Cis-1,4-Polymerization of Isoprene Catalyzed by 1,3-Bis(2-pyridylimino)isoindoline-Ligated Rare-Earth-Metal Dialkyl Complexes

The two rare-earth-metal dialkyl complexes 1 and 2 (1, Ln = Sc; 2, Ln = Lu) were obtained via the same acid-base reaction between 1,3-bis(2-pyridylimino)isoindoline (BPI) ligand and rare-earth-metal trialkyl complexes. These complexes 1 and 2 were structurally characterized by X-ray diffraction. Both in the solid state and in the solution state, the mononuclear Sc dialkyl complex 1, containing one monoanionic tridentate C₂-symmetric pincer-type BPI ligand, adopts a distorted-trigonal-bipyramidal configuration. When the Sc center of complex 1 was replaced by the larger Lu center, the intramolecular proton transfer from the isoindoline nitrogen atom to one of the imine nitrogen atoms could be observed in the tautomeric BPI ligand, which served as a monoanionic tetradentate ligand bridging two Lu centers and finally afforded the binuclear Lu dialkyl complex 2 with a cage-like symmetrical structure in the solid state. However, this binuclear Lu complex 2 could dissociate into a mononuclear structure in the solution state similar to the case for the scandium complex 1 since the same C₂ symmetry was also observed in the ¹H and ¹³C NMR spectra of the lutetium complex 2 in C₆D₆. In the presence of cocatalyst borate and AlⁱBu₃, these complexes 1 and 2 exhibited high activities (up to 1.9 × 10⁶ (g of polymer)/(mol_Ln h)) and high cis-1,4-selectivities (>99%) in the polymerization of isoprene in toluene, affording the cis-1,4-polyisoprenes with high molecular weights (M_n up to 610000 g mol^-1) and narrow to moderate molecular weight distributions (M_w/M_n = 1.26-2.08).