Experimental and Computational X-ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged MnIV Dimers Relevant to Redox-Active Metalloproteins

The protonation state of oxo bridges in nature is of profound importance for a variety of enzymes, including the Mn4CaO5 cluster of photosystem II and the Mn2O2 cluster in Mn catalase. A set of dinuclear bis-mu-oxo-bridged Mn-IV complexes in different protonation states was studied by K beta emission spectroscopy to form the foundation for unraveling the protonation states in the native complex. The valence-to-core regions (valence-to-core XES) of the spectra show significant changes in intensity and peak position upon protonation. DFT calculations were performed to simulate the valence-to-core XES spectra and to assign the spectral features to specific transitions. The K beta(2,5) peaks arise primarily from the ligand 2p to Mn Is transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite K beta '' peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated mu-oxo bridges dominate this spectral region. The energies of the K beta '' features differ by similar to 3 eV and thus are well resolved in the experimental spectra. Additionally, our work explores the chemical resolution limits of the method, namely, whether a mixed (mu-O)(mu-OH2) motif can be distinguished from a symmetric (mu-OH)(2) one. The results reported here highlight the sensitivity of K beta valence-to-core XES to single protonation state changes of bridging ligands, and form the basis for further studies of oxo-bridged polymetallic complexes and metalloenzyme active sites. In a complementary paper, the results from X-ray absorption spectroscopy of the same Mn-IV dimer series are discussed.