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-ty pe reagents, which depend on O-2/Zn to mediate oxidation of hydrocarbons. T he reagents investigated derive from the use of iron complexes with the ani on of the weak carboxylic acid Me3CCO2H (pivalic acid (PivH)) in pyridine/P ivH. In these solutions, the known compound [Fe3O(O2CCMe3)(6)(py)(3)] is re duced by Zn to generate yellow-green [Fe-II(O2CCMe3)(2)(py)(4)], which read ily reverts to [Fe3O(O2CCMe3)(6)(py)(3)], and eventually to [Fe3O(O4CCMe3)( 6)(py)(3)](+), upon exposure to dioxygen. Air three species are equally wel l suited to mediate Gif-like oxygenation of substrates supported by O-2/Zn. [(Fe3O)-O-III(O2CCMe3)(6)(L)(3)](+) (L = H2O, py) is converted by H2O2 to afford the hexairon(III) peroxo compounds [Fe-6(O-2)(O)(2)(O2CCMe3)(12)(L)( 2)] (L = Me3CCO2H, py), which feature a [Fe-6(eta (2)-mu (4)-O-2)(mu (3)-O) (2)] core previously documented in the closely related [Fe-6(O-2)(O)(2)(O2C Ph)(12)(H2O)(2)]. A similar peroxo species, [Fe-6(O-z)(O)(2)(O2CCMe3)(2)(O2 CCF3)(10)(H2O)(2)], is Obtained upon replacing all pivalate ligands by trif luoroacetate groups with the exception of those pivalates that bridge betwe en the two [Fe3O(O2CCF3)(5)(H2O)](2+) units. The structure of the [Fe-6(O-2 )(O)(2)] care in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe-6(O-2)(O)(2)(O2CCMe3)(12)(H O2CCMe3)(2)] With NaBH4 generates [Na2Fe4(O)(2)(O2CCNe3)(10)(L)(L')] (L = C H3CN, L' = Me2CO; L = L' = Me3CCO2H), which feature a [Na2Fe4(O)(2)] con po ssessing a bent butterfly conformation of the [Fe-4(O)(2)] unit. Oxidation of the same peroxo complex by Ce-IV or NOBF4 regenerates the ore-bridged [F e3O(O2CCMe3)(6)(solv)(3)](+) (solv = EtOH, H2O, thf). Employment of the ste ric ally encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe-4 (O)(2)(O2CCMe3)(8)(2-Me-5-Etpy)(2)], which can be readily transformed upon treatment with H2O2 to the asymmetric peroxo complex [Fe-6(O-2)(O)(2)(O2CCM e3)(12)(2-Me-5 -Etpy)(2)]. The peroxo-containing complexes oxidize bath cis -stilbene and adamantane in either benzene or py/PivH, but only under force ful conditions and at very low yields. The low reactivity and high selectiv ity (tert/sec = 8) obtained in the oxidation of adamantane suggests that th e present type of peroxo species is not directly involved in catalytic Gif- type oxygenations of adamantane.