Probing the Reactivity of Redox-Active 2-Aminophenolates on Iron Complexes of a Carbanionic N3C Donor Ligand

Three iron(II)-2-aminophenolate complexes [(L-1)Fe-II(4-tBu-HAP)] (2), [(L-1)Fe-II(HAP)] (3), and [(L-1)Fe-II(4-NO2-HAP)] (4) [L-1 = tris(2-pyridylthio)methanido anion, 4-tBu-HAP = 2-amino-4-tert-butylphenolate, HAP = 2-aminophenolate, and 4-NO2-HAP = 2-amino-4-nitrophenolate] were isolated and characterized to study their reactivity towards dioxygen. The complexes were synthesized from the iron(II)-acetonitrile complex [(L-1)Fe-II(CH3CN)(2)](+) (1) using substituted 2-aminophenolates. Single crystal structure of 4 reveals that the mononuclear central iron atom is coordinated by a tetradentate N3C ligand and a monoanionic bidentate 4-NO2-HAP giving rise to a distorted octahedral coordination arrangement. The six-coordinate iron(II) complexes, 2 and 4, react with dioxygen to form the corresponding iron(III)-2-iminobenzosemiquinonato radical complexes, [(L-1)Fe-III(4-tBu-ISQ)](+) (2(Ox)) and [(L-1)Fe-III(4-NO2-ISQ)] (4(Ox)). Complex 3, on the other hand, undergoes an oxidative C-S bond forming reaction between HAP and pyridine-2-thiolate (PyS) to afford [(L-1)Fe-III(ISQ)](+) (3(Ox)) [ISQ = 5-(2-thiopyridyl)-2-iminobenzosemiquinonato radical]. In each case, the reaction proceeds via a low-spin iron(III)-2-amidophenolate intermediate species. All the complexes exhibit strong iron-carbon bonding interaction. The bond parameters of the oxidized complexes, obtained from the single crystal structures, clearly establish the presence of ligand-centered radical.