Copolymerization of CHO/CO2 catalyzed by a series of aluminum amino-phenolate complexes and insights into structure–activity relationships

Abstract

Two series of monometallic aluminum complexes were prepared and characterized by elemental analyses, ¹H and ¹³C{¹H} NMR spectroscopy, and X-ray crystallography: Al[L]X, where [L] = dimethylaminoethylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate) and X = Cl, OEt, and Al[L]₂Cl, where [L] = 6-{[(2R,6R)-2,6-dimethyl-4-morpholino]methylene}-2,4-bis(tert-butyl)phenolate or 6-(piperidinomethylene)-2-(tert-butyl)-4-(methyl)phenolate. All the complexes, including the previously reported morpholinyl complex Al[L]Cl, where [L] = 4-(2-aminoethyl)morpholinylamino-N,N-bis(2-methylene-4,6-tert-butylphenolate), were tested as catalysts for copolymerization of cyclohexene oxide and CO₂ in the presence and absence of PPNCl. When coupled with 1 equiv. PPNCl, the complexes exhibit similar activities and the best selectivity for poly(cyclohexenecarbonate) vs. the cyclic product, cyclohexene carbonate, was obtained with the morpholinyl complex (ca. 90%) whereas significantly lower selectivities (<1–63%) were obtained with the other complexes. Preliminary DFT calculations investigating this difference in selectivity were carried out by analyzing the aluminum partial atomic charges in the Al-carbonate intermediates.