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

Abstract

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 C 2-chiral bis(oxazoline) ligand ( R2,R3BOX: R 2 = (4 S)- tBu, R 3 = H ( a); R 2 = (4 S)-Ph, R 3 = H ( b); R 2 = (4 R)-Ph, R 3 = (5 S)-Ph ( c)) with Zn(R 1) 2 (R 1 = Et ( 1), Me ( 2)) led to the heteroleptic three-coordinate complexes ( R2,R3BOX)ZnR 1, 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 ZnEt 2 led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC–C 6F 5(H))ZnEt 2 4e, respectively, in high yield. In situ combinations of selected complexes 1a– c, 3d and 4e with B(C 6F 5) 3 (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3–5.9 10 6 g polym. mol Zn −1 h −1) and high molecular weight ( M n up to 132 10 3 g mol −1) for CHO polymerization. Although the in situ B(C 6F 5) 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%).