Synthesis and characterization of neutral and cationic aluminum complexes supported by a furfuryl-containing aminophenolate ligand for ring-opening polymerization of ε-caprolactone

The synthesis, structural characterization and reactivity of aluminum complexes supported by a novel tetradentate aminophenolate ligand containing furfuryl groups (LH), LAlMe2 (1), LAlMeCl (2) and LAlMeOtBu (3), are described. The molecular structures of ligand LH and complexes 1–3 are determined by X-ray structural analysis. Complexes 1–3 contain a four-coordinated mononuclear aluminum center. Activation of complex 1 with either B(C6F5)3 or [Ph3C][B(C6F5)4] afforded the corresponding cationic complex, [LAlMe][MeB(C6F5)3] or [LAlMe][B(C6F5)4], respectively. All cationic complexes were stable at room temperature in the absence of an external Lewis base over a week. The cationic complex [LAlMe][MeB(C6F5)3] decomposed upon heating at 70 °C, giving a neutral LAlMe(C6F5) complex. Complexes 1–3 were inactive for the ring-opening polymerization (ROP) of e-caprolactone (CL) at room temperature. However, only the cationic aluminum complex [LAlMe][MeB(C6F5)3] in the presence of benzyl alcohol was found to be active in the ROP of CL at room temperature in a well-behaved manner, giving a first-order reaction with respect to [CL].