Reactions of 4-methylidene-bis(1-phenyl-3-methylpyrazol-5-one) with trimethylaluminum: Synthesis, structure and catalysis for the ring-opening polymerization of ε-caprolactone

Aluminum complexes bearing ligand O,O-bidentate dipyrazolate are synthesized and structurally characterized. The reaction of 4-methylidene-bis(1- phenyl-3-methylpyrazol-5-one) (MBP-H) with AlMe₃ (1.2 molar equiv) in toluene yields [(MBP)AlMe₂] (1) as the four-coordinate monomeric aluminum complex. Tri-adduct complex [Al(MBP)₃] (2) results from treatment of AlMe₃ with MBP-H (3 molar equiv) in toluene in a high yield. In the presence of 9-anthracenemethanol, mono-adduct aluminum complex 1 catalyzes efficiently the ring-opening polymerization of ε-caprolactone (ε-CL) in a controlled manner, yielding PCLs with the expected molecular weights and narrow polydispersity indices (PDIs).