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

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 the 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.