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.