Mn oxidation states in tri- and tetra-nuclear Mn compounds structurally relevant to photosystem II: Mn K-edge X-ray absorption and Kβ X-ray emission spectroscopy studies

Abstract

X-Ray spectroscopy is used to examine the effect of the manganese oxidation state for a series of Mn model compounds. Sensitive to Mn oxidation and structural symmetry, X-ray absorption and emission spectroscopy (XAS and XES) provide complementary insights. However, few benchmark examples of complexes with similar structures but in different oxidation states are available to evaluate data from unknown structures like the oxygen evolving complex (OEC) of Photosystem II (PSII). This study examines two types of compounds prepared in a variety of Mn oxidation states and which possess chemical structures with Mn–Mn interactions (∼2.7 Å and ∼3.3 Å) that have been observed in the OEC. Model complexes with core compositions Mn₃O and Mn₄O₂ contain combinations of Mn in either a reduced (II) or oxidized (III) state. Within each set of compounds, complexes with higher Mn oxidation states have absorption K-edge energy values that are higher (1.6–2.2 eV) than those of their more reduced counterparts. This trend is accordingly reversed in the Kβ emission spectroscopy where the first moment energy values are lower (0.09–0.12 eV) for compounds with higher Mn oxidation states. We will discuss in detail, how these trends can be quantitatively used to characterize the effects of the Mn oxidation state as well as the surrounding ligand environment on the observed X-ray spectra. The results are discussed with respect to previously obtained data on different S-states of the OEC.