Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides |
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Authors: | Nicolas Merle |
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Affiliation: | Department of Chemistry, University of Bergen, Allégaten 41, N 5007 Bergen, Norway |
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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 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%). |
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Keywords: | Zinc Bis(Oxazoline) N-Heterocyclic carbene Ring-opening polymerization Epoxides |
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