Synthesis, structure and catalytic activity of tridentate, base-functionalized β-ketiminate zinc complexes in ROP of lactide

Scheiper, Christoph; Dittrich, Dennis GND; Wölper, Christoph GND; Bläser, Dieter; Roll, Joachim ORCID; Schulz, Stephan GND

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 14, 610 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.

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Scheiper, C., Dittrich, D., Wölper, C., Bläser, D., Roll, J., Schulz, S., 2020. Synthesis, structure and catalytic activity of tridentate, base-functionalized β-ketiminate zinc complexes in ROP of lactide. https://doi.org/10.1002/ejic.201301462
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