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- t Bu 2 C 6 H 2 O-(2-C(N-R)═N-R}H 2 ({LON R }H 2, R = i Pr, cyclohexyl (Cy), 2,6- i Pr 2 C 6 H 3 (Ar)) were synthesized from the corresponding carbodiimines and 2-bromo-2,4-(di- tert -butyl)phenol. Pro-ligands {LON i Pr }H 2 and {LON Ar }H 2 were metalated by 2 equiv of n BuLi to provide the corresponding dilithium salts {LON i Pr }Li 2 ( 1 ) and {LON Ar }Li 2 ( 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 YCl 3 afforded the chloro complex [{LON i Pr }YCl] n ( 3 ), which was, in turn, converted into the corresponding amide {LON i Pr }YN(SiMe 3 ) 2 ( 6 ) by reaction with MN(SiMe 3 ) 2 (M = Li, Na). Similar reactions between 2 and YCl 3, followed by recrystallization from DME, led systematically to the isolation of the monoprotonated product, that is, phenoxy-amidino complex {LO H N Ar }YCl 2 (DME) ( 5 ). Amine elimination reactions between {LON i Pr }H 2 or {LON Cy }H 2 and Ln[N(SiMe 3 ) 2 ] 3 afforded the corresponding phenoxy-amidinate amides {LON R }LnN(SiMe 3 ) 2 (Ln = Y, R = i Pr, 6; R = Cy, 8; Ln = Nd, R = Cy, 9 ), whereas the same reaction between{LON Ar }H 2 and Y[N(SiMe 3 ) 2 ] 3, under various conditions, always yielded the homoleptic tris(phenoxy-amidinate) complex {LO H N Ar } 3 Y ( 11 ). Bimetallic “ ate ”-complexes {LON R } 2 Ln 2 Me 4 Li 2 (TMEDA) 2 of yttrium ( 12 and 13 ), neodymium ( 14 ), samarium ( 15 ), and {LON i Pr } 2 Yb 2 Me 2 (OH) 2 Li 2 (TMEDA) 2 ( 16 ) were prepared by alkane elimination of the corresponding pro-ligand and [Li(TMEDA)][LnMe 4 ] 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 ( P r up to 76%) polymers with high molecular weights ( M n up to 158 800 g·mol –1 ), but broad molecular weight distributions ( M w / M n = 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 ( M w / M n = 1.11–1.38).