Rare-earth metal complexes bearing electrophilic and nucleophilic carbon centres and their unique reactivity patterns towards pyridine derivatives

Abstract

The rare-earth metal dialkyl complexes (κ²-L¹)RE(CH₂SiMe₃)₂·(THF)₂ [RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e)] (L¹ = 1-(2-N-C₅H₁₀NCH₂CH₂)-3-(2,6-ⁱPr₂C₆H₃N[double bond, length as m-dash]CH)-C₈H₄N) and the rare-earth metal monoalkyl complexes (κ²-L¹)₂RE(CH₂SiMe₃)·(THF) _n [n = 0, RE = Lu(2a), Yb(2b); n = 1, Er(2c), Y(2d), Dy(2e)], (κ²-L²)₂RE(CH₂SiMe₃)·THF [RE = Yb(3a), Er(3b), Y(3c), Dy(3d), Gd(3e)] (L² = 1-(2-N-C₅H₁₀NCH₂CH₂)-3-(AdN[double bond, length as m-dash]CH)-C₈H₄N) (Ad = adamantyl, C₁₀H₁₅) have been synthesized and fully characterized. These complexes feature chelate ligands having a conjugated system (-C[double bond, length as m-dash]C-C[double bond, length as m-dash]N) with an sp² carbon, which enables both electrophilic and nucleophilic carbon centres to be directly connected to the highly electrophilic rare-earth metal ions. The reactions of these complexes with different pyridine derivatives have been systematically investigated with the discovery of reactivity patterns distinct from those of previously reported transition metal complexes. These unusual reactivity patterns include consecutive C-H activation/1,1-migratory insertion/C-N and C-H activation, C-H activation/1,1-migratory insertion/C-N bond activation, C-H activation/1,1-migratory insertion, and homolytic redox reactions. DFT calculation results together with experimental evidences support the key mechanistic proposal that the indol-2-yl carbon of the ligands exhibits electrophilic carbene character accounting for the subsequent 1,1-migratory insertion reaction after C-H activation.