Well-Defined Indol-2-yl Electrophilic Carbene-Based NCN and NCO Pincer-Ligated Rare-Earth Metal Chlorides as Precatalysts for the Synthesis of 1,4- <i>cis</i> -Polyisoprenes with Ultrahigh Molecular Weights

The rare-earth metal electrophilic carbenes are extremely active and difficult to synthesize because the rare-earth metal ion has no d-electron to donate to the empty p-orbital of the electrophilic carbene to form π-backbonding. The catalysis of this kind of complex is rarely developed. Here, novel rare-earth metal chlorides [( κ 3 NCN - L 1 )RECl(THF)(μ-Cl)] 2 (RE = Y( 1 ), Dy( 2 ), Gd( 3 )) ( L 1 = 1-(2-N-C 5 H 10 NCH 2 CH 2 )-3-(2,6- i Pr 2 C 6 H 3 N=CH)C 8 H 4 N) in dimeric form, and ( κ 3 NCO - L 2 )RECl 2 (THF) 2 (RE = Lu( 4 ), Yb( 5 ), Er( 6 ), Y( 7 ), Dy( 8 ), Gd( 9 )) ( L 2 = 1-CH 3 OCH 2 CH 2 -3-(2,6- i Pr 2 C 6 H 3 N=CH)C 8 H 4 N) in monomeric form bearing the indol-2-yl electrophilic carbene-based nonsymmetric NCN and NCO pincer ligands, were precisely synthesized. DFT calculations and 13 C{ 1 H} NMR results provide evidence that the indol-2-yl carbon of the ligands exhibits an electrophilic carbene character. These complexes serve as precatalysts for isoprene polymerization, featuring general compatibility with any combination of the common cocatalysts ([Ph 3 C][B(C 6 F 5 ) 4 ], B(C 6 F 5 ) 3, and [PhNHMe 2 ][B(C 6 F 5 ) 4 ]) and alkylaluminum compounds AlR 3 (R = Me, Et, i Bu). They also exhibit high catalytic activity in isoprene polymerization, yielding polymers with high 1,4- cis -selectivity (up to 99.6%) and ultrahigh number-average molecular weights ( M n up to 144.85 × 10 4 g·mol –1 ). Additionally, the results reveal that catalyst activity is determined by the substituents of the ligands, coordinated THF, and coordination modes of the complexes.