Synthetic, Spectroscopic and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o-Aminophenol Dioxygenases

Authors

Michael M. Bittner, Marquette UniversityFollow
David S. Krause, Ursinus College
Sergey V. Lindeman, Marquette UniversityFollow
Cordina V. Popescu, Ursinus CollegeFollow
Adam T. Fiedler, Marquette UniversityFollow

Document Type

Article

Publication Date

7-15-2013

Source Publication

Chemistry - A European Journal

Source ISSN

0947-6539

Abstract

The oxidative CC bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an Fe^II/O₂/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe(Tp)(^tBuISQ)] (2 a; where Tp=hydrotris(3,5-diphenylpyrazol-1-yl)borate and ^tBuISQ is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2 a and its one-electron oxidized derivative [3 a]⁺. In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (TIP). The isomer shifts of about 0.97 mm s⁻¹ obtained through Mössbauer experiments confirm that 2 a (and its TIP-based analogue [2 b]⁺) contain Fe^II centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the Fe^II–ISQ complexes yields complexes ([3 a]⁺ and [3 b]²⁺) with electronic configurations between the Fe^III–ISQ and Fe^II–IBQ limits (IBQ=iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed.

Comments

Accepted version. Chemistry - A European Journal, Vol. 19, No. 29 (July 2013): 9686-9698. DOI. © 2013 Wiley-VCH Verlag. Used with permission.

Recommended Citation

Bittner, Michael M.; Krause, David S.; Lindeman, Sergey V.; Popescu, Cordina V.; and Fiedler, Adam T., "Synthetic, Spectroscopic and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o-Aminophenol Dioxygenases" (2013). Chemistry Faculty Research and Publications. 365.
https://epublications.marquette.edu/chem_fac/365