Living 3,4-polymerization of isoprene by cationic rare earth metal alkyl complexes bearing iminoamido ligands

Treatment of rare earth metal trialkyl complexes Ln(CH₂SiMe ₃)₃(THF)₂ (Ln = Sc, Lu, and Y) with 1 equiv of α-diimine ligands 2,6-R₂C₆H₃N=CH-CH= NC₆H₃R₂-2,6 (R = ⁱPr, Me) affords straightforwardly monoanionic iminoamido rare earth metal dialkyl complexes [2,6-R₂C₆H₃N-CH₂-C(CH ₂SiMe₃)=NC₆H₃R₂-2,6] Ln(CH₂SiMe₃)₂(THF) (1: Ln = Sc, R = ⁱPr; 2: Ln = Lu, R = ⁱPr; 3: Ln = Y, R = ⁱPr; 4: Ln = Sc, R = Me; 5: Ln = Lu, R = Me; 6: Ln = Y, R = Me) in 65-85% isolated yields. X-ray analyses show these complexes have decreasing steric hindrance in the coordination spheres of the metal centers in the order 1 > 2 > 3 > 4 > 5 > 6. A mechanism involving intramolecular alkyl and hydrogen migration is supported on the basis of DFT calculations to account for ligand alkylation. Activated by [Ph₃C][B(C₆F₅) ₄], all of these iminoamido rare earth metal dialkyl complexes are active for living polymerization of isoprene, with activity and selectivity being significantly dependent on the steric hindrance around the metal center to yield homopolyisoprene materials with different microstructures and compositions. The sterically crowded complexes 1-3 give a mixture of 3,4- and trans-1,4-polyisoprenes (3,4-selectivities: 48-82%, trans-1,4-selectivities: 50-17%), whereas the less sterically demanding complexes 4-6 show high 3,4-selectivities (3,4-selectivities: 90-100%). In the presence of 2 equiv of AlⁱBu₃, the complexes 1-6/activator systems exhibit higher activities and 3,4-selectivities in the living polymerization of isoprene. A similar structure-reactivity relationship in polymerization catalysis can be also observed in these ternary systems. A possible mechanism of the isoprene polymerization processes is proposed on the basis of the DFT calculations.