Alpha- and beta-cyclodextrin rotaxanes of mu-bis(4-pyridyl)bis[pentacyanoferrate(II)] complexes.

Abstract

The kinetics and mechanism of the self-assemblies of alpha- and beta-cyclodextrin (CD) [2]rotaxanes, [(NC)5Fe-[pyRpy.CD]Fe(CN)5]6-, containing pentacyanoferrate(II)-stoppered 4,4'-bis(pyridyl) threads pyRpy (R = -CH = CH-, -N=N-, -CH=N-N=CH-, and -C(CH3)=N-N=C(CH3)-) have been investigated in aqueous solution by using visible and 1H NMR spectroscopy. The rotaxanes may be formed rapidly by the addition of the [Fe(CN)5OH2]3- ion to the CD-included pyRpy thread or slowly by the addition of an excess of CD to the dimeric [(NC)5Fe(pyRpy)Fe(CN)5]6- complex. In the latter method, the mechanism involves a rate-determining dissociation of a [Fe(CN)5]3- center to form the monomeric complex, which subsequently includes the coordinated pyRpy in the CD cavity to yield the semirotaxane, which is rapidly recomplexed by the [Fe(CN)5OH2]3- ion, generating the [2]rotaxane. Rate and activation parameters and CD inclusion stability constants have been determined for the ligand substitution reactions involving the formations and dissociations of the semirotaxanes and rotaxanes. The extents of the decreases in the formation (kf) and dissociation (kd) rate constants upon CD inclusions of the free and coordinated ligands, respectively, are related to the natures of the CD hosts and the R linkage on the pyRpy guests. The semirotaxanes and rotaxanes exhibit significant bathochromic shifts in their visible MLCT transitions compared with the corresponding monomeric and dimeric iron complexes. A correlation between the extent of the decrease in kd and the change in the MLCT energy upon alpha-CD inclusions of [Fe(CN)5L]3-, where L is an aromatic N-heterocyclic 4-Rpy or pyRpy ligand, has been observed.