Electron paramagnetic resonance study of the S = ½ ground state of a radiolysis-generated manganese(III)–trimanganese(IV) form of [Mn^IV₄O₆(bipy)₆]⁴⁺ (bipy = 2,2′-bipyridine). Comparison with the photosynthetic Oxygen Evolving Complex †

Abstract

γ-Ray irradiation at liquid nitrogen temperature of a dimethylformamide solution of the tetranuclear complex [Mn^IV₄O₆(bipy)₆]⁴⁺ (bipy = 2,2′-bipyridine) allowed the generation of the first mixed-valence tetranuclear system containing Mn^III and Mn^IV ions and exhibiting a S = ½ ground state. The X-band EPR spectrum of this tetranuclear system has been obtained. Simulations have been undertaken and the Mn hyperfine coupling tensors determined clearly show a Mn^IIIMn^IV₃ composition for the EPR active species. A general approach for the analysis of the isotropic components of the Mn hyperfine tensors is presented in detail. This allowed the determination of the spin projection value for each Mn site. A three J coupling scheme assuming that the linear topology of the starting compound remains is able to reproduce these spin projection values if and only if the Mn^III ion is located at a terminal position in a N₄O₂ environment. The EPR signal of this [Mn₄O₆(bipy)₆]³⁺ species is compared with the multiline signal observed in the S₂ state of the photosynthetic Oxygen Evolving Complex.

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