Synthesis and Characterization of Organo-Rare-Earth Metal Monoalkyl Complexes Supported by Carbon σ‑Bonded Indolyl Ligands: High Specific Isoprene 1,4-Cis Polymerization Catalysts

A series of N-protected 3-imino-functionalized indolyl ligands 1-R-3-(R′NCH)­C8H5N [R = Bn, R′ = 2,6-iPr2C6H3 (HL1); R = CH3, R′ = 2,6-iPr2C6H3 (HL2); R = Bn, R′ = tBu (HL3)] and 1-CH3-2-(2,6-iPr2C6H3NCH)­C8H5N (HL4) was prepared via reactions of N-protected indolyl aldehydes with corresponding amines.
The C–H σ-bond metathesis followed by alkane elimination
reactions between RE­(CH2SiMe3)3(thf)2 and HL1–HL3 afforded the carbon σ-bonded indolyl-ligated
rare-earth metal monoalkyl complexes. Reactions of RE­(CH2SiMe3)3(thf)2 with 2 equiv of HL1 or HL2 gave the carbon σ-bonded indolyl-ligated rare-earth
metal monoalkyl complexes L12RECH2SiMe3 (RE = Y­(1),
Er­(2), Dy­(3)) and L22RECH2SiMe3 (RE =
Y­(5), Er(6), Dy­(7), Yb­(8)),
while reaction of Yb­(CH2SiMe3)3(thf)2 with 2 equiv of HL1 afforded
the ytterbium dialkyl complex L1Yb­(CH2SiMe3)2(thf)2 (4). Reactions of RE­(CH2SiMe3)3(thf)2 with HL3 gave
the tris­(heteroaryl) rare-earth metal complexes L33RE (RE = Y­(9), Er­(10)). In the presence of cocatalysts, the rare-earth metal
monoalkyl complexes initiated isoprene polymerization with a high
activity (90% conversion of 1000 equiv of isoprene in 25 min) producing
polymers with high regio- and stereoselectivity (1,4-cis polymers
up to 99%).