Bistable copper complexes of bis-thia-bis-quinoline ligands

The new ligands R,R-trans-S,S-bis[methyl(2'-quinolyl)]-1,2-dithiacyclohexane, cis-S,S-bis[methyl(2'-quinolyl)]-1,2-dithiacyclohexane, and 1,6-bis(2'-quinolyl)-2,5-dithiahexane have been synthesized and their complexes with Cu-I and Cu-II prepared. The ligand/metal systems are bistable, as the complexes with copper in both its oxidation states are stable under the same conditions as solids and in solution. The crystal and molecular structure of [Cu-I(1,6-bis(2'-quinolyl)-2,5-dithiahexane)]ClO4 has been determined by X-ray diffraction and reveals that the complex is monomeric, with the ligand folding around the Cu+ cation, imparting to it a tetrahedral coordination. UV-vis, MS-ESI, and NMR data indicate that the same is found for the Cu-I complexes of all three ligands. Also, the Cu-II complexes are monomeric, but with a square arrangement of the ligands around Cu2+. On changing the oxidation state, the change in the geometrical arrangement is fast and complete in less than 80 ms, as demonstrated by cyclic voltammetry experiments. In the CV profiles, the oxidation and reduction events take place at separated E, and E-red values, with no return wave even at the fastest scan rates. In the E-ox-E-red interval (which ranges from 450 to 650 mV, depending on the ligand), the ligand/copper system can thus exist in one of its two states, depending on its history, and thus display electrochemical hysteretical behavior. The electrochemical cycle leading from the tetrahedral [Cu-I(ligand)](+) to the square [Cu-II(ligand)](2+) complex (and vice versa) is reversible and repeatable without degradation, as checked by coupled UV-vis-controlled potential coulometry experiments.