Aluminum complexes derived from a hexadentate salen-type Schiff base: synthesis, structure, and catalysis for cyclic carbonate synthesis

Abstract

Different aluminum complexes were synthesized by the reaction of aluminum alkyls with a hexadentate salen-type Schiff base. The reaction of N,N′-bis(3,5-di-tert-butylsalicylidene)-2,2′-(ethylenedioxy)dianiline (LH₂) with one equiv. of AlMe₃ in toluene at 100 °C proceeded by methane elimination to produce the intermediate methyl complex [AlMeL] (1), and then subsequent intramolecular methyl migration to give the aluminum complex [AlL′] (2) [L′ = (2-O-3,5-^tBu₂C₆H₂)CHNC₆H₄OCH₂CH₂OC₆H₄NCH(Me)(2′-O-3′,5′-^tBu₂C₆H₂)]. The reaction of the same ligand with AlEt₃ under the same experimental conditions involved ethane elimination, ethylene elimination and intramolecular hydrogen migration, and led to the complex [AlL′′] (3) [L′′ = (2-O-3,5-^tBu₂C₆H₂)CHNC₆H₄OCH₂CH₂OC₆H₄NCH₂(2′-O-3′,5′-^tBu₂C₆H₂)]. However, the interaction of two equivalents of AlMe₃ and AlEt₃ afforded the corresponding binuclear complexes [(AlMe₂)₂L] (4) and [(AlEt₂)₂L] (5), respectively, and no methyl or hydrogen migration was found. The solid-state structures of aluminum complexes 1–3 were determined by single-crystal X-ray diffraction. It was found that complexes 2–5 show a very effective catalytic activity for the cycloaddition of epoxides and CO₂ in the presence of NBu₄Br as a cocatalyst at atmospheric pressure.