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

Lewis acid−base adducts of the alumazene [2,6-(<i>i</i>-Pr)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>NAlMe]<sub>3</sub> (<b>1</b>) with pyridine (py) and 4-dimethylaminopyridine (dmap) were synthesized and structurally characterized:  <b>1</b>(py)<sub>2</sub> (<b>2</b>), <b>1</b>(py)<sub>3</sub> (<b>3</b>), <b>1</b>(dmap)<sub>2</sub> (<b>4</b>), and <b>1</b>(py)(dmap) (<b>5</b>). The bisadducts <b>2</b>, <b>4</b>, and <b>5</b> form the <i>trans</i> isomers. The trisadduct <b>3</b> exhibits an unexpected <i>cis</i><i>−</i><i>cis</i> 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 <b>3</b> is labile and converts to <b>2</b>. The thermodynamics of the adduct formation has been investigated experimentally and theoretically. Thermodynamic characteristics of the <b>1</b>(py)<i><sub>n</sub></i> (<i>n</i> = 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 <b>1</b>(py)<i><sub>n</sub></i> and model complexes [HAlNH]<sub>3</sub>(py)<i><sub>n</sub></i> (<i>n</i> = 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.