of Photosystem II

Biochemical, spectroscopic, structural, and computational studies have provided insight into the biological water oxidation by the Oxygen Evolving Complex [OEC] of Photosystem II. Nevertheless, the atomic level mechanism of O₂ formation remains a matter of debate. Mechanistic proposals have been guided by the spectroscopic characterization of reaction intermediates. A key intermediate in the catalytic cycle, the S₂ state, features two characteristic EPR signals at g = 2 and g = 4.1, with the relative intensity of the two signals being condition-dependent. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure — electronic structure studies remain rare. In general, studies that probe the effect of small structural changes on the spectroscopic and magnetic properties of synthetic model clusters of S-states are rare, likely due to the synthetic challenges of accessing a series of isolable clusters that are suitable for comparisons. In this chapter, structurally characterized tetranuclear complexes with Mn oxidation states relevant to S₀–S₃ states are surveyed. In particular, magnetic and spectroscopic studies of a series of [Mn^(III)Mn₃ ^(IV)O₄] cuboidal complexes show that the electronic structure of tetranuclear Mn complexes is highly sensitive to small geometric changes and the nature of the bridging ligands. Thus, the deduction of the geometry of OEC S-state intermediates based on spectroscopic features needs to be complemented with appropriate structural determination