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Mononuclear Iron‐(hydro/semi)quinonate Complexes Featuring Neutral and Charged Scorpionates: Synthetic Models of Intermediates in the Hydroquinone Dioxygenase Mechanism
Author(s) -
Baum Amanda E.,
Lindeman Sergey V.,
Fiedler Adam T.
Publication year - 2016
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201501380
Subject(s) - chemistry , deprotonation , electron paramagnetic resonance , protonation , ligand (biochemistry) , tautomer , dioxygenase , hydroquinone , photochemistry , stereochemistry , valence (chemistry) , denticity , crystallography , crystal structure , organic chemistry , ion , biochemistry , physics , receptor , nuclear magnetic resonance , enzyme
Neutral and anionic scorpionate ligands have been employed to generate active‐site models of hydroquinone dioxygenases (HQDOs). While the nonheme Fe center in nearly all HQDOs is coordinated to one Asp (or Glu) and two His residues, 1,2‐gentisate dioxygenase (GDO) is unique in featuring a three His triad instead. A synthetic GDO model was therefore prepared with the neutral tris(4,5‐diphenyl‐1‐methylimidazol‐2‐yl)phosphine ( Ph2 TIP) ligand. The gentisate substrate was mimicked with the bidentate ligand 2‐(1‐methylbenzimidazol‐2‐yl)hydroquinonate (BIHQ). X‐ray diffraction analysis of the resulting complex, [Fe( Ph2 TIP)(BIHQ)]OTf ( 1a ), revealed a distorted square‐pyramidal geometry. Structural and electrochemical data collected for 1a were compared to those previously reported for [Fe( Ph2 Tp)(BIHQ)] ( 1b ), which features an anionic hydridotris(3,5‐diphenylpyrazol‐1‐yl)borate ( Ph2 Tp) ligand. Oxidation of 1a and 1b provides the corresponding Fe III complexes ( 2a / 2b ) and the crystal structure of 2b is reported. Both complexes undergo reversible deprotonation to yield the brown chromophores, 3a and 3b . Detailed studies of 3a and 3b with spectroscopic (UV/Vis absorption, EPR, resonance Raman) and computational methods determined that each complex consists of a high‐spin Fe II center ferromagnetically coupled to a p ‐semiquinonate radical (BISQ). The (de)protonation‐induced valence tautomerization described here resembles key steps in the putative HQDO mechanism.