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 mix-solvents 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.