Yttrium complexes supported by linked bis(amide) ligand: synthesis, structure, and catalytic activity in the ring-opening polymerization of cyclic esters.

A series of new yttrium complexes supported by the bulky enediamido dianionic ligand DAB(2-) (DAB(2-) = (2,6-C(6)H(3)iPr(2))NC(Me)=C(Me)N(2,6-C(6)H(3)iPr(2))(2-)), that is, {DAB}Y(THF)(2)(mu-Cl)(2)Li(THF)(2) (1), {DAB}Y(OtBu)(THF)(DME) (2), and {{DAB}Y(BH(4))(2)}{Li(DME)(3)} (3), was synthesized by salt metathesis. The complexes were isolated after recrystallization in 73, 66, and 52% yield, respectively, and characterized in solution by NMR and in the solid state by single-crystal X-ray diffraction studies. In complex 1, the DAB(2-) ligand is bonded to the metal center via two covalent YN bonds, while in complexes 2 and 3 additional eta(2)-coordination of the C=C bond to the metal atom is observed, both in solution and in the solid state. The tert-butoxide and borohydride complexes 2 and 3 act as monoinitiators for the room temperature ring-opening polymerization of racemic lactide and beta-butyrolactone, providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (M(w)/M(n) = 1.15-1.82). Effectively immortal ROP of lactide with as many as 50 equiv of isopropanol per metal center was performed using complex 2 as the catalyst.