Anilido-oxazoline-ligated rare-earth metal complexes: synthesis, characterization and highly cis-1,4-selective polymerization of isoprene

Abstract

Anilido-oxazoline-ligated rare-earth metal dialkyl complexes have been synthesized and structurally characterized. The complexes exhibited strong fluorescence emissions and good catalytic performance on isoprene polymerization with high cis-1,4-selectivity. The treatment of anilido-oxazoline precursors ortho-C₆H₄[NH(2,6-R¹₂C₆H₃)][CNC(R²,R³)CH₂O] (R¹ = R² = R³ = Me (HL¹); R¹ = ⁱPr, R² = R³ = Me (HL²); R¹ = R² = ⁱPr, R³ = H (HL³); and R¹ = ⁱPr, R² = R³ = H (HL⁴)) with an equimolar amount of Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Sc, Y) afforded rare-earth metal complexes L^1–4-Ln(CH₂SiMe₃)₂(THF)_n (L¹-Sc (1), n = 1; L²-Sc (2), n = 0; L¹-Y (3), n = 1; L²-Y (4), n = 1; L³-Y (5), n = 1; and L⁴-Y (6), n = 1) in good yields. The complexes are stable in both the solid state and solution. Single crystal X-ray diffraction study showed that complexes 1, 3 and 4 exhibit a distorted trigonal bipyramidal configuration, while complex 2 is pseudo-tetrahedral without coordinated THF. The luminescence properties of complexes 1–4 were investigated and the emission maxima were found in the range of 465–477 nm. DFT and TD-DFT studies were carried out to explore their characteristic electronic structures and gain insight into their optical properties. Upon activation with organic borates, the reported complexes exhibited high activity and cis-1,4-selectivity for isoprene polymerization. The nature of the central metal and substituent groups in oxazoline have an influence on the cis-1,4-selectivity.