Bis(imino)aryl NCN Pincer Aluminum and Zinc Complexes: Synthesis, Characterization, and Catalysis on l-Lactide Polymerization

Reactions of 2,6-(ArNCH)2C6H3Li with AlEt2Cl afford a number of NCN pincer aluminum complexes (2,6-(ArNCH)2C6H3)AlEt2 (Ar = Ph (1), 2,6-Me2C6H3 (2), 2,6-Et2C6H3 (3), 2,6-iPr2C6H3 (4)). Similar reactions of 2,6-(ArNCH)2C6H3Li with ZnEtCl produce bisligated zinc complexes (2,6-(ArNCH)2C6H3)2Zn (Ar = Ph (5), 2,6-Me2C6H3 (6), 2,6-Et2C6H3 (7)) and monoligated NCN pincer zinc complex (2,6-(ArNCH)2C6H3)ZnEt (Ar = 2,6-iPr2C6H3 (8)). All complexes were characterized by 1H and 13C NMR spectroscopy, and the molecular structures of complexes 3, 4, 6, 7, and 8 were determined by X-ray crystallography. The X-ray diffraction analysis reveals that both complexes 3 and 4 adopt a distorted trigonal-bipyramidal geometry around the aluminum central metal with three carbon atoms in the equator and the two imine nitrogen atoms in the apical positions. Complexes 6 and 7 adopt a distorted tetrahedral geometry around their zinc metal centers, while complex 8 adopts a square-planar geometry around its metal center. All these Al and Zn complexes are efficient initiators for l-lactide ring-opening polymerization in the presence of benzyl alcohol, and the polymerization reaction takes place in an immortal manner. The productivity of the Zn complexes is generally higher than that of the Al complexes under similar conditions.

2,6-二(芳基亚胺甲基)苯基锂（2,6-(ArN=CH)₂C₆H₃Li）与二乙基氯化铝（AlEt₂Cl）发生反应，可得到一系列NCN钳形铝配合物(2,6-(ArN=CH)₂C₆H₃)AlEt₂，其中芳基（Ar）分别为苯基（Ph，配合物1）、2,6-二甲基苯基（2,6-Me₂C₆H₃，配合物2）、2,6-二乙基苯基（2,6-Et₂C₆H₃，配合物3）和2,6-二异丙基苯基（2,6-iPr₂C₆H₃，配合物4）。2,6-(ArN=CH)₂C₆H₃Li与乙基氯化锌（ZnEtCl）的类似反应则生成双配体锌配合物(2,6-(ArN=CH)₂C₆H₃)₂Zn（Ar分别为Ph，配合物5；2,6-二甲基苯基，配合物6；2,6-二乙基苯基，配合物7）以及单配体NCN钳形锌配合物(2,6-(ArN=CH)₂C₆H₃)ZnEt（Ar为2,6-二异丙基苯基，配合物8）。所有配合物均通过氢谱（¹H NMR）和碳谱（¹³C NMR）进行结构表征，配合物3、4、6、7和8的分子结构经X射线单晶衍射（X-ray crystallography）分析确定。X射线衍射结果显示，配合物3和4的中心铝原子均采用畸变三角双锥配位几何，赤道平面由三个碳原子占据，两个亚胺氮原子位于轴向位置。配合物6和7的中心锌原子采用畸变四面体配位几何，而配合物8的中心锌原子则呈平面正方形配位构型。上述所有铝系和锌系配合物均可在苄醇存在下作为L-丙交酯开环聚合的高效引发剂，且聚合反应以不死聚合（immortal polymerization）方式进行。在相同反应条件下，锌系配合物的催化产率通常高于铝系配合物。