X-ray Emission Spectroscopy of Biomimetic Mn Coordination Complexes

Scott C Jensen; Katherine M Davis; Brendan Sullivan; Daniel A Hartzler; Gerald T Seidler; Diego M Casa; Elina Kasman; Hannah E Colmer; Allyssa A Massie; Timothy A Jackson; Yulia Pushkar

doi:10.1021/acs.jpclett.7b01209

X-ray Emission Spectroscopy of Biomimetic Mn Coordination Complexes

J Phys Chem Lett. 2017 Jun 15;8(12):2584-2589. doi: 10.1021/acs.jpclett.7b01209. Epub 2017 May 30.

Authors

Affiliations

¹ Department of Physics and Astronomy, Purdue University , West Lafayette, Indiana 47907, United States.
² Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
³ Department of Physics, University of Washington , Seattle, Washington 98195, United States.
⁴ Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
⁵ Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas , Lawrence, Kansas 66045, United States.

PMID: 28524662
DOI: 10.1021/acs.jpclett.7b01209

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.