PUB - Publikationen an der Universität Bielefeld

The ligand H(6)ioan has been used to synthesize the three dinuclear complexes [(ioan)(MnTiIV)-Ti-II], [(ioan)(FeTiIV)-Ti-II], and [(ioan)(FeTiIV)-Ti-III](+). The face-sharing bridging mode of the three phenolates provides short M-Ti-IV distances of approximate to 3.0 angstrom. Mossbauer spectra of [(ioan)(FeTiIV)-Ti-III](+) 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)(FeTiIV)-Ti-II]. EPR spectroscopy demonstrates sizable zero-field splittings of the S=5/2 spin systems of [(ioan)(MnTiIV)-Ti-II] (D=0.246 cm(-1)) and [(ioan)(FeTiIV)-Ti-III](+) (D<-1 cm(-1)) that can be related to enforced covalency of the M-O-ph bonds. [(ioan)(FeTiIV)-Ti-III](+) exhibits a reversible reduction at -0.26 V vs. Fc(+)/Fc demonstrating the facile accessibility of Fe-III and Fe-II. In contrast to an irreversible oxidation in [(ioan)(NiTiIV)-Ti-II] at 0.78 V vs. Fc(+)/Fc, the reversible oxidation at 0.25 V vs. Fc(+)/Fc in [(ioan)(MnTiIV)-Ti-II] indicates even the access of Mn-III. These results indicate that pentanuclear complexes [(ioan)(FeMMMFe)-M-1-M-2-Fe-1(ioan)](n+) are meaningful targets to access electron delocalization in mixed-valence systems over five ions due to the facile accessibility of both Fe-II and Fe-III in the terminal positions. This study provides important local spin-Hamiltonian and Mossbauer parameters that will be essential for the understanding of the potentially complicated electronic structure in the anticipated pentanuclear complexes.