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

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 amine (3) reacted in THF, at -40 degrees 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 degrees 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 C-13 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 degrees C. High molecular weight polymers of broad molecular weight distributions were obtained in good yields.