Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac-lactide

Abstract

A series of Schiff base bridged bis(indolyl) ligands were developed for aluminum chemistry. The reactions of AlEt₃ or AlMe₃ with the Schiff base bridged bis(indolyl) proligands R₁(–NCHC₈H₅NH)₂ (R₁ = –CH₂CH₂– (H₂L¹); –CH₂CH₂CH₂– (H₂L²); –CH₂CMe₂CH₂– (H₂L³); rac-Cy (H₂L⁴); and R,R-Cy (H₂L⁵)) were studied leading to the synthesis of a series of aluminum alkyl complexes L¹AlEt (1)–L⁵AlEt (5) and L³AlMe (3b) in good yields, while the reaction of H₂L³ with Al(OⁱPr)₃ gave the aluminum alkoxide complex L³AlOⁱPr (3a). These aluminum complexes were characterized by spectroscopic methods and elemental analyses. The solid state structures of the aluminum complexes 1–5 and 3a were confirmed by the X-ray diffraction study. X-ray analyses revealed that the aluminum centre in these complexes is five-coordinated. The coordination geometry is between square pyramidal and trigonal bipyramidal. In the presence of 1 equiv. of isopropanol, the aluminum alkyl complexes exhibited notable activity towards the ring-opening polymerization of rac-lactide at 70 °C in toluene, with good control over molecular weights and dispersities. The substituents and the length of the bridging part between the two Schiff base nitrogen atoms have an influence on either the tacticity of isolated polymers or the rate of polymerization. The kinetics of complex L³AlOⁱPr (3a) in C₆D₆ was also investigated, and the experimental results revealed that the rate of polymerization was first-order with respect to rac-lactide.