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3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare Earth Metal Precursors: Switching of Stereo Selectivity and Mechanism
journal contribution
posted on 2014-08-12, 00:00 authored by Bo Liu, Lei Li, Guangping Sun, Jingyao Liu, Meiyan Wang, Shihui Li, Dongmei CuiThe 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.