Synthesis and Characterization of Dinuclear Salan Rare-Earth Metal Complexes and Their Application in the Homo- and Copolymerization of Cyclic Esters

Four rare-earth-metal aryloxo complexes stabilized by a tetradentate Salan ligand were prepared, and their catalytic properties for the (co)polymerization of lactides and ε-caprolactone were elucidated. The proton-exchange reactions of (C 5 H 5 ) 3 Ln(THF) with the Salan ligand N, N ′-(CH 2 Ph) 2 - N, N ′-[CH 2 (2-OH-C 6 H 2 -Me 2 -3,5)] 2 (LH 2 ) in a 1:1 molar ratio, and subsequently with 1 equiv of p -methylphenol, gave the rare-earth-metal aryloxides [LLn(OC 6 H 4 -4-CH 3 )(THF) n ] 2 [ n = 0 and Ln = Y ( 1 ), Sm ( 2 ), and Nd ( 3 ); n = 1 and Ln = La ( 4 )] in good isolated yields. These complexes were fully characterized by elemental analysis, IR, and NMR spectroscopy (for complexes 1 and 4 ). Solid-state structures of complexes 1 – 4 were confirmed by single-crystal X-ray diffraction analysis. Complexes 1 – 4 have dinuclear solid-state structures, with a Ln 2 O 2 core bridging the Salan ligands. The coordination geometry around each of the metals is a slightly distorted octahedron in complexes 1 – 3, whereas it is a capped trigonal prism in complex 4 . It was found that complexes 1 – 4 can initiate efficiently the homopolymerization of l -lactide ( l -LA) and rac -lactide ( rac -LA) at 30 °C in tetrahydrofuran. The increasing activity of these complexes is in agreement with increasing ionic radii. A kinetic study revealed that seven-coordinated lanthanum complex 4 is more active for rac -LA polymerization compared with l -LA. A further study revealed that complex 4 was also an efficient initiator for the random copolymerization of l -LA and ε-caprolactone with the simultaneous addition of these two monomers, and the T g values of the copolymers obtained increase linearly from −30.2 to +38.3 °C with an increase of the percentage of LA units. A mechanism study revealed that transesterification plays a crucial role in the formation of a random copolymer.