Syntheses of bis(pyrrolylaldiminato)aluminum complexes for the polymerisation of lactide

Abstract

Seven bis(pyrrolylaldiminato)aluminum methyl complexes were synthesized from the reactions of AlMe3 and two equiv. of the corresponding pyrrolylaldimine ligands. The ligands were modified to have different steric hindrances (C6H5 (1), 2,6-Me2C6H3 (2), 2,4,6-Me3C6H2 (3), 2,6-Et2C6H3 (4) and 2,6-(i)Pr2C6H3 (5)) and electronic contributions (4-CF3C6H4 (6) and 4-OMeC6H4 (7)). Crystal structures of complexes 3-7 were determined and shown to have distorted trigonal bipyramidal geometry (4, 6, 7) and intermediates between trigonal bipyramidal and square pyramidal geometries (3 and 5). The rotation around the N-C(aryl) bond was fast for ligands having small ortho substituents and became slower as the size of the substituents increased. Polymerisations of l-lactide using complexes 1-7 and benzyl alcohol as an initiator were carried out giving the rate dependence on steric hindrance (5 < 4 < 3 < 2 < 1) and electronic contribution (6 < 7 < 1). Larger substituents and electron withdrawing groups were found to suppress the polymerisation rates. Despite having C2 symmetry in the crystal structures of compounds 3-7, only slight enhancement for isotactic enchainment was found in the polymerisation of rac-lactide.

Transition Metal Complexes Containing C2‐Symmetric Bis(imidazolin‐2‐imine) Ligands Derived from a 1‐Alkyl‐3‐arylimidazolin‐2‐ylidene

“…0.5 for the complexes [(BL Dipp, Me )ZnCl 2 ] ( 8 ) [(BL Me )ZnCl 2 ] ( 11 )4d reveals that the stereochemistry of the resulting polymer is not influenced by the presence of the potentially C 2 ‐symmetric ligand in 8 25. Similar results were reported for aluminum complexes with bidentate N‐donor ligands, which induced only slight enhancement for isotactic enchainment despite C 2 symmetry in the crystal structures 26…”

Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency

“…[27,28] The influence of regioisomeric ligand substituents upon initiation and propagation remains less systematically explored, in spite of evidence that the substituent position alters the catalyst activity and polymerisation control. [26,29] Cyclic ester ROP is typically initiated by metal-alkoxide species, often formed in situ through alcoholysis of a metal-alkyl precursor. While metal-chloride complexes may offer benefits of improved air-stability compared to the metal-alkyl analogues, aluminium chloride complexes are typically poor initiators for LA ROP.…”

Hydrosilylation in Aryliminopyrrolide‐Substituted Silanes

“…The most widely studied main‐group aryliminopyrrolide complexes are based on aluminum . In particular, these Al‐complexes find usage as catalysts in lactide ring‐opening polymerization (ROP) and guanylation . Anderson et al.…”