Synthesis and structures of tridentate ketoiminate zinc complexes bearing trifluoromethyl substituents that act as l-lactide ring opening polymerization initiators

Abstract

A series of NNO ketoimines bearing trifluoromethyl substituents were synthesized from the Schiff base condensation of 1,3-diketones (1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, 4,4,4-trifluoro-1-phenyl-1,3-butanedione, and 1,1,1-trifluoro-2,4-pentanedione, 1,1,1,5,5,5-hexafluoroacetylacetone) and 8-aminoquinoline or 8-amino-2-methylquinoline and isolated in 40-70% yield. The ketoimines were combined with zinc bis-(trimethylsilyl)amide to prepare a zinc amide complex in 41% yield or were combined with zinc bis-(trimethylsilyl)amide and 2,6-di-tert-butylphenol to prepare zinc phenoxide complexes in 81-94% yield. The ketoimines and zinc complexes were characterized with (1)H, (13)C, and (19)F NMR, absorbance spectroscopy, mass spectrometry, elemental analysis and X-ray crystallography. The mononuclear solid state structures of the zinc amide and phenoxide complexes showed tridentate coordination of the zinc center by the ketoiminate and monodentate coordination by the amide or phenoxide. The zinc complexes were assessed for their ability to catalyze the ring opening polymerization (ROP) of L-lactide into poly-lactic acid (PLA) with some complexes reaching 100% conversion in 3 h. As the monomer to catalyst ratio increased, the molecular weight of the isolated polymeric material increased in a nearly linear fashion while retaining a narrow molecular weight distribution. Homonuclear decoupled (1)H NMR spectra of the isolated polymeric material showed the retention of stereochemistry in the isotactic poly-L-lactic acid. Kinetic studies, where the substituents on the ketoiminate and quinolyl moiety were varied, showed that lower electron density on the Zn metal center yielded lower ROP catalytic activity than their electron rich counterparts. The complexes are proposed to use the coordination-insertion mechanism for living polymerization of L-lactide.