Synthesis and structural characterization of well-defined anionic aluminium alkoxide complexes supported by NON-type diamido ether tridentate ligands and their use for the controlled ROP of lactide

Abstract

The tridentate proligands (RNH-o-C₆H₄)₂O (1a, R = C₅H₉; 1b, R = Cy) were found to readily react with LiAlH₄ to yield the corresponding lithium aluminium dihydrido salt species [η²-N,N-{(RN-o-C₆H₄)₂O}AlH(μ-H)Li(THF)]₂ (2a, R = C₅H₉; 2b, R = Cy) in 50% and 42% yield, respectively. The solid-state structure of both complexes 2a and 2b were determined by X-ray crystallographic studies. Compounds 2a and 2b readily react with one equivalent of benzaldehyde to afford the corresponding mono-benzyloxide species η²-N,N-{RN-o-C₆H₄)₂O}Al(H)(μ -OCH₂Ph)Li(THF)₂ (4a, R = C₄H₉; 4b, R = Cy), as confirmed by X-ray studies in the case of 4b. In a similar manner, when compounds 2a and 2b are reacted with two equivalents of benzaldehyde the bis-benzyloxide derivatives 5a and 5bη³-N,N,O-{RN-o-C₆H₄)₂O}Al(μ -OCH₂Ph)₂Li(THF)₂ (5a, R = C₄H₉; 5b, R = Cy) may be prepared. While the lithium Al mono-alkoxide species 4a–b are inactive in the ring-opening polymerization (ROP) of lactide, their bis-alkoxide Al analogues 5a–b polymerize rac-lactide and (S)-lactide at room temperature, which is rather uncommon for Al-based alkoxide systems. Kinetic studies of the lactide ROP initiated by compound 5a suggest a strong preference for racemic enchainment during the ROP chain growth; the resulting PLAs are however moderately heterotactic due to detrimental transesterification processes occurring as the chain grows.