Pyrazolyl-based zinc(II) carboxylate complexes: synthesis, characterization and catalytic behaviour in ring opening polymerization of ε-caprolactone and D,L-lactide

Abstract

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L1) and bis(3,5-dimethylpyrazol-1-yl)methane (L2), react with [Zn(OAc)2] and 4-OH-C6H4COOH, 2-Cl-C6H4COOH, 3,5-(NO2)2-C6H3COOH and C6H5COOH to form the corresponding zinc(II) complexes [Zn(4-OH-C6H4COO)2L1] (1), [Zn(2-Cl-C6H4COO)2L1] (2), [Zn{3,5-(NO2)2-C6H3COO}2L1] (3), [Zn4(C6H5COO)8(L1)2]n (4) and [Zn{3,5-(NO2)2-C6H3COO}2L2] (5). All five zinc complexes were characterized by a combination of NMR and IR spectroscopy, mass spectrometry and elemental analysis. Complexes 4 and 5 were further characterized by single-crystal X-ray diffractometry. Complex 4 is polymeric, with two different pentacoordinate Zn centres in two different coordination environments. Complex 5 has a distorted tetrahedral geometry around the zinc. These Zn(II) complexes (1–5) successfully initiate the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and D,L-lactide at elevated temperatures, and under solvent-free conditions and in toluene, respectively. The polycaprolactone (PCL) and poly(D,L-lactide) (PLA) obtained are of moderate molecular weights and medium polydispersity indices. The end groups of the polymers, determined by MALDI-ToF MS, are mainly hydroxyl groups and the stereochemistry of PLA is predominantly isotactic as determined with 13C{1H} NMR spectroscopy.

Keywords

• Pyrazole
• Zinc
• Ring opening polymerization
• ε-Caprolactone
• D,L-lactide