Alumazene Adducts with Pyridines:  Synthesis, Structure, and Stability Studies

Lewis acid−base adducts of the alumazene [2,6-(i-Pr)₂C₆H₃NAlMe]₃ (1) with pyridine (py) and 4-dimethylaminopyridine (dmap) were synthesized and structurally characterized:  1(py)₂ (2), 1(py)₃ (3), 1(dmap)₂ (4), and 1(py)(dmap) (5). The bisadducts 2, 4, and 5 form the trans isomers. The trisadduct 3 exhibits an unexpected cis−cis isomer and can be prepared only in the presence of excess py. The planarity of the alumazene ring is lost upon coordination of the Lewis base molecules. A comparison of the Al−N(base) bond distances and pyramidality at Al suggests the higher basicity of dmap. NMR spectroscopy confirms stability to dissociation of the bisadducts in solution while the trisadduct 3 is labile and converts to 2. The thermodynamics of the adduct formation has been investigated experimentally and theoretically. Thermodynamic characteristics of the 1(py)_n (n = 2, 3) dissociation reactions in the temperature range 25−200 °C have been derived from the vapor pressure−temperature dependence measurements by the static tensimetric method. In all experiments, excess py was employed. Quantum chemical computations at the B3LYP/6-31G* level of theory have been performed for the 1(py)_n and model complexes [HAlNH]₃(py)_n (n = 1−3). Obtained results indicate that for the gas phase adducts upon increasing the number of py ligands the donor−acceptor Al−N(py) distance increases in accord with decreasing donor−acceptor bond dissociation energies.