Thiophene-NPN Ligand Supported Rare-Earth Metal Bis(alkyl) Complexes. Synthesis and Catalysis toward Highly <i>trans</i>-1,4 Selective Polymerization of Butadiene

A series of new rare-earth metal bis(alkyl) complexes [L1−3Ln(CH2SiMe3)2(THF)n] (L1 = MeC4H2SCH2NC6H4(Ph)2PNC6H2Me3-2,4,6: Ln = Sc, n = 1 (1a); Ln = Lu, n = 1 (1b); L2 = MeC4H2SCH2NC6H4(Ph)2PNC6H3Et2-2,6: Ln = Sc, n = 1 (2a); Ln = Lu, n = 1 (2b); Ln = Y, n = 1 (2c); L3 = MeC4H2SCH2NC6H4(Ph)2PNC6H3iPr2-2,6: Ln = Sc, n = 0 (3a)) and L4Sc(CH2SiMe3)2(THF) (4a) (L4 = C6H5CH2NC6H4(Ph)2PNC6H3Et2-2,6) have been prepared by reaction of rare-earth metal tris(alkyl)s with the corresponding HL1−4 ligands via alkane elimination. Complexes 1a, 1b, 2a−2c, and 4a are monomeric with a coordinating THF molecule. Each metal ion is coordinated by a NPN ligand, two trans-located alkyl groups, and a THF molecule, forming a distorted trigonal-bipyramidal geometry. Complex 3a is THF-free, adopting a distorted tetrahedron geometry. In combination with AlR3 and borate, these complexes have shown medium activity and good trans-1,4 selectivity for the polymerization of butadiene. The resultant polymer has moderate molecular weight (Mn = 10 000−18 000) with narrow molecular weight distribution (Mw/Mn < 1.6) and trans-1,4 regularity varying from 49.2% up to 91.3%. The catalyst performances are strongly dependent on the ortho substituent of the N-aryl ring and the presence of the thiophene moiety of the ligands and the type of aluminum alkyls and the lanthanide metal used. The scandium complex 2a displays the highest trans-1,4 selectivity.