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-aminoethyl)­morpholinylamino-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, 1H, 13C­{1H}, 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 ε-caprolactone, producing polycaprolactone with narrow dispersity (D̵ = 1.00–1.05). Employing glycidol as a co-initiator furnished polymers with narrow dispersity (D̵ = 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 1H 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.