Synthesis, structural characterization and reactivity of aluminium complexes supported by benzotriazole phenoxide ligands: air-stable alumoxane as an efficient catalyst for ring-opening polymerization of l-lactide

Abstract

Aluminium complexes bearing sterically bulky benzotriazole-phenoxide ligands are synthesized and characterized structurally. The reaction of 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (^CMe2PhBTP-H) or 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (^t-BuBTP-H) with AlMe₃ (1.2 molar equiv.) in toluene yields [(^CMe2PhBTP)AlMe₂] (1) or [(^t-BuBTP)AlMe₂] (2) as a four-coordinated monomeric aluminium complex. Compound 1 reacts further with ^CMe2PhBTP-H in a stoichiometric proportion, affording penta-coordinated monomeric aluminium methyl complex [(^CMe2PhBTP)₂AlMe] (3). Complex 3 is also obtained directly upon treatment of AlMe₃ with ^CMe2PhBTP-H (two molar equiv.) in refluxing toluene in high yield. In the presence of H₂O (half a molar equiv.), hydrolysis of 3 in a mixed solvent of THF and toluene at ambient temperature affords [{(^CMe2PhBTP)₂Al}₂(μ-O)] (4), in which the oxo ligand acts as a chelating group linearly bridging two aluminium centers. Air-stable alumoxane 4 is an efficient catalyst for the ring-opening polymerization of L-lactide (L-LA) in the presence of 9-anthracenemethanol (9-AnOH). Complex 4 catalyzes the polymerization of L-LA in a controlled manner, yielding PLLAs with the expected molecular weights and narrow polydispersity indices (PDIs).