Pyrrolide-Supported Lanthanide Alkyl Complexes. Influence of Ligands on Molecular Structure and Catalytic Activity toward Isoprene Polymerization

The N,N -bidentate ligands 2-{( N -2,6-R)iminomethyl)}pyrrole ( HL 1, R = dimethylphenyl; HL 2, R = diisopropylphenyl) have been prepared. HL 1 reacted readily with 1 equiv of lanthanide tris(alkyl)s, Ln(CH 2 SiMe 3 ) 3 (THF) 2, affording lanthanide bis(alkyl) complexes L 1 Ln(CH 2 SiMe 3 ) 2 (THF) n ( 1a, Ln = Lu, n = 2; 1b, Ln = Sc, n = 1) via alkane elimination. Reaction of the bulky ligand HL 2 with 1 equiv of Ln(CH 2 SiMe 3 ) 3 (THF) 2 gave the bis(pyrrolylaldiminato) lanthanide mono(alkyl) complexes L 2 2 Ln(CH 2 SiMe 3 )(THF) ( 2a, Ln = Lu; 2b, Ln = Sc), selectively. The N,N -bidentate ligand HL 3, 2-dimethylaminomethylpyrrole, reacted with Ln(CH 2 SiMe 3 ) 3 (THF) 2, generating bimetallic bis(alkyl) complexes of central symmetry ( 3a, Ln = Y; 3b, Ln = Lu; 3c, Ln = Sc). Treatment of the N,N,N,N -tetradentate ligand H 2 L 4, 2,2‘-bis(2,2-dimethylpropyldiimino)methylpyrrole, with equimolar Lu(CH 2 SiMe 3 ) 3 (THF) 2 afforded a C 2 -symmetric binuclear complex ( 4 ). Complexes 3a, 3b, 3c, and 4 represent rare examples of THF-free binuclear lanthanide bis(alkyl) complexes supported by non-cyclopentadienyl ligands. All complexes have been tested as initiators for the polymerization of isoprene in the presence of AlEt 3 and [Ph 3 C][B(C 6 F 5 ) 4 ]. Complexes 1a, 1b, and 3a show activity, and 1b is the most active initiator, whereas 2a, 2b, 3b, 3c, and 4 are inert. The microstructure of the resultant polyisoprene has a cis -1,4 or trans -1,4 configuration depending on the initiator applied.