Chloroyttrium 2‑(1-(Arylimino)alkyl)quinolin-8-olate Complexes: Synthesis, Characterization, and Catalysis of the Ring-Opening Polymerization of ε‑Caprolactone

Stoichiometric reactions of YCl3(THF)3 with
potassium 2-((arylimino)­methyl)­quinolin-8-olates or 2-(1-(arylimino)­ethyl)­quinolin-8-olates
in THF solution gave the mononuclear LYCl2(DMSO)2 complexes 1–5 in the presence of
DMSO and a representative dinuclear complex 6 in the
absence of DMSO. All yttrium complexes were fully characterized by
NMR measurements and elemental analysis, and the crystal structures
of complexes 1 and 4–6 were determined by single-crystal X-ray diffraction. The structures
indicate coordination number seven around the yttrium center and pentagonal
bipyramidal geometries. The complexes all feature diapical YCl2 moieties and one tridentate organic ligand in the equatorial
plane. Upon reaction of the yttrium precatalysts 1–6 with LiCH2Si­(CH3)3 alone
or with LiCH2Si­(CH3)3 together with
BnOH, the ring-opening polymerization (ROP) of ε-caprolactone
(ε-CL) occurred with high efficiency. Depending on conditions,
the ROP of ε-CL produced polycaprolactone with narrow molecular
distribution and in a living manner. Theoretical studies of the chlorine/CH2SiMe3 and Me3SiCH2/BnO ligand
exchange reactions suggest that the replacement of the apical ligands
can proceed without significantly affecting the equatorial ligands.
These results suggest that one of the apical Y–CH2SiMe3 bonds within the LY­(CH2SiMe3)2 intermediate catalyzes the polymerization in the BnOH-free
process. Most polymers generated by BnOH-assisted catalysis possess Mn values that are similar to Mn,cal values based on Y–OBn, suggesting that one
apical Y–OBn bond of the diapical LY­(OBn)­(CH2SiMe3) intermediate catalyzes most or all of the ring polymerization
of ε-CL.