Synthesis, characterization and reactivity of single-site aluminium amides bearing benzotriazole phenoxide ligands: catalysis for ring-opening polymerization of lactide and carbon dioxide/propylene oxide coupling.

New aluminium complexes containing bis-BTP ligands (BTP = N,O-bidentate benzotriazole phenoxide) were synthesized and structurally characterized. Amine elimination of Al(NMe2)3 with (R)BTP-H ligands ((CMe2Ph)BTP-H = 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, (t-Bu)BTP-H = 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol and (TMCl)BTP-H = 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol) (2.0 mol equiv.) in toluene or hexane afforded the penta-coordinated single-site amidoaluminium complexes [((R)BTP)2Al(NMe2)] (R = CMe2Ph for 1; R = t-Bu for 2; R = TMCl for 3) in satisfactory yields. With the addition of H2O (0.5 molar equiv.), the hydrolysis of Al amides 2 and 3 in a mixed solvent of THF/toluene at 25 °C produced oxo-bridged bimetallic aluminium complexes [{((R)BTP)2Al}2(μ-O)] (R = t-Bu for 4 and R = TMCl for 5) in ≥70% yield. According to single crystal X-ray diffraction studies, complex 2 shows a monomeric Al(iii) amide with bis((t-Bu)BTP) ligands and one -NMe2 group, whereas alumoxane 4 is a dinuclear species, in which the bonding mode of the Al-O-Al moiety from μ2-oxo assumes a linear type. Catalysis for ring-opening polymerization of lactide (LA) and CO2/propylene oxide (PO) coupling was systematically studied. Single-site Al amide 3 is an efficient initiator for LA polymerizations with a living character; the polymerization displays a first-order dependence on the concentration of l-LA. Bimetallic BTP-ligated alumoxane 5 is an active catalyst (TOF: 120 h(-1)) for the coupling of CO2 with PO in the presence of n-Bu4NBr to give propylene carbonate under mild conditions.