Neutral and Cationic Tetracoordinated Aluminum Complexes Featuring Tridentate Nitrogen Donors: Synthesis, Structure, and Catalytic Activity for the Ring-Opening Polymerization of Propylene Oxide and (d,l)-Lactide
journal contributionposted on 15.07.1998, 00:00 by Norbert Emig, Hanh Nguyen, Harald Krautscheid, Régis Réau, Jean-Bernard Cazaux, Guy Bertrand
The dilithium salts of N,N‘,N‘‘-tris(trimethylsilyl)diethylenetriamine (1), N-methyl-N‘,N‘‘-bis(trimethylsilyl)diethylenetriamine (2), and N-methyl-N‘,N‘‘-bis(diisopropyl)diethylenetriamine (3) reacted in THF, at −40 °C, with AlCl3 affording monomeric aluminum chloride derivatives 4 (55% yield), 5 (76% yield), and 6 (71% yield), respectively. Addition of 1 equiv of AlMe3 to the amine 2 gave rise to the five-coordinate dimethylaluminum derivative 9 (67% yield); heating a toluene solution of 9 overnight at 80 °C induces a loss of methane giving rise to the four-coordinate methylaluminum compound 10 (50% yield). The amine 2 also reacted at low temperature with LiAlH4 affording derivative 11 (38% yield). Compounds 4−6 reacted subsequently in toluene with 1 equiv of HCl and 1 equiv of AlCl3 to afford the first chiral tetracoordinated aluminum cations 12 (94% yield), 13 (93% yield), and 14 (90% yield), respectively. Single-crystal X-ray diffraction studies of derivatives 4, 5, 8, 10, 11, 12, and 15 have been carried out. They revealed that the tridentate nitrogen donors enforce an approximately trigonal-monopyramidal coordination geometry for neutral and cationic four-coordinate aluminum complexes. All cationic aluminum derivatives 12−14 and the neutral aluminum chloride 4 brought about the oligomerization of propylene oxide (PO). Low molecular weight polymers having narrow molecular weight distributions were obtained. The 13C NMR spectra of these polymers indicate that they consist exclusively of head-to-tail linkages and that these macromolecules are rich in meso diad and isotactic triad sequences. Due to the presence of a free axial site, a new mechanism for PO polymerization is proposed. Methyl- and hydridoaluminum derivatives 10 and 11, as well as aluminum alkoxide, prepared in situ by reacting derivative 13 with PO, initiated the polymerization of (d,l)-lactide in benzene at 80 °C. High molecular weight polymers of broad molecular weight distributions were obtained in good yields.