Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies

Area of Science:

• Coordination Chemistry
• Organometallic Chemistry
• Polymer Chemistry

Background:

• Calix[4]arenes are versatile macrocyclic hosts with tunable properties.
• Zinc complexes are widely studied for their catalytic applications.
• Ring-opening polymerization (ROP) is a key method for synthesizing polyesters.

Purpose of the Study:

• To synthesize and characterize novel trinuclear zinc calix[4]arene complexes.
• To investigate the catalytic activity of these complexes in the ring-opening polymerization of ε-caprolactone.
• To compare the catalytic performance of different zinc complexes.

Main Methods:

• Synthesis of trinuclear zinc calix[4]arene complexes via reactions involving zinc precursors and substituted calix[4]arenes.
• Structural characterization using X-ray crystallography.
• Evaluation of catalytic activity in ring-opening polymerization (ROP) of ε-caprolactone at 90 °C.
• Kinetic studies to determine reaction rates and orders.

Main Results:

• Isolation and structural elucidation of six trinuclear zinc calix[4]arene complexes (1-6) with near-linear Zn3 motifs.
• Complexes 1-5 feature a central tetrahedral Zn flanked by trigonal bipyramidal Zn centers; complex 6 has a central distorted octahedral Zn flanked by trigonal planar and tetrahedral Zn centers.
• All synthesized complexes exhibited moderate to good activity in ε-caprolactone ROP, producing low to medium molecular weight polyesters.
• Kinetic studies revealed a specific activity order: I > 4 > 6 ≈ 2 ≈ 1 > 3.

Conclusions:

• The synthesized trinuclear zinc calix[4]arene complexes are effective catalysts for ε-caprolactone ROP.
• Structural variations in the zinc complexes influence their catalytic activity.
• The study provides insights into the structure-activity relationships of zinc-based ROP catalysts.