Synthesis, Structure, and Catalytic Activity of Tridentate, Base-Functionalized β-Ketiminate Zinc Complexes in Ring-Opening Polymerization of Lactide

• Published in: European journal of inorganic chemistry 2014-05, Vol.2014 (13), p.2230-2240
• Main Authors: Scheiper, Christoph, Dittrich, Dennis, Wölper, Christoph, Bläser, Dieter, Roll, Joachim, Schulz, Stephan
• Format: Article
• Language: English
• Subjects: N,O ligands; O ligands; Polylactide; Ring-opening polymerization; Tridentate ligands; Zinc
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201301462

Zinc complexes [LZnR] containing tridentate β‐ketiminate ligands with an additional side‐arm donor {referred to as [NNO] ligands; L = L1 = OC(Me)C(H)C(Me)NCH2CH2CH2NMe2, R = Me (1), Et (2), Cl (3); L = L2 = OC(Me)C(H)C(Me)NCH2CH2NMe2, R = Me (6), Et (7), Cl (9)} as well as cationic derivatives with a weakly coordinating anion [LZn][Al(OC(CF3)3)4] [L = L1 (4), L = L2 = (10)], a base‐stabilized complex [L2Zn(dmap)Et] (8; dmap = 4‐(dimethylamino)pyridine), and a base‐free complex without a side‐arm donor [L3ZnEt] [12, L3 = OC(Me)C(H)C(Me)N‐2,6‐iPr2C6H3] were synthesized and characterized by single‐crystal X‐ray diffraction (1, 2, 3, 8, 9, 11, and 12). The ring‐opening polymerization (ROP) of rac‐lactide was studied with the zinc complexes 1–4, 6–10 and 12, mixtures of 7 and Lewis bases [7/1,8‐diazabicycloundec‐7‐ene (7/DBU; 15), 7/tBuPy (16)], and 12/DBU (17). Lithium‐β‐ketiminates L1Li (5), L2Li (11), DBU (13), and a mixture of L2Li/DBU (14) were also investigated. All compounds except for 3, 4, 10, and 12 are active rac‐lactide polymerization catalysts by a slightly modified activated‐monomer mechanism. The catalytic activity of the complexes in rac‐lactide polymerization reactions at ambient temperature increases with the addition of Lewis bases to the β‐ketiminate zinc compounds. The highest activity was found for the mixture 7/DBU (15), which converted 200 equiv. of rac‐lactide into polylactide within 1.5 min at ambient temperature. Several NNO zinc complexes were synthesized and structurally characterized by single‐crystal X‐ray diffraction. In addition, their catalytic activity in the ring‐opening polymerization (ROP) of lactide is presented. The polymerization occurs through a modified activated‐monomer mechanism.