Photo-induced oxidation of a dinuclear Mn₂^II,II complex to the Mn₂^III,IV state by inter- and intramolecular electron transfer to Ru^IIItris-bipyridine

To model the structural and functional parts of the water oxidizing complex in Photosystem II, a dimeric manganese(II,II) complex (1) was linked to a ruthenium(II)tris-bipyridine (Ru^II(bpy)₃) complex via a substituted L-tyrosine, to form the trinuclear complex 2 [J. Inorg. Biochem. 78 (2000) 15]. Flash photolysis of 1 and Ru^II(bpy)₃ in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by Ru^III(bpy)₃ from the Mn₂^II,II dimer, which then attained the Mn₂^III,IV oxidation state. In a similar experiment with compound 2, the dinuclear Mn complex reduced the photo-oxidized Ru moiety via intramolecular electron transfer on each photochemical event. From EPR it was seen that 2 also reached the Mn₂^III,IV state. Our data indicate that oxidation from the Mn₂^II,II state proceeds stepwise via intermediate formation of Mn₂^II,III and Mn₂^III,III. In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn₂^{II,III/III,III} oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn₂^III,IV, this suggests that water is essential for the formation of the Mn₂^III,IV oxidation state. Compound 2 is a structural mimic of the water oxidizing complex, in that it links a Mn complex via a tyrosine to a highly oxidizing photosensitizer. Complex 2 also mimics mechanistic aspects of Photosystem II, in that the electron transfer to the photosensitizer is fast and results in several electron extractions from the Mn moiety.