Synthesis of aluminum complexes supported by 2-(1,10-phenanthrolin-2-yl)phenolate ligands and their catalysis in the ring-opening polymerization of cyclic esters

Abstract

A series of aluminum complexes supported by 2-(1,10-phenanthrolin-2-yl)phenolate ligands were synthesized and characterized. The reaction of o-bromophenol with n-butyllithium followed by nucleophilic addition to the phenanthroline and oxidation process gave the ligand precursors 2-(1,10-phenanthrolin-2-yl)phenol derivatives 1a–1d. Treatment of 1a–1d with an equiv. of AlR₃ (R = Et, Buⁱ) afforded the corresponding aluminum aryloxides 2a–2e. Treatment of 2b with 2 equiv. of benzyl alcohol formed complex 2f. All new compounds were characterized by ¹H and ¹³C NMR spectroscopy and elemental analyses. The structure of complex 2c was further characterized by single crystal X-ray diffraction techniques. Complexes 2a–2e/BnOH and 2f were active to catalyze the ring-opening polymerization of ε-caprolactone, leading to polycaprolactone with good molecular weight control and relatively narrow molecular weight distribution. Complexes 2a–2e/BnOH and 2f catalyzed the polymerization of rac-lactide and 2b, 2c, 2e and 2f were demonstrated to lead to isotactic enriched polylactides. The above catalysts also catalyzed the ROP of rac-β-butyrolactone and led to atactic poly(hydroxybutyrate). Complex 2f also catalyzed block copolymerization of the cyclic esters to form PCL-b-PHB, PCL-b-PLA, and PHB-b-PLA block copolymers.