Zero‐Field Splittings and Redox Potentials in an Isostructural Series of Dinuclear FeIITiIV, FeIIITiIV, and MnIITiIV Complexes with a Face‐Sharing Bridging Motive

Abstract

AbstractThe ligand H6ioan has been used to synthesize the three dinuclear complexes [(ioan)MnIITiIV], [(ioan)FeIITiIV], and [(ioan)FeIIITiIV]+. The face‐sharing bridging mode of the three phenolates provides short M‐TiIV distances of ≈3.0 Å. Mössbauer spectra of [(ioan)FeIIITiIV]+ show a magnetically split six‐line spectrum at 3 K in zero magnetic field demonstrating a slow magnetic relaxation. Magnetic measurements provide a zero‐field splitting of |D|=5 cm−1 in [(ioan)FeIITiIV]. EPR spectroscopy demonstrates sizable zero‐field splittings of the S=5/2 spin systems of [(ioan)MnIITiIV] (D=0.246 cm−1) and [(ioan)FeIIITiIV]+ (D<−1 cm−1) that can be related to enforced covalency of the M‐Oph bonds. [(ioan)FeIIITiIV]+ exhibits a reversible reduction at −0.26 V vs. Fc+/Fc demonstrating the facile accessibility of FeIII and FeII. In contrast to an irreversible oxidation in [(ioan)NiIITiIV] at 0.78 V vs. Fc+/Fc, the reversible oxidation at 0.25 V vs. Fc+/Fc in [(ioan)MnIITiIV] indicates even the access of MnIII. These results indicate that pentanuclear complexes [(ioan)FeM1M2M1Fe(ioan)]n+ are meaningful targets to access electron delocalization in mixed‐valence systems over five ions due to the facile accessibility of both FeII and FeIII in the terminal positions. This study provides important local spin‐Hamiltonian and Mössbauer parameters that will be essential for the understanding of the potentially complicated electronic structure in the anticipated pentanuclear complexes.