title: Synthesis and structures of bis-ligated zinc complexes supported by tridentate ketoimines that initiate L-lactide polymerization.
abstract: Eight bis-ligated, homoleptic, zinc complexes were synthesized through the reaction of NNO Schiff base ketoimines bearing varying substituents with diethyl zinc in an inert atmosphere glovebox at room temperature and isolated in 62-95% yield. The complexes were characterized with (1)H, (13)C, and (19)F nuclear magnetic resonance spectroscopy, absorbance spectroscopy, high resolution mass spectrometry, elemental analysis, and single crystal X-ray crystallography. The complexes were shown to adopt distorted octahedral coordination geometry around zinc. The (1)H and (19)F NMR spectra of complexes 1-7 showed stable zinc coordination at 300 K while the effect of steric encumbrance and two trifluoromethyl groups in complex 8 was investigated with variable temperature NMR. The bis-ligated zinc complexes were effective initiators for the ring opening polymerization of L-lactide into poly-L-lactic acid (PLLA). With [L-lac]/[Zn complex] = 50, the bis-ligated zinc complexes yielded percentage conversion of 14-98% with polymerization times varying from 15-1440 min, where the longest reaction times were required when two trifluoromethyl groups were present. The addition of 4-fluorophenol co-catalyst resulted in up to a 5-fold increase in the percentage conversion in toluene solution and up to a 14-fold increase in bulk melt polymerization with reductions in the poly-dispersity index values for the isolated PLLA. Addition of 4-fluorophenol to complex 1 was studied with (1)H and (19)F NMR and appeared to yield an in situ generated zinc alkoxide complex.
authors.affiliation: Department of Chemistry, Pepperdine University, 24255 Pacific Coast Highway, Malibu, CA 90263, USA. Joseph.fritsch@pepperdine.edu.
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Gerling
authors.forename: Nomaan M
authors.initials: NM
authors.lastname: Rezayee
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold
authors.forename: David B
authors.initials: DB
authors.lastname: Green
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Fritsch
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004146:Dioxanes / Q000737:chemistry*; D007097:Imines / Q000737:chemistry*; D019344:Lactic Acid / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D011091:Polyesters; D058105:Polymerization; D011108:Polymers / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D015032:Zinc / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D004146:Dioxanes; D007097:Imines; D011091:Polyesters; D011108:Polymers; D019344:Lactic Acid; C033616:poly(lactide); C091880:dilactide; D015032:Zinc