Intramolecular Electron Transfer from Mn or Ligand Phenolate to Photochemically Generated RuIII in Multinuclear Ru/Mn Complexes. Laser Flash Photolysis and EPR Studies on Photosystem II Models

In a mononuclear Mn IV and a trinuclear Mn II complex, the ligands of which contain electron-rich phenols (coordinated to the Mn('s)) and covalently attached ruthenium(II) 2,2‘-trisbipyridyl(=bpy)-type groups, intramolecular electron transfer (ET) from the phenolate ligand (in the mononuclear Mn IV complex) or from a Mn II (in the trinuclear Mn II complex) to the photochemically (λ exc = 455 nm) generated Ru III takes place with k ≥ 5 × 10 7 s - 1, giving rise to the corresponding phenoxyl radical (complexed to Mn IV ) or to Mn III, respectively. Thus, in the trinuclear Mn II complex, the source of the electron that reduces the photogenerated Ru III (bpy • - ) moiety is a Mn II, in contrast to the situation with the mononuclear Mn IV complex, where the electron stems from a phenolate. The half-life of the coordinated phenoxyl-type Ru(bpy)/Mn complex (as produced in the presence of [Co III (NH 3 ) 5 Cl] 2+ ) is of the order 0.5−1 ms. The Ru(bpy) compound containing three (phenolate-ligated) Mn II atoms is the first example of a photochemically induced intramolecular ET from a multinuclear Mn cluster to an attached “sensitizer”, and the Ru complex containing one (phenolate-ligated) Mn IV is the first case of an ET from a synthetic Mn IV -coordinated phenolate to a photochemically produced oxidant (Ru III ).