Mononuclear Iron‐(hydro/semi)quinonate Complexes Featuring Neutral and Charged Scorpionates: Synthetic Models of Intermediates in the Hydroquinone Dioxygenase Mechanism

Abstract

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-2yl)phosphine ( Ph2 TIP) ligand. The gentisate substrate was mimicked with the bidentate ligand 2-(1-methylbenzimidazol-2yl)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 re- [a]

Electronic, Magnetic, and Redox Properties and O2 Reactivity of Iron(II) and Nickel(II) o-Semiquinonate Complexes of a Tris(thioether) Ligand: Uncovering the Intradiol Cleaving Reactivity of an Iron(II) o-Semiquinonate Complex

“…The increase in χ T as the temperature was raised from 50 to 300 K is likely due to the thermal population of the S = 5 / 2 excited state. Interestingly, Fiedler’s mononuclear five-coordinate iron(II) p -semiquinonate complexes exhibit ferromagnetic coupling between the high-spin iron(II) center and the p -semiquinonateradical to yield an S = 5 / 2 ground state, as indicated by electron paramagnetic resonance (EPR) and DFT data 57 , 58 . The ferromagnetic coupling observed in the iron(II) p -semi-quinonate complexes (DFT-computed J values of ~65 cm −1 , H = −2 JS 1 ∙ S 2 ) was attributed to the orthogonal orientation of the magnetic orbitals.…”

Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes

“…145,146,147,148,149,150,151 Studies of relevant model complexes have confirmed that such an intermediate is feasible due to the "noninnocent" nature of dioxolene ligands. 152,153,154,155,156,157 However, spectroscopic studies of Fe-HPCD employing mutant enzyme and/or unactivated substrate have failed to detect a catalytically viable Fe(II)/SQ species, 158,159,160 and a recent density functional theory (DFT) analysis of the iron-based mechanism favored a superoxo-Fe(III)-catecholate description instead. 161 Thus, the precise electronic structure of the M/O2 adduct remains a matter of dispute, and we will return to this topic below.…”

Mimicking the Aromatic‐Ring‐Cleavage Activity of Gentisate‐1,2‐Dioxygenase by a Nonheme Iron Complex

“…Fiedlersg roup has recently shown that mononuclear iron(III) 2-(1-methylbenzimidazol-2-yl)hydroquinonate complexes of tridentate facial N 3 ligands undergo deprotonation to generate the corresponding iron(II) semiquionate radical species. [26] Thes uperoxide radical (II)c an attack the C1 carbon of the carboxysemiquinonato group to form an alkylperoxide intermediate (III). [27] The para-hydroxy group on the aromatic ring then participates in directing the heterolytic O À Oc leavage of the alkylperoxo intermediate (III)t hrough Criegee type rearrangement to form an anhydride intermediate (IV).…”