Aluminum Complexes of Bidentate Fluorinated Alkoxy-Imino Ligands: Syntheses, Structures, and Use in Ring-Opening Polymerization of Cyclic Esters.

International audienceThe coordination chem. of bidentate fluorinated alkoxy-imino ligands onto Al(III) centers was studied. The proligands (CF3)2C(OH)CH2C(R1):NR2 (1, \ONR1,R2\H; R1 = Ph, R2 = Bn (a, Bn = CH2Ph), Ph (c), Cy (d, Cy = cyclohexyl); R1 = Me, R2 = Bn (b)) react selectively with AlMe3 (0.5 or 1.0 equiv) and AlMe2(OiPr) or Al(OiPr)3 (0.5 equiv) to give the corresponding monoligand compds. \ONR1,R2\AlMe2 (2a-d) and the bis-ligand compds. \ONR1,R2\2AlMe (3a-d) and \ONR1,R2\2Al(OiPr) (4a-c). X-ray diffraction studies revealed that \ONPh,Bn\AlMe2 (2a), \ONMe,Bn\AlMe2 (2b), \ONMe,Bn\2AlMe (3b), \ONPh,Ph\2AlMe (3c), \ONMe,Bn\2Al(OiPr) (4b), and \ONPh,Ph\2Al(OiPr) (4c) all adopt a mononuclear structure in the solid state; four-coordinate \ONR1,R2\AlMe2 and five-coordinate \ONR1,R2\2AlMe and \ONR1,R2\2Al(OiPr) feature resp. distorted-tetrahedral and trigonal-bipyramidal geometries. The 1H, 13C\1H\, and 19F\1H\ NMR data indicate that the structures obsd. in the solid state are retained in CD2Cl2 or C6D6 soln. at room temp. The binary systems \ONR1,R2\AlMe2 (2)/BnOH and discrete \ONR1,R2\2Al(OiPr) (4) are effective catalysts for the controlled ROP of ε-caprolactone and rac-lactide, both in bulk molten monomer and in toluene soln./slurry. In contrast to the case of aluminum complexes having a bridged tetradentate fluorinated dialkoxy-diimino ligand that provides isotactic-enriched polylactides, the unbridged compds. \ONR1,R2\2Al(OiPr) (4) produce atactic PLAs. The key element which appears to be at the origin of the absence of stereocontrol is the lack of bridge between the two imino-alkoxy moieties, possibly via a decrease in the rigidity of the compds. and/or a different positioning of N,O vs. N,N heteroatoms in axial sites. [on SciFinder(R)