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

Abstract

A series of N-protected 3-imino-functionalized indolyl ligands 1-R-3-(R'N═CH)C8H5N [R = Bn, R' = 2,6-(i)Pr2C6H3 (HL(1)); R = CH3, R' = 2,6-(i)Pr2C6H3 (HL(2)); R = Bn, R' = (t)Bu (HL(3))] and 1-CH3-2-(2,6-(i)Pr2C6H3N═CH)C8H5N (HL(4)) 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 HL(1)-HL(3) afforded the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes. Reactions of RE(CH2SiMe3)3(thf)2 with 2 equiv of HL(1) or HL(2) gave the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes L(1)2RECH2SiMe3 (RE = Y(1), Er(2), Dy(3)) and L(2)2RECH2SiMe3 (RE = Y(5), Er(6), Dy(7), Yb(8)), while reaction of Yb(CH2SiMe3)3(thf)2 with 2 equiv of HL(1) afforded the ytterbium dialkyl complex L(1)Yb(CH2SiMe3)2(thf)2 (4). Reactions of RE(CH2SiMe3)3(thf)2 with HL(3) gave the tris(heteroaryl) rare-earth metal complexes L(3)3RE (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%).

Reactivity of 3-Imino-Functionalized Indoles with Rare-Earth-Metal Amides: Unexpected Substituent Effects on C–H Activation Pathways and Assembly of Rare-Earth-Metal Complexes

“…From Table 1, it is found that the Ho−C1 bond lengths, 2.551(5) and 2.652(5) Å, and the Er−C1 bond lengths, 2.538 (3) and 2.621(3) Å, are close to those of the Y−C1 bond lengths of 2.551(8) and 2.665(8) Å and the Yb−C1 bond lengths of 2.506(12) and 2.654(10) Å, 19 which is consistent with their ionic radii (Ho 3+ ≈ Y 3+ > Er 3+ > Yb 3+ ). However, Er−C1 bond lengths of complex 1b are significantly longer than the bond lengths of Er1−C(sp 2 ), 2.457(3) and 2.466(4) Å, of carbon σbonded indolyl supported rare-earth-metal alkyl complex L 2 ErCH 2 SiMe 3 (L = 1-Me-3-(2,6-iPr 2 C 6 H 3 NCH)C 8 H 5 N), 18 which might be due to the bridging μ 2 -η 1 :η 1 mode.…”

“…19 This result is different from our recent findings that the reactions of rare-earth-metal alkyls Yb(CH 2 SiMe 3 ) 3 (THF) 2 with 3-(tertbutylimino)indole 3-(tBuNCH)C 8 H 4 NH (L 1 ) produced dinuclear ytterbium complexes having indolyl ligands in μη 2 :η 1 :η 1 hapacities through alkyl insertion, while no C−H bond activation has been observed in the process. 17a We have also found that the reactions of 1-alkyl-3-imino-functionalized indoles with RE(CH 2 SiMe 3 ) 3 (THF) 2 provided the carbon σbonded indolyl-supported rare-earth-metal alkyl complexes through sp 2 C−H activation, 18 which is believed to be produced via an imino-coordination-promoted process. It has also been proposed that the activation of the α-C−H bond of pyridine 8,9 or imidazole 20 The anilido group of D then abstracted a proton from the released amine HN(SiMe 3 ) 2 to give the aniline-coordinated hexanuclear rare-earth-metal complexes 5 and lithium amide LiN(SiMe 3 ) 2 .…”

“…We also found that reactions of 2- or 3-imino-functionalized indoles with rare-earth-metal alkyls RE­(CH 2 SiMe 3 ) 3 (THF) 2 afforded rare-earth-metal alkyl complexes with different hapaticities, from which highly efficient catalysts for selective isoprene 1,4- cis polymerization were identified . Moreover, the reactions of 1-alkyl-3-imino-functionalized indoles with RE­(CH 2 SiMe 3 ) 3 (THF) 2 involving sp 2 C–H bond activation produced the rare-earth-metal complexes having carbon σ-bonded indolyl ligands, which could also function as highly efficient isoprene 1,4- cis polymerization catalysts . In our previous communication, we found that the reactions of 3-( tert- butylimino)­indole with rare-earth-metal amides [(Me 3 Si) 2 N] 3 RE­(μ-Cl)­Li­(THF) 3 produced novel tetranuclear rare-earth-metal complexes {[η 1 :(μ 2 -η 1 :η 1 ):η 1 -3-( t BuNCH)­C 8 H 4 N]­RE III 2 (μ 2 -Cl) 2 (THF)­[N­(SiMe 3 ) 2 ]­(η 1 :η 1 -[μ-η 5 :η 2 - 3-( t BuNCH)­C 8 H 5 N] 2 Li)} 2 (RE = Y, Yb) via sp 2 C–H activation, in which the unusual 1,2-dianion indolyl ligand was found to be bonded with the central metal in a novel η 1 :(μ 2 -η 1 :η 1 ) fashion .…”

“…17 Moreover, the reactions of 1-alkyl-3-imino-functionalized indoles with RE-(CH 2 SiMe 3 ) 3 (THF) 2 involving sp 2 C−H bond activation produced the rare-earth-metal complexes having carbon σbonded indolyl ligands, which could also function as highly efficient isoprene 1,4-cis polymerization catalysts. 18 In our previous communication, we found that the reactions of 3-(tertbuty lim ino )ind ol e wit h rare-eart h-met al amid es…”

Transformation of the sp2 Carbanion to Carbene with Subsequent 1,1-Migratory Insertion and Nucleophilic Substitution in Rare-Earth Metal Chemistry

“…15 It is also different from the results of reactions of 1-alkyl-3-iminofunctionalized indoles with RE(CH 2 SiMe 3 ) 3 (THF) 2 producing only the 2-indolyl complexes. 16 All complexes were characterized by spectroscopic methods and elemental analyses, and their structures were further confirmed by single-crystal X-ray crystallographic analyses. The diamagnetic complex 1a was characterized by nuclear magnetic resonance (NMR) spectroscopy.…”