Side-Arm Assisted Anilido-Imine Based Rare-Earth Metal Complexes for Isoprene Stereoselective Polymerization

Anilido-imine ligands o-C(6)H(4)(NHAr(1))(CH=NAr(2)), in which Ar(1) is 2,6-diisopropylbenzyl group and Ar(2) contains fluorine (HL(1)) or methoxyl (HL(2)) group on ortho-position of phenyl substituent, were synthesized for constructing rare-earth metals based complexes of 1a–1c (HL(1) based Sc, Lu, Y) and 2a–2c (HL(2) based Sc, Lu, Y). Based on their NMR spectra and X-ray single-crystal structures, the side-arm group of -F and -OMe is identified to chelate to the corresponding central metal. The twisted angles between two planes formed by chelated heteroatoms (N, N, F for HL(1) and N, N, O for HL(2)) are observed, in which the largest dihedral angle (53.3°) for HL(1)-Y and the smallest dihedral angle (44.32°) for HL(2)-Sc are detected. After being activated by Al(i)Bu(3) and [Ph(3)C][B(C(6)F(5))(4)], these catalysts showed great activity for isoprene polymerization. Bearing the same ligand HL(1), smaller scandium based complex 1a and middle size of lutetium based 1b provided lower cis-1,4-selectivity (57.3% and 64.2%), larger yttrium complex 1c displayed high cis-1,4-selectivity (84%). Chelating by crowded HL(2), small size of scandium complex 2a provided impressive trans-1,4-selectivity (93.0%), middle lutetium based 2b displayed non-selectivity and larger yttrium complex 2c showed clear cis-1,4-selectivity (83.3%). Moreover, 2a/Al(i)Bu(3) system showed the quasi-living chain transfer capability.