Synthesis and Characterization of Bidentate Rare-Earth Iminophosphorane o-Aryl Complexes and Their Behavior As Catalysts for the Polymerization of 1,3-Butadiene

O-Aryllithium complexes are easily prepared from stable aminophosphonium salts, and their coordination to rare-earth metals was studied. The ligand to metal ratio in the formed complexes was shown to depend exclusively on the substituent on the nitrogen atom of the ligand. Aryllithium derivatives 3a and 3b, exhibiting bulky groups (SiMe3 and tBu, respectively), gave monocoordinated yttrium complexes 4a-Y and 4b-Y. On the other hand, with aryllithium 3a, possessing an isopropyl at nitrogen, complexes of YIII, NdIII, and GdIII with a 2:1 ligand to metal ratio could be obtained. Finally with less hindered ligands such as 6c, featuring an n-butyl substituent, triscoordinated Y, Nd, and La complexes were accessible. X-ray crystal structures have been obtained with all three stoichiometries. These complexes were employed as catalyst precursors for 1,3-butadiene polymerization using various activators. Yttrium complexes were found ineffective, but some neodymium complexes achieved highly selective polymerization of 1,3-butadiene, giving up to 95% of 1,4-cis-polybutadiene albeit with mild activity.