Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C2-chiral bis(oxazoline) ligand (R2,R3BOX: R2 = (4S)-tBu, R3 = H (a); R2 = (4S)-Ph, R3 = H (b); R2 = (4R)-Ph, R3 = (5S)-Ph (c)) with Zn(R1)2 (R1 = Et (1), Me (2)) led to the heteroleptic three-coordinate complexes (R2,R3BOX)ZnR1, 1a–c and 2a, which were isolated in 92–96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed and synthesized: the 1,3-bis(2-pyridylmethyl)imidazolinium salt (d) and the protected NHC adduct 2-(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine (e). The reaction of ligands d and e with ZnEt2 led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC–C6F5(H))ZnEt24e, respectively, in high yield. In situ combinations of selected complexes 1a–c, 3d and 4e with B(C6F5)3 (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3–5.9 106 gpolym. molZn−1 h−1) and high molecular weight (Mn up to 132 103 g mol−1) for CHO polymerization. Although the in situ B(C6F5)3-activated zinc species were not isolated, the sterically demanding BOX ligands (1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42–88%).