Aluminum Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands: Syntheses, Structures, and Use in Ring-Opening Polymerization of Cyclic Esters

The coordination chemistry of new fluorinated dialkoxy-diimino ligands onto Al(III) centers has been studied. Diimino-diols (CF3)(2)C(OH)(CR2C)-C-1(R-2)=N- R-3-N=C(R-2)(CR2C)-C-1(OH)(CF3)(2) ({(ONNO)-N-Et}H-2, R-1 = H, R-2 = Me, R-3 = C2H4; {(ONNO)-N-Cy}H-2, R-1 = H, R-2 = Me, R-3 = trans-1,2-cyclohexyl; {(ONNO)-O-Me-N-Et}H-2, R-1 = Me, R-2 = iPr, R-3 = C2H4) react selectively with AlMe3, AlMe2Cl, and Al(OiPr)(3) to give the corresponding complexes {(ONNO)-N-Et}AlX (X = Me, 1; Cl, 2; OiPr, 3), {(ONNO)-N-Cy}AlX (X = Me, 5; Cl, 6; OiPr, 7), and {(ONNO)-O-Me-N-Et}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 H-1, C-13, and F-19 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 E-caprolactone and racemic lactide, giving polymers with high molecular weights (M-n up to 37 500 g mol(-1)) and relatively narrow polydispersities (M-w/M-n = 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 (P-meso = 0.87).