Alkane Functionalization at Nonheme Iron Centers. Stoichiometric Transfer of Metal-Bound Ligands to Alkane

A series of [Fe^IIIX₂L]⁺complexes (L = TPA, tris(2-pyridylmethyl) amine, or NTB*, tris(N-ethylbenzimidazol- 2-ylmethyl)amine; X = Br, Cl, or N₃) has been synthesized and examined for their ability to activate alkyl hydroperoxides for the functionalization of alkanes at room temperature. The crystal structures of [FeCl₂(TPA)]ClO₄ (2) and [Fe₂-OCl₂(TPA)₂] (ClO₄)₂(6) were determined, while other complexes were characterized by their visible and NMR spectra. Treatment of the mononuclear complexes with a stoichiometric amount of alkyl hydroperoxide in the presence of cyclohexane affords halocyclohexane in good yield. The production of bromo- and azidocyclohexane was correlated with the disappearance of the characteristic LMCT bands of the mononuclear catalysts, which were converted into catalytically inactive (µ-oxo)diferric species. The yield of haloalkane increased to 100% based on complex when an excess of alkyl hydroperoxide was used, but it was not affected by the addition of excess halide. The formation of haloalkane was inhibited by the presence of dimethyl sulfide (forming DMSO), but was unaffected by the addition of 4-methyl-2,6-di-tert-butylphenol, suggesting the involvement of a two-electron oxidant in the reaction mechanism. The selectivities of the catalysts for cyclohexane and adamantane functionalization were significantly affected by the nature of the tripodal ligand and the bound halide but not by the alkyl group on the hydroperoxide. We thus propose the active species to be [O=Fe(L)(X)]²⁺, which may be related to the transient intermediate generated from the reaction of Fe₂O(TPA)₂(ClO₄)₄ with H₂O₂ (Leising et al. J. Am. Chem. Soc. 1991, 113, 3988–3990). In the proposed mechanism, [0=Fe(L)(X)]²⁺ abstracts hydrogen from the alkane and then transfers the bound halide to the incipient alkyl radical. Such an oxidative ligand transfer reaction has been proposed for the mechanism of thiazolidine ring formation in penicillin biosynthesis by the nonheme iron enzyme isopenicillin N synthase.