Synthesis, Characterization, and Reactivity of Ferrous and Ferric Oxo/peroxo Pivalate Complexes in Relation to Gif-type Oxygenation of Substrates


This study examines structural features and aspects of reactivity of Gif-type reagents, which depend on O2/Zn to mediate oxidation of hydrocarbons. The reagents investigated derive from the use of iron complexes with the anion of the weak carboxylic acid Me3CCO2H (pivalic acid (PivH)) in pyridine/PivH. In these solutions, the known compound [Fe3O(O2CCMe3)6) (py)3] is reduced by Zn to generate yellow-green [FeII(O2CCMe3)2 (py)4], which readily reverts to [Fe3O(O2CCMe3)6(py)3], and eventually to [Fe3O(O2CCMe3)6 (py)3]+, upon exposure to dioxygen. All three species are equally well suited to mediate Gif-like oxygenation of substrates supported by O2/Zn. [FeIII3O(O2 CCMe3)6(L)3]+ (L = H2O, py) is converted by H2O2 to afford the hexairon(III) peroxo compounds [Fe6(O2)(O)2(O2 CCMe3)12(L)2] (L = Me3CCO2H, py), which feature a [Fe62μ 4-O2) (μ 3-O)2] core previously documented in the closely related [Fe6(O2)(O)2 (O2CPh)12(H2O)2]. A similar peroxo species, [Fe6(O2)(O)2(O2 CCMe3)2-(O2 CCF3)10(H2O)2], is obtained upon replacing all pivalate ligands by trifluoroacetate groups with the exception of those pivalates that bridge between the two [Fe3O(O2CCF3)5 (H2O)]2+ units. The structure of the [Fe6(O2)(O)2] core in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe6(O2)(O)2(O2 CCMe3)12(HO2CCMe3)2] with NaBH4 generates [Na2Fe4(O)2(O2 CCMe3)10(L)(L′ )] (L = CH3CN, L′ = Me2CO; L = L′ = Me3CCO2H), which feature a [Na2Fe4(O)2] core possessing a bent butterfly conformation of the [Fe4(O)2] unit. Oxidation of the same peroxo complex by CeIV or NOBF4 regenerates the oxo-bridged [Fe3O(O2CCMe3)6 (solv)3]+ (solv = EtOH, H2O, thf). Employment of the sterically encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe4(O)2(O2CCMe3)8(2-M e-5-Etpy)2], which can be readily transformed upon treatment with H2O2 to the asymmetric peroxo complex [Fe6(O2)(O)2(O2 CCMe3)12(2-Me-5-Etpy)2] . The peroxocontaining complexes oxidize both cis-stilbene and adamantane in either benzene or py/PivH, but only under forceful conditions and at very low yields. The low reactivity and high selectivity (tert/sec = 8)obtained in the oxidation of adamantane suggests that the present type of peroxo species is not directly involved in catalytic Gif-type oxygenations of adamantane.



Keywords and Phrases

Dyes, Reagents, Indicators, Markers And Buffers; Etilefrine; Ferric Ion; Ferrous Ion; Iron Complex; Oxide; Peroxide; Pivalic Acid; chemical Structure; Complex Formation; Conformation; Crystal Structure; Kinetics; Oxidation; Reaction Analysis; Structure Activity Relation; Synthesis; Etilefrine; Ferric Compounds; Ferrous Compounds; Indicators And Reagents; Kinetics; Models, Molecular; Molecular Conformation; Oxides; Peroxides; Structure-Activity Relationship

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