Flexible Coordination of the Bis(amino-oxazoline) Ligand in Rare-Earth Metal Complexes: Synthesis, Structure, and Their Reactivity and Polymerization Performance.

Mononuclear rare-earth metal alkyl complexes supported by tetradentate dianionic bis(amino-oxazoline) ligands have been synthesized, and their reactivity toward small molecules and catalytic performance on ring-opening polymerization have been studied. Treatment of Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln = Sc, Y; THF = tetrahydrofuran) with the bis(amino-oxazoline) proligand H 2 L afforded the corresponding rare-earth metal monoalkyl complexes L-Ln(CH 2 SiMe 3 )(THF) x (Ln = Sc, x = 0 ( 1 ); Ln = Y, x = 1 ( 2 )). The isopropyl-substituted Sc alkyl complex L'-Sc(CH 2 SiMe 3 ) ( 3 ) and the analogue Y silylamide complex L-Y[N(SiHMe 2 ) 2 ] ( 4 ) have been prepared by a similar method. Complexes 1 and 2 were stable in solution at room temperature but transformed gradually at elevated temperature to give a nucleophilic addition product for Sc ( 5 ) and an oxazoline ring-opened dimeric complex for Y ( 6 ). Reactions of 1 with elemental sulfur and selenium each led to insertion of one chalcogen into the Sc-C bond, and the corresponding six-coordinate mononuclear chalcogenolate complexes L-Sc(ECH 2 SiMe 3 )(THF) (E = S ( 7 ), Se ( 8 )) were isolated. Treatment of 1 with an equimolar amount of aniline yielded the Sc anilide complex L-Sc(NHC 6 H 5 ) ( 9 ), whereas the reaction of 1 with [NHEt 3 ][BPh 4 ] afforded the Sc ion-pair [L-Sc][BPh 4 ] ( 10 ), which upon recrystallization led to formation of a THF-solvated product [L-Sc(THF)][BPh 4 ] ( 11 ). Single-crystal X-ray diffraction analyses of complexes 1 - 3 , 7 - 9 , and 11 revealed the flexible coordination capability of the tetradentate bis(amino-oxazoline) ligand of upholding a mononuclear metal center via a torsion of the diaminobiphenyl axis. Complexes 1 - 4 were active catalysts for initiating the ring-opening polymerization of rac -lactide with good activity (TOF up to 3204 h -1 ) and heteroselectivity ( P r = 0.65-0.71). This study highlights the applicability of the well-defined tetradentate bis(amino-oxazoline) ligands for mononuclear rare-earth metal complexation and shed light on the new potential of rare-earth metal catalysts bearing this type of easily derivatizable polydentate ligand.