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

投稿者: Yuma Morimoto

Article

カテコール、オルトアミノフェノール、オルトフェニレンジアミンで比較。O,Oより、N,Nのほうが、リガンドから電子が出やすいもよう。

† Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, United States
‡ Department of Chemistry, Ursinus College, Collegeville, Pennsylvania 19426, United States
Inorg. Chem., Article ASAP
DOI: 10.1021/ic403126p
Publication Date (Web): April 3, 2014
Copyright © 2014 American Chemical Society
*E-mail: adam.fiedler@marquette.edu. (A.T.F), *E-mail: cpopescu@ursinus.edu. (C.V.P) Fax: (+1) 414-288-7066.

Synopsis

The O2 reactivities of three mononuclear Fe(II) complexes containing “noninnocent” catecholate, o-aminophenolate, and o-phenylenediamine ligands have been examined with spectroscopic, computational, and kinetic methods. Depending on the identity of the bidentate ligand, the O2 reaction triggers iron-based oxidation, ligand-based oxidation, or a combination of both processes. The ligands are capable of donating protons as well as electrons, and the interplay between these two factors influences rates of reaction with O2.

Abstract

Abstract Image
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–), 4,6-di-tert-butyl-2-aminophenolate (tBu2APH–), 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.—Eur. J. 2013, 19, 9686–9698]. 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, Mössbauer, 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–), 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– 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–)]+ (3ox) to [Fe3+(L•–)]2+ (2ox) to [Fe2+(L0)]2+ (4ox). Notably, the O2 reaction rates vary by a factor of 105across 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.
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