CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly <i>cis</i>-1,4-Selective Polymerization of Isoprene and Active Species

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)LnBr2(THF) ((PBNHC) = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; Ln = Sc (1), Y (2), La (3), Nd (4), Sm (5), Gd (6), Dy (7), Ho (8), Tm (9), Lu (10)) have been synthesized. Upon activation with AlR3 (R = Me, Et, iBu) and [Ph3C]+[B(C6F5)4]−, complexes 2, 4, 6, 7, and 8 exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) toward the polymerization of isoprene, although complexes 1, 3, 5, 9, and 10 were inert. The selectivity was not affected by the nature of the central metal and AlR3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H)2AliBu2]+ was identified as the active species according to NMR spectroscopic analysis.