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

The S-2 state of the OxygenEvolving Complex(OEC) ofPhotosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signalsattributed to distinct structures based on computation. Five-coordinateMn(III) centers are proposed in these species but are absentin available spectroscopic model complexes. Herein, we report thesynthesis, crystal structure, electrochemistry, SQUID magnetometry,and EPR spectroscopy of a (MnMnIV)-Mn-III O-3(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 withwater results in a spin state change to S = 1/2.These results demonstrate that coordination number, without dramaticchanges within the Mn4O4 core, has a substantialeffect on spectroscopy.

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This study reports the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a (MnMnIV)-Mn-III O-3(4) cuboidal complex featuring five-coordinate Mn-III. The 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 has a substantial effect on spectroscopy, even without dramatic changes within the Mn4O4 core.