Synthesis, structural characterization and catalytic evaluation of the ring-opening polymerization of discrete five-coordinate alkyl aluminium complexes

Abstract

Five-coordinate alkyl aluminium complexes [AlR(κ(2)-pbpam)2] (R = Me 1, Et 2), [AlR(κ(2)-sbpam)2] (R = Me 3, Et 4) and [AlR{κ(2)-(S)-mbpam}2] (R = Me 5, Et 6), -pbpam [pbpam = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate], sbpam [sbpam = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] and (S)-mbpam [(S)-mbpam = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamidate] were obtained by an alkane elimination route involving the reaction of the previously reported acetamide heteroscorpionate precursors with 0.5 equiv. of the corresponding AlR3. In the solid state, complexes 1-6 adopt a distorted trigonal bipyramidal structure with the heteroscorpionate ligands arranged in a κ(2)-NO coordination mode. The molecular structures of 1-6 in solution were studied by VT NMR spectroscopy and a fluxional exchange between coordinated and noncoordinated pyrazole rings was observed. This process led to interconversion between the different isomers. Compounds 4 and 6 were used as precursors for the synthesis of the aryloxide aluminium compounds [Al(OR)(κ(2)-sbpam)2] (7) and [Al(OR){κ(2)-(S)-mbpam}2] (8) (R = 2,6-Me2C6H3O) by reaction with the corresponding 2,6-dimethylphenol. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 2, 6 and 7 were also established. Five-coordinate compounds 1-8 were evaluated as initiators in the ring-opening polymerization of rac-lactide. Compound 5 was also evaluated in the presence of a co-initiator. Finally, a study of the block and random copolymerization of ε-caprolactone and L-lactide was carried out.

Alternating Copolymerization of Epoxides and Anhydrides Catalyzed by Aluminum Complexes

“…On the other hand, the catalytic activities of ethyl derivatives 2 and 4 are slightly higher than the methyl complexes 1 and 3 which have been previously observed for scorpionate aluminum complexes. 53,55,58 Finally, a reaction control using TBAB as a catalyst showed that the cocatalyst by itself could catalyze the reaction but less efficiently. NMR studies confirmed that the presence of TBAB is crucial in order to increase the selectivity toward the synthesis of polyester (Figure 1).…”

Use of Metal Catalysts Bearing Schiff Base Macrocycles for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…When these systems were employed for the ROP of rac-lactide, best conversions were achieved at 110 °C with the ratio 100:1:1 (rac-LA:Al:BnOH) over 12 h. For organoaluminium systems, the ROP of lactide often requires more robust conditions than those employed for ε-CL [32]. However, there was no clear evidence of an enhanced performance for the tetranuclear versus the dinuclear systems.…”

“…However, there was no clear evidence of an enhanced performance for the tetranuclear versus the dinuclear systems. For the latter, the ethyl derivatives were slightly more active than the methyl derivatives; the same When these systems were employed for the ROP of rac-lactide, best conversions were achieved at 110 • C with the ratio 100:1:1 (rac-LA:Al:BnOH) over 12 h. For organoaluminium systems, the ROP Catalysts 2017, 7, 165 6 of 11 of lactide often requires more robust conditions than those employed for ε-CL [32]. However, there was no clear evidence of an enhanced performance for the tetranuclear versus the dinuclear systems.…”

“…Furthermore, the systems 27 and 29 exhibit an induction period that, following a 15%-20% Scheme 7. Zinc complexes (27)(28)(29)(30)(31)(32).…”

Mechanistic Insight into the Ring-Opening Polymerization of ε-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts

“…Therefore, development of efficient synthetic strategies becomes of great interest to chemists. Among these strategies, ring-opening polymerization (ROP) catalyzed by the less toxic and inexpensive aluminum complexes is one of the useful and well-controlled methods [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. The ring-opening polymerization catalyzed by an organometallic complex usually proceeds through a coordination-insertion mechanism [31,32].…”