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 [(<i>κ</i>³_NCN-<b>L</b>^<b>1</b>)RECl(THF)(μ-Cl)]₂ (RE = Y(<b>1</b>), Dy(<b>2</b>), Gd(<b>3</b>)) (<b>L</b>^<b>1</b> = 1-(2-N-C₅H₁₀NCH₂CH₂)-3-(2,6-^<i>i</i>Pr₂C₆H₃N=CH)C₈H₄N) in dimeric form, and (<i>κ</i>³_NCO-<b>L</b>^<b>2</b>)RECl₂(THF)₂ (RE = Lu(<b>4</b>), Yb(<b>5</b>), Er(<b>6</b>), Y(<b>7</b>), Dy(<b>8</b>), Gd(<b>9</b>)) (<b>L</b>^<b>2</b> = 1-CH₃OCH₂CH₂-3-(2,6-^<i>i</i>Pr₂C₆H₃N=CH)C₈H₄N) in monomeric form bearing the indol-2-yl electrophilic carbene-based nonsymmetric NCN and NCO pincer ligands, were precisely synthesized. DFT calculations and ¹³C{¹H} 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 ([Ph₃C][B(C₆F₅)₄], B(C₆F₅)₃, and [PhNHMe₂][B(C₆F₅)₄]) and alkylaluminum compounds AlR₃ (R = Me, Et, ^<i>i</i>Bu). They also exhibit high catalytic activity in isoprene polymerization, yielding polymers with high 1,4-<i>cis</i>-selectivity (up to 99.6%) and ultrahigh number-average molecular weights (<i>M</i>_n up to 144.85 × 10⁴ 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.