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Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S2 State of the Oxygen Evolving
Angela A Shiau1, Heui Beom Lee1, Paul H Oyala1
1Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E California Blvd MC 127-72, Pasadena, California 91125, United States.
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 MnIII 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 MnIIIMnIV3O4 cuboidal complex featuring five-coordinate MnIII. 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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