Alumazene Adducts with Pyridines: Synthesis, Structure, and Stability Studies
journal contributionposted on 09.07.2007, 00:00 by Jiri Löbl, Alexey Y. Timoshkin, Trinh Cong, Marek Necas, Herbert W. Roesky, Jiri Pinkas
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.