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

The coordination chemistry of bidentate fluorinated alkoxyimino ligands onto Al(III) centers has been studied. The proligands (CF3)(2)C(OH)CH2C(R-1)=N-R-2 ({ONR1,R2}H; R-1 = Me, Ph; R-2 = Ph, CH2Ph, cyclohexyl; la-d) 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 compounds {ONR1,R2}AlMe2 (2a-d) and the bis-ligand compounds {ONR1,R2}(2)AlMe (3a-d) and {ONR1,R2)(2)Al(OiPr) (4a-c). X-ray diffraction studies revealed that {ONPh,Bn}AlMe2 (2a), {ONMe,Bn}AlMe2 (2b), {ONMe,Bn}(2)AlMe (3b), {ONPh,Ph}(2)AlMe (3c), {ONMe,Bn}(2)Al(Oipr) (4b), and {ONPh,Ph)(2)Al(OiPr) (4c) all adopt a mononuclear structure in the solid state; four-coordinate {ONR1,R2}AlMe2 and five-coordinate {ONMe,Bn}(2)Al(OiPr) feature respectively distorted-tetrahedral and trigonal-bipyramidal geometries. The H-1, C-13{H-1), and F-19{H-1) NMR data indicate that the structures observed in the solid state are retained in CD2Cl2 or C6D6 solution at room temperature. The binary systems {ONR1,R2}AlMe2 (2)/BnOH and discrete {ONR1,R2}(2)Al(OiPr) (4) are effective catalysts for the controlled ROP of epsilon-caprolactone and rac-lactide, both in bull( molten monomer and in toluene solution/slurry. In contrast to the case for {(ONNO)-N-R}Al(OiPr), having a bridged tetradentate fluorinated dialkoxy-diimino ligand that provides isotactic-enriched polylactides, the unbridged compounds {ONR1,R2}(2)Al(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 compounds and/or a different positioning of N,O vs N,N heteroatoms in axial sites.