Polymethylated [Fe(η6-arene)2]2+ dications: methyl-group rearrangements and application of the EINS mechanism

Abstract

Reactions between the methylated arenes ArMe_n [where ArMe_n = C₆Me_nH_(6-n), and n = 1–6] and FeCl₂ in heptane at 90 °C in the presence of anhydrous AlCl₃ give, for the arenes with n = 1–5, extensive isomerisations and disproportionations involving the methyl groups on the arene rings, and the formation of mixtures of [Fe(ArMe_n)₂]²⁺ dications that defy separation into pure species. GC-MS studies of AlCl₃/mesitylene and AlCl₃/durene reactions in the absence of FeCl₂ (90 °C, 2 h) allow quantitative assessments of the rearrangements, and the EINS mechanism (electrophile-induced nucleophilic substitution) is applied to rationalise the phenomena. By contrast, ArMe_n / FeCl₂ /AlCl₃ reactions in heptane for 24–36 h at room-temperature proceed with no rearrangements, allowing the synthesis of the complete series of pure [Fe(ArMen)]²⁺ cations in yields of 48–71%. The pure compounds are characterised by ¹H NMR spectroscopy and electrospray-ionization mass-spectrometry (ESI-MS), and the structures of [Fe(m-xylene)₂][PF₆]₂ and [Fe(durene)₂][PF₆]₂ are established by single-crystal X-ray diffraction analyses.