Synthesis of N-Methyl-o-phenylenediamine-Bridged Bis(phenolato) Lanthanide Alkoxides and Their Catalytic Performance for the (Co)Polymerization of rac-Butyrolactone and l-Lactide.

Abstract

A series of lanthanide alkoxo complexes supported by ONNO salalen ligands were synthesized and characterized. A one-pot reaction of LH2 (L = (2-O-C6H2-tBu2-3,5)CH═N-C6H4-N(CH3)CH2(2-O-C6H2-tBu2-3,5)) with LnCp3(THF) in a 1:1 molar ratio followed by the addition of 1 equiv of ROH (R = Bn, iPr, and CF3CH2), afforded the dimeric lanthanide alkoxo complexes [LLn(μ-OCH2Ph)]2 [Ln = Lu (1), Yb (2), Sm (3), Nd (4)], [L2Yb(μ-OiPr)]2 (5), and [L2Yb(μ-OCH2CF3)]2 (6) in good isolated yields. All these lanthanide complexes were characterized by elemental analysis and FT-IR spectroscopy. In addition, complex 1 has been characterized by NMR spectroscopy. Single-crystal X-ray diffraction analysis of complexes 1, 2, 5, and 6 showed that these lanthanide alkoxo complexes are dimeric in the solid state. Complexes 1-6 showed good activity toward the homopolymerization of rac-butyrolactone (rac-BBL) to give atactic PHB, and ionic radii of central metals have profound influence on the polymerization. The polymerization behavior of l-lactide (l-LA) initiated by complex 2 was also explored. The kinetic study revealed that the polymerizations of rac-BBL and l-LA initiated by salalen lanthanide akoxide are first order for both the monomer and the initiator concentrations. Furthermore, it was found that complexes 1 and 2 showed good activity in the copolymerization of l-LA and rac-BBL, affording gradient copolymers.