S = 3 Ground State for a Tetranuclear MnIV4O4 Complex Mimicking the S3 State of the Oxygen-Evolving Complex.

The S3 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 MnIV4 core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S3 state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S3 state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a MnIV4O4 cuboidal complex as a spectroscopic model of the S3 state. Results show that this MnIV4O4 complex has an S = 3 ground state with isotropic 55Mn 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 MnIV centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S2 state to the S3 state. This same spin state change is observed following oxidation of the previously reported MnIIIMnIV3O4 cuboidal complex to the MnIV4O4 complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.