Synthesis and Reactivity of Rare-Earth-Metal Monoalkyl Complexes Supported by Bidentate Indolyl Ligands and Their High Performance in Isoprene 1,4-cis Polymerization

A series of novel rare-earth-metal monoalkyl complexes incorporating partially rotation restricted [N,N]-bidentate indolyl ligands were synthesized and characterized, and their reactivities and catalytic activities were investigated. Treatment of [RE­(CH₂SiMe₃)₃(thf)₂] with 1 equiv of 2-[(<i>N</i>-2,6-diisopropylphenyl)­iminomethyl)]­indole (2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­C₈H₅NH) in toluene at room temperature afforded the rare-earth-metal monoalkyl complexes [η¹:η¹-2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­Ind]₂RE­(CH₂SiMe₃)­(thf) (Ind = indolyl; RE = Yb (<b>1</b>), Er (<b>2</b>), Y (<b>3</b>), Dy (<b>4</b>), Gd (<b>5</b>)) and the samarium complex [η¹:η¹-2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­Ind]₃Sm (<b>6</b>) via alkane elimination in good yields. Treatment of complex <b>2</b> or <b>3</b> with 1 equiv of PhSiH₃ in toluene at 80 °C for 12 h afforded the dinuclear complexes {[μ-η⁶:η¹:η¹-2-(2,6-^<i>i</i>Pr₂C₆H₃NCH₂)­Ind]­RE­[2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­Ind]}₂ (Ind = indolyl, RE = Er (<b>7</b>), Y (<b>8</b>)) in good isolated yields. Treatment of complex <b>2</b> or <b>3</b> with 1 equiv of amidine (2,6-^<i>i</i>Pr₂C₆H₃)­NCHNH­(2,6-^<i>i</i>Pr₂C₆H₃) in toluene produced the corresponding complexes [η¹:η¹-2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)­C₈H₅N]₂RE­[(2,6-^<i>i</i>Pr₂C₆H₃)­NCHN­(2,6-^<i>i</i>Pr₂C₆H₃)] (RE = Er (<b>9</b>), Y (<b>10</b>)) possessing the amidinate ligand [(2,6-^<i>i</i>Pr₂C₆H₃N)₂CH]⁻. The molecular structures of all complexes were determined by X-ray crystallography. The monoalkyl complexes <b>1</b>–<b>5</b> were tested as isoprene polymerization initiators. Among the complexes investigated, the optimum combination <b>5</b>/Al^<i>i</i>Bu₃/[Ph₃C]­[B­(C₆F₅)₄] displayed a high catalytic activity in isoprene polymerization, producing polymers with an extremely high 1,4-cis selectivity (up to 99%), a high number-average molecular weight (<i>M</i>_n = 7.2 × 10⁵), and a narrow molecular weight distribution (PDI = 1.34) at an isoprene to initiator molar ratio of 6000:1