Aluminum Complexes Bearing Aminoquinoline Ligands with Different Pendant Donors: Remarkable Impact of Hemilability on the Ring-Opening Polymerization of ε-Caprolactone

Hemilabile ligands are highly promising components of advanced catalysts because their switchable coordination modes can trigger novel activation and stabilization mechanisms. Their increasing use has led to the successful development of stable, active, and selective catalysts. In this study, new aluminum complexes bearing aminoquinoline ligands with different potential hemilabile thiophenyl (Al1), phenoxyl (Al2), or benzyl (Al3) donor arms have been synthesized and used for the ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL). X-ray diffraction analysis revealed that the distance between the Al center and the pendant donor decreased in the order Al3 > Al1 > Al2, consistent with the P-31 NMR spectroscopic evaluations of their relative donor capacities by the Gutmann-Beckett method. ROP of epsilon-CL under otherwise identical conditions revealed that Al1 with a hemilabile thiophenyl donor displayed both higher activity and stability than the complexes with a phenoxyl (Al2) or benzyl (Al3) group. The catalytic properties of the Al complexes were further enhanced by fine-tuning the S substituents. Remarkably, ultrahigh-molecular-weight polycaprolactones (PCLs; M-n up to 41.2 x 10(4) g<middle dot>mol(-1)) were readily synthesized, and immortal catalysis with a catalyst loading as low as 0.01 mol % relative to monomer and a large excess BnOH (up to 100 equiv to Al) was achieved as a result of hemilabile ligand coordination.