Lanthanide chloride complexes of amine-bis(phenolate) ligands and their reactivity in the ring-opening polymerization of ε-caprolactone

Reaction of two equivalents of n-BuLi with sterically demanding amine-bis(phenol) compounds, H2O2NN'(R) (Me2NCH2CH2N{CH2-3,5-R-2-C6H2OH}(2); R = t-Bu or t-Pe (tert-pentyl)) yields isolable lithium complexes, Li-2(O2NN'(R)), in good yields. Upon reaction with one equivalent of LnCl(3)(THF)(x), the lithium salts afford rare earth amine-phenolate chloride complexes in good yields, Ln(O2NN'(R))Cl(THF); Ln = Y, Yb, Ho, Gd, Sm, Pr. Crystals of Y(O2NN'(t-Bu))Cl(THF), 1, and Sm(O2NN'(t-Bu))Cl(DME), 2, suitable for single crystal X-ray crystallographic analysis were obtained. In contrast to previously reported [{Gd(O2NN'(t-Pe))(THF)(mu-Cl)}(2)] and related La and Sm complexes, these species are monomeric. 1 contains Y in a distorted octahedral environment bonded to two amine, two phenolate, one THF and one chloride donor. 2 contains Sm in a distorted capped trigonal prismatic environment bonded to two amine, two phenolate, two DME oxygens and one chloride donor. The Ln(O2NN'(t-Pe))Cl(THF) complexes were active initators for the controlled ring-opening polymerization of epsilon-caprolactone with a tendency to form low molecular weight cyclic polyesters (M-n 3000-5000). The conversion rates, although slower than related amido and alkyl species, were different for monomeric and dimeric initiators. The size of the metal centre also affected the conversions and the molecular weights achieved.