Mn IV 4 O 4 Model of the S 3 Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand.

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different S n -state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn IV 4 core corresponding to the S 3 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 Mn IV 4 O 4 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 Mn III Mn IV 3 /Mn IV 4 redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated Mn IV 4 O 4 complex matches the acetate and amidate variants, in corroboration with the Mn IV 4 assignment of the S 3 state of the OEC.