Zinc Complexes Supported by Maltolato Ligands: Synthesis, Structure, Solution Behavior, and Application in Ring-Opening Polymerization of Lactides

A series of novel zinc alkoxides supported by chelating maltolato (MalO; MalOH = maltol) ligands were successfully synthesized and characterized. Reaction of MalOH with ZnEt2 (3:4) gives a trinuclear cluster [Zn3(Et)2(MalO)4] (1), which spontaneously disproportionates in solution to mononuclear species [Zn­(MalO)2] (1a) and [Zn­(Et)­(MalO)] (1b); (1a) and (1b) form [Zn­(MalO)2(py)] (2), [Zn­(Et)­(MalO)­(py)] (3), [Zn­(MalO)2]2 ((1a)2), [Zn­(MalO)­(OBn)]2 ((1c)2), and [Zn4(Et)2(OEt)2(MalO)4] (4) on addition of pyridine, benzyl alcohol (BnOH), or dry O2, respectively. Compounds 1, 2, (1a)2, and 4 were characterized by elemental analysis, NMR, ESI-MS, and single-crystal X-ray structural analysis. Variable-temperature NMR experiments showed that (1a)2 and (1c)2 are in equilibrium with the monomeric form in solution. The addition of l-lactide (l-LA) to a combination of 1 and 2 equiv of BnOH in dichloromethane at room temperature in different molar ratios leads to rapid and efficient generation of poly­(l-LA) with end-capped BnO groups. According to kinetic studies, propagation by [Zn­(OBn)­(MalO)] (1c) is first-order with respect to both the monomer and 1c concentrations; 1a in ring-opening polymerization of l-LA shows no activity. These results suggest a single-site active species in the ring-opening polymerization of l-LA.本研究成功合成并表征了一系列由螯合麦芽酚根（MalO；MalOH = 麦芽酚）配体支撑的新型烷氧基锌化合物。将MalOH与二乙基锌（ZnEt₂）按3:4的摩尔比反应，可得到三核簇合物[Zn₃(Et)₂(MalO)₄]（1），该簇合物在溶液中会自发歧化生成单核物种[Zn(MalO)₂]（1a）与[Zn(Et)(MalO)]（1b）。分别向体系中加入吡啶、苯甲醇（BnOH）或干燥氧气，1a与1b可对应生成不同产物：加入吡啶得到[Zn(MalO)₂(py)]（2）与[Zn(Et)(MalO)(py)]（3）；加入苯甲醇得到二聚体[Zn(MalO)₂]₂（(1a)₂）与二聚体[Zn(MalO)(OBn)]₂（(1c)₂）；加入干燥氧气得到四核簇合物[Zn₄(Et)₂(OEt)₂(MalO)₄]（4）。采用元素分析、核磁共振波谱（NMR）、电喷雾电离质谱（ESI-MS）以及单晶X射线衍射结构分析对化合物1、2、(1a)₂与4进行了表征。变温核磁共振实验表明，(1a)₂与(1c)₂在溶液中可与单体形式保持动态平衡。将L-丙交酯（l-LA）按不同摩尔比加入到1与2当量苯甲醇（BnOH）的二氯甲烷混合体系中，于室温下反应，可快速高效地生成端基为苄氧基（BnO）的聚L-丙交酯。动力学研究表明，[Zn(OBn)(MalO)]（1c）引发的链增长过程对单体与1c的浓度均为一级反应；而1a对L-丙交酯的开环聚合无催化活性。上述结果表明，L-丙交酯开环聚合的活性中心为单一位点活性物种。