Photo-induced oxidation of a dinuclear Mn2II,II complex to the Mn2III,IV state by inter- and intramolecular electron transfer to RuIIItris-bipyridine

Abstract

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 (RuII(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 RuII(bpy)3 in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by RuIII(bpy)3 from the Mn2II,II dimer, which then attained the Mn2III,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 Mn2III,IV state. Our data indicate that oxidation from the Mn2II,II state proceeds stepwise via intermediate formation of Mn2II,III and Mn2III,III. In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn2II,III/III,III oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn2III,IV, this suggests that water is essential for the formation of the Mn2III,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.