ELECTRON-PARAMAGNETIC-RESONANCE STUDY OF THE S=1 2 GROUND-STATE OF A RADIOLYSIS-GENERATED MANGANESE(III)-TRIMANGANESE(IV) FORM OF [MN-IV-O-4(6)(BIPY)(6)](4+) (BIPY=2,2'-BIPYRIDINE) - COMPARISON WITH THE PHOTOSYNTHETIC OXYGEN-EVOLVING-COMPLEX/

gamma-Ray irradiation at liquid nitrogen temperature of a dimethylform amide solution of the tetranuclear complex [(Mn4O6)-O-IV(bipy)(6)](4+) (bipy = 2,2'-bipyridine) allowed the generation of the first mixed-va lence tetranuclear system containing Mn-III and Mn-IV ions and exhibit ing a S = 1/2 ground state. The X-band EPR spectrum of this tetranucle ar system has been obtained. Simulations have been undertaken and the Mn hyperfine coupling tensors determined clearly show a (MnMn3IV)-Mn-I II, composition for the EPR active species. A general approach for the analysis of the isotropic components of the Mn hyperfine tensors is p resented in detail. This allowed the determination of the spin project ion value for each Mn site. A three J coupling scheme assuming that th e linear topology of the starting compound remains is able to reproduc e these spin projection values if and only if the Mn-III ion is locate d at a terminal position in a N4O2 environment. The EPR signal of this [Mn4O6(bipy)(6)](3+) species is compared with the multiline signal ob served in the S-2 state of the photosynthetic Oxygen Evolving Complex.