Synthesis and characterization of amine-bridged bis(phenolate)lanthanide alkoxides and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone.

A series of neutral lanthanide alkoxides supported by an amine-bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of (C(5)H(5))(3)Ln(THF) with amine-bridged bis(phenol) LH(2) [L = Me(2)NCH(2)CH(2)N{CH(2)-(2-OC(6)H(2)Bu(t)(2)-3,5)}(2)] in a 1:1 molar ratio in THF for 1 h and then with 1 equiv each of 2,2,2-trifluoroethanol, benzyl alcohol, and 2-propanol gave the neutral lanthanide alkoxides LLn(OCH(2)CF(3))(THF) [Ln = Y (1), Yb (2), Er (3), Sm (4)], LY(OCH(2)Ph)(THF) (5), and LY(OPr(i))(THF) (6), respectively. These lanthanide alkoxides are sensitive to moisture, and the yttrium complex [(LY)(2)(μ-OPr(i))(μ-OH)] (7) was also isolated as a byproduct during the synthesis of complex 6. Complexes 1-6 were well characterized by elemental analyses and IR and NMR spectroscopy in the cases of complexes 1 and 4-6. The definitive molecular structures of all of these complexes were determined by single-crystal X-ray analysis. It was found that complexes 1-6 can initiate efficiently the ring-opening polymerization of rac-LA and rac-BBL in a controlled manner. For rac-LA, polymerization gave polymers with very narrow molecular weight distributions (PDI ≤ 1.12) and very high heterotacticity (P(r) up to 0.99). The observed activity-increasing order is in agreement with the order of the ionic radii, whereas the order for stereoselectivity is in the reverse order. For rac-BBL polymerization, the resultant polymers have narrow molecular distributions (PDI ≤ 1.26) and high syndiotacticity (P(r) up to 0.83). It is worth noting that the activity-decreasing order Yb > Er > Y >> Sm is observed for rac-BBL polymerization, which is opposite to the order of ionic radii and to the order of activity for rac-LA polymerization. The ionic radii of lanthanide metals have no obvious effect on the stereoselectivity for rac-BBL polymerization, which is quite different from that for rac-LA polymerization. End-group analysis of the oligomer of rac-BBL suggested that elimination side reactions occurred slowly in these systems, which led to chain cleavage and the formation of crotonate (and carboxy) end groups.