Molecular and Electronic Structures of Dinuclear Iron Complexes Incorporating Strongly Electron-Donating Ligands: Implications for the Generation of the One- and Two-Electron Oxidized Forms

The ligands (Lt-Bu2)2−, (LMe2)2−, and (LCl2)2− have been employed for the synthesis of the dinuclear FeIII complexes [Lt-Bu2Fe(μ-O)FeLt-Bu2], [LMe2Fe(μ-O)FeLMe2], and [LCl2Fe(μ-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 FeIII−Ooxo and FeIII−OPh 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 FeIII−Ooxo bonding. Spectroelectrochemical characterization demonstrates that [Lt-Bu2Fe(μ-O)FeLt-Bu2] can be oxidized reversibly at +0.27 and +0.44 V versus Fc+/Fc, whereas [LMe2Fe(μ-O)FeLMe2] and [LCl2Fe(μ-O)FeLCl2] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc+/Fc, respectively. UV−vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mössbauer spectroscopy show that the successive oxidations of [Lt-Bu2Fe(μ-O)FeLt-Bu2] are ligand-centered leading to the monophenoxyl radical complex [•Lt-Bu2FeIII(μ-O)FeIIILt-Bu2]+ (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [•Lt-Bu2FeIII(μ-O)FeIII•Lt-Bu2]2+. Both products are unstable in solution and decay by cleavage of an FeIII−Ooxo bond. The two-electron oxidized species is more stable because of two equally strong FeIII−Ooxo bonds, whereas in the singly oxidized species the FeIII−Ooxo 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]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]−.