Orientational Isomers of Cyclodextrin Semirotaxanes and Rotaxanes With Organic and Transition Metal Complex Stoppers

The novel asymmetric dicationic ligands Quin(CH₂)₁₀tbp]²⁺, Quin(CH₂)₁₀bpy]²⁺, and Lut(CH₂)₁₀bpy]²⁺ (Quin⁺ = quinuclidinium, Lut = 3,5-lutidinium, tbp⁺ = 4-tert-butylpyridinium, and bpy⁺ = 4,4′-bipyidinium) form 2]semirotaxanes with α- and β-cyclodextrins in aqueous solution, with the cyclodextrin passage possible only over the 4-tert-pyridinium or bipyridinium end groups to yield two orientational isomers. The kinetics of the formation and dissociation of the kinetically and thermodynamically preferred orientational isomers of the Quin(CH₂)₁₀tbp]²⁺2]semirotaxane with α-CD have been investigated by ¹H NMR spectroscopy. Complexation of the free nitrogen on the 4,4′-bipyridinium end groups of the R(CH₂)₁₀bpy]²⁺ ligands by the aquapentacyanoferrate(II) ion results in the formation of the corresponding 2]rotaxanes.