Syntheses of lanthanide monochloride and monoborohydride complexes supported by bridged bis(guanidinate) ligand and the use of borohydride complexes in polymerization of cyclic esters

Reaction of LnCl3 with bridged bis(guanidinate) lithium salt LLi2 (1) (L = i Pr(Me3Si)NC(Ni Pr)N(CH2)3NC(Ni Pr)N(SiMe3)i Pr) afforded the monochlorides LLnCl(THF)2 (Ln = Nd (2), Yb (3), Y (4)) and LYbCl(DME) (5) (DME = 1,2-dimethoxyethane). Complexes 2–5 were structurally characterized. Treatment of the chlorides with NaBH4 gave the monoborohydride complexes LLnBH4(DME) (Ln = Nd (6), Yb (7), Y (8)). X-ray crystal structure analyses of 6–8 revealed each complex was monomer with the central metal ligated by one L, one η 3-BH4 group and one DME. Complexes 6–8 initiate ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL), l-lactide (l-LA) and rac -lactide (rac -LA) as single-site initiators. The highest activity and the best molecular mass control for all the three monomers were found for the Nd system 6. The ROP of ɛ-CL in toluene with 6 gave the polymers with the View the MathML sourceM¯nexp in good agreement with the View the MathML sourceM¯ntheo (View the MathML sourceM¯n = the number averaged molar mass) at the molar ratio of monomer to 6 up to 600 and moderate molar mass distributions (View the MathML sourceM¯w/M¯n=1.43-1.52) (View the MathML sourceM¯w = the weight averaged molar mass). The ROP of l-LA with 6 in THF afforded the polymers with controlled molar mass and relatively narrow View the MathML sourceM¯w/M¯n. Complexes 6–8 are active in the ROP of rac-LA affording poly(LA) with a slight heterotactic bias (Pr = 0.62–0.72).