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 (CF 3 ) 2 C(OH)CR 1 2 C(R 2 )═N−R 3 −N═C(R 2 )CR 1 2 C(OH)(CF 3 ) 2 ( {ON Et NO}H 2, R 1 = H, R 2 = Me, R 3 = C 2 H 4; {ON Cy NO}H 2, R 1 = H, R 2 = Me, R 3 = trans -1,2-cyclohexyl; { Me ON Et NO}H 2, R 1 = Me, R 2 = i Pr, R 3 = C 2 H 4 ) react selectively with AlMe 3, AlMe 2 Cl, and Al(O i Pr) 3 to give the corresponding complexes {ON Et NO}AlX (X = Me, 1; Cl, 2; O i Pr, 3 ), {ON Cy NO}AlX (X = Me, 5; Cl, 6; O i Pr, 7 ), and { Me ON Et NO}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 CD 2 Cl 2 solution by 1 H, 13 C, and 19 F 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-O i Pr complexes 3 and 7 are effective initiators for the ring-opening polymerization of ε-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).