Rare-earth complexes supported by an ansa-bis(amidinate) ligand with a rigid o-phenylene linker: synthesis, structure, and catalytic activity for polymerization of cyclic esters.

A bis(amidinate) ligand with a rigid o-phenylene linker C6H4-1,2-[NC(tBu)NH(C6H5)]2 (1) was successfully employed for the syntheses of rare-earth amides 2-5 in good yields. The structures of the amides were established by X-ray diffraction analysis and revealed that the ligand framework coordinated to the lanthanide atoms in different fashions depending on the central ion size. Larger lanthanide metal ions La3+ and Nd3+ prefer to form dinuclear complexes with one linked bis(amidinate) ligand as a bridge, while the smaller sized Y3+ and Er3+ favor the formation of mononuclear complexes. These complexes exhibited good catalytic activity for ring-opening polymerization of ε-caprolactone and l-lactide and the mononuclear complexes display higher activity.