X-ray Emission Spectroscopy of Biomimetic Mn Coordination Complexes

Abstract

Understanding the function of Mn ions in biological and chemical redox catalysis requires precise knowledge of their electronic structure. X-ray emission spectroscopy (XES) is an emerging technique with a growing application to biological and biomimetic systems. Here, we report an improved, cost-effective spectrometer used to analyze two biomimetic coordination compounds, [Mn^IV(OH)₂(Me₂EBC)]²⁺ and [Mn^IV(O)(OH)(Me₂EBC)]⁺, the second of which contains a key Mn^IV═O structural fragment. Despite having the same formal oxidation state (Mn^IV) and tetradentate ligands, XES spectra from these two compounds demonstrate different electronic structures. Experimental measurements and DFT calculations yield different localized spin densities for the two complexes resulting from Mn^IV-OH conversion to Mn^IV═O. The relevance of the observed spectroscopic changes is discussed for applications in analyzing complex biological systems such as photosystem II. A model of the S₃ intermediate state of photosystem II containing a Mn^IV═O fragment is compared to recent time-resolved X-ray diffraction data of the same state.