Controlled synthesis of mononuclear or binuclear aryloxo ytterbium complexes supported by β-diketiminate ligand and their activity for polymerization of ε-caprolactone and L-lactide

The reaction of monomeric [(DIPPh)(2)nacnac]YbCl(2)(THF)(2) ((DIPPh)(2)nacnac = N,N-diisopropylphenyl-2,4-pentanediimineanion) with NaOAr (OAr = 2,6-diisopropylphenoxide) in THF afforded the mononuclear complexes [(DIPPh)(2)nacnac]Yb(OAr)Cl(THF) 1 and [(DIPPh)(2)nacnac]Yb(OAr)(2) 2 depending on the molar ratio of dichloride to sodium salt, while the same reaction with NaOAr' (OAr' = 2,6-dimethylphenoxide) in toluene yielded the binuclear complex [{(DIPPh)(2)nacnac}Yb(OAr')](2)(mu-Cl)(2) 3. Treatment of [(DIPPh)(2)nacnac]Yb(THF)(mu-Cl)(3)Yb(Cl)[(DIPPh)(2)nacnac] with NaOAr and NaOAr', respectively, in toluene yielded selectively the corresponding binuclear complexes {[(DIPPh)(2)nacnac]Yb(OAr)(mu-Cl)(3)Yb[(DIPPh)(2)nacnac](THF)} 4 and {[(DIPPh)(2)nacnac]Yb(OAr')(mu-Cl)(3)Yb[(DIPPh)(2)nacnac](THF)} 5 in high yields. All complexes were structurally characterized. Complex 2 was found to be a highly active initiator for both polymerizations of epsilon-caprolactone (epsilon-CL) and L-lactide. All monoaryloxide complexes can initiate the polymerization of epsilon-CL in a controlled manner giving polymers with narrow molecular weight distributions (M(w)/M(n) around 1.06) with relatively low activity relative to complex 2.