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 O 2/Zn to mediate oxidation of hydrocarbons. The reagents investigated derive from the use of iron complexes with the anion of the weak carboxylic acid Me 3CCO 2H (pivalic acid (PivH)) in pyridine/PivH. In these solutions, the known compound [Fe 3O(O 2CCMe 3) 6) (py) 3] is reduced by Zn to generate yellow-green [Fe II(O 2CCMe 3) 2 (py) 4], which readily reverts to [Fe 3O(O 2CCMe 3) 6(py) 3], and eventually to [Fe 3O(O 2CCMe 3) 6 (py) 3] +, upon exposure to dioxygen. All three species are equally well suited to mediate Gif-like oxygenation of substrates supported by O 2/Zn. [Fe III 3O(O 2 CCMe 3) 6(L) 3] + (L = H 2O, py) is converted by H 2O 2 to afford the hexairon(III) peroxo compounds [Fe 6(O 2)(O) 2(O 2 CCMe 3) 12(L) 2] (L = Me 3CCO 2H, py), which feature a [Fe 6(η 2μ 4-O 2) (μ 3-O) 2] core previously documented in the closely related [Fe 6(O 2)(O) 2 (O 2CPh) 12(H 2O) 2]. A similar peroxo species, [Fe 6(O 2)(O) 2(O 2 CCMe 3) 2-(O 2 CCF 3) 10(H 2O) 2], is obtained upon replacing all pivalate ligands by trifluoroacetate groups with the exception of those pivalates that bridge between the two [Fe 3O(O 2CCF 3) 5 (H 2O)] 2+ units. The structure of the [Fe 6(O 2)(O) 2] core in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe 6(O 2)(O) 2(O 2 CCMe 3) 12(HO 2CCMe 3) 2] with NaBH 4 generates [Na 2Fe 4(O) 2(O 2 CCMe 3) 10(L)(L′)] (L = CH 3CN, L′ = Me 2CO; L = L′ = Me 3CCO 2H), which feature a [Na 2Fe 4(O) 2] core possessing a bent butterfly conformation of the [Fe 4(O) 2] unit. Oxidation of the same peroxo complex by Ce IV or NOBF 4 regenerates the oxo-bridged [Fe 3O(O 2CCMe 3) 6 (solv) 3] + (solv = EtOH, H 2O, thf). Employment of the sterically encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe 4(O) 2(O 2CCMe 3) 8(2-M e-5-Etpy) 2], which can be readily transformed upon treatment with H 2O 2 to the asymmetric peroxo complex [Fe 6(O 2)(O) 2(O 2 CCMe 3) 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.



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