Electrochemical study of the cyclodextrin encapsulation of a macrocyclic nickel complex

Abstract

The influence of cyclodextrin complexation on the electrochemistry of a macrocyclic Ni(II) complex has been studied. Both β-CD and γ-CD form 1:1 inclusion complexes with the Ni(II) complex. The interaction of the Ni(II) complex with γ-CD is stronger than with β-CD. The formation constant values show that the reduced form of the nickel complex (Ni(I)) is bound more strongly into the cavity of β-CD and γ-CD when compared to the oxidized form (Ni(II)). The Ni(II) complex experiences a less polar microenvironment which results in a positive shift in the peak potential. When the size and hydrophobicity of the supporting electrolyte molecules (NaClO₄, TMA⁺, TEA⁺ and TBA⁺) are increased, the interaction between the CD and Ni(II) complex decreases due to the inclusion of the supporting electrolyte molecule into the cavity of γ-CD. The formation constants calculated in the presence of different supporting electrolytes decrease with the increase in the size of the supporting electrolyte. Inclusion of both the supporting electrolyte and the Ni(II) complex into the cavity of γ-CD (ternary inclusion complex) is observed. β-CD forms a weak complex with Ni(II) when NaClO₄ is used as supporting electrolyte and the complexation was not observed when TBA⁺ is used as supporting electrolyte. The inclusion of a butyl chain of the TBA⁺ molecule into the cavity of β-CD hinders the entry of the Ni(II) complex into the cavity of β-CD.