Toward Models for the Full Oxygen-Evolving Complex of Photosystem II by Ligand Coordination To Lower the Symmetry of the Mn_3CaO_4 Cubane: Demonstration That Electronic Effects Facilitate Binding of a Fifth Metal

Abstract

Synthetic model compounds have been targeted to benchmark and better understand the electronic structure, geometry, spectroscopy, and reactivity of the oxygen-evolving complex (OEC) of photosystem II, a low-symmetry Mn_4CaO_n cluster. Herein, low-symmetry Mn^(IV)_3GdO_4 and Mn^(IV_)3CaO_4 cubanes are synthesized in a rational, stepwise fashion through desymmetrization by ligand substitution, causing significant cubane distortions. As a result of increased electron richness and desymmetrization, a specific μ_3-oxo moiety of the Mn_3CaO_4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn_3CaAgO_4 cluster that models the topology of the OEC by displaying both a cubane motif and a “dangler” transition metal. The present synthetic strategy provides a rational roadmap for accessing more accurate models of the biological catalyst.