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 (RNSNdipp)­Ln­(CH2SiMe3)2(THF)n (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe2, Ln = Lu (1d), n = 1) were synthesized by treatment of the ion-pairs [Ln­(CH2SiMe3)2(THF)x]­[BPh4] with equimolar amount of the ligand lithium salts (RNSNdipp)­Li­(THF)2 (NSNdipp = S­(NC6H4iPr2-2,6)2). Addition reaction between lutetium tris­(alkyl)­s, Ln­(Z)3(THF)n and NSNdipp gave the corresponding dialkyl complexes (ZNSNdipp)­Lu­(Z)2(THF)n (Z = CH2SiMe3, n = 1 (1e); Z = o-CH2C6H4NMe2, n = 0 (1f)). Deprotonation of β-imidophosphonamido ligands H–NPNdipp and H–NPNEt (NPNdipp = Ph2P­(NC6H3iPr2-2,6)2, NPNEt = PPh2(NC6H3iPr2-2,6)­(NC6H4-Et-2)) with Lu­(CH2SiMe3)3(THF)2 yielded the corresponding dialkyl complexes (NPNdipp)­Lu­(CH2SiMe3)2(THF) (2) and (NPNEt)­Lu­(CH2SiMe3)2(THF) (3). All the complexes had been structurally well-defined, and 1a, 1b, 1e, 2, and 3 were further characterized by X-ray diffraction analysis where the almost planar NSN rare-earth metal unit is Cs (or pseudo Cs) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe2NH]­[B­(C6F5)4] and AliBu3, 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 mmmm > 99% have been achieved with complexes 1 depending on the electronics of the sulfur substituents to give crystalline polyisoprene with the highest Tm (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%, rr = 66%, mmmm = 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.