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Abstract

The active site of the photosystem II (PSII) also called the Oxygen-Evolving-Center (OEC) - an inorganic oxygen-bridged cluster of four manganese and one calcium ions (Mn₄CaO₅) stabilized by carboxylate ligands - catalyzes the four electron oxidation of water into dioxygen. The recent crystal structure of PSII at the 1.9 Å resolution evidences for the first time the presence of four "water" molecules (of unknown protonated states) directly bound to the cluster (two coordinated to the Ca and the other two to one Mn in close proximity), that could act as substrates for dioxygen formation. To date, less than ten examples of synthetic hetero-nuclear manganese-calcium complexes habe been isolated and their studies in solution (stability/reactivity) are still very limited. Only two examples reproduce the cubane unit of the OEC.
The goal of our work is to develop a new family of homo- and heteronuclear Mn/Ca complexes of low nuclearity incorporating aquo ligands, as found in the OEC, and to deeply investigate their electronic structure by state-of-the-art Mn and Ca X-ray absorption (XAS) and emission (XES) techniques, combined to theoretical chemistry. Calcium K-Edge XAS and XES spectroscopies remain largely unexplored. Thus, the investigation of these well-characterized model Ca complexes will provide essential insights for the understanding of more complex systems such as the Mn₄Ca cluster of the OEC.
To access these complexes, we have initiated the synthesis of a family of tripodic ligands including the pyridine-carboxylate motif.
From these ligands, new Ca and heteronuclear Mn/Ca complexes have been successfully synthesized. Figure 1 displays the X-ray structures of two selected complexes. In both complexes, the calcium ion is coordinated to water molecules and carboxylate ligands, as in the natural cluster.
Preliminary results obtained by XAS Ca spectroscopy will be also presented.