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

[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)₅OH ₂]^3- ion.