Aluminum Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands: Syntheses, Structures and Use in Ring-Opening Polymerization of Cyclic Esters
Résumé
The coordination chemistry of new fluorinated dialkoxy-diimino ligands onto Al(III) centers has been studied. Diimino-diols (CF3)2C(OH)CR12C(R2)═N−R3−N═C(R2)CR12C(OH)(CF3)2 ({ONEtNO}H2, R1 = H, R2 = Me, R3 = C2H4; {ONCyNO}H2, R1 = H, R2 = Me, R3 = trans-1,2-cyclohexyl; {MeONEtNO}H2, R1 = Me, R2 = iPr, R3 = C2H4) react selectively with AlMe3, AlMe2Cl, and Al(OiPr)3 to give the corresponding complexes {ONEtNO}AlX (X = Me, 1; Cl, 2; OiPr, 3), {ONCyNO}AlX (X = Me, 5; Cl, 6; OiPr, 7), and {MeONEtNO}AlX (X = Me, 8; Cl, 9) with concomitant alkane or alcohol elimination, respectively. Single-crystal X-ray diffraction studies revealed that complexes 2, 5, 7, 8, and 9 are mononuclear in the solid state with distorted trigonal-bipyramidal to square-pyramidal geometries. Complexes 1−9 were also characterized in CD2Cl2 solution by 1H, 13C, and 19F NMR spectroscopy. Only one symmetric isomer was observed for complexes 1−7, but mixtures of two isomers (one symmetric, one dissymmetric on the NMR time scale) were observed for complexes 8 and 9. The Al-OiPr complexes 3 and 7 are effective initiators for the ring-opening polymerization of ε-caprolactone and racemic lactide, giving polymers with high molecular weights (Mn up to 37 500 g mol−1) and relatively narrow polydispersities (Mw/Mn = 1.08−1.91). The PLAs produced, both under slurry or melt conditions, with either achiral complex 3 or chiral complex 7, have a highly isotactic-enriched stereoblock microstructure (Pmeso = 0.87).