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

This study describes the 02 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 ((TIP)-T-Ph2) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-buty1-2-amino-phenolate ((tBu2)ApH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe2+((TIP)-T-Ph2)((tBu)CatH)]OTf, [3]OTf and [Fe2+((TIP)-T-ph2)((tBu)pDA)]- (OTf)(2), [4](OTf)(2) are described here, whereas [Fe2+((TIP)-T-Ph2)-((tBu2)APH)]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 O-2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mossbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O-2 to yield the ferric-catecholate species [Fe3+((TIP)-T-Ph2)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to [Fe2+((TIP)-T-ph2)((tBu)D1BQ](2+) give (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2]+ with 02 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (201) is best described as [Fe3+((TIP)-T-Ph2)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe3+(L2-)](+) (3(ox)) to [Fe3+(L center dot-)](2+) (2(ox)) to [Fe2+(L-0)](2+) (4(ox)). Notably, the O-2 reaction rates vary by a factor of 10(5) 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 O-2 and participate in H-atom transfer reactions. We conclude that the trend in O-2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.