Synthesis and Calorimetric, Spectroscopic, and Structural Characterization of Isocyanide Complexes of Trialkylaluminum and Tri-tert-butylgallium

Addition of tert-butylisocyanide or 2,6-dimethylphenylisocyanide to a solution of trialkylaluminum or trialkylgallium results in formation of complexes R3M·CNtBu (M = Al, R = Me (1), Et (2), iBu (3), tBu (4); M = Ga, R = tBu (9)) or R3M·CN­(2,6-Me2C6H3) (M = Al, R = Me (5), Et (6), iBu (7), tBu (8); M = Ga, R = tBu (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 (1H, 13C) 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: tBu3Al ≫ iBu3Al ≥ Me3Al ≈ Et3Al ≫ tBu3Ga. In the absence of oxygen and protic reagents, the reported complexes do not undergo insertion or elimination reactions upon heating their benzene-d6 solutions to 80 °C.