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

Al complexes containing phenoxy-imine ligands of type, Me2Al[O-2-R-1-6-((RN)-N-2=CH)C6H3] [R-1 = Me, R-2 = 2,6-(Pr2C6H3)-Pr-i (1a), Bu-t (1b); R-1 = Bu-t, R-2 = 2,6-(Pr2C6H3)-Pr-i (2a), Bu-t (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-R-1-6-((RN)-N-2=CH)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[mu(2)-O-2-((RN)-N-2=CH)C6H4](AlMe3) [R-2 = 2,6-(Pr2C6H3)-Pr-i (3a), Bu-t (3b)] were prepared exclusively by reaction of AlMe3 with 2-(R2N= CH)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 epsilon-caprolactone (CL) in the presence of (BuOH)-Bu-n (1.0 equiv. to Al), and their catalytic activities were strongly influenced by the imino substituent (R-2). The efficient ROP has been achieved using the C6F5 analogue (2g), with the ROP taking place in a living manner.