Complexation in Pseudorotaxanes Based on α-Cyclodextrin and Different α,ω-Diaminoalkanes by NMR Diffusion Measurements

Abstract

The interactions of 1,4-diaminobutane (1), 1,6-diaminohexane (2), 1,8-diaminooctane (3), 1,10-diaminodecane (4), and 1,12-diaminododecane (5) with α-cyclodextrin (α-CD) were studied in aqueous solutions by NMR diffusion measurements before and after protonation. The correlation between the association constant and the length of the alkyl chain of the diamine unit was studied. The assumption that protonation on the amino groups can be used as a stopper and, as a result, to convert the pseudorotaxanes into rotaxanes was tested. In addition, other factors that can affect the pseudorotaxane stability, such as the effects of temperature, were tested. On the basis of these measurements, the following conclusions could be reached: (1) The association constant increases with the increase in the alkyl chain length. (2) For the salts (2b−5b), both in neutral and in acidic solutions, the binding constants increase as the number of CH₂ units increases. (3) The association constants of the complexes of the diaminoalkane salts and α-CD are lower than those of the corresponding neutral diaminoalkanes. (4) This difference between the binding constants of the diaminoalkanes and their respective salts decreases as the chain length increases. (5) By examining the effects of temperature on the ¹H NMR spectra, it was found that after addition of DCl the energy barrier for the threading−dethreading process of the salt of 5a is larger than that for the salt of 4a.