Four- and five-coordinate aluminum ketiminate complexes: Synthesis, characterization, and ring-opening polymerization

A series of aluminum complexes featuring with the ketiminate ligand, OCMeCHCMeNHAr (Ar = 2,6-ⁱPr₂C₆H₃, 1), have been prepared and characterized spectroscopically and structurally. Reactions of 1 with trialkylaluminum in 1:1 or 1:2 molar ratio generate four- and five-coordinated aluminum complexes (OCMeCHCMeNAr)AlR₂ (R = Me (2); R = Et (3)) and (OCMeCHCMeNAr)₂AlR (R = Me (4); R = Et (5)) in high yields. Similarly, reaction of AlCl₃ with 1 or 2 equiv of the lithiated 1 in toluene afforded bis(ketiminate) aluminum chloride complex, (OCMeCHCMeNAr)₂AlCl (6) or (OCMeCHCMeNAr)AlCl₂ (7). Surprisingly, reacting 6 with 1 equiv of AgBF₄ in methylene/acetonitrile mixsolvents generates (OCMeCHCMeNAr)₂AlF (8) in moderate yield. The structures of complexes 2-6 and 8 have been determined by X-ray crystallography. Complexes 2 and 3 both exhibit tetrahedron structures with the aluminum atom surrounded by oxygen and nitrogen atoms of chelating ketiminate and two alkyl groups. The mono- and bis-ketiminate aluminum complexes 2 - 5 have shown moderate activity toward the ring-opening polymerization of ε-caprolactone.