Kinetic and spectroscopic studies on α- and β-cyclodextrin rotaxanes with (µ-N,N′- bis(4-pyridinylmethylene)-α,ω-alkanediimine)bis[pentacyanoferrate(II)] threads

Abstract

[2]Pseudorotaxanes have been prepared by threading N,N′-bis(4-pyridinylmethylene)-1,2-ethanediimine (L2), -1,4-butanediimine (L4), and -1,6-hexanediimine (L6) ligands through α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD), and have subsequently been converted to the corresponding [2]rotaxane species by coordinating bulky [Fe(CN)5]3– end groups. The stability constants for the [2]pseudorotaxanes were determined by 1H NMR chemical shift titrations and increase with the polymethylene chain length n. The rate constants for both the formation of the [Fe(CN)5(Ln)]3– complexes from the [Fe(CN)5OH2]3– ion and Ln, and the rate constants for the dissociation of Ln from the metal complexes, exhibit significant diminutions in the presence of α- and β-CD, owing to inclusions of the free and coordinated ligands, respectively. The lability of the iron(II)–pyridine bonds also permits the spontaneous self-assembly of the [2]rotaxane upon the addition of cyclodextrin to the iron dimer complexes. The mechanism for this process involves the rate-determining dissociation of a [Fe(CN)5]3– unit from [(NC)5Fe(Ln)Fe(CN)5]6–, followed by CD inclusion of the Ln ligand to form a semirotaxane, and subsequent recomplexation by the [Fe(CN)5OH2]3– ion. Key words: cyclodextrins, rotaxanes, pentacyanoferrate(II), ligand substitution, kinetics.