Neutral and Cationic Alkyl and Amido Group 3 Metal Complexes of Amidine-Amidopyridinate Ligands: Synthesis, Structure, and Polymerization Catalytic Activity
Résumé
Alkane and amine elimination reactions between the amidine-aminopyridine proligands (NR1NNR1CMeNR2)H [R1 = R2 = Me (1a), R1 = H, R2 = Me (1b), R1 = Me, R2 = iPr (1c)] and the precursors [Ln(CH2SiMe3)3(thf)2] (Ln = Sc and Y) and [Ln{N(SiHMe2)2}3(thf)x] (Ln = Sc, Y, and La) have been investigated. Bulky 1c and [Ln(CH2SiMe3)3(thf)2] (Ln = Sc and Y) selectively afforded dialkyl complexes 2c-Sc and 2c-Y. Yttrium and lanthanum diamido complexes 3a-Y, 3b-Y, 3c-Y, and 3c-La were synthesized and isolated in high yields; however, similar aminolysis reactions to prepare the scandium analogues were unsuccessful. Detailed 29Si NMR spectroscopic studies combined with X-ray crystallography revealed the presence of β-Si–H···Ln agostic interactions in the diamido complexes. Cationic monoamido complex 4a-Y was obtained from the reaction of 3a-Y with [Et3NH]+[BPh4]–. Both the dialkyl and diamido complexes, activated with [Ph3C]+[B(C6F5)4]–/AliBu3 or [PhNMe2H]+[B(C6F5)4]–/AliBu3, were found to be moderately active at 60 °C in the trans-1,4-selective polymerization of isoprene, and the diamido complexes promoted the ring-opening polymerization of rac-lactide with moderate activities.