Rare-Earth Metal Alkyl Complexes Featuring Bridged Bis(β-Diketiminato) Ligand: Synthesis, Structure, and Catalytic Activity for Intramolecular Hydroamination Reaction

Abstract

A series of rare-earth metal alkyl complexes containing chiral cyclohexyl-bridged bis(β-diketiminate) ligands have been synthesized, with a general chemical formula of {Cy[NC (Me)CHC (Me)NAr]₂}RECH₂SiMe₃ [Cy=(1R, 2R)-(-)-1,2-cyclohexyl, Ar=2,6-ⁱPr₂C₆H₃, RE=Dy(1), Er(2), Yb(3), Y(4)]. These compounds were prepared in good yields by the reaction of RE[CH₂SiMe₃]₃(THF)₂ with Cy[NHC (Me)CHC (Me)NAr]₂ (H₂L). Comprehensive characterizations of all compounds were achieved through spectroscopic methods and elemental analysis. The structures of Compounds 1–4 were determined by single-crystal x-ray diffraction analysis, and Compound 4 was further characterized by hydrogen-1 (¹H) NMR and carbon-13 (¹³C) NMR spectroscopy. The catalytic performance of these complexes was investigated, and their ability to catalyze the hydroamination/cyclization reaction of aminoalkenes to afford the corresponding cyclic amines was proven. The resulting heterocyclic compounds were predominantly Markovnikov addition products. The catalytic efficiency of different catalysts was not significantly affected by central metal, and over 90% conversion could be achieved with a catalyst loading as low as 3%. However, the catalytic hydroamination/cyclization reaction to six-membered ring products was found to be more challenging compared to the formation of five-membered rings.