Mono-aluminum, di-magnesium and tri-zinc complexes supported by bisphenolate ligand: Synthesis, characterization and catalytic studies for ring-opening polymerization of cyclic esters

Mono-aluminum, di-magnesium and tri-zinc complexes coordinated by a Biphen ^2- ligand (Biphen-H ₂ = 3,3′-di-tert-butyl-5, 5′,6,6′-tetramethylbiphenyl-2,2′-diol) were synthesized and structurally characterized by X-ray single crystal determinations. The reaction of Biphen-H ₂ with AlMe3 (1.0 mol equiv.) in THF produced the tetra-coordinated monomeric aluminum complex [Al(Biphen)Me(THF)] (1), but treatment of Biphen-H ₂ with Mg ⁿBu ₂ under the same stoichiometric proportion in THF gave the dimeric magnesium complex [Mg(μ-Biphen)(THF)] ₂ (2). Tri-zinc [Zn ₃(μ, μ-Biphen) ₂Et ₂(THF) ₂] (3) resulted from treatment of Biphen-H ₂ with ZnEt ₂ (1.5 mol equiv) in THF; it consists of two different kinds of four-coordinated Zn centers. All of metal complexes are coordinated by at least one neutral THF coligand, indicating each metal center can behave as an active site to activate monomers during ring-opening polymerization (ROP). Catalysis for ROP of ε-caprolactone (ε-CL) or β-butyrolactone (β-BL) of complexes 1-3 are investigated, and the comparative studies of ε-CL polymerization are also discussed.