Synthesis, spectroscopy, and crystallography of mononuclear, five-coordinate aluminum complexes that act as cyclic ester polymerization initiators

Abstract

Three ketoimines with pendant quinolyl moieties and a series of five-coordinate ketoiminate aluminum complexes were prepared and characterized spectroscopically and crystallographically. The synthesis of the ketoimines completed a series of ten regioisomers for which the product ratio varied between the product resulting from quinoline addition adjacent to the alkyl/aryl group or adjacent to the trifluoromethyl substituent. Quantitation of the ketoimine regioisomer ratios with 19F NMR demonstrated the role of steric encumbrance in the nucleophilic attack mechanistic step of the Schiff base condensation reaction and was further supported by linear free energy relationships with the Charton steric parameter and DFT calculations. Eight five-coordinate aluminum complexes were prepared in 57–92% yield from ketoimines and tris(2,6-dimethylphenoxide) aluminum dimer and characterized with 1H, 13C, 19F, and 27Al nuclear magnetic resonance spectroscopy, UV–visible absorbance spectroscopy, elemental analysis, and single-crystal X-ray crystallography. These mononuclear complexes differ from previously reported aluminum ion-pair complexes due to the symmetric cleavage of tris-dimethylphenoxide aluminum dimer by THF and were shown to have distorted trigonal bipyramidal coordination geometries (τ5 = 0.73–0.89). In a preliminary study, the aluminum complexes were demonstrated to be effective initiators for ring-opening polymerization of ε-caprolactone (εCL) to poly-caprolactone (PCL) and l-lactide (LA) to poly-lactic acid (PLA), reaching 77–99% conversion in 2 h for εCL and 10 h for LA at 100 °C.