Redox-Triggered Helicity Inversion in Chiral Cobalt Complexes in Combination with H+ and NO3– Stimuli

Three chiral ligands with variable denticity, H2L2–H2L4, conjugated by N,N′-ethylenebis­[N-methyl-(S)-alanine] and an ortho-heterosubstituted aromatic amine, were newly synthesized as analogues of previously reported H2L1. Four contracted-Λoxo cobalt­(III) complexes [Co­(L)]+ with left-handed helical structure of Λ4Δ2 configuration were prepared by one-electron oxidation of the corresponding contracted-Λred cobalt­(II) complexes [Co­(L)], which were generated from chiral ligands and Co­(ClO4)2·6H2O or Co­(CF3SO3)2·5.2H2O in the presence of an organic base. Although the prepared cobalt­(III) complexes were very inert and kinetically stable against protonation and NO3– complexation, cobalt­(III) reduction in the presence of CF3SO3H and/or Bu4NNO3 allowed immediate changing of their three-dimensional structures from the contracted-Λoxo form to the extended-Λ [Co­(H2L)­Y2]n+ (Y = solvent and/or anion, n = 0–2) form with left-handed helicity or to the extended-Δ [Co­(H2L)­(NO3)]+ form with right-handed helicity via N– to O–amide coordination switching. Both extended forms were contracted to the original Λoxo form by oxidation of the cobalt­(II) center in the presence of an organic base. Thus, redox reactions triggered dynamic helicity inversion of the chiral cobalt complexes, via multiple molecular motions consisting of relaxation/compression, extension/contraction, and helicity inversion motions in combination with deprotonation/protonation of amide linkages and NO3– anion complexation.