Dioxygen Reactivity of Biomimetic Fe(II) Complexes with Noninnocent Catecholate, o-Aminophenolate, and o-Phenylenediamine Ligands

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• 作者：Michael M. Bittner ; Sergey V. Lindeman ; Codrina V. Popescu ; Adam T. Fiedler
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：April 21, 2014
• 年：2014
• 卷：53
• 期：8
• 页码：4047-4061
• 全文大小：655K
• 年卷期：v.53,no.8(April 21, 2014)
• ISSN：1520-510X

文摘

This study describes the O₂ reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (^Ph2TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate (^tBuCatH^{鈥?/sup>), 4,6-di-tert-butyl-2-aminophenolate (tBu2APH鈥?/sup>), or 4-tert-butyl-1,2-phenylenediamine (tBuPDA). The preparation and X-ray structural characterization of [Fe2+(Ph2TIP)(tBuCatH)]OTf, [3]OTf and [Fe2+(Ph2TIP)(tBuPDA)](OTf)₂, [4](OTf)₂ are described here, whereas [Fe2+(Ph2TIP)(tBu2APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.鈥擡ur. J. 2013, 19, 9686鈥?698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O₂, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, M枚ssbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O₂ to yield the ferric-catecholate species [Fe3+(Ph2TIP)(tBuCat)]+ (3ox), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4]2+ experiences a two-electron (2e鈥?/sup>), ligand-based oxidation to give [Fe2+(Ph2TIP)(tBuDIBQ)]2+ (4ox), where DIBQ is o-diiminobenzoquinone. The reaction of [2]+ with O₂ is also a 2e鈥?/sup> process, yet in this case both the Fe center and tBu2AP ligand are oxidized; the resulting complex (2ox) is best described as [Fe3+(Ph2TIP)(tBu2ISQ)]+, where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe3+(L2鈥?/sup>)]+ (3ox) to [Fe3+(L鈥⑩€?/sup>)]2+ (2ox) to [Fe2+(L0)]2+ (4ox). Notably, the O₂ reaction rates vary by a factor of 105 across the series, following the order [3]+ > [2]+ > [4]2+, even though the complexes have similar structures and Fe3+/2+ redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O₂ and participate in H-atom transfer reactions. We conclude that the trend in O₂ reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.}