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

Abstract

The dehydrogenation of pyrrolyl-functionalized secondary amines initiated by rare-earth metal amides was systematically studied. Reactions of the rare-earth metal amides [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with pyrrolyl-functionalized secondary amines 2-^tBuNHCH₂-5-R-C₄H₂NH (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-C₄H₂N]₂REN(SiMe₃)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-C₄H₂N]Er[2-^tBuNCH-5-^tBuC₄H₂N]₂ClLi₂(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)C₄H₃NH with yttrium amide [(Me₃Si)₂N]₃Y(μ-Cl)Li(THF)₃ further proved that pyrrolyl-amino ligands were transferred to pyrrolyl-imino ligands. Treatment of 2-(^tBuNHCH₂)C₄H₃NH (1) with excess (Me₃Si)₂NLi gave the only pyrrole deprotonated product {[η⁵:η²:η¹-2-(^tBuNHCH₂)C₄H₃N]Li₂N(SiMe₃)₂}₂ (5), indicating that LiN(SiMe₃)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 [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with 1 equiv. of more bulky pyrrolyl-functionalized secondary amine 2-[(2,6-ⁱPr₂C₆H₃)NHCH₂](C₄H₃NH) (6) in toluene afforded the only amine and pyrrole deprotonated dinuclear rare-earth metal amido complexes {(μ–η⁵:η¹):η¹-2-[(2,6-ⁱPr₂C₆H₃)NCH₂]C₄H₃N]LnN(SiMe₃)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.