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Synthetic, Spectroscopic, and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o ‐Aminophenol Dioxygenases
Author(s) -
Bittner Michael M.,
Kraus David,
Lindeman Sergey V.,
Popescu Codrina V.,
Fiedler Adam T.
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201300520
Subject(s) - chemistry , density functional theory , ligand (biochemistry) , bond cleavage , phosphine , raman spectroscopy , radical ion , resonance raman spectroscopy , stereochemistry , catalysis , crystallography , photochemistry , medicinal chemistry , ion , computational chemistry , organic chemistry , receptor , optics , biochemistry , physics
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 /O 2 /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( Ph 2Tp)( t Bu ISQ)] ( 2 a ; where Ph 2Tp=hydrotris(3,5‐diphenylpyrazol‐1‐yl)borate and t Bu ISQ 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 ( Ph 2TIP). The isomer shifts of about 0.97 mm s −1 obtained through Mössbauer experiments confirm that 2 a (and its Ph 2TIP‐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.