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

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The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ligands (RNSN dipp )Ln(CH 2 SiMe 3 ) 2 (THF) n (R = Ph, Ln = Lu ( 1a ), n = 1, Y ( 1b ), n = 2, Sc ( 1c ), n = 1; R = PhNMe 2, Ln = Lu ( 1d ), n = 1) were synthesized by treatment of the ion-pairs [Ln(CH 2 SiMe 3 ) 2 (THF) x ][BPh 4 ] with equimolar amount of the ligand lithium salts (RNSN dipp )Li(THF) 2 (NSN dipp = S(NC 6 H 4 i Pr 2 -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) n (Z = CH 2 SiMe 3, n = 1 ( 1e ); Z = o -CH 2 C 6 H 4 NMe 2, n = 0 ( 1f )). Deprotonation of β-imidophosphonamido ligands H–NPN dipp and H–NPN Et (NPN dipp = Ph 2 P(NC 6 H 3 i Pr 2 -2,6) 2, NPN Et = PPh 2 (NC 6 H 3 i Pr 2 -2,6)(NC 6 H 4 -Et-2)) with Lu(CH 2 SiMe 3 ) 3 (THF) 2 yielded the corresponding dialkyl complexes (NPN dipp )Lu(CH 2 SiMe 3 ) 2 (THF) ( 2 ) and (NPN Et )Lu(CH 2 SiMe 3 ) 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 C s (or pseudo C s ) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe 2 NH][B(C 6 F 5 ) 4 ] and Al i Bu 3, 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 T 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%, 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.