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

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• 作者：Julia B. H. Strautmann ; Carl-Georg Freiherr von Richthofen ; Gabriele Heinze-Bru虉ckner ; Serena DeBeer ; Eberhard Bothe ; Eckhard Bill ; Thomas Weyhermu虉ller ; Anja Stammler ; Hartmut Bo虉gge ; Thorsten Glaser
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：January 3, 2011
• 年：2011
• 卷：50
• 期：1
• 页码：155-171
• 全文大小：1446K
• 年卷期：v.50,no.1(January 3, 2011)
• ISSN：1520-510X

文摘

The ligands (L^t-Bu₂)^{2鈭?/sup>, (LMe₂)2鈭?/sup>, and (LCl₂)2鈭?/sup> have been employed for the synthesis of the dinuclear FeIII complexes [Lt-Bu₂Fe(渭-O)FeLt-Bu₂], [LMe₂Fe(渭-O)FeLMe₂], and [LCl₂Fe(渭-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鈭扥oxo and FeIII鈭扥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 FeIII鈭扥oxo bonding. Spectroelectrochemical characterization demonstrates that [Lt-Bu₂Fe(渭-O)FeLt-Bu₂] can be oxidized reversibly at +0.27 and +0.44 V versus Fc+/Fc, whereas [LMe₂Fe(渭-O)FeLMe₂] and [LCl₂Fe(渭-O)FeLCl₂] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc+/Fc, respectively. UV鈭抳is, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and M枚ssbauer spectroscopy show that the successive oxidations of [Lt-Bu₂Fe(渭-O)FeLt-Bu₂] are ligand-centered leading to the monophenoxyl radical complex [鈥?/sup>Lt-Bu₂FeIII(渭-O)FeIIILt-Bu₂]+ (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [鈥?/sup>Lt-Bu₂FeIII(渭-O)FeIII鈥?/sup>Lt-Bu₂]2+. Both products are unstable in solution and decay by cleavage of an FeIII鈭扥oxo bond. The two-electron oxidized species is more stable because of two equally strong FeIII鈭扥oxo bonds, whereas in the singly oxidized species the FeIII鈭扥oxo bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [Lt-Bu₂FeIII]+ and [Lt-Bu₂FeIV鈺怬], while the decay of the dication yields [鈥?/sup>Lt-Bu₂FeIII]2+ and [Lt-Bu₂FeIV鈺怬]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl₆]鈭?/sup>, leads to the formation of [FeIIICl₄]鈭?/sup>.}