Light-induced multistep oxidation of dinuclear manganese complexes for artificial photosynthesis

Two dinuclear manganese complexes, [Mn2BPMP(@m-OAc)2].ClO4 (1, where BPMP is the anion of 2,6-bis{[N,N-di(2-pyridinemethyl)amino]methyl}-4-methylphenol) and [Mn2L(@m-OAc)2].ClO4 (2, where L is the trianion of 2,6-bis{[N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2-pyridinemethyl)amino]met hyl}-4-methylphenol), undergo several oxidations by laser flash photolysis, using rutheniumII-tris-bipyridine (tris(2,2-bipyridyl)dichloro-ruthenium(II) hexahydrate) as photo-sensitizer and penta-amminechlorocobalt(III) chloride as external electron acceptor. In both complexes stepwise electron transfer was observed. In 1, four Mn-valence states from the initial Mn2II,II to the Mn2III,IV state are available. In 2, three oxidation steps are possible from the initial Mn2III,IIIstate. The last step is accomplished in the Mn2IV,IV state, which results in a phenolate radical.For the first time we provide firm spectral evidence for formation of the first intermediate state, Mn2II,III, in 1 during the stepwise light-induced oxidation. Observation of Mn2II,III is dependent on conditions that sustain the @m-acetato bridges in the complex, i.e., by forming Mn2II,III in dry acetonitrile, or by addition of high concentrations of acetate in aqueous solutions. We maintain that the presence of water is necessary for the transition to higher oxidation states, e.g., Mn2III,III and Mn2III,IV in 1, due to a bridging ligand exchange reaction which takes place in the Mn2II,III state in water solution. Water is also found to be necessary for reaching the Mn2IV,IV state in 2, which explains why this state was not reached by electrolysis in our earlier work (Eur. J. Inorg. Chem (2002) 2965).In 2, the extra coordinating oxygen atoms facilitate the stabilization of higher Mn valence states than in 1, resulting in formation of a stable Mn2IV,IV without disintegration of 2. In addition, further oxidation of 2, led to the formation of a phenolate radical (g=2.0046) due to ligand oxidation. Its spectral width (8 mT) and very fast relaxation at 15 K indicates that this radical is magnetically coupled to the Mn2IV,IV center