NNN-Tridentate Pyrrolyl Rare-Earth Metal Complexes: Structure and Catalysis on Specific Selective Living Polymerization of Isoprene

The acid–base reactions of NNN-tridentate pyrrolyl ligands (HL 1: 2,5-bis((pyrrolidin-1-yl)methylene)-1 H -pyrrole; HL 2: 2,5-bis((piperidino)methylene)-1 H -pyrrole) with rare-earth metal tris(alkyl)s, Ln(CH 2 SiMe 3 ) 3 (THF) 2, afforded the corresponding bis(alkyl) complexes L 1 Ln(CH 2 SiMe 3 ) 2 (THF) x (Ln = Sc, x = 0 ( 1a ); Ln = Y, x = 1 ( 1b ); Ln = Lu, x = 1 ( 1c )), L 2 Sc(CH 2 SiMe 3 ) 2 ( 2a ), and L 2 2 Ln 2 (CH 2 SiMe 3 ) 4 (Ln = Y ( 2b ); Lu ( 2c )) in moderate to high yields. X-ray diffraction analysis revealed that the scandium complexes 1a and 2a are THF solvent-free monomers where the ligands coordinate to the Sc 3+ ion in a κ 1:κ 2 mode, while the yttrium and lutetium complexes 1b and 1c have the same ligand coordination geometry to that of the scandium complex but are one-THF solvates; complex 2b, however, is a dimer bridged by two anionic L 2 fragments that coordinate to the two yttrium ions in mixed η 5:η 5 /κ 1:κ 1 coordination modes. Upon activation with an organoborate, all these complexes initiated the controlled polymerization of isoprene. In general, complexes 2a – c, bearing ligand L 2, exhibited higher activity than the analogous complexes 1a – c, attached to the L 1 ligand. Complex 2b, in which the L 2 ligand adopts the mixed η 5 /κ 1 coordination mode, showed the highest activity and livingness mode toward the polymerization of isoprene with high cis -1,4-selectivity (94.1%), and both scandium complexes 1a and 2a exhibited high 3,4-selectivity (87%) irrespective of the ligand type; in contrast, the lutetium complexes initiated the atactic isoprene polymerization. The influences of thell ligand structural factors, the coordination solvent, and the central metal ion on the catalytic activity and selectivity are discussed.