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) diethylenetriami ne (1), ethyl-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 chlorid e derivatives 4 (55% yield), 5 (76% yield), and 6 (71% yield), respect ively. Addition of 1 equiv of AlMe3 to the amine 2 gave rise to the fi ve-coordinate dimethylaluminum derivative 9 (67% yield); heating a tol uene solution of 9 overnight at 80 degrees C induces a loss of methane giving rise to the four-coordinate methylaluminum compound 10 (50% yi eld). The amine 2 also reacted at low temperature with LiAlH4 affordin g derivative 11 (38% yield). Compounds 4-6 reacted subsequently in tol uene with 1 equiv of HCl and 1 equiv of AlCl3 to afford the first chir al tetracoordinated aluminum cations 12 (94% yield), 13 (93% yield), a nd 14 (90% yield), respectively. Single-crystal X-ray diffraction stud ies of derivatives 4, 5, 8, 10, 11, 12, and 15 have been carried out. They revealed that the tridentate nitrogen donors enforce an approxima tely trigonal-monopyramidal coordination geometry for neutral and cati onic four-coordinate aluminum complexes. All cationic aluminum derivat ives 12-14 and the neutral aluminum chloride 4 brought about the oligo merization of propylene oxide (PO). Low molecular weight polymers havi ng narrow molecular weight distributions were obtained. The C-13 NMR s pectra of these polymers indicate that they consist exclusively of hea d-to-tail linkages and that these macromolecules are rich in meso diad and isotactic triad sequences. Due to the presence of a free axial si te, a new mechanism for PO polymerization is proposed. Methyl- and hyd ridoaluminum derivatives 10 and 11, as well as aluminum alkoxide, prep ared in situ by reacting derivative 13 with PO, initiated the polymeri zation of (D,L)-lactide in benzene at 80 degrees C. High molecular wei ght polymers of broad molecular weight distributions were obtained in good yields.