Moessbauer study of the hyperfine and magnetic properties of organo-di-iron electron reservoirs containing a polyaromatic bridging ligand

Organo di-iron electron reservoirs Fe(Cp*)2(Ar)n+, with n=2, 1, 0, where Cp* is C5(CH3)5 and where Ar are the following bridges: biphenyl, dihydrophenanthrene, triphenylene, have been studied by Moessbauer spectroscopy in the solid state. Complexes with n=2, with 36e- in the coordination spheres of the metals, exhibit the usual diamagnetic behaviour of 18e-, FeII mono-iron systems. Complexes with n=1, 37e-, are delocalized mixed valence (FeIIFeI) with a spin 1/2; the magnetic hyperfine interaction, measured under an external field, shows equal delocalization of the 37th e- on the two iron centers and the two bridging carbon atoms of the biphenylene. Complexes with n=0, formally with 38e-, have a practically temperature-independent quadrupole splitting, and isomer shift values which contrast with the expected behaviour of independent FeI, 19e- centers. This indicates that the 37th and 38th electrons are mostly located on the polyaromatic bridge. Spectra obtained in an external field show a negligible magnetic hyperfine interaction and support this conclusion. In the case of biphenyl and dihydrophenanthrene bridges, this electron localization can be related to a strong intramolecular chemical coupling, evidenced by other spectroscopic and X-ray data. (orig.)