Synthesis and characterization of calcium complexes: Efficient catalyst for ring-opening polymerization of L-lactide

Abstract

Four calcium complexes coordinated with N,N,O-tridentated pyrazolonate (1–2) or Schiff base (3–4) ligands have been prepared. These reactions can be monitored by 1H NMR spectroscopic studies with the chemical shift from 2.20 to 2.36 ppm for 1, and 2.22 to 2.28 ppm for 2 for the NMe2 group and the proton peak of the imino group shifted from 8.23 ppm to 7.95 ppm for 3, and 8.52 ppm to 8.14 ppm for 4. Single crystal X-ray studies reveal that complex 1 bears a mononuclear feature with a hexa-coordinated calcium metal center bonded to two NNO-tridentate ligands. Variable temperature dependent 1H NMR spectroscopic studies of 3 indicate that the nitrogen atom of NMe2 group dissociates slowly in solution ranging from − 20 to 80 °C. In the presence of alcohol, all of these complexes show good catalytic activity toward ring-opening polymerization of L-lactide in a well-controlled manner, yielding polymers with expected molecular weights and narrow polydispersity indexes. The activity of these complexes is affected by the basicity of donor atoms.