Magnesium and zinc complexes containing pendant pyrazolyl–phenolate ligands as catalysts for ring opening polymerisation of cyclic esters

• Mg and Zn complexes bearing pendant pyrazolyl–phenolate ligands are reported.
• Mg complexes with bulkier ligands show better activities in ROP of ε-caprolactone.
• Less steric demanding Zn complex shows better activities in ROP of l-lactide.

Reactions of HOPhtBu(CH2)Pz {4,6-di-tert-butyl-2-(pyrazol-1-yl-methyl)phenol} or HOPhtBu(CH2)PzMe {4,6-di-tert-butyl-2-(3,5-dimethylpyrazol-1-yl-methyl)phenol} with 1.1 equivalent of MgnBu2 in toluene gave the binuclear compounds (nBuMgOPhtBu(CH2)Pz)2 (1) or (nBuMgOPhtBu(CH2)PzMe)2 (2). Treatment of HOPhtBu(CH2)Pz or HOPhtBu(CH2)PzMe with 1.1 equivalent of ZnEt2 in n-hexane yielded binuclear compounds (EtZnOPhtBu(CH2)Pz)2 (3) and (EtZnOPhtBu(CH2)PzMe)2 (4). Colourless crystals of (ClZnOPhtBu(CH2)Pz)2 (5) were obtained via the recrystallisation of 3 from a concentrated dichloromethane solution at room temperature. Alternative route for the synthesis of 5 can be achieved via the reaction of HOPhtBu(CH2)Pz with 1.1 equivalent of n-butyllithium in diethylether followed by the treatment of 1.1 equivalent of ZnCl2 in tetrahydrofuran gave compound 5 as binuclear chloride complex. The compounds 2 and 5 were crystallographically characterised. The catalytic activities of 1–5 towards the ring opening polymerisation reactions of cyclic esters in the presence of benzyl alcohol were under investigation.

A family of magnesium or zinc alkyl complexes containing pendant pyrazolyl-phenolate ligands exhibits efficient catalytic activities for the ring opening polymerisation of ε-carprolactone or l-lactide in the presence of benzyl alcohol under mild condition.Figure optionsDownload as PowerPoint slide