Confirmation of Bridging <i>N</i>-Indolides in 3-Methylindole and Di- and Tri(3-methylindolyl)methane Complexes of Dimethyl-, Diethyl-, and Diisobutylaluminum

Reactions of 3-methylindole with one equivalent of R3Al produce the N-indolide-bridged dimers [(3-CH3C8H5N)AlR2]2 (R = Me (3a), Et (3b), iBu (3c)). NMR spectroscopy (1H, 13C) demonstrates that compounds 3a, 3b, and 3c each exist in solution as a mixture of interconverting syn and anti isomers. Reactions of di(3-methylindolyl)methane or tri(3-methylindolyl)methane with one equivalent of Me3Al or Et3Al similarly produce the N-indolide-bridged compounds [(Ph)HC(3-CH3C8H4NAlR2)2] (R = Me (4)) or [HC(3-CH3C8H4NAlR2)3] (R = Me (5a), Et (5b)), respectively. Reaction of tri(3-methyl-2-indolyl)methane with one equivalent of iBu3Al yielded the N-indolide-bridged compound [HC(3-CH3C8H4NAliBu2)2(3-CH3C8H4NAl(H)iBu)] (6), apparently the result of β-hydrogen elimination of isobutene from one isobutyl group per complex. The molecular structures of compounds 3a, 4, and 5a were determined by X-ray crystallography. Crystallographic data confirmed the presence of μ2-η1:η1-N-3-methylindolide groups in each compound, the first examples of bridging N-indolide groups in compounds of the group 13 elements.