Experimental study reveals modest activity and instability of zirconium calixarene complexes during lactide polymerization, suggesting ligand flexibility limits catalytic control.
A series of zirconium(IV) complexes with a bis(benzylamino)-p-tert-butylcalix[4]arene ligand, LZrX2, were synthesized and structurally characterized via single-crystal X-ray diffraction and NMR. The obtained complexes proved to be unexpectedly susceptible to protonation because of the low acidity of the ligand. A high flexibility of the calix[4]arene ligand allowed either distorted-octahedral coordination or coordination numbers greater than 6. Complexes with different ancillary ligands X showed modest activity and poor stereocontrol toward rac-lactide polymerization and proved to be unstable under polymerization conditions.
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Cardoso et al. (2026) studied this question.
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