Aluminium, gallium and indium complexes supported by a chiral phenolato-prolinolato dianionic ligand

Abstract

Congeneric complexes (S)-{ÔO}MCH2SiMe3 (M=Al, 3; Ga, 5; In, 6) of the triel metals supported by an enantiomerically pure phenolato-alkoxo {ÔO}2- dianionic tridentate ligand derived from prolinol, along with their chloro derivatives, have been prepared and characterised. The aluminium-alkyl species (S)-{ÔO}AlMe and 3 form four-coordinate complexes with slightly distorted tetrahedral geometries, whereas the geometry in the Lewis acidic five-coordinate (S)-{ÔO}GaCl·THF is a distorted trigonal bipyramid. These alkyl complexes do not react cleanly, if at all, with protic sources. The indium(III) compound 6, which is for instance inert towards iPrOH or BnOH even after hours at 70°C, catalyses at 95°C the controlled, immortal ring-opening polymerisation of racemic lactide (up to 1000 equivalents) in the presence of excess BnOH as a chain transfer agent. It affords atactic, monodisperse polylactides with predictable molecular weights.