Artificial Mn₄SrO₄-Cluster Mimicking the Structural Changes of the Photosynthetic Oxygen-Evolving Center

The oxygen-evolving center (OEC) of photosystem II (PSII) is characterized by a unique Mn₄CaO₅- or Mn₄SrO₅-cluster. Understanding the structure-function relationship and the catalytic mechanism of the OEC has been hindered by the lack of a rational model that precisely mimics both the static and dynamic structures of this biological cluster. Herein, we report a series of synthetic Mn₄SrO₄-clusters that closely mimic the main metal-oxide core, peripheral coordination sphere, redox properties, and the oxidation states of the four Mn ions in the Sr²⁺-containing OEC. Crystal structural measurements demonstrate that the presence of additional neutral ligands on Sr²⁺ of the S₁ state Mn₄SrO₄-cluster can significantly modify the geometric conformation of the cluster, whereas the oxidation states and the dominant antiferromagnetic interactions of four Mn ions are largely undisturbed. EPR investigations and DFT calculations on the S₂ state Mn₄SrO₄-cluster demonstrate that the presence of additional neutral ligands can significantly affect the magnetic interactions of the cluster, converting the high-spin state giving rise to a g ≈ 4 EPR signal into a low-spin state with a g = 2 multiline EPR signal. Mass spectroscopic measurements show that a Mn₄SrO₅-cluster can be generated in solution from the synthetic Mn₄SrO₄-cluster. These observations provide chemical insights into the functional role of the redox-inactive metal ion (calcium or strontium), dynamic structural changes, and catalytic mechanism of its biological counterpart.