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The S 3 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 IV  4 core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S 3 state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S 3 state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn IV  4 O 4 cuboidal complex as a spectroscopic model of the S 3 state. Results show that this Mn IV  4 O 4 complex has an S = 3 ground state with isotropic 55 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 IV centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S 2 state to the S 3 state. This same spin state change is observed following oxidation of the previously reported Mn III Mn IV  3 O 4 cuboidal complex to the Mn IV  4 O 4 complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.

journal of the american chemical society

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