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

A series of aluminum methyl and chloride complexes bearing 2­(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-R-phenolate or 2­(N-morpholinyl)-2-methylene-4-R′-6-R-phenolate ([ONER1,R2]-) {[R1 = tBu, R2 = Me, E = NMe (L1); R1= R2 = tBu, E = NMe (L2); R1 = R2 = tBu, E = O (L3)} ligands were synthesized and characterized through elemental analysis, 1H, 13C­{1H}, and 27Al NMR spectroscopy, and X-ray crystallography. Reactions of AlMe3 with two equivalents of L1H-L3H gave {[ONER1,R2]2AlMe} (1–3), while reaction of Et2AlCl with two equivalents of L1H and L3H afforded {[ONER1,R2]2AlCl} (4 and 5) as monometallic complexes. The catalytic activity of complexes 1–3 toward ring-opening polymerization (ROP) of ε-caprolactone was assessed. These complexes are more active than analogous Zn complexes for this reaction but less active than the Zn analogues for ROP of rac-lactide. Characteristics of the polymer as well as polymerization kinetics and mechanism were studied. Polymer end-group analyses were achieved using 1H NMR spectroscopy and MALDI-TOF MS. Eyring analyses were performed, and the activation energies for the reactions were determined, which were significantly lower for 1 and 2 compared with 3. This could be for several reasons: (1) the methylamine (NMe) group of 1 and 2, which is a stronger base than the ether (O) group of 3, might activate the incoming monomer via noncovalent interactions, and/or (2) the ether group is able to temporarily coordinate to the metal center and blocks the vacant coordination site toward incoming monomer, while the amine cannot do this. Preliminary studies using 4 and 5 toward copolymerization of cyclohexene oxide with carbon dioxide have been performed. 4 was inactive and 5 afforded polyether carbonate (66.7% epoxide conversion, polymer contains 54.0% carbonate linkages).