Chlorine K-edge X-ray absorption spectroscopy as a probe of chlorine-manganese bonding: Model systems with relevance to the oxygen evolving complex in photosystem II

For a series of Mn-Cl coordination complexes the chlorine K-edge X-ray absorption spectra (XAS) have been measured to probe chloride-manganese bonding. Results show that in a series of mononuclear (single Mn) compounds with Cl covalently bound to Mn, the intensity of the pre-edge feature increases in the order Mn-II < Mn-III < Mn-IV. For a series of distorted cubane complexes (containing 4 Mn) with bridging and terminal Cl Ligands, three pre-edge features are seen. The pre-edge peak at lowest energy increases in intensity as more terminal Cl are included in the compounds. The origin of these features in the edge spectra is described in terms of the symmetry and ligand field splittings expected for the Mn complexes. The Mn model systems studied here have potential application to the oxygen evolving complex (OEC) in Photosystem II (PSII), where direct structural evidence for a halide binding site on the Mn cluster in one or more S-states of the Kok cycle would be informative.