Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C-N Bond Cleavage Reactions Induced by C-H Bond Activations

Abstract

Five bis(quinolylmethyl)-(1H-indolylmethyl)amine (BQIA) compounds, that is, {(quinol-8-yl-CH₂ )₂ NCH₂ (3-Br-1H-indol-2-yl)} (L¹ H) and {[(8-R³ -quinol-2-yl)CH₂ ]₂ NCH(R² )[3-R¹ -1H-indol-2-yl]} (L^2-5 H) (L² H: R¹ =Br, R² =H, R³ =H; L³ H: R¹ =Br, R² =H, R³ =iPr; L⁴ H: R¹ =H, R² =CH₃ , R³ =iPr; L⁵ H: R¹ =H, R² =nBu, R³ =iPr) were synthesized and used to prepare calcium complexes. The reactions of L^1-5 H with silylamido calcium precursors (Ca[N(SiMe₂ R)₂ ]₂ (THF)₂ , R=Me or H) at room temperature gave heteroleptic products (L^{1, 2} )CaN(SiMe₃ )₂ (1, 2), (L^{3, 4} )CaN(SiHMe₂ )₂ (3 a, 4 a) and homoleptic complexes (L^{3, 5} )₂ Ca (D3, D5). NMR and X-ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C-Si, Ca⋅⋅⋅H-Si or Ca⋅⋅⋅H-C agostic interactions. Unexpectedly, calcium complexes ((L^3-5 )CaN(SiMe₃ )₂ ) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C-N bond cleavage processes as a consequence of intramolecular C-H bond activation, leading to the exclusive formation of (E)-1,2-bis(8-isopropylquinol-2-yl)ethane.

Keywords: C−H activation; C−N bond cleavage; N ligands; calcium complexes; ring-opening polymerization.