THE SELF-ASSEMBLY OF ALPHA-CYCLODEXTRIN ROTAXANES OF MU-(1,1''-(ALPHA,OMEGA-ALKANEDIYL)BIS-(4,4-BIPYRIDINIUM))BIS[PENTACYANOFERRATE-(II)] COMPLEXES

The rapid, quantitative complexation reactions of labile [Fe(CN)5-OH2]3- ions ions with alpha-cyclodextrin (alpha-CD) threaded 1,1''-(alpha,omega-alkanediyl)-bis(4,4'-bipyridinium) dicationic ligands yield stable, self-assembled metal rotaxane complexes, [(NC)5Fe{bpy(CH2)(n)bpy.alpha-CD}Fe(CN)5]4- (n = 8-12), in aqueous solution. The {bpy(CH2)(n)bpy.alpha-CD}2+ inclusion complexes and the metal rotaxanes have been characterized by their H-1 and C-13 NMR spectra, which display pairs of proton and carbon resonances for symmetry-related nuclei upon inclusion in the asymmetric alpha-CD cavity. The stability constants and the associated thermodynamic parameters for the {bpy(CH2)(n)bpy.alpha-CD}2+ inclusion complexes have been determined by H-1 NMR. The kinetics of the ligand substitution processes involved in the self-assembly of the metal rotaxane complexes from alpha-CD and [(NC)5Fe(bpy(CH2)(n)bpy)Fe(CN)5]4- have been measured using visible and H-1 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.alpha-CD}Fe(CN)5]-, to yield the rotaxane.