Iron-Mediated Coupling of Two Ethyl Anions to Form a 2-Butyne Ligand

The σ‐aryliron complexes [5CpFe(C6H3iPr2‐2,6)] (2, 5Cp = η5‐C5iPr5) and [4CpFe(C6H3iPr2‐2,6)] (6, 4Cp = η5‐C5HiPr4) are available from the halides [5CpFe(μ‐Br)]2 (1) and [4CpFe(μ‐Br)]2 (4) and have been used for σ/π rearrangement reactions with trialkylaluminum compounds. Whereas trimethylaluminum can be added to 2 and 6 to form the aryltrimethylaluminate complexes [5CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlMe3] (3) and [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlMe3] (7) in clean σ/π rearrangement reactions, the addition of tripropylaluminum to 6 afforded a mixture containing the aryl(bromo)dipropylaluminate complex [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)AlBr(C3H7)2] (8). With triethylaluminum, a similar product mixture was obtained and could be separated to yield the aryl(bromo)diethylaluminate [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6) AlBr(C2H5)2] (9), the aryltriethylaluminate [4CpFe(μ,η6:η1‐C6H3iPr2‐2,6)Al(C2H5)3] (10), the arene complex [4CpFe(η6‐C6H4iPr2‐1,3)]Br (11), and the 2‐butyne complex [(4CpFe)2(μ,η2:η2‐MeCCMe)] (12). The formation of 12 involves the coupling of two ethyl groups with the loss of four hydrogen atoms and could also be observed in reactions of the bromide dimer 4 or the σ aryl complex 6 with ethylmagnesium bromide. The aluminates 3, 7, 8, and 10, the arene sandwich 11, the 2‐butyne complex 12, as well as the alkylcyclopentadienyliron(II) halides [5CpFe(μ‐Br)]2 (1), [4CpFe(μ‐Br)]2 (4), and [4CpFe(μ‐I)]2 (5) have been crystallographically characterized. With ethylmagnesium bromide, the high‐spin iron(II) aryl complex [(C5HiPr4)Fe(C6H3‐2,6‐iPr2)] or the half‐sandwich complex [(C5HiPr4)Fe(μ‐Br)]2 forms the dinuclear complex [{(C5HiPr4)Fe}2(μ,η2:η2‐C2Me2)], a rare example of a metallatetrahedrane with a metal–metal triple bond.