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?

Dalton Transactions Pub Date: 2013-06-06 DOI: 10.1039/C3DT51003D

Abstract

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 RBTP-H ligands (CMe2PhBTP-H = 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, t-BuBTP-H = 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol and TMClBTP-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 [(RBTP)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 [{(RBTP)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-BuBTP) 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.

Graphical abstract: 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
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