Mn4IVO4 Model of the S3 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 Sn-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the Mn-4(IV) 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 (Mn4O4)-O-IV 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 (MnMn3IV)-Mn-III/Mn-4(IV) redox couple by up to & SIM;760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated (Mn4O4)-O-IV complex matches the acetate and amidate variants, in corroboration with the Mn-4(IV) assignment of the S-3 state of the OEC.

Automated summary

This article reports the synthesis and characterization of a stable (Mn4O4)-O-IV cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand improves the stability and shifts the reduction potential of the complex.