Aluminium alkyl complexes supported by [OSSO] type bisphenolato ligands: synthesis, characterization and living polymerization of rac-lactide

Abstract

Aluminium alkyl complexes [(OSSO)AlR] (1–3: R = Me, Et) were isolated in good yields from the protonolysis reaction of AlR₃ with the corresponding tetradentate 1,ω-dithiaalkanediyl-bridged bisphenols (1,4-dithiabutanediyl-bis(6-tert-butyl-4-methylphenol), etbmpH₂; ortho-xylylenedithio-bis(6-tert-butyl-4-methylphenol), xytbmpH₂). The monomeric structures of all three complexes were confirmed by X-ray diffraction studies. Complexes 1 and 2 have an isotypic packing arrangement. The aluminium center is coordinated by the etbmp ligand and one alkyl group with distorted trigonal bipyramidal geometry. Complex 3 shows C_s symmetry with square pyramidal geometry around the metal center. Substitution reaction of complex 1 with trityl alcohol gave the monomeric alkoxide complex [(etbmp)Al(OCPh₃)] 4, which has a similar trigonal bipyramidal geometry around the aluminium atom as complex 1. In the presence of isopropanol, complexes 1–3 initiated the living ring-opening polymerization of rac-lactide (PDI = 1.03–1.06, M_w/M_n). The ligand structure influenced the tacticity of the obtained polymer, with complex 3 giving heterotactic-enriched polylactides.