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.