Rare-Earth Complexes with Multidentate Tethered Phenoxy-Amidinate Ligands: Synthesis, Structure, and Activity in Ring-Opening Polymerization of Lactide

New multidentate tethered amidine-phenol pro-ligands {4,6-tBu2C6H2O-(2-C(N-R)N-R}H2 ({LONR}H2, R = iPr, cyclohexyl (Cy), 2,6-iPr2C6H3 (Ar)) were synthesized from the corresponding carbodiimines and 2-bromo-2,4-(di-tert-butyl)phenol. Pro-ligands {LONiPr}H2 and {LONAr}H2 were metalated by 2 equiv of nBuLi to provide the corresponding dilithium salts {LONiPr}Li2 (1) and {LONAr}Li2 (2), which were authenticated by elemental analysis, X-ray crystallography, and NMR spectroscopy. Three different approaches were explored to coordinate these (pro)ligands onto rare earths: salt metathesis, and amine and methane elimination reactions. The salt metathesis reaction between 1 and YCl3 afforded the chloro complex [{LONiPr}YCl]n (3), which was, in turn, converted into the corresponding amide {LONiPr}YN(SiMe3)2 (6) by reaction with MN(SiMe3)2 (M = Li, Na). Similar reactions between 2 and YCl3, followed by recrystallization from DME, led systematically to the isolation of the monoprotonated product, that is, phenoxy-amidino complex {LOHNAr}YCl2(DME) (5). Amine elimination reactions between {LONiPr}H2 or {LONCy}H2 and Ln[N(SiMe3)2]3 afforded the corresponding phenoxy-amidinate amides {LONR}LnN(SiMe3)2 (Ln = Y, R = iPr, 6; R = Cy, 8; Ln = Nd, R = Cy, 9), whereas the same reaction between{LONAr}H2 and Y[N(SiMe3)2]3, under various conditions, always yielded the homoleptic tris(phenoxy-amidinate) complex {LOHNAr}3Y (11). Bimetallic “ate”-complexes {LONR}2Ln2Me4Li2(TMEDA)2 of yttrium (12 and 13), neodymium (14), samarium (15), and {LONiPr}2Yb2Me2(OH)2Li2(TMEDA)2 (16) were prepared by alkane elimination of the corresponding pro-ligand and [Li(TMEDA)][LnMe4] complex. Both amido and methyl “ate”- complexes were shown by X-ray diffraction studies to be dimeric in the solid state. The multidentate nature of the ligands in these dimeric species generates a cis/trans isomerism related to the nitrogen atoms in the nonsymmetrically coordinated amidinate fragments. Amido complexes 6, 8, and 9 are effective initiators for the ring-opening polymerization (ROP) of racemic lactide (rac-LA), giving atactic or heterotactic-enriched (Pr up to 76%) polymers with high molecular weights (Mn up to 158 800 g·mol–1), but broad molecular weight distributions (Mw/Mn = 1.5–2.8). An effective immortal ROP of rac-LA was feasible by combining complex 6 with 5–50 equiv of isopropanol or benzyl alcohol, affording PLAs with well-controlled molecular weights and narrow polydispersities (Mw/Mn = 1.11–1.38).