Isoprene Polymerization with Iminophosphonamide Rare-Earth-Metal Alkyl Complexes: Influence of Metal Size on the Regio- and Stereoselectivity

The protonolysis reaction of beta-iminophosphonamine ligand (NPNdipp = Ph2P((NC6H3Pr2)-Pr-i-2,6)(2)) with one equivalent of rare-earth-metal tris(alkyl)s afforded the corresponding bis(alkyl) complexes NPN(dipp)Ln-(CH2SiMe3)(2)(THF) (Ln = Sc (1), Lu (2), Y (3), Er (4)). The bis(4-methylbenzyl) complexes NPN(dipp)Ln(CH2Ph-4Me)(2)(THF) (Ln = Nd (5), La (6)) were prepared by treatment of the tris(4-methylbenzyl) compounds Ln(CH2Ph-4-Me)(3)(THF)(3) with beta-iminophosphonamine ligand. The small-size rare-earth-metal-based complexes 1-4 upon activation with (AlBu3)-Bu-i and [Ph3C] [B(C6F5)(4)] showed high 3,4-selectivities up to 98.1% for isoprene polymerization. When the larger size rare-earth-metal-based 4-methylbenzyl complexes 5 and 6 were employed instead, moderate 3,4-selectivities were obtained since the opening coordination environment facilitated the 1,4-enchainment (Nd3+: 76.1%; La3+: 62.9%). Replacing (AlBu3)-Bu-i by AlEt3, the 5 and 6 systems exhibited high activity and excellent trans-1,4 selectivity for both isoprene (96.5%, 0 degrees C) and butadiene (92.8%, 20 C) polymerizations.