Mononuclear Iron-(hydro/semi)quinonate Complexes Featuring Neutral and Charged Scorpionates: Synthetic Models of Intermediates in the Hydroquinone Dioxygenase Mechanism
Format of Original
European Journal of Inorganic Chemistry
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 (Ph2TIP) 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(Ph2TIP)(BIHQ)]OTf (1a), revealed a distorted square-pyramidal geometry. Structural and electrochemical data collected for 1a were compared to those previously reported for [Fe(Ph2Tp)(BIHQ)] (1b), which features an anionic hydridotris(3,5-diphenylpyrazol-1-yl)borate (Ph2Tp) ligand. Oxidation of 1a and 1b provides the corresponding FeIII 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 FeII 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.