Electrochemical and H-1 NMR studies on the complexation of hexacyanoferrate(II) anions by aminocyclodextrins

The complexation of the hexacyanoferrate(II) anion by three modified cyclodextrins [hexakis(6-deoxy-6-amino-)-alpha-CD (1), heptakis(6-deoxy-6-amino)-beta-CD (2), and octakis(6-deoxy-6-amino)-gamma-CD (3)] has been studied by cyclic voltammetry and H-1 NMR in moderately acidic aqueous media. The binding constants of hexacyanoferrate(II) with the three hosts were estimated by computer simulation of the voltammetric data, and were found to fall in the ranges 3 x 10(5)-10(6) dm(3) mol(-1) and 2-5 x 10(3) dm(3) mol(-1) for the reduced and oxidized forms of the anion, respectively. The reduced form (-4 charge) of the guest is always more strongly bound to any of the positive charged aminocyclodextrins than the oxidized form (-3 charge). This finding, along with the magnitude of the Gibbs energies of complexation, suggests that the binding interactions are essentially electrostatic in nature. Based on the H-1 NMR spectroscopic data, complexation seems to take place by partial inclusion of the anionic guest inside the cavity of the CD receptors.