Syntheses of Rare-Earth Metal Alkyl Complexes Bearing a Dianionic α‑Iminopyridyl Ligand and Their Catalytic Activities toward Polymerization of 2‑Vinylpyridine

The
rare-earth metal alkyl complexes bearing a dianionic α-iminopyridyl
ligand {μ-η2:σ1:κ1:κ1-2-[O­(CH2CH2)2NCH2CH2NCH]­C5H4N}2RE2(CH2SiMe3)2(THF)n (n = 2,
RE = Gd­(2a), Dy­(2b), Y­(2c); n = 0, RE = Lu­(2e), Sc­(2f)) and
{μ-η2:σ1:κ1:κ1-2-[O­(CH2CH2)2NCH2CH2NCH]­C5H4N}­{μ-η4:η2:κ1-2-[O­(CH2CH2)2NCH2CH2NCH]­C5H4N}­Er2(CH2SiMe3)2(THF) (2d) were prepared through reactions
of RE­(CH2SiMe3)3(THF)2 with 2-aminomethylpyridine 2-[O­(CH2CH2)2NCH2CH2NHCH2]­C5H4N (1). In the process, the reactions underwent
double deprotonation of the functionalized donor group 2-aminomethylpyridine.
Reaction of Sc­(CH2SiMe3)3(THF)2 with compound 1 in a shortened reaction time
afforded the scandium dialkyl complex {2-[O­(CH2CH2)2NCH2CH2NCH2]­C5H4N}­RE­(CH2SiMe3)2 (3f) which would slowly transform into 2f at room
temperature, indicating that the formation of dianionic α-iminopyridine
went through the continual deprotonation reaction of the functionalized
donor group 2-aminomethylpyridine. The α-iminopyridyl rare-earth
metal alkyl complexes exhibited high catalytic activities toward polymerization
of 2-vinylpyridine with high iso-selectivity. The polymerization mechanism
and the origins of stereoselectivity was deduced by DFT calculations.