PUB - Publikationen an der Universität Bielefeld

The ligands (Lt-Bu2)(2-), (L-Me2)(2-), and (L-Cl2)(2-) have been employed for the synthesis of the dinuclear Fe-III complexes [Lt-Bu2Fe(mu-O)FeLt-Bu2],[(LFe)-Fe-Me2(mu-O)FeLMe2], and [(LFe)-Fe-Cl2(mu-O)FeLCl2]. The strongly electron-donating groups (tert-amines and phenolates) were chosen to increase the electron density at the coordinated ferric ions and thus to facilitate the oxidation of the complexes, with the possibility of fine-tuning the electronic structures by variation of the remote substituents. Molecular structures established in the solid (by single-crystal X-ray diffraction) and in solution (by X-ray absorption spectroscopy) show that the Fe ions are five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation. Spectroscopic and magnetic characterization establishes the highly covalent nature of the Fe-III-O-oxo and Fe-III-O-Ph bonds. The variations in the donor capabilities of the phenolates (due to changes in the remote substituents) are compensated for by a flexible electron donation of the Fe-III-O-oxo bonding. Spectroelectrochemical characterization demonstrates that [Lt-Bu2Fe(mu-O)FeLt-Bu2] can be oxidized reversibly at +0.27 and +0.44 V versus Fe+/Fe, whereas [(LFe)-Fe-Me2(mu-O)FeLMe2] and [(LFe)-Fe-Cl2(mu-O)FeLCl2] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fe+/Fe, respectively. UV-vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mossbauer spectroscopy show that the successive oxidations of [Lt-Bu2Fe(mu-O)FeLt-Bu2] are ligand-centered leading to the monophenoxyl radical complex [center dot(Lt-Bu2FeIII)-(mu-O)(FeLl-Bu2)-L-III](+) with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [(Lt-Bu2FeIII)-L-center dot(mu-O)(FeLt-Bu2)-L-III center dot](2+). Both products are unstable in solution and decay by cleavage of an Fe-III-O-oxo bond. The two-electron oxidized species is more stable because of two equally strong Fe-III-O-oxo bonds, whereas in the singly oxidized species the Fe-III-O-oxo bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [(Lt-Bu2FeIII)](+) and [(Lt-Bu2FeIV)=O], while the decay of the dication yields [(Lt-Bu2FeIII)-L-center dot](2+) and [(Lt-Bu2FeIV)=O]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl6](-), leads to the formation of [FeIIICl4](-).