A high-throughput approach to lanthanide complexes and their rapid screening in the ring opening polymerisation of caprolactone

Abstract

Libraries of lanthanide complexes supported by nitrogen and oxygen containing ligands have been synthesised using a high-throughput approach. The complexes were employed in the ring-opening polymerisation of ε-caprolactone, in some cases giving polycaprolactone of controlled molecular weight and narrow polydispersity. The libraries, based on twenty-one ligands and eight lanthanide reagents, were developed in order to determine the best combination of lanthanide metal and ligand. They were prepared via transamination reactions of [Ln{N(SiMe₃)₂}₃] complexes with tetradentate dianionic ligands containing oxygen and nitrogen donors. ¹H NMR spectroscopy was used to screen polymerisation activity. The steric demand of the ligand has a significant effect on the polymerisation process, as do the type of nitrogen donor and the size of the central Ln³⁺ ion. Ligands containing aryl rings with bulky substituents such as tert-pentyl groups afforded species capable of performing controlled polymerisation of caprolactone, whereas less bulky groups such as methyl were not effective. Yttrium and mid-sized lanthanides such as samarium showed increased activity compared with the larger lanthanides, lanthanum and praseodymium, and the smaller lanthanides like ytterbium. X-ray crystal structures of a sterically demanding chelating amine-bis((2-hydroxyaryl)methyl) ligand (1) and a chloride bridged dinuclear gadolinium complex (2) are reported. The centrosymmetric molecule (2) contains gadolinium in distorted capped trigonal prismatic environments bonded to two amine, two phenolate, one THF and two chloride donors.