Inorganic Chemistry, Vol.36, No.4, 729-736, 1997
Kinetic and Spectroscopic Studies on Alpha-Cyclodextrin Rotaxanes with Pentacyano(Cyanopyridinium)Ferrate(II) Stoppers
[2]Pseudorotaxanes have been prepared by threading linear chains of the type [R(CH2)(n)R](2+) (where R=3- or 4-cyanopyridine and n=9 or 10) through alpha-cyclodextrin (alpha-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)x10(-2) s(-1) at 25 degrees C for [(NC)(5)Fe(4CNpyr(CH2)(9)4CNpyr)Fe(CN)(5)](4-)). The stability constants for the alpha- and beta-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 H-1 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(CH2)(n)CNpyr)](-) complexes exhibited significant diminutions in the presence of alpha- and beta-CD, owing to inclusions of the free and coordinated cationic ligands.
Keywords:LIGAND SUBSTITUTION-REACTIONS;DINUCLEAR COBALT(III) COMPLEXES;VIOLOGEN LINKED COMPOUNDS;BETA-CYCLODEXTRIN;CHAIN-LENGTH;PENTACYANOFERRATE(II) COMPLEXES;STEREOSELECTIVE FORMATION;EXCHANGE PROPERTIES;MOLECULAR SHUTTLE;ELECTRON-TRANSFER