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^dipp)­Ln­(CH₂SiMe₃)₂(THF)_<i>n</i> (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 = PhNMe₂, Ln = Lu (<b>1d</b>), <i>n</i> = 1) were synthesized by treatment of the ion-pairs [Ln­(CH₂SiMe₃)₂(THF)_<i>x</i>]­[BPh₄] with equimolar amount of the ligand lithium salts (RNSN^dipp)­Li­(THF)₂ (NSN^dipp = S­(NC₆H₄^<i>i</i>Pr₂-2,6)₂). Addition reaction between lutetium tris­(alkyl)­s, Ln­(Z)₃(THF)_n and NSN^dipp gave the corresponding dialkyl complexes (ZNSN^dipp)­Lu­(Z)₂(THF)_<i>n</i> (Z = CH₂SiMe₃, <i>n</i> = 1 (<b>1e</b>); Z = <i>o</i>-CH₂C₆H₄NMe₂, <i>n</i> = 0 (<b>1f</b>)). Deprotonation of β-imidophosphonamido ligands H–NPN^dipp and H–NPN^Et (NPN^dipp = Ph₂P­(NC₆H₃^<i>i</i>Pr₂-2,6)₂, NPN^Et = PPh₂(NC₆H₃^<i>i</i>Pr₂-2,6)­(NC₆H₄-Et-2)) with Lu­(CH₂SiMe₃)₃(THF)₂ yielded the corresponding dialkyl complexes (NPN^dipp)­Lu­(CH₂SiMe₃)₂(THF) (<b>2</b>) and (NPN^Et)­Lu­(CH₂SiMe₃)₂(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>_s (or pseudo <i>C</i>_s) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe₂NH]­[B­(C₆F₅)₄] and Al^<i>i</i>Bu₃, 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>_m (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