Mnᴵⱽ₄O₄ Model of the S₃ Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand

Abstract

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different Sn-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mnᴵⱽ₄ core corresponding to the S₃ state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued. Herein, we report the synthesis, electrochemistry, SQUID magnetometry, and electron paramagnetic resonance spectroscopy of a stable Mnᴵⱽ₄O₄ cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand shifts the reduction potential of the MnᴵᴵᴵMnᴵⱽ₃/Mnᴵⱽ₄ redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated Mnᴵⱽ₄O₄ complex matches the acetate and amidate variants, in corroboration with the MnIV4 assignment of the S₃ state of the OEC.

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