title: Reactivity of an iron-oxygen oxidant generated upon oxidative decarboxylation of biomimetic iron(II) α-hydroxy acid complexes.
abstract: Three biomimetic iron(II) α-hydroxy acid complexes, [(Tp(Ph2))Fe(II)(mandelate)(H2O)] (1), [(Tp(Ph2))Fe(II)(benzilate)] (2), and [(Tp(Ph2))Fe(II)(HMP)] (3), together with two iron(II) α-methoxy acid complexes, [(Tp(Ph2))Fe(II)(MPA)] (4) and [(Tp(Ph2))Fe(II)(MMP)] (5) (where HMP = 2-hydroxy-2-methylpropanoate, MPA = 2-methoxy-2-phenylacetate, and MMP = 2-methoxy-2-methylpropanoate), of a facial tridentate ligand Tp(Ph2) [where Tp(Ph2) = hydrotris(3,5-diphenylpyrazole-1-yl)borate] were isolated and characterized to study the mechanism of dioxygen activation at the iron(II) centers. Single-crystal X-ray structural analyses of 1, 2, and 5 were performed to assess the binding mode of an α-hydroxy/methoxy acid anion to the iron(II) center. While the iron(II) α-methoxy acid complexes are unreactive toward dioxygen, the iron(II) α-hydroxy acid complexes undergo oxidative decarboxylation, implying the importance of the hydroxyl group in the activation of dioxygen. In the reaction with dioxygen, the iron(II) α-hydroxy acid complexes form iron(III) phenolate complexes of a modified ligand (Tp(Ph2)*), where the ortho position of one of the phenyl rings of Tp(Ph2) gets hydroxylated. The iron(II) mandelate complex (1), upon decarboxylation of mandelate, affords a mixture of benzaldehyde (67%), benzoic acid (20%), and benzyl alcohol (10%). On the other hand, complexes 2 and 3 react with dioxygen to form benzophenone and acetone, respectively. The intramolecular ligand hydroxylation gets inhibited in the presence of external intercepting agents. Reactions of 1 and 2 with dioxygen in the presence of an excess amount of alkenes result in the formation of the corresponding cis-diols in good yield. The incorporation of both oxygen atoms of dioxygen into the diol products is confirmed by (18)O-labeling studies. On the basis of reactivity and mechanistic studies, the generation of a nucleophilic iron-oxygen intermediate upon decarboxylation of the coordinated α-hydroxy acids is proposed as the active oxidant. The novel iron-oxygen intermediate oxidizes various substrates like sulfide, fluorene, toluene, ethylbenzene, and benzaldehyde. The oxidant oxidizes benzaldehyde to benzoic acid and also participates in the Cannizzaro reaction.
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science (IACS) , 2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India.
authors.forename: Sayantan
authors.initials: S
authors.lastname: Paria
authors.forename: Sayanti
authors.initials: S
authors.lastname: Chatterjee
authors.forename: Tapan Kanti
authors.initials: TK
authors.lastname: Paine
mesh_terms: D005296:Ferrous Compounds / Q000737:chemistry*; D008401:Gas Chromatography-Mass Spectrometry; D006880:Hydroxy Acids / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D066244:Proton Magnetic Resonance Spectroscopy; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D005296:Ferrous Compounds; D006880:Hydroxy Acids; D016877:Oxidants; D007501:Iron; D010100:Oxygen