Addition of tert-butylisocyanide or 2,6-dimethylphenylisocyanide to a solution of trialkylaluminum or trialkylgallium results in formation of complexes R(3)M·C≡N(t)Bu (M = Al, R = Me (1), Et (2), (i)Bu (3), (t)Bu (4); M = Ga, R = (t)Bu (9)) or R(3)M·C≡N(2,6-Me(2)C(6)H(3)) (M = Al, R = Me (5), Et (6), (i)Bu (7), (t)Bu (8); M = Ga, R = (t)Bu (10)), respectively. Complexes 1, 4, 5, and 8-10 are isolated as solids, whereas the triethylaluminum and triisobutylaluminum adducts 2, 3, 6, and 7 are viscous oils. Complexes 1-10 were characterized by NMR ((1)H, (13)C) and IR spectroscopies, and the molecular structures of 4, 5, and 8-10 were also determined by X-ray crystallography. The frequency of the C≡N stretch of the isocyanide increased by 58-91 cm(-1) upon complexation, consistent with coordination of the isocyanide as a σ donor. Enthalpies of complex formation for 1-10 were determined by isothermal titration calorimetry. Enthalpy data suggest the following order of decreasing Lewis acidity: (t)Bu(3)Al ≫ (i)Bu(3)Al ≥ Me(3)Al ≈ Et(3)Al ≫ (t)Bu(3)Ga. In the absence of oxygen and protic reagents, the reported complexes do not undergo insertion or elimination reactions upon heating their benzene-d(6) solutions to 80 °C.

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Reactions of 3-methylindole with one equivalent of R 3 Al produce the N-indolide-bridged dimers ). NMR spectroscopy ( 1 H, 13 C) 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 Me 3 Al or Et 3 Al similarly produce the N-indolide-bridged compounds [(Ph)HC(3-CH 3 C 8 H 4 NAlR 2 ) 2 ] (R = Me (4)) or [HC(3-CH 3 C 8 H 4 NAlR 2 ) 3 ] (R =Me (5a), Et (5b)), respectively. Reaction of tri(3-methyl-2-indolyl)methane with one equivalent of i Bu 3 Al yielded the N-indolide-bridged compound [HC(3-CH 3 C 8 H 4 NAl i Bu 2 ) 2 (3-CH 3 C 8 H 4 NAl(H) i Bu)] ( 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 μ 2 -η 1 :η 1 -N-3-methylindolide groups in each compound, the first examples of bridging N-indolide groups in compounds of the group 13 elements.

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