Dehydrogenation of secondary amines: Synthesis, and characterization of rare-earth metal complexes incorporating imino- or amido-functionalized pyrrolyl ligands

摘要

The dehydrogenation of pyrrolyl-functionalized secondary amines initiated by rare-earth metal amides was systematically studied. Reactions of the rare-earth metal amides (Me_3Si)_2N_3RE(μ-Cl) Li(THF)_3 with pyrrolyl-functionalized secondary amines 2- ~tBuNHCH_2-5-R-C_4H_2NH (R = H (1), R = ~tBu (2)) led to dehydrogenation of the secondary amines with isolation of imino-functionalized pyrrolyl rare-earth metal complexes 2- ~tBuNCH-5-R-C_4H_2N_2REN(SiMe _3)_2 (R = H, RE = Y (3a), Dy (3b), Yb (3c), Eu (3d); R = tBu, RE = Y (4a), Dy (4b), Er (4c)). The mixed ligands erbium complex 2-~tBuNCH2-5-tBu-C_4H _2NEr2-~tBuNCH-5-~tBuC_4H _2N_2ClLi_2(THF) (4c′) was isolated in a short reaction time for the synthesis of complex 4c. Reaction of the deuterated pyrrolyl-functionalized secondary amine 2-(~tBuNHCHD)C _4H_3NH with yttrium amide (Me_3Si) _2N_3Y(μ-Cl)Li(THF)_3 further proved that pyrrolyl-amino ligands were transferred to pyrrolyl-imino ligands. Treatment of 2-(~tBuNHCH_2)C_4H_3NH (1) with excess (Me_3Si)_2NLi gave the only pyrrole deprotonated product {η~5:η~2:η~1-2-(~tBuNHCH_2)C_4H_3NLi _2N(SiMe_3)_2}_2 (5), indicating that LiN(SiMe_3)_2 could not dehydrogenate the secondary amines to imines and rare-earth metal ions had a decisive effect on the dehydrogenation. The reaction of the rare-earth metal amides (Me _3Si)_2N_3RE(μ-Cl)Li(THF)_3 with 1 equiv. of more bulky pyrrolyl-functionalized secondary amine 2-(2,6- ~iPr_2C_6H_3)NHCH_2(C _4H_3NH) (6) in toluene afforded the only amine and pyrrole deprotonated dinuclear rare-earth metal amido complexes {(μ- η~5:η~1):η~1-2-(2,6- ~iPr_2C_6H_3)NCH_2C _4H_3NLnN(SiMe_3)_2}_2 (RE = Nd (7a), Sm (7b), Er (7c)), no dehydrogenation of secondary amine to imine products were observed. On the basis of experimental results, a plausible mechanism for the dehydrogenation of secondary amines to imines was proposed.