Synthesis, structural characterization and catalytic evaluation of the ring-opening polymerization of discrete five-coordinate alkyl aluminium complexes

Abstract

Five-coordinate alkyl aluminium complexes [AlR(κ²-pbpam)₂] (R = Me 1, Et 2), [AlR(κ²-sbpam)₂] (R = Me 3, Et 4) and [AlR{κ²-(S)-mbpam}₂] (R = Me 5, Et 6), –pbpam [pbpam = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate], sbpam [sbpam = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] and (S)-mbpam [(S)-mbpam = (S)-(−)-N-α-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] were obtained by an alkane elimination route involving the reaction of the previously reported acetamide heteroscorpionate precursors with 0.5 equiv. of the corresponding AlR₃. In the solid state, complexes 1–6 adopt a distorted trigonal bipyramidal structure with the heteroscorpionate ligands arranged in a κ²-NO coordination mode. The molecular structures of 1–6 in solution were studied by VT NMR spectroscopy and a fluxional exchange between coordinated and noncoordinated pyrazole rings was observed. This process led to interconversion between the different isomers. Compounds 4 and 6 were used as precursors for the synthesis of the aryloxide aluminium compounds [Al(OR)(κ²-sbpam)₂] (7) and [Al(OR){κ²-(S)-mbpam}₂] (8) (R = 2,6-Me₂C₆H₃O) by reaction with the corresponding 2,6-dimethylphenol. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 2, 6 and 7 were also established. Five-coordinate compounds 1–8 were evaluated as initiators in the ring-opening polymerization of rac-lactide. Compound 5 was also evaluated in the presence of a co-initiator. Finally, a study of the block and random copolymerization of ε-caprolactone and L-lactide was carried out.