Novel, selective and co-operative assembly of cyclodextrins around [1,8-bis(pyridin-2-yl)-3,6-dithiaoctane]copper(II)

Abstract

The redox chemistry of [CuL]²⁺[L = pdto = 1,8-bis(pyridin-2-yl)-3,6-dithiaoctane, bbdo = 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane, pttn = 1,9-bis(pyridin-2-yl)-2,5,8-trithianonane or pttu = 1,11-bis(pyridin-2-yl)-3,6,9-trithiaundecane] in the presence of α-, β- and γ-cyclodextrins (cd) in aqueous solution has been extensively investigated by cyclic and differential pulse voltammetric techniques. The addition of cyclodextrins to the complexes results in a substantial decrease in peak currents rather than in peak potentials. The i_pa rather than i_pc or ΔE_p or E_½ is very sensitive to the variation in the cyclodextrin concentration. The couple Cu^II–Cu^I of [Cu(pdto)]²⁺ tends to become reversible, as shown by the decrease in ΔE_p and that of i_pa/i_pc towards unity. Plots of i_pa, i_pc, E_pa and ΔE_pvs. the number of moles of cyclodextrin show sharp inflections, interestingly at 5, 4 and 3 mol of α-, β- and γ-cd respectively. These limiting values do not correspond to the usual inclusion complex formation by cyclodextrins but to the formation of novel and regular arrays around the complex, the number of molecules in the array being dictated by the size of the cyclodextrin. This also illustrates the prevention of adsorption of [Cu(pdto)]⁺ on the glassy carbon electrode. For the other complexes the changes in redox properties in the presence of cyclodextrins are not as regular and significant. Plots of changes in i_pa and i_pcvs. cyclodextrin concentration give Hill's coefficients greater than unity (1.3–2.1). The values of K₊/K₂₊ for all the complexes and K_a(K₂₊) for the complex formation of [Cu(pdto)]²⁺ with cyclodextrins have been determined and discussed. Significant reduction or enhancement in ε_max values has been observed both for the ligand-field and charge-transfer bands in the presence of all three cyclodextrins.