KINETIC AND SPECTROSCOPIC STUDIES ON ALPHA-CYCLODEXTRIN ROTAXANES WITH PENTACYANO(CYANOPYRIDINIUM)FERRATE(II) STOPPERS

[2]Pseudorotaxanes have been prepared by threading linear chains of th e 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 t he corresponding [2]rotaxane species by coordinating bulky [Fe(CN)(5)] (3-) end groups. The lability of the iron(II)-cyanopyridinium bonds al so permits the spontaneous rotaxane self-assembly upon cyclodextrin ad dition to the iron dimer complexes. The mechanism for this process inv olves 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 in clusion complexes; {AD-CNpyr . CD}(+) and [Fe(CN)(5){AD-CNpyr . CD}](2 -) (AD-CNpyr(+)=1-adamantan-1'-yl-3- and -4-cyanopyridinium), have bee n determined by H-1 NMR spectroscopy and ligand substitution kinetic s tudies. The rate constants for the ligand substitution reactions of th e [Fe(CN)(5)(AD-CNpyr)](2-) and [Fe(CN)(5)(CNpyr(CH2)(n)CNpyr)](-) com plexes exhibited significant diminutions in the presence of alpha- and beta-CD, owing to inclusions of the free and coordinated cationic lig ands.