Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S 2 State of the Oxygen Evolving Complex.

The S 2 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 3 O 4 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 Mn 4 O 4 core, has a substantial effect on spectroscopy.