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 <i>N</i>-protected 3-imino-functionalized indolyl ligands 1-R-3-(R′NCH)­C₈H₅N [R = Bn, R′ = 2,6-^<i>i</i>Pr₂C₆H₃ (<b>HL</b>^<b>1</b>); R = CH₃, R′ = 2,6-^<i>i</i>Pr₂C₆H₃ (<b>HL</b>^<b>2</b>); R = Bn, R′ = ^<i>t</i>Bu (<b>HL</b>^<b>3</b>)] and 1-CH₃-2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­C₈H₅N (<b>HL</b>^<b>4</b>) was prepared via reactions of <i>N</i>-protected indolyl aldehydes with corresponding amines. The C–H σ-bond metathesis followed by alkane elimination reactions between RE­(CH₂SiMe₃)₃(thf)₂ and <b>HL</b>^<b>1</b><b>–HL</b>^<b>3</b> afforded the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes. Reactions of RE­(CH₂SiMe₃)₃(thf)₂ with 2 equiv of <b>HL</b>^<b>1</b> or <b>HL</b>^<b>2</b> gave the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes <b>L</b>^<b>1</b>₂RECH₂SiMe₃ (RE = Y­(<b>1</b>), Er­(<b>2</b>), Dy­(<b>3</b>)) and <b>L</b>^<b>2</b>₂RECH₂SiMe₃ (RE = Y­(<b>5</b>), Er(6), Dy­(<b>7</b>), Yb­(<b>8</b>)), while reaction of Yb­(CH₂SiMe₃)₃(thf)₂ with 2 equiv of <b>HL</b>^<b>1</b> afforded the ytterbium dialkyl complex <b>L</b>^<b>1</b>Yb­(CH₂SiMe₃)₂(thf)₂ (<b>4</b>). Reactions of RE­(CH₂SiMe₃)₃(thf)₂ with <b>HL</b>^<b>3</b> gave the tris­(heteroaryl) rare-earth metal complexes <b>L</b>^<b>3</b>₃RE (RE = Y­(<b>9</b>), Er­(<b>10</b>)). 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%)