Dialkyllanthanide Complexes Containing New Tridentate Monoanionic Ligands with Nitrogen Donors

Three new tridentate NNN ligand precursors, CH3C(2,6-(iPr)2C6H3NH)CHC(CH3)(NCH2CH2−D) (D = NMe2, NEt2, N((CH2CH2)2CH2)), were synthesized. Subsequent metalations with in situ generated Ln(CH2SiMe3)3(THF)n (Ln = Nd, Sm, Y, Lu) provided six solvent-free dialkyllanthanide complexes. Five of the lanthanide complexes were characterized by single-crystal X-ray diffraction, which showed that the pendant arm D bonds to the lanthanide ion in the solid state. The NMR spectra of these complexes in C6D6 showed that such coordination is retained in solution. These dialkyllanthanide complexes show high activities for the ring-opening polymerization of ϵ-caprolactone, in which narrow-polydispersity polymers were produced. The size of the pendant arm has a significant effect on the molecular weight of the polymer obtained. In comparison to the Y complex with a −NMe2 group, the Y complexes with −NEt2 and −N((CH2CH2)2CH2) groups yield much higher molecular weight polymer (60 000 vs 20 000).