3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare Earth Metal Precursors: Switching of Stereo Selectivity and Mechanism

The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ligands (RNSN<sup>dipp</sup>)­Ln­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF)<sub><i>n</i></sub> (R = Ph, Ln = Lu (<b>1a</b>), <i>n</i> = 1, Y (<b>1b</b>), <i>n</i> = 2, Sc (<b>1c</b>), <i>n</i> = 1; R = PhNMe<sub>2</sub>, Ln = Lu (<b>1d</b>), <i>n</i> = 1) were synthesized by treatment of the ion-pairs [Ln­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF)<sub><i>x</i></sub>]­[BPh<sub>4</sub>] with equimolar amount of the ligand lithium salts (RNSN<sup>dipp</sup>)­Li­(THF)<sub>2</sub> (NSN<sup>dipp</sup> = S­(NC<sub>6</sub>H<sub>4</sub><sup><i>i</i></sup>Pr<sub>2</sub>-2,6)<sub>2</sub>). Addition reaction between lutetium tris­(alkyl)­s, Ln­(Z)<sub>3</sub>(THF)<sub>n</sub> and NSN<sup>dipp</sup> gave the corresponding dialkyl complexes (ZNSN<sup>dipp</sup>)­Lu­(Z)<sub>2</sub>(THF)<sub><i>n</i></sub> (Z = CH<sub>2</sub>SiMe<sub>3</sub>, <i>n</i> = 1 (<b>1e</b>); Z = <i>o</i>-CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>NMe<sub>2</sub>, <i>n</i> = 0 (<b>1f</b>)). Deprotonation of β-imidophosphonamido ligands H–NPN<sup>dipp</sup> and H–NPN<sup>Et</sup> (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>, NPN<sup>Et</sup> = PPh<sub>2</sub>(NC<sub>6</sub>H<sub>3</sub><sup><i>i</i></sup>Pr<sub>2</sub>-2,6)­(NC<sub>6</sub>H<sub>4</sub>-Et-2)) with Lu­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> yielded the corresponding dialkyl complexes (NPN<sup>dipp</sup>)­Lu­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (<b>2</b>) and (NPN<sup>Et</sup>)­Lu­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(THF) (<b>3</b>). All the complexes had been structurally well-defined, and <b>1a</b>, <b>1b</b>, <b>1e, 2</b>, and <b>3</b> were further characterized by X-ray diffraction analysis where the almost planar NSN rare-earth metal unit is <i>C</i><sub>s</sub> (or pseudo <i>C</i><sub>s</sub>) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe<sub>2</sub>NH]­[B­(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] and Al<sup><i>i</i></sup>Bu<sub>3</sub>, all these complexes exhibited high 3,4-regioselectivity (ranging from 91% to >99%) for the polymerization of isoprene. Moreover, the excellent isospecific selectivity up to <i>mmmm</i> > 99% have been achieved with complexes <b>1</b> depending on the electronics of the sulfur substituents to give crystalline polyisoprene with the highest <i>T</i><sub>m</sub> (170 °C) reported to date. The NPN-bidentate β-imidophosphonamide ligated rare-earth metal complexes provide both high syndio- and iso- 3,4-selectivities (3,4 > 99%, <i>rr</i> = 66%, <i>mmmm</i> = 96%) depending on the frameworks, steric environment and geometry of the ligands. The regio- and stereo- selective mechanisms proceeded in these systems were explicated by DFT simulation.