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 CuI
and CuII 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 [CuI(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 CuI complexes of all three ligands. Also, the CuII
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 Eox
and Ered values, with no return wave even at the fastest scan rates. In the Eox−Ered 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 [CuI(ligand)]+ to the square [CuII(ligand)]2+ complex (and vice versa) is reversible and repeatable without
degradation, as checked by coupled UV−vis-controlled potential coulometry experiments.