Bimetallic aluminum complexes bearing β-pyridyl-enamino ligands: synthesis, characterization and their applications in the ring-opening polymerization of ε-caprolactone

A series of bridged β-pyridyl-enamino dinuclear aluminum complexes 1–6 have been prepared in high yields, where complexes 1, 2, 3, 5 and 6 were characterized by single-crystal X-ray diffraction. The distance between two Al centers in 1–3 was in the range of 3.7 to 5.8 Å, which is far shorter than that in 5 and 6 (about 8.0 Å). All of these aluminum complexes were used as initiators for the ring-opening polymerization of ε-caprolactone in the presence of benzyl alcohol. Complexes 1–3 exhibited higher catalytic activity compared to complexes 4–6 under the same conditions, indicating that the distance between the Al atoms affects the catalytic activity. Complexes 3 and 5 with Al-Me groups exhibit higher catalytic activity than complexes 4 and 6 with Al-Et groups. This is likely due to the larger steric hindrance or the electron-donating group weakens the Lewis acidity of aluminum, which results in a reduced reaction rate between the complexes and benzyl alcohol. Herein, there are two factors affecting the polymerization activity in the ROP of ε-caprolactone: one is the distance between the two Al centers in bimetallic systems; the other is the alkyl groups on the Al atoms. The MALDI-TOF mass spectrum of the polymers revealed that the ring-opening polymerization of ε-caprolactone catalyzed by these catalysts yields linear polycaprolactones with controlled molecular weights, narrow distributions and end group fidelity.