Magnesium complexes containing biphenyl-based tridentate imino-phenolate ligands for ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate
作者:Wei Yi、Haiyan Ma
DOI:10.1039/c3dt53513d
日期:——
A series of racemic 2-[(2â²-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1HâL4H) were reacted with Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L1â4)MgN(SiMe3)2·(THF)n (1, R1 = tBu, R2 = Me, n = 1; 2, R1 = R2 = CMe2Ph, n = 0; 3, R1 = CPh3, R2 = tBu, n = 1; 4, R1 = Br, R2 = tBu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1â4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that Pr values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygenâacyl bond (Xreg = 0.65â0.86).
一系列外消旋的2-[(2′-(二甲氨基)联苯-2-基亚氨基)甲基]-4-R2-6-R1-酚(L1H–L4H)与Mg[N(SiMe3)2]2}2反应,生成四个非配位镁络合物(L1–4)MgN(SiMe3)2·(THF)n(1,R1 = tBu,R2 = Me,n = 1;2,R1 = R2 = CMe2Ph,n = 0;3,R1 = CPh3,R2 = tBu,n = 1;4,R1 = Br,R2 = tBu,n = 0),这些络合物已被全面表征。X射线结构分析显示,复合物1具有单体结构,而复合物4呈二聚体结构,具有C2对称性,其中两个金属中心由配体的两个酚氧原子桥接。这些络合物中镁中心的配位几何形状可以最好地描述为畸变的四面体几何形状。非配位络合物1–4有效地催化了外消旋乳酸和α-甲基三亚甲基碳酸酯(α-MeTMC)的开环聚合,并且在2-丙醇存在下,聚合过程更易于控制。一般而言,在酚氧基团上引入一个笨重的邻位取代基会提高相应镁络合物的催化活性以及立体或区域选择性。对所得到的PLA的微观结构分析显示,Pr值从0.46到0.81不等,具体取决于催化剂和聚合条件。对于外消旋α-MeTMC,使用1H和13C NMR光谱的详细分析表明,α-MeTMC在最受阻的氧-酰基键处优先开环(Xreg = 0.65–0.86)。