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 1H, 13C, and 19F 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 1H and 19F 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 1H and 19F NMR and appeared to yield an in situ generated zinc alkoxide complex.
在室温的惰性气氛手套箱中,通过含有不同取代基的 NNO 席夫碱酮
亚胺与
二乙基锌的反应,合成了八种双配位同型
锌络合物,并以 62-95% 的收率进行了分离。利用 1H、13C 和 19F 核磁共振谱、吸光度光谱、高分辨率质谱、元素分析和单晶 X 射线晶体学对这些复合物进行了表征。结果表明,这些配合物在
锌周围呈扭曲的八面体配位几何形状。配合物 1-7 的 1H 和 19F NMR 光谱显示,
锌在 300 K 时配位稳定,而配合物 8 中的立体包络和两个三
氟甲基的影响则通过变温 NMR 进行了研究。双配位
锌配合物是 L-内酰胺开环聚合成聚
L-乳酸(P
LA)的有效
引发剂。当[L-
LAc]/[
锌络合物] = 50 时,双配位
锌络合物的转化率为 14-98%,聚合时间为 15-1440 分钟不等,当存在两个三
氟甲基时,反应时间最长。添加
4-氟苯酚助催化剂后,
甲苯溶液中的转化率提高了 5 倍,大量熔融聚合的转化率提高了 14 倍,但分离的聚
乳酸的聚分散指数值却降低了。用 1H 和 19F NMR 研究了向复合物 1 添加
4-氟苯酚的情况,结果显示生成了一种原位生成的烷
氧化锌复合物。