Synthesis of Bimetallic Bis(phenolate) N-Heterocyclic Carbene Lanthanide Complexes and Their Applications in the Ring-Opening Polymerization of l-Lactide

Abstract

The bis(phenolate) N-heterocyclic carbene (NHC) lanthanide complexes [Ln(L)2M] (L = 1,3-bis[O-4,6-di-tBu-C6H2-2-CH2][C(NCH2CH2N)], M = Li, Ln = Nd (1a); M = K, Ln= Nd (2a), Sm (2b), La (2c), Y (2d)) were synthesized by the reaction of L with KN(SiMe3)2 and Nd[N(SiMe3)2]3-μ-Cl-Li(THF)3 or Ln[N(SiMe3)2]3 (Ln = Nd, Sm, La, Y). The imidazolinium-bridged bis(phenolate) lanthanide complexes [Ln2(L3)3Cl3] (L3 = 1,3-bis[O-4,6-di-tBu-C6H2-2-CH2][CH(NCH2CH2N)]+Cl–, Ln = Sm (3a), Dy (3b), Er (3c)) were synthesized by the amine elimination reaction of L3 with Ln[N(SiMe3)2]3 or Ln[N(SiMe3)2]3-μ-Cl-Li(THF)3. All complexes were characterized by elemental analysis. Complex 1a, 2a,c, and 3a–c were characterized by X-ray crystallography, and complexes 2c,d were characterized by 1H and 13C NMR spectroscopy. The NHC complexes were efficient initiators for the ring-opening polymerization (ROP) of l-lactide (l-LA), with 2a giving the best catalytic activity. Imidazolinium-bridged lanthanide complexes 3a–c were inert in the ROP of l-LA. The influences of ligand structural factors and the cooperative effects between metals and the reaction medium on the catalytic activity were discussed for the ROP of l-LA.