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 c yclodextrins [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 aq ueous media. The binding constants of hexacyanoferrate(II) with the th ree hosts were estimated by computer simulation of the voltammetric da ta, 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 t he anion, respectively. The reduced form (-4 charge) of the guest is a lways more strongly bound to any of the positive charged aminocyclodex trins than the oxidized form (-3 charge). This finding, along with the magnitude of the Gibbs energies of complexation, suggests that the bi nding interactions are essentially electrostatic in nature. Based on t he H-1 NMR spectroscopic data, complexation seems to take place by par tial inclusion of the anionic guest inside the cavity of the CD recept ors.