Biomimetic approach to the oxygen evolving center: resonance Raman investigation of a manganese mu-oxo dimer in three oxidation states.

A biologically relevant dinuclear manganese mono-mu-oxo complex with a bound phenolate ligand in three oxidation states, (III,III), (III,IV) and (IV,IV), was studied using resonance Raman spectroscopy. Depending upon the excitation frequency, phenolate vibrations or mu-oxo vibrations were enhanced, which allowed us to assign the UV-visible absorption spectra. In the case of the mixed valence species (III,IV), the mu-oxo vibration at 854 cm-1 has been assigned by isotopic substitution (H2(18)O) to nu as(Mn-O-Mn). This preferential enhancement of the asymmetric vibration stresses the asymmetric character of the bridge.