Electron paramagnetic resonance study of the S = ½ ground state of a radiolysis-generated manganese(III)–trimanganese(IV) form of [MnIV4O6(bipy)6]4+ (bipy = 2,2′-bipyridine). Comparison with the photosynthetic Oxygen Evolving Complex†
γ-Ray irradiation at liquid nitrogen temperature of a dimethylformamide solution of the tetranuclear complex [MnIV4O6(bipy)6]4+ (bipy = 2,2′-bipyridine) allowed the generation of the first mixed-valence tetranuclear system containing MnIII and MnIV 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 MnIIIMnIV3 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 MnIII 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.