Photo-induced oxidation of a dinuclear Mn-2(II,II) complex to the Mn-2(III,IV) state by inter- and intramolecular electron transfer to Ru-III tris-bipyridine

摘要

To model the structural and functional parts of the water oxidizing complex in Photosystem 11, a dimeric manganese(II,11) 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), in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by Ru-III(bpy), from the Mn-2(II,II) dimer, which then attained the Mn-2(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 Mn-2(III,IV) state. Our data indicate that oxidation from the Mn-2(II,II) state proceeds stepwise via intermediate formation of Mn-2(II,III) and Mn-2(III,III). In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn-2(II,III/III,III) oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn-2(III,IV), this suggests that water is essential for the formation of the Mn-2(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 11, in that the electron transfer to the photosensitizer is fast and results in several electron extractions from the Mn moiety.