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

The coordination chemistry of bidentate fluorinated alkoxy-imino
ligands onto Al­(III) centers has been studied. The proligands (CF3)2C­(OH)­CH2C­(R1)N–R2 ({ONR1,R2}­H; R1 = Me, Ph; R2 = Ph, CH2Ph, cyclohexyl; 1a–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}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
{ONMe,Bn}2Al­(OiPr) feature
respectively distorted-tetrahedral and trigonal-bipyramidal geometries.
The 1H, 13C­{1H}, and 19F­{1H} 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}2Al­(OiPr) (4) are effective catalysts for the controlled ROP of ε-caprolactone
and rac-lactide, both in bulk molten monomer and
in toluene solution/slurry. In contrast to the case for {ONRNO}­Al­(OiPr), having a bridged tetradentate fluorinated
dialkoxy-diimino ligand that provides isotactic-enriched polylactides,
the unbridged compounds {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 compounds and/or a different
positioning of N,O vs N,N heteroatoms in axial sites.