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 [Fe^II(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. [Fe^III₃O(O₂CCMe₃)₆(L)₃]⁺ (L = H₂O, py) is converted by H₂O₂ to afford the hexairon(III) peroxo compounds [Fe₆(O₂)(O)₂(O₂CCMe₃)₁₂(L)₂] (L = Me₃CCO₂H, py), which feature a [Fe₆(η²-μ₄-O₂)(μ₃-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(O₂CCMe₃)₂(O₂CCF₃)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(O₂CCMe₃)₁₂(HO₂CCMe₃)₂] with NaBH₄ generates [Na₂Fe₄(O)₂(O₂CCMe₃)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 Ce^IV 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-Me-5-Etpy)₂], which can be readily transformed upon treatment with H₂O₂ to the asymmetric peroxo complex [Fe₆(O₂)(O)₂(O₂CCMe₃)₁₂(2-Me-5-Etpy)₂]. The peroxo-containing 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.