Coordination Number in High-Spin–Low-Spin Equilibrium in Cluster Models of the S₂ State of the Oxygen Evolving Complex

Abstract

The S₂ state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate Mn^(III) centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a Mn^(III)Mn^(IV)₃O₄ cuboidal complex featuring five-coordinate Mn^(III). This cluster displays a spin ground state of S = 5/2, while conversion to a six-coordinate Mn upon treatment with water results in a spin state change to S = 1/2. These results demonstrate that coordination number, without dramatic changes within the Mn₄O₄ core, has a substantial effect on spectroscopy.

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