Reactions of (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline with diethylzinc: Synthesis, characterization and catalytic studies for ring-opening polymerization of ε-caprolactone

Zinc complexes supported by NN-bidentate AA^Bn ligand (AA ^Bn-H = (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline) are synthesized and fully characterized. The reaction of diethyl zinc (ZnEt ₂) with AA^Bn-H (1.1 mol equiv) yields three-coordinated mono-adduct zinc complex [(AA^Bn)ZnEt] (1). However, treatment of AA^Bn-H with ZnEt₂ in the same stoichiometric proportion in the presence of 4-dimethylaminopyridine (DMAP, one equiv) affords [(AA ^Bn)Zn(DMAP)Et] (2) as the tetra-coordinated monomeric zinc complex. Bis-adduct complex [(AA^Bn)₂Zn] (3) results from treatment of ZnEt₂ with AA^Bn-H (two equiv) in refluxing toluene in high yield. Complexes 1 and 3 are efficient catalysts for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) in the presence of alcohol. The "controlled" character of Zn complex 1 also enabled preparation of the PEG-b-PCL copolymer.