Synthesis, Characterization of Rare‐Earth Metal Chlorides Bearing Indolyl‐Based NCN Pincer Ligand and Their Catalytic Activity toward 1,4‐cis Polymerization of Isoprene

Abstract

Main observation and conclusionA novel indolyl‐based NCN pincer ligand precursor HL (HL = (1‐(2‐(CH2)4NCH2CH2)‐3‐(2,6‐iPr2C6H3N=CH)C8H5N)) was rationally and precisely designed, and successfully prepared in a high yield. Lithiation of HL with nBuLi in n‐hexane, followed by treatment with RECl3 in THF led to the isolation of rare‐earth metal chlorides in monomeric form (κ3‐L)RECl2(THF) (RE = Sc(2), Yb(3)), and dimeric form [(κ3‐L)RECl(THF)(μ‐Cl)]2 (RE = Er(4), Y(5)) bearing the novel indolyl‐based NCN pincer ligand in designed pincer ligation depending on different metal ionic radii; and the chloride bridged rare‐earth metal and lithium mixed complexes in form (κ2‐L)2RE(μ‐Cl)2Li(THF)2 (RE = Y(6), Dy(7). All complexes were fully characterized and their structures were determined. The diamagnetic complexes 2, 5 and 6 were furtherly characterized by the NMR spectroscopic method. The catalytic activity studies revealed that the central metal ions, borates, aluminum alkyls, and ligation of the central metal ions have influences on catalytic activity, and regio‐ and stereoselectivity for the polymerization of isoprene. With cooperation of the cocatalysts, the pincer‐type yttrium chloride 5 initiated the isoprene polymerization with a high activity producing polymers with high regio‐ and stereoselectivity (1,4‐cis polymers up to 99.6%), and the Mn as high as 8.76 × 105.