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.

Graphical abstractA 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 full-size imageDownload as PowerPoint slide