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

Abstract

A series of aluminum methyl and chloride complexes bearing 2­(<i>N</i>-piperazinyl-<i>N</i>′-methyl)-2-methylene-4-R′-6-R-phenolate or 2­(<i>N</i>-morpholinyl)-2-methylene-4-R′-6-R-phenolate ([ONE^R1,R2]-) {[R¹ = ^<i>t</i>Bu, R² = Me, E = NMe (<b>L1</b>); R¹= R² = ^<i>t</i>Bu, E = NMe (<b>L2</b>); R¹ = R² = ^<i>t</i>Bu, E = O (<b>L3</b>)} 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 <b>L1</b>H-<b>L3</b>H gave {[ONE^R1,R2]₂AlMe} (<b>1</b>–<b>3</b>), while reaction of Et₂AlCl with two equivalents of <b>L1</b>H and <b>L3</b>H afforded {[ONE^R1,R2]₂AlCl} (<b>4</b> and <b>5</b>) as monometallic complexes. The catalytic activity of complexes <b>1</b>–<b>3</b> 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 <i>rac</i>-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 <b>1</b> and <b>2</b> compared with <b>3</b>. This could be for several reasons: (1) the methylamine (NMe) group of <b>1</b> and <b>2</b>, which is a stronger base than the ether (O) group of <b>3</b>, 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 <b>4</b> and <b>5</b> toward copolymerization of cyclohexene oxide with carbon dioxide have been performed. <b>4</b> was inactive and <b>5</b> afforded polyether carbonate (66.7% epoxide conversion, polymer contains 54.0% carbonate linkages)