Characterization and properties of the copper(II/I) complexes of macrocyclic hexathiaether ligand [21]aneS6

The copper(II/I) complexes of hexathiaether macrocyclic ligand, 1,4,8,11,15,18-hexathiacyclohenicosane (21]aneS₆), were synthesized, and characterized by electrochemical and spectroscopic techniques. Cyclic voltammetric studies indicate that Cu(21]aneS₆)^2+/+ forms a reversible one-electron redox couple. The electrochemical potential obtained for Cu(21]aneS₆)^2+/+ (E^f = 0.89 V, against SHE) was found to be the highest potential reported to date for a Cu^2+/+ macrocyclic system in aqueous solution. By employing the Nernst equation, we can infer that the practical upper limit for formal potential of Cu(II/I)L systems maybe close to this high value. Stability constant data obtained for these complexes indicate that Cu(21]aneS₆)⁺is 12 orders of magnitude greater in stability than that of Cu(21]aneS₆)²⁺ indicating the favorable nature of this large macrocyclic ligand towards formation of Cu(I) complexes over Cu(II) complexes. Crystal structure of Cu(21]aneS₆)⁺ ( Fig. 2) shows that four sulfurs adjacent to one another are coordinated to Cu⁺ ion in this complex. Bond angles and distances calculated for the crystal indicate that it is a distorted tetrahedron, a geometry commonly encountered by Cu(I) complexes. This is the first report of synthesis and characterization of a metal coordinated 21]aneS₆ complex.