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.