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,1\1]-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-[(N-2,6-diisopropylphenyl)-iminomethyl)]indole (2-(2,6-(Pr2C6H3N)-Pr-i=CH)C8H5NH) in toluene at room temperature afforded the rare-earth-metal monoalkyl complexes [eta(1):eta(1)-2-(2,6-(Pr2C6H3N)-Pr-i=CH)Ind](2)RE(CH2SiMe3)(thf) (Ind = indolyl; RE = Yb (1), Er (2), Y (3), Dy (4), Gd (5)) and the samarium complex [eta(1):eta(1)-2-(2,6-(Pr2C6H3N)-Pr-i=CH)Ind](3)Sm (6) via alkane elimination in good yields. Treatment of complex 2 or 3 with 1 equiv of PhSiH3 in toluene at 80 degrees C for 12 h afforded the dinuclear complexes {[mu-eta(6):eta(1):eta(I)-2-(2,6-(Pr2C6H3NCH2)-Pr-i)Ind]RE[2-(2,6-(Pr2C6H3N)-Pr-i=CH)Ind]}(2) (Ind = indolyl, RE = Er (7), Y (8)) in good isolated yields. Treatment of complex 2 or 3 with 1 equiv of amidine (2,6-(Pr2C6H3)-Pr-i)N=CHNH(2,6-(Pr2C6H3)-Pr-i) in toluene produced the corresponding complexes [eta(1):eta(1)-2-(2,6-(Pr2C6H3N)-Pr-i=CH)C8H5M2RE[(2,6-(Pr2C6H3)-Pr-i)N=CHN(2,6-(Pr2C6H3)-Pr-i)] (RE = Er (9), Y (10)) possessing the amidinate ligand [(2,6-iPr(2)C(6)H(3)N)(2)CH](-). The molecular structures of all complexes were determined by X-ray crystallography. The monoalkyl complexes 1-5 were tested as isoprene polymerization initiators. Among the complexes investigated, the optimum combination 5/Al'Bu-3/[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 (M-n = 7.2 x 10(5)), and a narrow molecular weight distribution (PDI = 1.34) at an isoprene to initiator molar ratio of 6000:1.