The self-assembly of α-cyclodextrin rotaxanes of μ-(1,1″-(α,ω-alkanediyl)bis-(4,4′-bipyridinium))bis[pentacyanoferrate-(II)] complexes

Abstract

The rapid, quantitative complexation reactions of labile [Fe(CN)₅-OH₂]^3- ions with α-cyclodextrin (α-CD) threaded 1,1″-(α,ω-alkanediyl)-bis(4,4′-bipyridinium) dicationic ligands yield stable, self-assembled metal rotaxane complexes, [(NC)₅Fe{bpy(CH₂) _n bpy•α-CD}Fe(CN)₅]^4- (n=8–12), in aqueous solution. The {bpy(CH₂)_nbpy•α-CD}²⁺ inclusion complexes and the metal rotaxanes have been characterized by their ¹H and ¹³C NMR spectra, which display pairs of proton and carbon resonances for symmetry-related nuclei upon inclusion in the asymmetric α-CD cavity. The stability constants and the associated thermodynamic parameters for the {bpy(CH₂) _n bpy•α-CD}²⁺ inclusion complexes have been determined by ₁H NMR. The kinetics of the ligand substitution processes involved in the self-assembly of the metal rotaxane complexes from α-CD and [(NC)₅Fe(bpy(CH₂) _n bpy)Fe(CN)₅]^4- have been measured using visible and ¹H NMR spectroscopy. The kinetic and activation parameters are consistent with a rate-limiting dissociation of a [Fe(CN)₅]^3- unit, which subsequently recomplexes with the semirotaxane species, [{bpy(CH₂) _n bpy•α-CD}Fe(CN)₅]⁻, to yield the rotaxane.