Ferrocene-Appended Cyclodextrins. The Effects of Temperature, Organic Solvent, Length of Spacer, and Cavity Size on the Complexation Behavior

Abstract

Abstract Inclusion phenomena of cyclodextrins (CyDs) bearing a ferrocene moiety, 1—4, which have different cavity sizes and spacer lengths, were investigated by circular dichroism (CD) spectroscopy in pure ethylene glycol (EG) and 20% EG aqueous solution. β-CyD derivatives, 1 and 3, and a γ-CyD derivative, 4, form intramolecular complexes, including the ferrocene moiety in the CyD cavity with orientation similar to that in corresponding native CyD–ferrocene complexes, while another γ-CyD derivative, 2, which has no spacer alkyl chain, takes various conformations. The host–guest binding constants of 1—4 for several guests revealed that the ferrocene moiety of 1—4 depresses the guest binding, acting as an intramolecular inhibitor in guest binding. In particular, the depression was remarkable for 3 and 4 and the spacer alkyl chain was suggested to stabilize the intramolecular complexes. The ferrocene modified CyDs exhibit similar trends in guest selectivity to those of native CyDs. The effects of EG content on the binding constants of 1 and 2 revealed that hydrophobic effect operated in the guest accommodation. Thermodynamic parameters for the binding processes of 1 and 2 for 1-adamantanol were obtained from the temperature variations in binding constants, the values being −21.9 kJ mol−1 for ΔH° and 2.76 J mol−1 K−1 for ΔS° for 1 and −25.1 kJ mol−1 for ΔH° and −36.1 J mol−1 K−1 for ΔS° for 2. These values indicated that the complexation of 1 and 2 for 1-adamantanol was governed both by hydrophobic effect and van der Waals interaction, and the latter was especially important in the case of 1 and 1-adamantanol.