Ternary inclusion complexes of cyclodextrins with spin-labeled indoles and hydrocarbons. The formation of 2: 1: 2 complexes and partial dissociation of spin-labeled guests in the presence of a hydrocarbon liquid phase

Abstract

The interaction of ternary complexes γ-or β-cyclodextrin (CD)-spin-labeled indole (pyrrolidine (1) or piperidine (2) derivative)-hydrocarbon (benzene or cyclohexane) with the liquid phase of these hydrocarbons gives rise to a new type of complexes (s) the ESR spectra of which are indicative of a much lower rotational mobility and its weaker temperature dependence for the spin-labeled guests compared with the initial ternary complexes (w). The formation of s-complexes is accompanied by a decrease in the proportion of the initial w-complexes. This gives rise to several isosbestic points in the ESR spectra, which indicates the formation of s-complexes from w-complexes. The rotational diffusion coefficient of 2 in s-complexes decreases 6–10-fold with respect to its value for w-complexes and the libration amplitude of 1 in the s-complexes decreases to 5–6° at 295 K. Transition between the two types of complexes occurs with characteristic times of 10³–10⁵ s and is reversible: upon the removal of the hydrocarbon, the proportion of the strongly immobilized s-signal decreases, while upon its addition, it increases again. These results indicate that the strongly immobilized ESR signals belong to 2: 1: 2 complexes, which are formed upon the insertion of the second hydrocarbon molecule into the initial ternary 2: 1: 1 w-complexes. According to PM3 quantum-chemical calculations, 2: 1: 2 complexes are stable in energy and the geometry of hydrocarbon arrangement depends on the structure of the spin-labeled guest.