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 DAB2− (DAB2− = (2,6-C6H3iPr2)NC(Me)C(Me)N(2,6-C6H3iPr2)2−), that is, {DAB}Y(THF)2(μ-Cl)2Li(THF)2 (1), {DAB}Y(OtBu)(THF)(DME) (2), and {{DAB}Y(BH4)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 DAB2− ligand is bonded to the metal center via two covalent YN bonds, while in complexes 2 and 3 additional η2-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 β-butyrolactone, providing atactic polymers with controlled molecular weights and relatively narrow polydispersities (Mw/Mn = 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.