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 [eta(2)-N,N-{(RN-o-C6H4)2O}AlH(mu-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 eta(2)-N,N-{RN-o-C6H4)2O}Al(H)(mu-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 5b eta(3)-N,N,O-{RN-o-C6H4)2O}Al( -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.