Quantum chemical studies on elementary fragments of three-coordinated methylaluminoxanes

The molecular structures and relative stabilities of methylaluminoxanes (MAO) with three-coordinated aluminum centers were investigated by quantum chemical Hartree–Fock, MP2, and B3LYP methods. Linear and cyclic fragments of MAO were studied as a function of the degree of oligomerisation. The molecular structures are determined by the strain due to ring formation and/or by the stabilizing π-bonding interactions between the oxygen lone pairs and vacant p-orbitals of aluminums. The latter results in short AlO distances, and in an AlOAl angle of 180° for linear MAO, with adjacent methyl groups perpendicular to each other. The relative stabilities of the cyclic MAO increase with the degree of oligomerisation. Smaller oligomers are destabilized due to the strain of the ring and reduced π-conjugation. Stabilities of the linear MAO show no variation with length of the fractions.