= 3 Ground State for a Tetranuclear MnO Complex Mimicking the S State of the Oxygen-Evolving Complex.

The S state is currently the last observable intermediate prior to O-O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn core with an = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a MnO cuboidal complex as a spectroscopic model of the S state. Results show that this MnO complex has an = 3 ground state with isotropic Mn hyperfine coupling constants of -75, -88, -91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer-monomer magnetic coupling model of pseudo-octahedral Mn centers. Importantly, the spin ground state changes from = 1/2 to = 3 as the OEC is oxidized from the S state to the S state. This same spin state change is observed following oxidation of the previously reported MnMnO cuboidal complex to the MnO complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.