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]} (L2-5 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 L1-5 H with silylamido calcium precursors (Ca[N(SiMe2 R)2 ]2 (THF)2 , R=Me or H) at room temperature gave heteroleptic products (L1, 2 )CaN(SiMe3 )2 (1, 2), (L3, 4 )CaN(SiHMe2 )2 (3 a, 4 a) and homoleptic complexes (L3, 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 ((L3-5 )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.