Isoprene Polymerization with Iminophosphonamide Rare-Earth-Metal Alkyl Complexes: Influence of Metal Size on the Regio- and Stereoselectivity
Abstract
The protonolysis reaction of β-iminophosphonamine ligand (NPN<sup>dipp</sup> = Ph<sub>2</sub>P(NC<sub>6</sub>H<sub>3</sub><sup><i>i</i></sup>Pr<sub>2</sub>-2,6)<sub>2</sub>) with one equivalent of rare-earth-metal tris(alkyl)s afforded the corresponding bis(alkyl) complexes NPN<sup>dipp</sup>Ln(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (Ln = Sc (<b>1</b>), Lu (<b>2</b>), Y (<b>3</b>), Er (<b>4</b>)). The bis(4-methylbenzyl) complexes NPN<sup>dipp</sup>Ln(CH<sub>2</sub>Ph-4-Me)<sub>2</sub>(THF) (Ln = Nd (<b>5</b>), La (<b>6</b>)) were prepared by treatment of the tris(4-methylbenzyl) compounds Ln(CH<sub>2</sub>Ph-4-Me)<sub>3</sub>(THF)<sub>3</sub> with β-iminophosphonamine ligand. The small-size rare-earth-metal-based complexes <b>1</b>–<b>4</b> upon activation with Al<sup><i>i</i></sup>Bu<sub>3</sub> and [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] showed high 3,4-selectivities up to 98.1% for isoprene polymerization. When the larger size rare-earth-metal-based 4-methylbenzyl complexes <b>5</b> and <b>6</b> were employed instead, moderate 3,4-selectivities were obtained since the opening coordination environment facilitated the 1,4-enchainment (Nd<sup>3+</sup>: 76.1%; La<sup>3+</sup>: 62.9%). Replacing Al<sup><i>i</i></sup>Bu<sub>3</sub> by AlEt<sub>3</sub>, the <b>5</b> and <b>6</b> systems exhibited high activity and excellent <i>trans</i>-1,4 selectivity for both isoprene (96.5%, 0 °C) and butadiene (92.8%, 20 °C) polymerizationsSimilar works
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