Kinetic and Spectroscopic Studies on α-Cyclodextrin Rotaxanes with Pentacyano(cyanopyridinium)ferrate(II) Stoppers

Abstract

[2]Pseudorotaxanes have been prepared by threading linear chains of the type [R(CH 2 ) n R] 2+ (where R = 3- or 4-cyanopyridine and n = 9 or 10) through α-cyclodextrin (α-CD), and subsequently converted to the corresponding [2]rotaxane species by coordinating bulky [Fe(CN) 5 ] 3 - end groups. The lability of the iron(II)-cyanopyridinium bonds also permits the spontaneous rotaxane self-assembly upon cyclodextrin addition to the iron dimer complexes. The mechanism for this process involves the rate-determining dissociation of a [Fe(CN) 5 ] 3 - unit ((7 ± 1) × 10 - 2 s - 1 at 25 °C for [(NC) 5 Fe(4CNpyr(CH 2 ) 9 4CNpyr)Fe(CN) 5 ] 4 - ). The stability constants for the α- and β-CD inclusion complexes; {AD-CNpyr·CD} + and [Fe(CN) 5 {AD-CNpyr·CD}] 2 - (AD-CNpyr + = 1-adamantan-1‘-yl-3- and -4-cyanopyridinium), have been determined by 1 H NMR spectroscopy and ligand substitution kinetic studies. The rate constants for the ligand substitution reactions of the [Fe(CN) 5 (AD-CNpyr)] 2 - and [Fe(CN) 5 (CNpyr(CH 2 ) n CNpyr)] - complexes exhibited significant diminutions in the presence of α- and β-CD, owing to inclusions of the free and coordinated cationic ligands.