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

Jose A Castro-Osma, Carlos Alonso-Moreno, Isabel Márquez-Segovia, Antonio Otero, Agustín Lara-Sánchez, Juan Fernández-Baeza, Ana M Rodríguez, Luis F Sánchez-Barba, Joaquín C García-Martínez

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Five-coordinate alkyl aluminium complexes [AlR(κ(2)-pbpam)2] (R = Me 1, Et 2), [AlR(κ(2)-sbpam)2] (R = Me 3, Et 4) and [AlR{κ(2)-(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 κ(2)-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)(κ(2)-sbpam)2] (7) and [Al(OR){κ(2)-(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.

Original languageEnglish
Pages (from-to)9325-9337
Number of pages13
JournalDalton Transactions
Issue number25
Early online date8 Feb 2013
Publication statusPublished - 7 Jul 2013


  • Aluminum
  • Caproates
  • Catalysis
  • Dioxanes
  • Lactones
  • Models, Molecular
  • Molecular Structure
  • Organometallic Compounds
  • Polymerization

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