Aluminum Complexes of Octahydrophenanthroline-Based Salophan Ligands: Coordination Chemistry and Activity in the Ring-Opening Polymerization of Lactide

Salophan ligands featuring the octahydrophenanthroline core are described for the first time. Two ligand precursors featuring either di-tert-butylphenolate or dichlorophenolate were prepared by 1,10-phenanthroline hydrogenation followed by reaction with bromomethyl-substituted phenols. The ligand precursors react with AlMe₃ to give the [{ONNO}Al-Me]-type complexes as single stereoisomers of averaged C_s-symmetry on the NMR timescale. The reactivity of the methylaluminum complexes depends strongly on the phenolate substituents: The complex of the bulky ligand is inert towards alcohols and inactive in the ring-opening polymerization of rac-LA, whereas the complex of the non-bulky ligand reacts readily with benzyl alcohol to yield the corresponding [{ONNO}Al-OBn] complex, which polymerizes rac-LA to give isotactically inclined PLA.