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

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.