Experimental and computational X-ray emission spectroscopy as a direct probe of protonation states in oxo-bridged Mn^IV 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-μ-oxo-bridged Mn^IV complexes in different protonation states was studied by Kβ 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 Kβ_2,5 peaks arise primarily from the ligand 2p to Mn 1s transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite Kβ″ peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated μ-oxo bridges dominate this spectral region. The energies of the Kβ″ features differ by ∼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 (μ-O)(μ-OH ₂) motif can be distinguished from a symmetric (μ-OH)₂ one. The results reported here highlight the sensitivity of Kβ 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.