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 ciscis 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.