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

Zinc complexes [LZnR] containing tridentate -ketiminate ligands with an additional side-arm donor {referred to as [NNO] ligands; L = L-1 = OC(Me)C(H)C(Me)NCH2CH2CH2NMe2, R = Me (1), Et (2), Cl (3); L = L-2 = 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 = L-1 (4), L = L-2 = (10)], a base-stabilized complex [(LZn)-Zn-2(dmap)Et] (8; dmap = 4-(dimethylamino)pyridine), and a base-free complex without a side-arm donor [(LZnEt)-Zn-3] [12, L-3 = OC(Me)C(H)C(Me)N-2,6-iPr(2)C(6)H(3)] 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--ketiminates (LLi)-Li-1 (5), (LLi)-Li-2 (11), DBU (13), and a mixture of (LLi)-Li-2/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 -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.