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Photosystem II, with its active center a CaMn$_4$O$_5$ cluster (OEC), is essential for photosynthesis and therefore O$_2$ production in nature [1]. The understanding of the electronic structure and properties of this complex plays an important role in designing artificial water-oxidizing complexes. During oxygen formation the OEC undergoes five distinct states called S$_0$-S$_4$ forming the Kok cycle. Despite detailed knowledge of S$_0$ through S$_3$ there is still a lack of information on S$_4$ due to challenges preparing OEC in this state [2]. However, two major competing models for S$_4$ have been proposed in the literature which involve distinctively different oxidation states namely Mn(IV) (and an oxygen radical) and Mn(V), respectively.
We performed X-ray absorption spectroscopy (XAS) in ion yield mode at the manganese L-edge and oxygen K-edge on a series of cryogenically cooled, mass-selected manganese oxide ions at 20 K.
Here, we report on Mn$_2$O$_3^+$ – a high-valent species with two μ-oxo bridges and a terminal oxo ligand, which forms a subunit of the OEC. Using XAS we find an unusual charge disproportionation in Mn$_2$O$_3^+$,where one manganese atom is in a high oxidation state, and stability of this complex in a H$_2$O ligand presence. The oxidation states were identified by by comparison to reference X Ray absorption spectra of other manganese compounds.
[1] N. Cox and et al. Electronic structure of the oxygen-evolving complex in Photosystem II prior to O–O bond formation. Science, 2014, 345, 804.
[2] J. Barber. A mechanism for water splitting and oxygen production in photosynthesis. Nature Plants, 2017, 3(4), 17041.
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