Synthesis of Al complexes containing phenoxy-imine ligands and their use as the catalyst precursors for efficient living ring-opening polymerisation of ε-caprolactone

Abstract

Al complexes containing phenoxy-imine ligands of type, Me₂Al[O-2-R¹-6-(R²NCH)C₆H₃] [R¹ = Me, R² = 2,6-ⁱPr₂C₆H₃ (1a), ^tBu (1b); R¹ = ^tBu, R² = 2,6-ⁱPr₂C₆H₃ (2a), ^tBu (2b), cyclohexyl (2c), adamantyl (2d), C₆H₅ (2e), 2,6-Me₂C₆H₃ (2f), C₆F₅ (2g)] have been prepared in high yields from AlMe₃ by treating with 1.0 equiv. of 2-R¹-6-(R²NCH)C₆H₃OH in n-hexane. Structures for 1a, 1b, 2a–e and 2g were determined by X-ray crystallography, and these complexes have a distorted tetrahedral geometry around Al; both the Al–O and the Al–N bond distances were influenced by substituents in both the aryloxo and the imino groups. Me₂Al[μ₂-O-2-(R²NCH)C₆H₄](AlMe₃) [R² = 2,6-ⁱPr₂C₆H₃ (3a), ^tBu (3b)] were prepared exclusively by reaction of AlMe₃ with 2-(R²NCH)C₆H₄OH, and these complexes form a distorted tetrahedral geometry around each Al centre with additional AlMe₃ coordinating to the oxygen in the phenoxy-imine ligand. Complexes 1a, 1b and 2a–g were tested as catalyst precursors for ring-opening polymerisation (ROP) of ε-caprolactone (CL) in the presence of ⁿBuOH (1.0 equiv. to Al), and their catalytic activities were strongly influenced by the imino substituent (R²). The efficient ROP has been achieved using the C₆F₅ analogue (2g), with the ROP taking place in a living manner.