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
(NPNdipp = Ph2P­(NC6H3iPr2-2,6)2) with one
equivalent of rare-earth-metal tris­(alkyl)­s afforded the corresponding
bis­(alkyl) complexes NPNdippLn­(CH2SiMe3)2(THF) (Ln = Sc (1), Lu (2),
Y (3), Er (4)). The bis­(4-methylbenzyl)
complexes NPNdippLn­(CH2Ph-4-Me)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 β-iminophosphonamine ligand.
The small-size rare-earth-metal-based complexes 1–4 upon activation with AliBu3 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 AliBu3 by AlEt3, 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.