Fluorenyl modified N-heterocyclic carbene ligated rare earth metal bis(alkyl) complexes, (Flu-NHC)Ln(CH 2 SiMe 3 ) 2 (Flu-NHC ) (C 13 H 8 CH 2 CH 2 (NCHCCHN)C 6 H 2 Me 3 -2,4,6); Ln ) Sc (1a); Ln ) Y (1b); Ln ) Ho (1c); Ln ) Lu (1d)), were synthesized and fully characterized by NMR and X-ray diffraction analyses. Complexes 1b-d with the activation of Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 4 ] exhibited high activity, medium syndiobut remarkably high 3,4-regio-selectivity, and the unprecedented livingness for the polymerization of isoprene. Such distinguished catalytic performances could be maintained under various monomer-to-initiator ratios (500-5000) and broad polymerization temperatures (25-80°C). The resultant polymers are crystalline, having syndiotactically enriched (racemic enchainment triad rr ) 50%, pentad rrrr ) 30%) 3,4-regulated (99%) microstructure and high glass-transition temperatures (40-49°C). In contrast, complex 1a was almost inert, while complexes bearing indenyl modified N-heterocyclic carbene moiety, (Ind-NHC)Ln(CH 2 SiMe 3 ) 2 (Ind-NHC ) C 9 H 6 CH 2 CH 2 (NCHCCHN)C 6 H 2 Me 3 -2,4,6; Ln ) Sc (2a); Ln ) Y (2b); Ln ) Ho (2c); Ln ) Lu (2d)), also showed low activity or selectivity. These differences in catalytic performance could be attributed mainly to the electronics and spacial sterics of the metal center of these precursors.

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)-LnBr 2 (THF) ((PBNHC) = 2,6-(2,4,6-Me 3 C 6 H 2 NCHCHNCCH 2 ) 2 C 6 H 3 ; 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 AlR 3 (R = Me, Et, i Bu) and [Ph 3 C] þ [B(C 6 F 5 ) 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 AlR 3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H) 2 Al i Bu 2 ] þ was identified as the active species according to NMR spectroscopic analysis.