Formation and structure of inclusion complexes involving α-cyclodextrin and alkyl(aqua)cobaloxime

Abstract

A series of inclusion complexes involving α-cyclodextrin (α-cd) and alkyl(aqua)cobaloximes has been synthesized for the first time and characterized by elemental analysis, one and two-dimensional 1H NMR spectroscopy. Their formation constants Ka were determined by quantitative 1H NMR methods. The crystal structures of α-cd–[Co(Hdmg)2Prn(H2O)](H2dmg = dimethylglyoxime) and its guest molecule [Co(Hdmg)2Prn(H2O)] were determined. According to both NMR and crystal structure data, 1 : 1 inclusion complexes of α-cd and alkylcobaloximes are formed, in which the alkyl groups of the guests are included into the cavity of α-cd. After formation of the inclusion complexes, variations in the bond lengths and angles around the cobalt atom, and conformation changes of the n-propyl(aqua)cobaloxime were observed. It is suggested that these changes in the guest molecule are probably due to the hydrophobic interaction between the alkyl groups and the α-cd cavities, and steric interaction between α-cd and Co(Hdmg)2. It has also been established that the stability of the inclusion complexes in aqueous solution is related to the length and the size of the alkyl groups, the Ka values varying in the order n-C5H11 Bun > Prn > Bui.