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

•Six new chiral binuclear aluminum complexes have been reported.•The steric effect of the ligand plays an important role in the complex formation.•The aluminum complexes are active catalysts for the polymerization of rac-lactide.

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.

Graphical abstractA new series of binuclear aluminum complexes have been prepared from chiral biaryl-based N2O ligands. The aluminum complexes are active catalysts for the polymerization of rac-lactide, leading to the heterotactic-rich polylactides.Figure optionsDownload full-size imageDownload as PowerPoint slide