Morpholine-Stabilized Cationic Aluminum Complexes and Their Reactivity in Ring-Opening Polymerization of ε-Caprolactone

The first structures exhibiting bidentate (N, O) chelation of a morpholine group to a p-block element (aluminum) have been prepared and characterized by X-ray diffraction methods: Al[L](+) [WCA](-), where [L] = 4-(2-amino ethyl)m orpholinylamino-N,N-bis (2-methylene-4,6-tert-butylphenolate) and [WCA](-) is a weakly coordinating anion. These compounds are easily synthesized by reacting Al[L]Cl with an equimolar amount of anhydrous Lewis acid and were characterized by elemental analyses, ESI-MS, MALDI-TOF MS, H-1, C-13{H-1}, and multinuclear NMR spectroscopy. DFT calculations showed that Al[L](+) cations containing bidentate NO coordination of the morpholine group are at least 21.1 kJ/mol more stable when compared to hypothetical monodentate (N bound) structures. When combined with protic co-initiators (EtOH, glycerol carbonate), the cationic complexes, where [WCA](-) = [GaCl4](-) or [InCl4](-), are living catalyst systems for the polymerization of E-caprolactone, producing polycaprolactone with narrow dispersity (D = 1.00-1.05). Employing glycidol as a co-initiator furnished polymers with narrow dispersity (D = 1.01-1.07) but experimental molecular weights diverged considerably from the calculated values. Similar reactivity toward ROP was observed for all complexes containing a stable [WCA](-) but where [WCA](-) = [AlCl4](-), upon combination with alcohols, alcoholysis was observed. Kinetic studies (Eyring analyses) allowed the determination of activation parameters, which were consistent with a coordination-insertion mechanism for the catalysts containing [WCA](-) = [GaCl4](-) or [InCl4](-). End group analyses using MALDI-TOF mass spectrometry and H-1 NMR spectroscopy showed hydroxyl and ester end groups within the polymer, corroborating the proposed mechanism. Stoichiometric reactions of EtOH, glycidol or tert-butyl alcohol with the complex, where [WCA](-) = [GaCl4](-), showed protonation of the ligand at the N-morpholine site, which leads to dissociation of this pendent group.