Synthesis, structure, and catalytic activity of binuclear aluminum complexes with chiral biaryl-based N2O ligands

Six new binuclear organoaluminum complexes (1)2Al2Me2 (7), {(2)AlMe}2 (8), {(3)AlMe}2 (9), {(4)AlMe}2 (10), {(5)AlMe}2 (11) and {(6)AlMe}2 (12) have been prepared in good yields by alkane elimination from AlMe3 and chiral biaryl-based N2O ligands, (S)-2-(pyrrol-2-methyleneamino)-2′-hydroxy-1,1′-binaphthyl (1H2), (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyrrol-2-methyleneamino)-2′-hydroxy-1,1′-binaphthyl (2H2), (S)-2-(pyrrol-2-methyleneamino)-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl (3H2), (S)-2-(pyridine-2-ylmethylamino)-2′-hydroxy-1,1′-binaphthyl (4H2), (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyridine-2-ylmethylamino)-2′-hydroxy-1,1′-binaphthyl (5H2), and (S)-2-(pyridine-2-ylmethylamino)-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl (6H2), which are derived from (S)-2-amino-2′-hydroxy-1,1′-binaphthyl or (S)-2-amino-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl, respectively. Complexes 7–12 have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. Complex 7 derived from 1H2, has a C2v quasi-symmetric N4AlO2AlMe2 core structure, while complexes 8 and 10 derived from 2H2 or 4H2, adopt a C2 quasi-symmetric MeN2AlO2AlN2Me core structure, indicating that the steric effect of the ligand plays an important role in the complex formation. Complexes 7–12 are active catalysts for the polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides.