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(CHSiMe)(THF) (Ln = Sc, Y; THF = tetrahydrofuran) with the bis(amino-oxazoline) proligand HL afforded the corresponding rare-earth metal monoalkyl complexes L-Ln(CHSiMe)(THF) (Ln = Sc, = 0 (); Ln = Y, = 1 ()). The isopropyl-substituted Sc alkyl complex L'-Sc(CHSiMe) () and the analogue Y silylamide complex L-Y[N(SiHMe)] () have been prepared by a similar method. Complexes and were stable in solution at room temperature but transformed gradually at elevated temperature to give a nucleophilic addition product for Sc () and an oxazoline ring-opened dimeric complex for Y (). Reactions of 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(ECHSiMe)(THF) (E = S (), Se ()) were isolated. Treatment of with an equimolar amount of aniline yielded the Sc anilide complex L-Sc(NHCH) (), whereas the reaction of with [NHEt][BPh] afforded the Sc ion-pair [L-Sc][BPh] (), which upon recrystallization led to formation of a THF-solvated product [L-Sc(THF)][BPh] (). Single-crystal X-ray diffraction analyses of complexes -, -, and 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 - were active catalysts for initiating the ring-opening polymerization of -lactide with good activity (TOF up to 3204 h) and heteroselectivity ( = 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.