β-Diketiminate aluminium complexes: synthesis, characterization and ring-opening polymerization of cyclic esters

Abstract

A series of aluminium alkyl complexes (BDI)AlEt(2) (3a-m) bearing symmetrical or unsymmetrical beta-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding beta-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize beta-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex "(BDI-2a)AlMeCl" (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe](2)(micro-O) (7) as characterized by X-ray diffraction methods. Complexes 3a-m and [(2,6-(i)Pr(2)C(6)H(3)NCMe)(2)HC]AlEt(2) (8) were found to catalyze the ring-opening polymerization (ROP) of epsilon-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (M(eta)) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of epsilon-caprolactone initiated by complexes 3a-m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66-3.74, M(w)/M(n)) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.

β‐Diketiminate Zinc Complexes for the Hydroamination of Alkynes

“…[28,36] The ligands of complexes 1-5 were pre-pared in one step by refluxing 2,4-pentadione with two equivalents of the corresponding aniline. The introduction of substituents at the N-aryl ring leads to altered catalytic properties of the β-diketiminate zinc complexes.…”

Aluminum Methyl and Chloro Complexes Bearing Monoanionic Aminephenolate Ligands: Synthesis, Characterization, and Use in Polymerizations

“…This observation is similar to that reported by Chmura et al 84 for Ti(IV) complexes of bis(phenolate)s, but in contrast to the aluminum systems reported by other authors where less sterically demanding ligands afforded more effective initiators. 85,86 However, by comparing the activity of the methyl complexes with the chloride complexes, the latter showed lower activity and produced polymers with different properties. This trend has also been observed by others.…”

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

“…This might cause broader dispersity values and higher molar masss (Table 1); such phenomena were reported in the literature without a detailed explanation. 80 Previously reported examples of TMC polymerizations by aluminum complexes following a coordination-insertion mechanism also showed a broad dispersity together with higher than expected molar masss. [83][84][85] However, if the metal catalysts reacted as Lewis acids and TMC was polymerized by an appropriate Lewis acid/ROH pair instead, the PTMC material could be chain-length controlled.…”