Toward Models for the Full Oxygen-Evolving Complex of Photosystem II by Ligand Coordination To Lower the Symmetry of the Mn₃CaO₄ 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 Mn4CaOn cluster. Herein, low-symmetry MnIV3GdO4 and MnIV3CaO4 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 μ3-oxo moiety of the Mn3CaO4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn3CaAgO4 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.

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…Since manganese was selected as one of the essential elements in Mn 4 Ca cluster for the natural OEC, many manganese-based complexes including homogeneous catalysts and heterogeneous catalysts (such as Mn oxide and Mn cluster) have been prepared and investigated in water oxidation [70][71][72][73][74]. As compared to the extensively studied manganese oxide, only a few molecular Mn-based catalysts were developed.…”

Synthetic Model of the Oxygen‐Evolving Center: Photosystem II under the Spotlight

“…With an asymmetrics tructure and severalh igh-valent metal ions, the synthesis of af unctionalO EC model is highly challenging for inorganic chemists. [18,19] Recently,aMn 4 Ca model compound of the OEC ( Figure 1B)w as reported by Zhang et al [8] In addition to the Mn 3 O 4 Ca cubane, this model includes the "dangler" Mn [6] (PDB ID:3 WU23,r endered with VMD)a nd B) its synthetic model [8] (Cambridge CrystallographicD atabase deposit number:1 042697, rendered with VMD). reportedt he synthesis of aM n 3 O 4 Ca cluster,w hich is structurally similar to the cubane part of the OEC.…”

Molecular Vibrations of an Oxygen‐Evolving Complex and Its Synthetic Mimic

“…[14,15] Molecular biomimetic modelsm oved as tep closer to PSII with the synthesis of asymmetricc lusters, that is, once an externalm anganese,w ith respect to ac ubane structure, was observed. Agapie and co-workersm ade af undamentalc ontribution in this direction by extensively exploring synthetic model complexes that mimicked the native OEC cluster:t he asymmetric Mn 3 CaO 4 cubane core of the OEC was indeedo btained in some synthetic complexes; [16] over several years, this was further refined to be increasingly similar to the OEC, [17,18] and a serieso fc uboidal Mn 4 complexes were recently reported as spectroscopicm odelso ft he S 2 state of the OEC. [19] Mukherjee et al obtained asynthetic complex containing the CaMn IV 3 CaO 4 core, in whicho ne Ca ion was attached to the Mn 3 CaO 4 cubane.…”