Lanthanide borohydride complexes supported by diaminobis(phenoxide) ligands for the polymerization of ∈-caprolactone and L- and rac-lactide

Reaction of Na2O2NN' [H2O2NN' = (2-C5H4N)CH2N{2-HO-3,5-(C6H2Bu2)-Bu-t}(2)] with M(BH4)(3)(THF)(3) afforded the dimeric, rare-earth borohydride compounds [M(O2NN')(mu-BH4)(THF)(n)](2) [M = Y-III, n = 0.5 (1-Y); M = Nd-III, n = 1 (1-Nd); M = Sm-III, n = 0 (1-Sm)]. For comparison the chloride analogues [M(O2NN')(mu-Cl)(THF)(n)](2) (2-M; M = La-III or Sm-III, n = 0; M = Will, n = 1) and the corresponding pyridine adducts [M(O2NN')(,U-X)(py)](2) [X = BH4 (3-M) or Cl (4-M); M = La-III, Nd-III, or Sm-III] were prepared and structurally characterized for 4-La. Compounds 1-M initiated the ring-opening polymerization of E-caprolactone. The best molecular weight control (suppression of chain transfer) for all three monomers was found for the samarium system 1-Sm. The most effective heterotactic enrichment (P-r) in the polymerization of rac-lactide was found for 1-Y (P-r = 87%). Compound 1-Nd catalyzed the block copolymerization of c-caprolactone and L- and rac-lactide provided that c-caprolactone was added first. Attempted block polymerization by the addition of L-lactide first, or random copolymerization of a ca. 1:1 mixture of is an element of-caprolactone and L-lactide, gave only a poly(L-lactide) homopolymer.