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

The protonolysis reaction of β-iminophosphonamine ligand (NPN dipp = Ph 2 P(NC 6 H 3 i Pr 2 -2,6) 2 ) with one equivalent of rare-earth-metal tris(alkyl)s afforded the corresponding bis(alkyl) complexes NPN dipp Ln(CH 2 SiMe 3 ) 2 (THF) (Ln = Sc ( 1 ), Lu ( 2 ), Y ( 3 ), Er ( 4 )). The bis(4-methylbenzyl) complexes NPN dipp Ln(CH 2 Ph-4-Me) 2 (THF) (Ln = Nd ( 5 ), La ( 6 )) were prepared by treatment of the tris(4-methylbenzyl) compounds Ln(CH 2 Ph-4-Me) 3 (THF) 3 with β-iminophosphonamine ligand. The small-size rare-earth-metal-based complexes 1 – 4 upon activation with Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 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 (Nd 3+: 76.1%; La 3+: 62.9%). Replacing Al i Bu 3 by AlEt 3, the 5 and 6 systems exhibited high activity and excellent trans -1,4 selectivity for both isoprene (96.5%, 0 °C) and butadiene (92.8%, 20 °C) polymerizations.