Synthesis of Al complexes containing phenoxy-imine ligands and their use as the catalyst precursors for efficient living ring-opening polymerisation of ε-caprolactone

Abstract

Al complexes containing phenoxy-imine ligands of type, Me2Al[O-2-R1-6-(R2N=CH)C6H3] [R1 = Me, R2 = 2,6-iPr2C6H3 (1a), tBu (1b); R1 = tBu, R2 = 2,6-iPr2C6H3 (2a), tBu (2b), cyclohexyl (2c), adamantyl (2d), C6H5 (2e), 2,6-Me2C6H3 (2f), C6F5 (2g)] have been prepared in high yields from AlMe3 by treating with 1.0 equiv. of 2-R1-6-(R2N=CH)C6H3OH in n-hexane. Structures for 1a, 1b, 2a-e and 2g were determined by X-ray crystallography, and these complexes have a distorted tetrahedral geometry around Al; both the Al-O and the Al-N bond distances were influenced by substituents in both the aryloxo and the imino groups. Me2Al[mu2-O-2-(R2N=CH)C6H4](AlMe3) [R2 = 2,6-iPr2C6H3 (3a), tBu (3b)] were prepared exclusively by reaction of AlMe3 with 2-(R2N=CH)C6H4OH, and these complexes form a distorted tetrahedral geometry around each Al centre with additional AlMe3 coordinating to the oxygen in the phenoxy-imine ligand. Complexes 1a, 1b and 2a-g were tested as catalyst precursors for ring-opening polymerisation (ROP) of epsilon-caprolactone (CL) in the presence of (n)BuOH (1.0 equiv. to Al), and their catalytic activities were strongly influenced by the imino substituent (R2). The efficient ROP has been achieved using the C6F5 analogue (2g), with the ROP taking place in a living manner.

Aluminum Alkyl Complexes Bearing Salicylaldiminato Ligands: Versatile Initiators in the Ring-Opening Polymerization of Cyclic Esters

“…The catalytic activity in the ring-opening polymerization (ROP) of ε-caprolactone initiated by 2, 3, 6, or 7 in combination with n BuOH increased in the order: 3 > 2 > 6 > 7, indicating that the imino substituent (R) plays an important role for the efficient ROP [43]. The role of the imino substituents for efficient and controlled ROP was further remarked in an extension of this study, showing that in the presence of C6F5 as imino substituent, the ROP of ε-caprolactone took place in a living manner [40]. Notably, the ROP reactions were accompanied by a certain degree of transesterification as side reaction except for the polymerization carried out in the presence of complexes bearing the C6F5 substituent.…”

An Unusual N‐Bridged (Amido)(hydrido)(phenoxido)aluminium Dinuclear Compound – The Role of Nitrogen Substituents in Determining Nuclearity: A Combined Experimental and Theoretical Study

“…For ligands containing both oxygen and nitrogen as potential bridging atoms, it is found that compounds having oxygen bridges between aluminium atoms [23,24,39,[47][48][49][50] predominate over those with nitrogen bridges. [37,51] Examples are known with both O and N atoms bridging in hydroxypyridine ligands (in an isolobal disposition to carboxylatebridged ligands) [25] or with an amidate group [52][53][54][55] between two aluminium atoms.…”

“…Bidentate N,O-donor ligands [20][21][22][23][24][25] constitute simple nonsymmetric chelating structures that impose control over the metal coordination geometry with a sterically demanding chelating structure. Phenoxidoimido [26][27][28] and the closely related phenoxido-amino bidentate systems [29][30][31] have been widely used for transition metals and to produce alkylaluminium complexes and their cationic derivatives, [32][33][34] which can be utilized as catalysts in ROP processes [24] of heterocyclic monomers and in ethylene polymerization reactions. [21,35] Here we report the synthesis and structural analysis of the reaction products from the treatment of iminophenol and aminophenol ligand precursors with alane AlH 3 ·NEtMe 2 to prepare derivatives of different nuclearity.…”

Review – recent development of ring-opening polymerization of cyclic esters using aluminum complexes

“…[98][99][100][101][102][103][104][105][106][107] Nomura [98] studied the aluminum complexes containing phenoxyimine ligand for initiating polymerization of ɛ-caprolactone and found that the aluminum complexes 71 exhibited living polymerization characteristics when low monomer conversion, increasing conversion resulted in widening the PDI of PCL. Redshaw [99] also investigated the aluminum complexes 72 and 73 for ROP of ɛ-caprolactone, these catalyst system showed produced polymers with 44640 g/mol molecular weight, but, 46% monomer conversion.…”