Electrochemical behaviour of certain biomimetic copper(II) complexes in aqueous and aqueous micellar solutions

Voltammetric studies of biomimetic Cu(II) complexes of bis(pyrid-2-yl)dithia (N₂S₂) and -trithia (N₂S₃) and bis(benzimidazol-2-yl)dithia ligands in aqueous and sodium dodecylsulphate (SDS, anionic), cetyltrimethylammonium bromide (CTAB, cationic) and Triton X-100 [α-[p-(1,1,3,3-tetramethylbutyl)phenyl]-w-hydroxypolyoxyethylene(9.5), nonionic] micellar solutions have been carried out on a glassy carbon electrode. The electronic spectral properties of Cu(II) complexes in micellar solutions are consistent with their incorporation into hydrophobic microenvironments. For most of the complexes, the adsorption of Cu(I) species observed in aqueous solution is absent and the reversibility of the Cu(II)/Cu(I) couple is increased in SDS and Triton X-100 micellar solutions. From the dependence of I_pc values on SDS concentration, the micellar binding constant K_M has been estimated. Ligand enlargement and the structure of the Cu(II) complexes and Cu(I) forms of complexes enhance the binding with micelles. Compared with aqueous isotropic solutions, the E_1/2 values in micellar solutions either decrease or increase depending upon the nature of the surfactant and the hydrophobicity and structure of the complexes. The ratio of equilibrium binding constants K₊/K₂₊ of Cu(I) and Cu(II) species does not follow the same trend in SDS, CTAB and Triton X-100 solutions, indicating that the interaction of the cationic complexes depends on a delicate balance between hydrophobic and electrostatic interactions. Interestingly, for complexes with the CuN₂S₂ chromophore the Cu(I)/Cu(0) couple is also observed in Triton X-100 micelles.