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

Abstract

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(CH2SiMe3)3(THF)2 (Ln = Sc, Y; THF = tetrahydrofuran) with the bis(amino-oxazoline) proligand H2L afforded the corresponding rare-earth metal monoalkyl complexes L-Ln(CH2SiMe3)(THF)x (Ln = Sc, x = 0 (1); Ln = Y, x = 1 (2)). The isopropyl-substituted Sc alkyl complex L'-Sc(CH2SiMe3) (3) and the analogue Y silylamide complex L-Y[N(SiHMe2)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(ECH2SiMe3)(THF) (E = S (7), Se (8)) were isolated. Treatment of 1 with an equimolar amount of aniline yielded the Sc anilide complex L-Sc(NHC6H5) (9), whereas the reaction of 1 with [NHEt3][BPh4] afforded the Sc ion-pair [L-Sc][BPh4] (10), which upon recrystallization led to formation of a THF-solvated product [L-Sc(THF)][BPh4] (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 (Pr = 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.