Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes—implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem II (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N3O3 in 1, N2O4 in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: MnII-(μ-OR)(μ-OCO)2-MnII⇔MnII-(μ-OR)(μ-OCO)2-MnIII⇒MnIII-(μ-OR)(μ-OCO)(μ-O)-MnIIIMnII-(μ-OR)(μ-OCO)2-MnII⇔MnII-(μ-OR)(μ-OCO)2-MnIII⇒MnIII-(μ-OR)(μ-OCO)(μ-O)-MnIII. In 2: MnII-(μ-OR)(μ-OCO)2-MnIII⇔MnIII-(μ-OR)(μ-OCO)2-MnIII⇒MnIII-(μ-OR)(μ-OCO)(μ-O)-MnIVMnII-(μ-OR)(μ-OCO)2-MnIII⇔MnIII-(μ-OR)(μ-OCO)2-MnIII⇒MnIII-(μ-OR)(μ-OCO)(μ-O)-MnIV. In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5–1 V. The second one-electron oxidation likely is coupled to μ-oxo-bridge (or μ-OH) formation which seems to counteract a further potential increase. In both complexes, μ-O(H) bridge formation is associated with a redox transition proceeding at ∼1 V, but the μ-O(H) bridge is observed at the Mn2III,III level in 1 and at the MnIII,IV level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential.