Group 3 Metal Complexes of Salen-like Fluorous Dialkoxy−Diimino Ligands: Synthesis, Structure, and Application in Ring-Opening Polymerization of rac-Lactide and rac-β-Butyrolactone

Abstract

The coordination chemistry of the dianionic fluorous dialkoxy−diimino ligand {OC(CF3)2CH2C(Me)═NCH2CH2N═C(Me)CH2C(CF3)2O}2− ({ONEtNO}2−) onto Y(III) and La(III) centers has been studied. The diimino−diol {ONEtNO}H2 reacts with 1 equiv of Y(N(SiHMe2)2)3·2THF to afford {ONEtNO}Y(N(SiHMe2)2)(THF) (1). Complex 1 slowly decomposes in solution, forming as yet undefined products, but could be stabilized by further reaction with 1 equiv of (R)-(+)-tert-butyl lactate, to give selectively [{ONEtNO}Y((R)-tBu-lactate)]2 (2) in 78% yield. A single-crystal diffraction study revealed that 2 is dinuclear in the solid state, with bridging μ,κ2O,O,O-lactate units and tetracoordinated {ONEtNO} units. The same geometry is retained in THF-d8 solution, as indicated by 1H, 13C, and 19F NMR spectroscopy. Reaction of {ONEtNO}H2 with 1 equiv of La(N(SiHMe2)2)3·2THF or La(N(SiMe3)2)3 systematically yielded {ONEtNO}La{ONEtNOH} (3), which was characterized by single-crystal X-ray diffraction and NMR studies. Complex 2 is an effective initiator for the controlled-“living” ring-opening polymerization of rac-lactide and rac-β-butyrolactone, giving atactic polymers with high molecular weights (Mn up to 55 000 g mol−1) that match well the calculated values and relatively narrow polydispersities (Mw/Mn = 1.06−1.60). The immortal ROP of rac-lactide appears feasible by combining complex 2 with 5 equiv of 2-propanol.